Abstract Juveniles of Stegastes nigricans occur in adultcolonies, solitarily, and occasionally in juvenile colonies.We concentrated on solitary juveniles and those in adultcolonies. We examined the costs and benefits of differentsettlement strategies, quantified the territory require-ments of adults, and investigated the process of how ju-veniles make the transition to adult territorial fish. Anadