Zoogeography of the coral reef fishes of the Socotra Archipelago

Journal of Biogeography (1998) 25, 919–933

Zoogeography of the coral reef fishes of the SocotraArchipelago

J. M. K Department of Biology, University of York, York, YO1 5 DD, U.K., e-mail: [email protected]

Abstract. Fish communities and habitats were studied at fish zoogeography of the region is found to be a distinctthe Socotra archipelago (Gulf of Aden, ≈12°N 54°E). south Arabian area, characterized by a ‘pseudo-high latitudeExtensive and unexpected hermatypic coral communities effect’ which results from seasonal cold water upwellingwere recorded, at the centre of a 2200 km gap in knowledge along the Arabian sea coasts of Yemen and Oman and theof species and habitat distributions which coincides with a Indian Ocean coast of Somalia. This south Arabian featurechange from a western Indian Ocean coral reef fauna to an is consistent across a wide range of fish families. It is mostArabian one. The fish assemblage associated with the pronounced in Oman and Yemen, and although it is theSocotra archipelago corals is predominantly south Arabian. dominant influence at Socotra it is slightly ‘diluted’ here byAn east African influence, minimal on the mainland coasts the east African influence. The south Arabian area wholly orof Arabia, is more evident here, and results in previously partly accounts for most of the major marine zoogeographicunrecorded sympatry between Arabian endemic species and features around Arabia, and is the principal featuretheir Indian Ocean sister taxa. A study of distributions of fragmenting Arabian coastal fish assemblages, andChaetodontidae (butterflyfishes) in the north-western Indian separating them from those of the wider Indo-west Pacific.Ocean reveals a number of distinct patterns, with a trend

Key words. Socotra, fishes, corals, south Arabia,for species replacement along a track from the northernRed Sea to the Indian Ocean. A major feature of the reef upwelling.

Arabian marine taxa, including corals (Scheer & Pillai, 1983;INTRODUCTION

Sheppard & Sheppard, 1991; Sheppard et al., 1992) andfishes (Klausewitz, 1972, 1989; Ormond & Edwards, 1987;The Socotra archipelago is situated to the east of the HornSheppard et al., 1992; Randall & Hoover, 1995). This paperof Africa, at the south-eastern end of the Gulf of Adendescribes the shallow sublittoral habitats and fish species(Fig. 1). The marine biogeography of these islands is ofassemblages of Socotra, Abd-al-Kuri, Darsa and Semha,particular interest for a number of reasons. First, coralthe four main islands of the Socotra archipelago. Thisreef habitats and fish species distributions are essentiallyinformation, and recent descriptions of the coastal fishes ofundescribed around the entire Horn of Africa, a 2200 kmOman (Randall, 1996) allows a reappraisal of the nature andlength of coast centred on the archipelago. This largelysignificance of marine zoogeographic barriers influencing theunstudied region coincides with a major change in coastalreef fish assemblages of Arabia.fish assemblages, from a characteristically western Indian

Several major marine zoogeographic features relevant toOcean fauna to one which is basically Arabian. Second, thesouthern Arabia and the Red Sea have been identified bycoasts of Somalia and southern Arabia are subjected toprevious authors. These are illustrated in Fig. 2, and include:seasonal cold water upwellings associated with the northern(i) a major barrier extending from the Horn of Africa north-Indian Ocean monsoon climate (Currie et al., 1973; Glynn,eastwards to the coasts of Iran/Pakistan (Klausewitz, 1972,1993). As a result almost the entire Somali coast is believed1989; Ormond & Edwards, 1987; Blum, 1989; Sheppardto have only scattered corals, with no significant coralet al., 1992); (ii) a centre of endemism along the southerngrowth from Djibouti until southern Somalia, and Kenyacoast of Oman (Randall & Hoover, 1995; Randall, 1996),(Sheppard & Sheppard, 1991; Sheppard et al., 1992). In thewhich may extend westwards into Yemen, where little workonly published study of the marine habitats of the Socotrahas been carried out; (iii) a Gulf of Aden/Red Sea barrier,archipelago, Scheer (1964, 971) described a macroalgaelocated variously at the Bab-al-Mandab, or within the Gulfdominated sublittoral community with only a few scatteredof Aden (Klausewitz, 1972, 1989; Briggs, 1974; Ormond &corals, on the basis of studies confined to the south coastEdwards, 1987; (iv) a barrier or region of vicariance insideof Abd-al-Kuri. This implies the presence of a 2200 km gapthe Red Sea at ≈20°N (Winterbottom, 1985; Blum, 1989;between east African and Arabian coral reef habitats. ThisRoberts et al., 1992). The lack of knowledge about shallowgap, and the cold upwelling, have been invoked as isolating

mechanisms promoting or maintaining endemism in many marine species assemblages and habitats along the Somali

1998 Blackwell Science Ltd 919

920 J. M. Kemp

FIG. 1. The location of the Socotra archipelago, showing positions of sites surveyed during the 1996 survey (numbered 1–20).

coast, both in the Gulf of Aden and the Indian Ocean, has between Arabian and Western Indian Ocean zoogeographicsubprovinces, but without explanation for this placement.been a cause of uncertainty about the nature and position

of these hypothesized barriers to dispersal. The only direct The marine environment of the entire Socotraarchipelago has been identified as having ‘very high priority’reference to the position of the Socotra archipelago in

relation to zoogeographic barriers is by Klausewitz (1989). for protection (Chiffings, 1995), on the basis of its locationat the boundary between the seasonal Somali upwelling toHe describes Socotra as lying to the north of a boundarythe south and the Gulf of Aden to the north. Improvedknowledge of the shallow marine habitats present at thearchipelago, and the fish species associated with them, willfill an important gap in available information about thezoogeography of western Indian Ocean and Arabian coralreef taxa, and will be significant for regional marineconservation planning.

METHODS

Sublittoral habitats were studied, by snorkelling or SCUBAdiving, at twenty coastal and near-shore sites throughoutthe Socotra archipelago, in March 1996 (Fig. 1). The fishcommunities at sixteen of these sites were assessed. The

FIG. 2. Major marine zoogeographic features in the southern Socotra survey was land-based, and all Socotra sites wereArabian and Red Sea region. (i) A major barrier between Arabia surveyed by snorkelling because transport of divingand the Indian Ocean. (ii) a centre of endemism on the Arabian equipment around the island proved impractical. Surveyssea coast of Oman. This probably extends to Yemen, Socotra and of Abd-al-Kuri, Semha and Darsa were undertaken from athe north coast of Somalia. (iii) A barrier between the Red Sea

boat, and the surveys of these islands include both snorkelledand southern Arabia, located variously in the Gulf of Aden, or atand dived sites. It was only possible to study three sites onthe Bab-al-Mandab. (iv) A barrier in the southern Red Sea, at

around 20°N. the south coast of Socotra, and one on the south coast of

Blackwell Science Ltd 1998, Journal of Biogeography, 25, 919–933

Coral reef fishes of Socotra 921

Semha. The south coasts of Darsa and Abd-al-Kuri were RESULTSnot visited.

This study includes data from four families of fishes, (a) Habitatswith an emphasis on the zoogeography of the

The climate of the Socotra archipelago is dominated by theChaetodontidae. The Chaetodontidae (butterflyfishes),northern Indian Ocean monsoons, the south-west monsoonPomacanthidae (angelfishes), Acanthuridae (surgeonfishes)from April to September and the north-east monsoon fromand Balistidae (triggerfishes) were selected as the primaryOctober to February. As a result the coastal and marinefamilies for study, for two reasons. Firstly, these groupsenvironments of the archipelago are subjected to a highlyhave relatively well-known Arabian distributions (Fraser-energetic regime for most of the year. In spite of this, coralBrunner, 1950; Burgess, 1978; Randall, 1983, 1996; Blum,communities, macroalgae, seagrasses and mangroves coexist1989; Barratt & Medley, 1990; Roberts et al., 1992), within close proximity. Two significant stands of mangroves are

the Gulf of Aden and Horn of Africa being the mostpresent at Socotra (monospecific Avicennia marina [Forsskal]

significant gap in knowledge. Secondly, they include aVierh). Turtle nesting beaches of at least regional significance

number of taxa endemic to the region, many of which haveare present at Abd-al-Kuri, and to a lesser extent at Socotra

clearly identifiable sister taxa in the adjacent Indian Ocean(Anonymous, 1996). Shallow marine habitats include

(Klausewitz, 1972, 1989; Burgess, 1978; Ormond & extensive areas of coral, providing habitat for abundantEdwards, 1987; Blum, 1989). coral associated fish species (Anonymous, 1996; Kemp,

Two types of site assessment were carried out, depending 1997).upon the amount of time available at any one site. These The morphology of the shallow sublittoral of thewere as follows. archipelago is characteristically a gently sloping or flat sea

bed, with a sandy gravel or rocky substrate. At sites where1. Qualitative assessment. Very limited time was available cliffs fall directly into the sea they rarely continue deeperat these sites, i.e. less than 15 min. A list of chaetodontid, than 5 m before reaching a level or nearly level sea floor.pomacanthid, acanthurid and balistid species was The exception to this is the Semha sublittoral, which iscompiled. steeper than that of the other islands. Hermatypic corals2. Quantitative assessment. Sites where between 1 and were present at all twenty sites studied in the present survey,4 h were spent in the water. A visual census transect of with extensive areas of moderate to high living coral coverchaetodontids and pomacanthids was carried out over (up to 85% but more typically 20% to 50%) recorded in250 m, following the depth contour, and all chaetodontids places on north coasts of all four islands. At sand beachand pomacanthids observed within 5 m either side of the sites on the north coast of Socotra and Abd-al-Kuri a sandycentral line of the transect identified and recorded. In inshore area frequently gives way to rocky substrate withaddition to the quantitative transect, an extended search > 30% living coral cover, at depths below 3–5 m. At southfor chaetodontid, pomacanthid, balistid and acanthurid coast sites the percentage coral cover was, with the exceptionspecies was carried out at these sites, and all species of south Semha, below 1%. Shallow benthic communitiesseen were recorded. Multiple quantitative transects were of the south coast of Socotra were dominated by macroalgae,carried out at two sites, where very different habitat resembling the south coast of Abd-al-Kuri as described bytypes were found in close proximity. The pomacanthid Scheer (1964; 1971), and areas of southern Oman (BarrattCentropyge multispinis Playfair and Gunther was not et al., 1984; Sheppard & Salm, 1988). On north coasts, incensused due to its cryptic colouring and behaviour, and contrast, macroalgae were much less apparent. The southwas simply recorded as present where it was observed. coast of Semha is dominated by large cliffs. Coral cover

here was low, and dense communities of smaller macroalgaeand invertebrates covered the rock surfaces. LargeWider species distributions presented in this paper aremacroalgae were much less in evidence here than on thebased on published records, on personal communicationssouth coast of Socotra.from colleagues, and on personal observations by the author.

Some records from the literature which are in doubt are notincluded, where species ranges are given without reference in

(b) Fish species assemblages and zoogeographyvarious publications, and require further corroboration.For example, Allen (1979) and Debelius (1993) record the Table 1 summarizes the species assemblages andpresence of Chaetodon nigropunctatus Sauvage (generally zoogeographic affinities of the four families of fishes studied.regarded as being endemic to the Arabian Gulf and Oman) Table 2 lists other fish species recorded during the recentin eastern Africa, without quoting sources. The same is true survey.of records of C. kleinii Bloch in the Red Sea (Burgess, 1978), A total of nineteen chaetodontid and pomacanthidwhich Allen (1979) and others do not record there. The species were recorded throughout the archipelago, of whichrelative abundance of C. kleinii at Socotra suggests that it three are endemic to Arabian seas at species or subspeciesmay occasionally occur around Arabia as isolated waifs, level. While these only constitute 17.6% of the eighteenbut considerable work in the region since 1978 has failed species recorded in quantitative survey transects (one speciesto record it there (Randall, 1983, 1994, 1996; Barratt & was only recorded outside transects), they numericallyMedley, 1990; Roberts et al., 1992; Randall & Hoover, heavily dominate the chaetodontid and pomacanthid

community, constituting 69.7% of all individuals recorded1995).


922 J. M. Kemp

TABLE 1. Species assemblages and zoogeographic affinities of the four families of fishes studied.

Family Species Distribution Abundance

Chaetodontidae Chaetodon collare Bloch NIO, SA UcChaetodon gardineri Norman NIO, SA CoChaetodon kleinii Bloch IWP CoChaetodon leucopleura Playfair NWIO, WI, SA CoChaetodon lineolatus Cuvier IWP, RS UcChaetodon lunula Lacepede IWP, SA CoChaetodon melannotus Bloch IWP, RS UcChaetodon melapterus Guichenot SA, AG, SRS V.AbChaetodon v pictus Fraser-Brunner SA, SRS V.AbChaetodon a. setifer Bloch IWP, SA UcChaetodon trifascialis Quoy & Gaimard IWP, SA, RS V.AbForcipiger sp. Brousonnet IWP, SRS RHeniochus acuminatus Linnaeus IWP, SA, AG Ab

Pomacanthidae Apolemichthys xanthotis Fraser-Brunner SA, RS AbCentropyge acanthops Norman WIO, SA UcCentropyge multispinis Playfair IWP, SA, RS UcPomacanthus imperator Bloch IWP, RS, SA AbPomacanthus semicirculatus Cuvier IWP, SA CoPomacanthus maculosus Forsskal WIO, SA,RS, AG UcPomacanthus asfur Forsskal RS, SA ∗

Acanthuridae Acanthurus dussumeri Valenciennes IWP, SA V.AbAcanthurus gahhm Forsskal RS, SA UcAcanthurus leucosternon Bennett IWP, SA CoAcanthurus lineatus Linnaeus IWP UcAcanthurus mata Cuvier IWP, SA, RS AbAcanthurus nigrofuscus Forsskal IWP, RS UcAcanthurus sohal Forsskal RS, SA, AG UcAcanthurus tennenti Gunther NIO, WIO, SA AbAcanthurus triostegus Linnaeus IWP CoCtenochaetus striatus Quoy & Gaimard IWP, SA, RS UcZebrasoma desjardini Bennett IO, RS, SA CoZebrasoma xanthurum Blyth RS, SA, AG V.AbNaso annulatus Quoy & Gaimard IWP UcNaso brachycentron Valenciennes IWP UcNaso brevirostris Valenciennes IWP, RS CoNaso fageni Morrow IWP UcNaso hexacanthus Bleeker IWP, RS UcNaso lituratus Bloch & Schneider IWP, RS, SA CoNaso unicornis Forsskal IWP, RS, SA Uc

Balistidae Balistapus undulatus Park IWP, RS, SA AbBalistoides conspicillum Bloch & Schneider IWP RBalistoides viridescens Bloch & Schneider IWP, RS RMelichthys indicus Randall & Klausewitz IWP, RS, SA V.AbOdonus niger Ruppell IWP, RS, SA, AG V.AbPseudobalistes flavimarginatus Ruppell IWP, RS UcPseudobalistes fuscus Bloch & Schneider IWP, RS RRhinecanthus assasi Forsskal RS, SA, AG CoSufflamen chrysopterus Bloch & Schneider IWP, SA AbSufflamen fraenatus Latrielle IWP, SA V.Ab

Distributions. IWP, Indo-west Pacific excluding Arabia; NIO, Northern Indian Ocean excluding Arabia; NWIO, North-western IndianOcean excluding Arabia; WI, western Indian Ocean excluding Arabia; IO, Indian Ocean excluding Arabia; SA, Southern Arabia and Gulfof Aden; RS, Red Sea; SRS, Southern Red Sea; AG, Arabian Gulf. ∗Ref. Debelius, 1993.

(n=1378). This is illustrated in Fig. 3, a bar chart of this behaviour in H. acuminatus in the Gulf of Oman, soidentification of H. diphreutes from the Socotra archipelagochaetodontid and pomacanthid abundances in all

quantitative transects. must be left open to question. Chaetodon jayakari Norman,present in the Gulf of Aden and at Oman, may be presentThe chaetodontid Heniochus diphreutes Jordan was

identified at one Abd-al-Kuri site, distinguished from H. here but will have been missed by this survey, being restrictedto water deeper than 20 m (Klausewitz & Fricke, 1985;acuminatus L. on the basis of behavioural traits

(planktivorous schooling). However, Randall (1996) reports Randall, 1996).



TABLE 2. Other fish species recorded during the recent survey.

Family Species Family Species

Stegastomatidae Stegastoma varium SebaRhincodontidae Rhincodon typus SmithCarcharhinidae Carcharinus longimanus Poey

Galeocerdo cuvier Peron and LesueurTriaenodon obesus Ruppell

Sphyrnidae. Sphyrna sp. 1.Sphyrna sp. 2.

Rhinobatidae Rhynchobatus djiddensis ForsskalDasyatidae Taeniura melanospila BleekerMyliobatidae Aetobatus narinari EuphrasenMobulidae Manta birostris Donndorf

Mobula sp.Muraenidae Echidna nebulosa Ahl

Gymnothorax chilospylus BleekerGymnothorax favagineus Bloch & SchneiderGymnothorax javanicus BleekerGymnothorax nudivomer Playfair & GuntherSiderea grisea Lacepede

Chanidae Chanos chanos ForsskalAntennariidae Antennarius sp.Holocentridae Myripristis murdjan Forsskal

Sargocentron spiniferum ForsskalSargocentron caudimaculatum Ruppell

Fistulariidae Fistularia commersoni RuppellScorpaenidae Pterois antennata Bloch

Pterois miles BennettScorpaenopsis diabolus Cuvier

Serranidae Pseudanthias squamipinnis PetersAethaloperca rogaa ForsskalCephalopholis argus Bloch & SchneiderCephalopholis miniata ForsskalCephalopholis sonnerati ValenciennesCephalopholis sexmaculata RuppellDermatolepis striolatus Playfair & GuntherEpinephelus fasciatus ForsskalEpinephelus flavocaeruleus LacepedeEpinephelus fuscoguttatus ForsskalEpinephelus multinotatus PetersEpinephelus tukula MorgansEpinephelus stoliczkae DayVariola louti Forsskal

Pseudochromidae Pseudochromis nigrovittatus BoulengerKuhliidae Kuhlia mugil Schneider.Priacanthidae Priacanthus blochii Bleeker

Priacanthus hamrur ForsskalCirrhitidae Paracirrhites forsteri Schneider

Cirrhitichthys oxycephalus BleekerCirrhitichthys calliurus Regan

Teraponidae Terapon jarbua ForsskalApogonidae Archamia fucata Cantor

Apogon aureus LacepedeApogon cyanosoma BleekerCheliodipterus macrodon LacepedeCheilodipterus quinquelineatus CuvierRhabdamia sp.

Carangidae Caranx ignobilis ForsskalCaranx melampygus CuvierCaranx sexfasciatus Quoy & GaimardCaranx lugubris PoeyTrachinotus blochii LacepedeScomberoides commersonianus Lacepede

Lutjanidae Lutjanus argentimaculatus ForsskalLutjanus bohar ForsskalLutjanus coeruleolineatus RuppellLutjanus ehrenbergi PetersLutjanus fulvus SchneiderLutjanus gibbus ForsskalLutjanus kasmira ForsskalLutjanus monostigma CuvierLutjanus rivulatus ForsskalMacolor niger Forsskal

Caesionidae Caesio xanthonota BleekerCaesio lunaris Cuvier

Haemulidae Diagramma pictum ThunbergPlectorhyncus gaterinus ForsskalPlectorhyncus playfairi PellegrinPlectorhyncus pictus TortonesePlectorhyncus schotaf Forsskal

Lethrinidae Lethrinus olivaceous ValenciennesLethrinus nebulosus ForsskalLethrinus mahsena ForsskalLethrinus variegatus ValenciennesMonotaxis grandoculis Forsskal

Sparidae Acanthopragus bifasciatus ForsskalDiplodus sargus Smith

Mullidae Mulloidichthys vanicolensis ValenciennesParupeneus barberinus LacepedeParupeneus bifasciatus LacepedeParupeneus cyclostomus LacepedeParupeneus forskalli Fourmanoir & GuezeParupeneus indicus ShawParupeneus macronema LacepedeParupeneus rubescens Lacepede

Pempheridae Pempheris oualensis CuvierPempheris sp

Monodactylidae Monodactylus argenteus LinnaeusKyphosidae Kyphosus vaigensis Quoy & Gaimard

Kyphosus cinerascens ForsskalEphippidae Platax orbicularis ForsskalPomacentridae Abudefduf sexfasciatus Lacepede

Abudefduf vaigiensis Quoy & GaimardAbudefduf sordidus ForsskalAbudefduf septemfasciatus CuvierAmphiprion sp.Amphiprion akallopisos Bleeker (refDebelius, 1993)Chromis dimidiata KlunzingerChromis flavaxilla RandallChromis weberi Fowler & BeanChrysiptera annulata PetersDascyllus carneus FischerDascyllus marginatus RuppellDascylus trimaculatus RuppellNeoglyphidodon melas CuvierPomacentrus caeruleus Quoy & GaimardPomacentrus leptus Randall & AllenPomacentrus trichrous Gunther

Labridae Anampses meleagrides ValenciennesBodianus macrognathos MorrisCheilinus fasciatus BlochCheilinus lunulatus ForsskalCheilinus trilobatus Lacepede

contd


924 J. M. Kemp

TABLE 2. contd

Family Species Family Species

Labridae Cheilo inermis ForsskalCoris africana SmithCoris aygula LacepedeCoris caudimacula Quoy & GaimardCoris frerei GuntherEpibulus insidiator PallasGomphosus caerulea LacepedeHalichoeres dussumieri ValenciennesHalichoeres hortulanus LacepedeHalichoeres iridis Randall & SmithHalichoeres marginatus RuppellHemigymnus fasciatus BlochHemigymnus melapterus BlochHologymnosus doliatus LacepedeLabroides bicolor Fowler & BeanLabroides dimidiatus ValenciennesThalassoma lunare LinnaeusThalassoma lutescens Lay & Bennett

Scaridae Chlorurus strongylocephalus BleekerScarus ferrugineus ForsskalScarus rubroviolaceus BleekerScarus sordidus Forsskal

Blenniidae Ecsenius pulcher MurrayPlagiotremus rhinorhynchos Bleeker

Gobiidae Amblyeleotris wheeleri Polunin & LubbockGobiodon axillaris De VisIstigobius decoratus Herre

Microdesmidae Gunnelichthys monostigma SmithSphyraenidae Sphyraena barracuda WalbaumSiganidae Siganus argenteus Quoy & Gaimard

Siganus sp.Zanclidae Zanclus cornutus LinnaeusMonacanthidae Aluterus scriptus Osbeck

Cantherhines sp.Ostraciidae Ostracion cubicus Linnaeus

Ostracion cyanurus RuppellOstracion meleagris Shaw & Nodder

Tetraodontidae Arothron hispidus LinnaeusArothron nigropunctatus Bloch & SchneiderCanthigaster valentini BleekerCanthigaster solandri RichardsonCanthigaster coronata Vaillant & Sauvage

Diodontidae Diodon liturosus ShawCyclichthys spilostylus Leis & Randall

FIG. 3. Bar chart of total abundances of chaetodontid and pomacanthid species recorded in quantitative surveys throughout the Socotraarchipelago in March 1996. Dark bars indicate Arabian endemics. Centropyge multispinis is not shown in this figure because it was notquantitatively surveyed.

DISCUSSION Socotra archipelago and continues south along the Africancoast. This pattern is reversed between April and Octoberduring the south-west monsoon, when the Somali CurrentCoral habitats of the Socotra Archipelagoflows north along the African coast and passes Socotra and

The islands are swept by two major currents through the south Arabia before turning eastwards in the Arabian Sea.course of the year (Fig. 4). From November to March Over this period an upwelling of cold nutrient-rich waterthe north-east Monsoon Current flows westwards across occurs along the Indian Ocean coast of Somalia, with waterthe Arabian Sea, from India to southern Arabia, where it temperatures as low as 13 °C (Currie et al., 1973). The

northwards flow of the Somali current means that the souththen flows south-west along the Arabian coast, passes the



FIG. 4. Prevailing currents in the north-western Indian Ocean. (i) November – March. (ii) April – October.

coasts of the Socotra archipelago are more directly exposed corals at most sites were Acropora Oken, with shallow sitesoften dominated by dense arborescent thickets, and deeperto the effects of this upwelling than the north coasts. During

the north-east monsoon the north coasts are exposed to the sites by large tabulate corals (Kemp, 1997). Hermatypiccorals at most sites were frequently diverse at the genericprevailing current, but the water temperatures are higher,

in the region of 27 °C. The sublittoral habitats of the Socotra level, in spite of the domination by Acropora in terms ofpercentage cover. High cover of soft corals was recorded atarchipelago include extensive coral communities present on

the northern coasts, sheltered from the cold water of the many deeper Abd-al-Kuri sites. Based on the findings ofthis survey, hermatypic coral communities may be verySomali current. These coral-rich areas provide habitat for

a characteristically south Arabian coral reef fish fauna. This extensive to the north of Socotra and Abd-al-Kuri, wherein places shallow waters extend several kilometres offshore.is the most south-easterly location of this type of community

so far recorded. These coral habitats constitute an important Structurally, the coral communities of the Socotraarchipelago resemble those of the southern Arabian coast.link between the coral reefs of Arabia and those of the

wider Indian Ocean. Sheppard & Salm (1988) described the coral communitiesof Oman as being affected by the presence of the ‘pseudo-The discovery of these coral communities at the Socotra

archipelago is surprising. Scheer (1971) recorded nine genera high latitude effect’ in the western Arabian Sea. This effectis comprised of low water temperatures and raised nutrientof hermatypic corals from the south coast of Abd-al-Kuri,

describing a sparsely scattered coral community in an area levels due to the coastal upwelling, and results in a relativedominance of large macroalgae, which compete with coralsdominated by macroalgae. In a synthesis of available

records, Rosen (1971) recorded eleven genera of hermatypic for light and space. This ‘pseudo-high latitude effect’ resultsin weakly developed reefs and reef framework at Oman,corals from the region of ‘Cape Gardafui – Socotra’. A

total of thirty-two genera of hermatypic corals were recorded with reduced coral diversity and a sparse coral cover undera canopy of macroalgae. This description fits the southin the present survey (Anonymous, 1996). An additional

genus (Herpolitha Eschscholz) was recorded by Rosen coast communities of Socotra more closely than those of thenorth, particularly with reference to a canopy of macroalgae,(1971), bringing the total number of genera to thirty-three,

approaching the level of generic diversity predicted for this which was almost entirely absent on the north coasts of theSocotra archipelago at the time of the recent survey. Theregion by Rosen (1971) and Veron (1995). This total is

considerably lower than the forty-seven genera recorded inference is that the pseudo-high latitude effect in Oman,and on the south coasts of the Socotra archipelago, is morefrom Oman (Sheppard & Salm, 1988), and more detailed

work in the archipelago will probably record a greater severe than that on the northern side of the archipelago.This is supported by records of Sea Surface Temperaturesdiversity of coral genera.

Biogenic reef structures are rare throughout the recorded over the period of 1980–96 (NOAA data, providedby the Climate Diagnostics Center (www.cdc.noaa.gov)archipelago, only being present at two sites studied, and of

very limited extent at these sites. Even at the highest cover Slutz et al., 1985, Woodruff et al., 1993), which reveals aconsistently lower temperature at the coast of southerncoral sites the corals were generally growing directly on

eroded ancient reef or nonreef substrate. The dominant Oman during the upwelling, relative to that at Socotra.


926 J. M. Kemp

The pronounced difference between the development ofcoral communities on the northern and southern coasts ofthe archipelago may be due to two factors; nutrients andtemperature. Raised nutrient levels negatively affect corals,largely through promotion of the growth of macroalgae,which is dominant on hard substrates of the south coast ofAbd-al-Kuri (Scheer, 1964) and of Socotra. Competition

FIG. 5. Dendrogram of chaetodontid assemblage relationships inbetween corals and macroalgae is known to be a factorthe north-western Indian Ocean and Arabian seas, produced usinglimiting coral growth in parts of the Arabian region,Wards Method of cluster analysis (see text). Cluster 1 includes

including the Arabian Gulf (Coles, 1988) and Oman northern, central and southern Red Sea, and the western Gulf of(Sheppard & Salm, 1988). The same is likely to be true at Aden (grouping communities across the Bab-al-Mandab), Clusterthe Socotra archipelago. 2 includes Oman, Socotra and the Arabian Gulf and Cluster 3

includes east Africa, the Seychelles and the Maldives.Optimal temperatures for coral growth are in the range25–29 °C (Wells, 1957; quoted in Rosen, 1971), andtemperatures below≈18 °C, a full 5 °C above the minimumfor the south-west monsoon upwelling off Somalia, areknown to severely inhibit coral growth and cause coral Indian Ocean region are summarized in Table 3. This reveals

a clear trend for species replacement along a track from themortality (Glynn & Stewart, 1973; Glynn & D’Croz, 1990).Rosen (1971) notes a relationship between minimum northern Red Sea to the Indian Ocean. Some authors

have suggested the Bab-al-Mandab as the location of aprevailing sea water temperature and generic diversity ofIndian Ocean corals, with diversity reduced at lower zoogeographic barrier (Briggs, 1974; Klausewitz, 1989). A

cluster analysis of chaetodontid species assemblages fromtemperatures. From Rosen’s figures a generic diversity ofthirty-three, as recorded at the Socotra archipelago, would seven Arabian regions and three northern and western

Indian Ocean regions (Fig. 5) reveals that, for theindicate a minimum prevailing temperature of 20–22 °C, afurther indication that the coral communities of the north Chaetodontidae at least, the Bab-al-Mandab is in fact of

minimal significance. Fig. 5 is based upon the data incoasts of the archipelago may be relatively sheltered fromthe effects of the cold Somali Current upwelling (but see Table 3, using Wards Method of hierarchical agglomerative

clustering, which calculates the squared Euclidean distanceSheppard et al. (1992) for comments on temperatureadaptation in Arabian corals). to the cluster means for each case, and sums them for all

cases. At each step the clusters which, when merged, giveGlynn (1993) reported extensive predation of Acroporain Oman by the Crown of Thorns starfish Acanthaster the smallest increase in the total sum of squared within-

cluster distances, are merged. The chaetodontid distributionplanci L., noting that Acanthaster has minimal impact uponPocillopora corals there. He suggested that periodic data produces three clusters, which are extremely robust

across different clustering methods. Cluster 1 includes theoutbreaks of this coral predator limit reef growth in theregion. No evidence of Acanthaster was found at the Socotra northern, central and southern Red Sea, and the western

Gulf of Aden, thus including species assemblages from botharchipelago during the present survey. The Acroporidae arerelatively scarce in Oman (Sheppard & Salm, 1988) but are sides of the Bab-al-Mandab. Cluster 2 includes southern

Oman, Socotra and the Arabian Gulf, and Cluster 3 includeswidely distributed throughout the Socotra archipelago, andvery dominant in many north coast coral communities. east Africa, the Seychelles and the Maldives. A lack of

adequate distributional data from the Gulf of Aden andExtensive monogeneric areas of Pocillopora Lamarck arerecorded from Oman (Sheppard & Salm, 1988; Glynn, 1993; southern Red Sea precludes carrying out a similar analysis

for the Pomacanthidae, Acanthuridae and Balistidae.Salm, 1993), but only scattered colonies of this genus wererecorded in the Socotra survey. The dominance of Acropora Fig. 6 summarizes a number of basic distributional

patterns in the Chaetodontidae, which become apparentover Pocillopora throughout the Socotra archipelago, andthe absence or rarity of Acanthaster, suggests that from the data in Table 3 and Fig. 5. These patterns are not

linked to obvious differences in dispersal ability or type ofAcanthaster is probably not a factor limiting reefdevelopment there. The complete absence of the Fungiidae spawning. All chaetodontids are believed to have a similar

extended planktonic dispersal phase as larvae (Burgess,at Oman, which were common at some sites at the Socotraarchipelago, where two genera were recorded (Anonymous, 1978; Allen, 1979). The most significant of the illustrated

distribution patterns for the fish assemblage of Socotra is1996), further illustrates the divergence between the twoareas. 6.iv, the south Arabian distribution. This is likely to be an

extension of the centre of endemism in Oman (solid blackarea in Fig. 6.iv) identified by Randall & Hoover (1995).

The coral reef fish communityThe distinctive south Arabian fish community, identified inFig. 5, is ‘diluted’ in areas near the margins (Socotra, westernThe Socotran reef fish community is essentially south

Arabian, the chaetodontid community of the archipelago Gulf of Aden), where some south Arabian species are absent(e.g. Chaetodon dialeucos Salm & Mee), and species fromis distinctively so (Fig. 5), and the majority of other species

recorded in the present survey are known to occur at Oman. adjacent areas are present (e.g. C.kleinii at Socotra,C.larvatus Cuvier, C.mesoleucos Forsskal, andMany of these species are endemic to Arabian seas.

Distributions of chaetodontid fishes in the north-western C.semilarvatus Cuvier at Djibouti and Aden). The Gulf of



TABLE 3. Chaetodontid species present in east Africa, the Seychelles and the Maldives, but not recorded in Arabia or Socotra.

SPECIES NRS CRS SRS WGf Om Sc A.Gf M/SL Sey E.Af

C.auriga auriga Forsskal Χ Χ ?C.paucifasciatus Ahl Χ Χ ? Β

C.fasciatus Forsskal Χ Χ Χ Χ

H.intermedius Steindachner Χ Χ Χ Χ

C.larvatus Cuvier Χ Χ Χ Χ Β

C.austriacus Ruppel Χ Χ Χ Β

C.semilarvatus Cuvier Χ Χ Χ Χ Β

C.mesoleucos Forsskal Χ Χ Χ

C.melapterus Guichenot Χ Χ Χ Χ Χ Β

C.v.pictus Fraser-Brunner Χ Χ Χ Χ Β

C.leucopleura Playfair Χ Χ Χ Χ Χ Χ

C.gardineri Norman Χ Χ Χ Χ

C.collare Bloch Χ Χ Χ

C.auriga setifer Linnaeus ? ? Χ Χ Χ Χ Χ

C.lunula Lacepede Χ Χ Χ Χ Χ

H.acuminatus Linnaeus Β Χ Χ Χ Χ Χ Χ

C.nigropunctatus Sauvage Χ Χ

C.dialeucos Salm & Mee Χ

H.zoster Bennett Χ Χ Χ Χ

C.kleinii Bloch Χ Χ Χ Χ

C.blackburni Desjardins Χ

C.dolosus Ahl Χ

C.rostratus Linnaeus Χ

C.triangulum Cuvier Χ Χ

F.longirostris Broussonet Χ Χ

C.zanzibariensis Playfair Χ Χ

C.bennetti Cuvier Χ Χ Χ

C.citrinellus Cuvier Χ Χ Χ

C.falcula Bloch Χ Χ Χ

C.guttattissimus Bennett Χ Χ Χ

C.madagascariensis Ahl Χ Χ Χ

C.meyeri Solander Χ Χ Χ

C.trifasciatus Park Χ Χ Χ

C.unimaculatus Bloch Χ Χ Χ

C.v.vagabundus Linnaeus Χ Χ Χ

C.xanthocephalus Bennett Χ Χ Χ

F.flavissimus Jordan & McGregor Β Β Χ Χ Χ

H.monoceros Cuvier Χ Χ Χ

C.melannotus Bloch Χ Χ Χ Χ Χ Χ Χ Χ

C.lineolatus Cuvier Χ Χ Χ Χ Χ Χ Χ Χ

C.trifascialis Quoy & Gaimard Χ Χ Χ Χ Χ Χ Χ Χ Χ

KEY. Χ, Present ; o, Waifs only (see text); ?, uncertain.NRS, Northern Red Sea; CRS, Central Red Sea; SRS, Southern Red Sea; WGf, Western Gulf of Aden; Sc, Socotra; Om, Oman A.Gf,

Arabian Gulf; M/SL, Maldives/Sri Lanka; Sey, Seychelles; E.Af, East Africa.

Oman and the Arabian Gulf are northern extensions of the absence of a number of species from southern Arabia andthe Socotra archipelago raises the possibility that Red Seasouth Arabian area.

Other patterns with particular significance to Socotra are populations of some Indo-west Pacific species may begenetically isolated.the following.

6.v. Indo-west Pacific, excluding southern Arabia. This 6.vii Indian Ocean and Socotra, with a minor extensionto southern Arabia. This distribution gives rise to sympatrypattern is significant for two reasons. First, it indicates that

the Socotra archipelago is at the margins of the southern between a number of Indian Ocean species and theirArabian endemic sister taxa, in families other than theArabian region, being home to a number of species which

do not inhabit southern Arabia in spite of being present in Chaetodontidae. Sympatry between the pomacentridsDascyllus carneus Fischer and D. marginatus Ruppell wasthe Red Sea and Indian Ocean. Second, it suggests that

the coral associated communities of Socotra may form observed on the north side of Darsa. This was the only siteat which D. carneus was recorded throughout the survey.an important link between Red Sea and Indian Ocean

populations of these species, possibly playing a role in Although both species were abundant at this site, therewas clear separation within the site. In the Acanthuridaemaintaining gene flow between the two regions;

6.vi Disjunct Red Sea/Indian Ocean distributions. The sympatry between Acanthurus sohal Forsskal and


928 J. M. Kemp

FIG. 6. Main distribution patterns for reef fishes in the North-western Indian Ocean and Arabian seas. (i) Northern and central Red Sea.(ii) Entire Red Sea. (iii) Pan Arabian, including Socotra. (iv) South Arabia, with a minor extension the southern Red Sea. The centre ofendemism in Oman identified by Randall and Hoover (1995) is represented by the solid black area. (v) Indo-west Pacific, excluding southernArabia. (vi) Disjunct Red Sea/Indian Ocean distributions. (vii) Indian Ocean and Socotra, with a minor extension to southern Arabia. (viii)Indian Ocean excluding Arabian seas.

Acanthurus lineatus L. also occurred at the archipelago. One and easily identified chaetodontid subspecies are endemicto Arabian seas. Chaetodon auriga auriga Forsskal isA. lineatus was observed at Socotra, and two individuals

were seen at one Abd-al-Kuri site. The two at Abd-al-Kuri endemic to the Red Sea (Allen, 1979). The widespread Indo-west Pacific subspecies Chaetodon auriga setifer Bloch iswere associated with a group of A. sohal. Acanthurus sohal

is an Arabian endemic, and A. lineatus is its putative Indo- present at both Socotra and Oman. Chaetodon vagabunduspictus Fraser Brunner, abundant at Socotra, is endemic toPacific sister species, and is not recorded from Arabia.

Identification of chaetodontids to subspecies is important southern Arabia (Fraser-Brunner, 1950; Burgess, 1978).C. vagabundus vagabundus Linnaeus is the widespreadin studies of Arabian marine zoogeography. Two distinctive



Indo-west Pacific sister taxon to this subspecies (see Gill, are referred to as waifs. For some species of significance tostudies of Arabian marine zoogeography, it is importantin press, for further comments on the zoogeographic

significance of sub-species identification). The pomacanthid to differentiate, where possible, between records which areprobably of waifs, and the presence of establishedassemblage of the Socotra archipelago is identical to that

of Oman (Randall, 1996), although a more pronounced populations. Examples relevant to Socotra includeChaetodon melapterus Guichenot, an obligate corallivoreeast African influence is arguably apparent, with three

individuals of the east African species Centropyge acanthops and the most abundant chaetodontid at hard coral sites inthe archipelago. Randall (1994) showed that the type localityNorman observed at a single Abd-al-Kuri site. At Oman

only a single individual has been recorded (Randall, 1996). for this species (Reunion) is probably an error. The onlyother non-Arabian record for the species is of a juvenileThe presence of the pomacentrid Dascyllus carneus in large

numbers at Darsa, and of Chaetodon kleinii, Balistoides from the Seychelles (Ahl, 1923). In the absence of furtherrecords the isolated Seychelles specimen should be regardedconspicillum Bloch and Schneider and Acanthurus lineatus

reflects a similar influence of the east African species as a waif, and not as being indicative of the existence ofany established populations of the species outside Arabianassemblage in the Pomacentridae, Chaetodontidae,

Balistidae and Acanthuridae. It is reasonable to assume seas. Another Arabian endemic, in spite of an isolatedrecord from the Indian Ocean, is the blue tang Zebrasomathat more detailed studies will show this influence to occur

across a broad range of families. Two consequences of xanthurum Blyth. A single adult has been recorded fromthe Maldives (Randall & Anderson, 1993).this are, firstly, a noticeable overlap between distinctively

Arabian species assemblages and those of the eastern Records of waifs within the Arabian region fall into twocategories. Firstly, species which are Arabian endemics butIndian Ocean and, secondly, the consequent occurrence of

previously unrecorded sympatry between Arabian endemic have restricted distributions around Arabia have beenrecorded in parts of Arabian seas which are beyond theirspecies and their more widely distributed Indian Ocean or

Indo-Pacific sister taxa. This type of sympatry appears to normal range, e.g. Chaetodon paucifasciatus Ahl at Djibouti(Fraser-Brunner, 1950), and Chaetodon austriacus Ruppell,be restricted to the Socotra archipelago, and given the small

number of taxonomic groups studied during the present C. semilarvatus and C. larvatus from Oman (Randall, 1996).Secondly, non-Arabian species which are common elsewheresurvey, and the relatively small number of sites visited, it is

probable that more instances will be recorded here in future. in the Indian Ocean but do not have established Arabianpopulations also occasionally occur as waifs around Arabia,The Indian Ocean influence is less evident in southern

Oman, with the fish community of the Socotra archipelago e.g. Centropyge acanthops, Acanthurus leucosternon Bennettand Ctenochaetus strigosus Bennett from Oman (Randall,including a number of widespread Indo-Pacific or Indian

Ocean species common in the Red Sea, but absent from 1996).southern Arabia (Fig. 6.v).

The factors limiting distributions of fishes in the SocotraZoogeographic barriersregion may differ between species, with recruits limiting

some, and habitat availability limiting others. The ArabianPicasso triggerfish Rhinecanthus assasi Forsskal is The Gulf of Aden

The Gulf of Aden acts as a barrier between north andmoderately uncommon throughout the archipelago, beingobserved at only two sites. A single adult was recorded at south, and between east and west. A number of species

which were moderately abundant at the Socotra archipelagoSocotra, but at Abd-al-Kuri a population of more thantwenty, including subadults and juveniles, was recorded at are not recorded from Arabia (e.g. Chaetodon kleinii and

Dascyllus carneus), or are only known to occur as waifs,one site. This group was in an area closely resembling thereef flat habitat this species is commonly associated with in (e.g. Acanthurus leucosternon, Centropyge acanthops). A

further five species of Acanthurinae, four Nasinae, and fivethe northern and central Red Sea, but which is rare in theSocotra archipelago. The presence of several juveniles here Balistidae which are not recorded from Oman were present

at Socotra. Significant differences between the nutrientsuggests that a lack of suitable habitat is the limiting factorfor this species at the archipelago, rather then a lack of regimes and/or water temperatures at Oman and Socotra,

indicated by a marked temporal difference in the growthrecruits. The same may be true of other species withsimilar habitat requirements. Conversely, in the anemone cycles of macroalgae in the two areas (Kemp, 1998), may

be a significant factor in promoting the divergence betweencommensal pomacentrid genus Amphiprion, recruits ratherthan habitat are probably the limiting factor. Only one fish assemblages of Socotra and southern Arabia.

The divergence between species assemblages in the eastAmphiprion species was recorded in the present survey. Thiswas uncommon at the archipelago, and fewer than 20% of and west of the Gulf of Aden is as striking as that between

north and south, although coral communities on the Somalisuitable anemones were occupied by fish.The planktonic dispersal stage in the life cycles of the coast of the Gulf of Aden may be more widespread

then previously believed (Obura, 1997). Seven species ofmajority of coral reef fish species (as eggs and/or larvae)means that the larval range of many reef fish is likely to be chaetodontid fishes are regarded as being of Red Sea origin

(Klausewitz, 1972, 1989; Ormond & Edwards, 1987; Blum,greater than that of the established adult populations (Leis,1991). Occasionally reef fish larvae will settle and survive 1989; Righton et al., 1996). Of these, five are abundant at

Djibouti in the western Gulf of Aden (Barratt & Medley,in an area outside the normal range of their species. Suchfish are rare, are usually solitary because of that rarity, and 1990), three are recorded as waifs from Oman (Randall,


930 J. M. Kemp

1996), and none were recorded at the Socotra archipelago. The east coast of SomaliaThe fish community of Socotra is strikingly different fromAll other chaetodontids found at Djibouti were present atthat of Kenyan reefs 1000 km to the south. Some ofSocotra. The Arabian endemics Chaetodon melapterus andthe most abundant species at Socotra, such as ChaetodonC. vagabundus pictus were the most abundant chaetodontidsmelapterus and Zebrasoma xanthurum, are not recorded inthroughout the Socotra archipelago (Fig. 3). Both areKenya. There are many more species where the opposite iscommon at Djibouti (Barratt & Medley, 1990) and alsothe case. Table 3 records eighteen chaetodontid speciesoccur in the south of the Red Sea (Randall, 1994; J. E.present in the adjacent areas of east Africa, the SeychellesRandall, personal communication) This suggests that theand the Maldives which have not been recorded from Arabiaeast–west barrier in the Gulf of Aden is less effective ator Socotra (Burgess, 1978; Allen, 1979; Blum, 1989).keeping south Arabian species out of the Red Sea and

The probable lack of significant coral habitats in thewestern Gulf of Aden than it is at keeping Red Sea species1000 km directly south of Socotra will restrict opportunitiesaway from Oman and Socotra. Many species occurringfor gradual ‘stepping-stone’ colonization northwards by eastaround southern Arabia and Socotra may be preventedAfrican species, necessitating dispersal in a single 1000 kmfrom colonizing northwards into the Red Sea by anotherleap to Socotra. The temporal coincidence of the coldfactor or factors, including clines in salinity and temperatureupwelling during the south-west monsoon and the only(Robinson 1979, quoted in Edwards, 1987; Roberts et al.,opportunity for northwards dispersal of larvae from east1992; Sheppard et al., 1992), and reef habitat changes insideAfrica on the Somali Current probably greatly reduces thethe Red Sea at around 20°N (Roberts et al., 1992). Thuschances of success in surviving this leap, and plays a majorthe divergence between the south Arabian and Red Sea fishrole in reducing dispersal to Socotra from east Africa. Thefaunas may be maintained by two barriers rather than apseudo-high latitude effect at the archipelago, although lesssingle Bab-al–Mandab barrier. One of these barriers ispronounced than it is in Oman, means that the marine

located in the western Gulf of Aden, restricting eastwardenvironment of Socotra may be marginal for postsettlement

colonization of Red Sea species. A second is located insurvival of many Indian Ocean species which do survive

the southern third of the Red Sea, preventing northward the journey north.colonization by southern Arabian species. According to this There are two possible explanations for the fact thathypothesis there should be a discernible region of overlap the populations of species such as Chaetodon kleinii andbetween southern Arabian and Red Sea species assemblages, Balistoides conspicillum at the Socotra archipelago are notin the far west of the Gulf of Aden and in the southern capable of stocking southern Arabia. Firstly, theRed Sea. This is supported by Randall’s (1994) records of reproductive success of such species at the archipelago maytwenty species in the southern Red Sea previously only be very low. Being at the north-western extreme of theirknown from east of the Bab-al-Mandab, and by the range, some of these species (e.g. B. conspicillum) arecharacteristically southern Red Sea assemblage at Djibouti relatively uncommon at the archipelago, and may have low(Fig. 5). Sympatry of south Arabian and Red Sea sister success finding a mate at spawning time (Pyle & Randall,species is also predicted, supported by observations of 1994). In combination with mortality approaching 100% insympatry of Chaetodon austriacus and C. melapterus in the the planktonic larval phase (Leis, 1991), this may accountsouthern Red Sea (Randall, 1994). for the absence or rarity of some species in Oman.

The Bab-al-Mandab was almost certainly the site of a Alternatively, the more pronounced upwelling and resultantsignificant Pleistocene vicariant event, which gave rise to a pseudo-high latitude effect in southern Arabia relative tonumber of species now endemic to the Red Sea or Arabia that at the Socotra archipelago may prevent postsettlement(Winterbottom, 1985; Klausewitz, 1989). However, a barrier survival of some species. This is likely to be the case for

the more abundant Socotra species absent from Arabia,probably no longer exists at the Bab-al-Mandab, thesuch as C. kleini.divergence between the Red Sea and south Arabian

Randall (1996) provides the first comprehensive accountassemblages now being maintained by a diffuse and probablyof the coastal fishes of Oman, in which he records theclinal barrier in the southern Red Sea south of 20°N, andpresence of some Indian Ocean taxa not previously knowna further barrier to the east of the Bab-al-Mandab. Theto reach Arabia, including butterflyfishes, angelfishes andidentification of the strait as the location of a modernsurgeonfishes. The present Socotra survey records additionalbarrier in some of the literature may be due to inadequatespecies characteristic of reefs to the south of the Somaliasampling in the southern Red Sea south of the Farasan andbarrier, but here occurring to the north. These recordsDahlak archipelagos, and in the Gulf of Aden. Randallindicate a slightly greater degree of permeability in both(1994) remarks that the south of the Red Sea is under-the Somali barrier and across the Gulf of Aden from northrepresented in surveys and collections, and that ‘ . . ..it isto south than was previously thought to be the case. Thereobvious that more fieldwork in the south [of the Red Sea]is a corresponding increase in the significance of the Gulfwill result in the discovery of new additions to the Red Seaof Aden as an east–west barrier, since it is this barrier whichfish fauna’. It is equally clear that the opposite is also theexcludes these species from the western Gulf of Aden andcase: more fieldwork in the Gulf of Aden will certainlythe southern Red Sea.result in the discovery of more species previously only

recorded from inside the Red Sea. Until such work is Disjunct Red Sea/Indian Ocean distributionsundertaken the location of a faunal break at the Bab-al- The recent survey was relatively limited in duration, so the

temptation to draw firm conclusions about the absence ofMandab, rather than either side of the strait, is unsupported.



any particular fish species from the archipelago must be although the southern end of the seasonal Somali upwelling,at ≈2°30N (Currie et al., 1973) may be significant. Theresisted. Nevertheless, the widespread Indo-west Pacific

angelfish Pygoplites diacanthus Boddaert is conspicuous in southern boundary of the south Arabian region will bedefined by the disappearance of south Arabian coral reefits absence from the Socotra survey data, and from Oman.

This is the most abundant large pomacanthid in the northern taxa, and the replacement of some species by their IndianOcean sister species. All of the distribution patternsand central Red Sea, and it is common at Djibouti (Barratt

& Medley, 1990; Roberts et al., 1992). Its absence from illustrated in Fig. 6, except for 6.i, may be partly or whollyexplained by the hypothesized south Arabian region. SpeciesOman and Socotra suggests that there is a significant gap

in its distribution, between the Indian Ocean and the Red with disjunct distributions, and those which are excludedfrom the Arabian region altogether, are the most constrainedSea. Other species which appear, on the basis of this limited

survey, to demonstrate similarly disjunct distributions by environmental conditions prevailing in the southernArabian region. These species are at one end of a continuuminclude the Pomacentrids Dascyllus aruanus L. and

Pomacentrus sulphureus Klunzinger. Red Sea and Indian from highly environmentally or ecologically constrained(stenoecious) species to less constrained (euryoecious)Ocean populations of species such as these may be

genetically isolated from each other. Colouration differences species. Species slightly less constrained by southern Arabianconditions include those which occur in the Red Sea, Socotrabetween Red Sea and Indo-Pacific populations of P.

diacanthus (Allen, 1979) provides some evidence that this and the Indian Ocean, but are absent at Oman wherethe pseudo-high latitude effect is most pronounced. Themay be the case, at least for this species. Chaetodon jayakari

also has an apparently disjunct distribution, although in chaetodontid subgenus Corallochaetodon Burgess illustratesthe hypothesized south Arabian region well. The Red Seathis case the gap is the length of the Red Sea rather than

around the Horn of Africa. Klausewitz & Fricke (1985) endemic species Chaetodon austriacus is unable to colonizeeffectively outside the northern and central Red Sea, in spitestate for this species that ‘Without any doubt there exists a

steady gene flow between populations of the Indian Ocean of the presence of waifs in southern Oman. Chaetodonmelapterus is restricted to the southern Arabian environmentand the Red Sea [Gulf of Aqaba]’. This type of statement

cannot be supported without either genetic studies, or new as illustrated in Fig. 6.iv, and the Indian Ocean speciesChaetodon trifasciatus Park is abundant in adjacent areasrecords filling in the distribution gap. In the absence of

evidence to the contrary such species should be regarded of the Indian Ocean, but is unknown from Arabia.as genetically isolated. It is important to adopt this‘precautionary’ approach in contexts which may have

CONCLUSIONSimplications for biodiversity conservation. At the extreme,sibling species, which are morphologically indistinguishable The coral communities of the Socotra archipelago most

closely resemble those of Oman (Sheppard & Salm, 1988;but genetically or behaviourally isolated, are known tooccur in many marine taxa (Knowlton, 1993). With high Glynn, 1993; Salm, 1993; Randall, 1996), although there

are some major differences. At the Socotra archipelago thelevels of endemism in many marine groups, the Arabianregion and the Red Sea are among the most likely areas dominance of Acropora corals, the presence of the

Fungiidae, the lack of extensive areas of Pocillopora, andin the Indo-Pacific for allopatric development of as-yetunrecognized sibling species. the apparent absence of Acanthaster outbreaks in the recent

past all suggest that different forces shape the coralcommunities in the two regions. Significant differences in

The Gulf of Aden and Somali barriers are thethe ecology of macroalgae provides further evidence for

boundaries of a southern Arabian regioncontrasting conditions in Socotra and southern Arabia.

Zoogeographic barriers identified in the Gulf of AdenThe two areas of vicariance described above, in the Gulf ofAden and along the Somali coast, are most parsimoniously and along the eastern Somali coast probably mark the

western and southern boundaries of a distinct south Arabianexplained as being the western and southern boundaries ofa distinct southern Arabian region. This approximates to region, with oceanographic conditions and resultant

habitats unsuitable for many species living in the Red SeaKlausewitz’s (1972) south Arabian section of the Arabiansubprovince, and is illustrated in Fig. 6iv. This region of and Indian Ocean to either side. It is probable that the

southern Arabian region hypothesized here is an extensionpseudo-high latitude effect corresponds to the Socotra-Oman cluster of chaetodontid assemblages in Fig. 5. It of the centre of endemism in Oman noted by Randall &

Hoover (1995). Lying near the southern edge of the southextends from the western Gulf of Aden to Socotra, andnorthwards along the coast of Arabia to include the whole Arabian influence, the Socotra archipelago is home to a

number of Indian Ocean species which are at the north-of the coast of Oman at least as far north as Ras al Hadd.The Arabian Gulf and Gulf of Oman form a northern western extreme of their ranges, and which are prevented

from colonizing Arabia itself by the increasing severity ofextension of the south Arabian environment, but with theadded influence of a number of species, such as Amphiprion the pseudo-high latitude effect to the north of Socotra. This

study indicates that the south Arabian region is the majorclarkii, which extend their distributions to Arabia via thenorthern coast of the Arabian Sea (India-Pakistan), rather feature fragmenting the fish assemblages of Arabia, and

isolating Arabia from the Indian Ocean. This hypothesisthan via Africa and the Gulf of Aden.How far the southern Arabian region extends to the unites features (i), (ii) and (iii) on Fig. 2 as three aspects of

a single phenomenon. The Somali upwelling to the southsouth of Socotra must remain conjectural at this stage,


932 J. M. Kemp

conservation and sustainable use of the biodiversity of Socotra.of Socotra, and the associated coral habitat gap, probably19th February – 20th March 1996. MacAlister Elliott andrestrict dispersal of a number of Indian Ocean speciesPartners Ltd, Lymington, UK.towards Socotra, but many Indian Ocean species which do

Barratt, L. & Medley, P. (1990) Managing multi-species ornamentalreach the archipelago as larvae will be unable to survivereef fisheries. Prog. Underwater Sci. 15, 55–72.postsettlement.

Barratt, L., Ormond, R.F.G., Campbell, A.C., Hiscock, S.,The distributions of the Chaetodontidae currently

Hogarth, P.J. & Taylor, J.D. (1984) Ecological studies of rockyprovide no evidence of a modern zoogeographic barrier shores of the south coast of Oman. Report to the Internationalat the Bab-al-Mandab. Identification of such a barrier is Union for Conservation of Nature and United Nations Environmentprobably due to inadequate survey and sampling in the Programme, Regional Seas Programme. Geneva.area. Survey work in the past few years, particularly by Blum, S.D. (1989) Biogeography of the Chaetodontidae: an analysisRandall (1994, 1996), already indicates that this may be the of allopatry among closely related species. Env. Biol. Fishes, 25,

9–31.case, and that divergence of species assemblages of the GulfBriggs, J.C. (1974) Marine zoogeography, 475 pp. McGraw-Hill,of Aden and Red Sea is likely to be due to a complex of

New York.two or more barriers. Considerably more work either sideBurgess, W.E. (1978) Butterflyfishes of the world: a monograph ofof the Bab-al-Mandab is required, to clarify this.

the family Chaetodontidae, 832 pp. TFH publications. NeptuneThe recent survey confirmed Chiffings’s (1995) assessmentCity.that the entire region should have a high priority for

Chiffings, A.W. (1995) Arabian Seas. A global representative systemprotection. The marine habitats of the archipelago are highly

of Marine Protected Areas, Vol. Three. Central Indian Ocean,varied and are currently in exceptionally good condition. Arabian Seas, East Africa and East Asian Seas (ed. by G.Kelleher,The coral communities of the Socotra archipelago, protected C. Bleakley and S. Wells), pp. 39–70. Great Barrier Reef Marinein the lee of the islands from the worst effects of the south- Park Authority / International Union for Conservation ofwest monsoon upwelling, are isolated habitat islands of Nature / World Bank.possibly great regional significance. This significance is two- Coles, S.L. (1988) Limitation on reef coral development in the

Arabian Gulf: temperature or algal competition. Proc. 6th Int.fold. Firstly, on the basis of the fish assemblages, the coralCoral Reef Symp., Australia, 3, 211–216.associated communities here are unique, representing a

Currie, R.J., Fisher, A.E. & Hargreaves, P.M. (1973) Arabian Seapronounced overlap between east African and ArabianUpwelling. The biology of the Indian Ocean (ed. by B. Zeitzschelassemblages. The fishes show a number of instances ofand S.A.Gerlach). Ecological Studies 3 (1.4), 37–52.sympatry between closely related species previously believed

Debelius, H. (1993) Indian Ocean tropical fish guide, 321 pp.to have allopatric distributions. This unique combinationAquaprint Verlags GmbH, Neu Isenberg, Germany.

of species is likely to occur in many marine groups. Secondly,Edwards, F.J. (1987) Red Sea climate and oceanography. Red Sea

many widespread species, found in both the Red Sea and (ed. by A.J.Edwards, S.M.Head), pp. 45–68. Pergamon Press,the Indian Ocean, are present at these islands. For these Oxford.species the populations of the Socotra archipelago may play Fraser-Brunner, A. (1950) Holacanthus xanthotis, sp. n. & othera significant role in maintaining gene flow between the Red chaetodont fishes from the Gulf of Aden. Proc. Zool. Soc. Lond.Sea and the Indian Ocean. 120, 43–48.

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Pearl Islands (Gulf of Panama) in relation to thermal conditions.of the JLB Smith Institute of Ichthyology, Grahamstown,Limnol.Oceanogr. 18, 367–379.South Africa, for discussions and comments. Dr Callum

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Kemp, J.M. (1998) The occurrance of Nizamuddinia zanardiniiand an anonymous referee both provided constructive(Schiffner) P.C.Silva (Phaeophyta: Fucales) at the Socotra

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Documents

Zoogeography of the coral reef fishes of the Socotra Archipelago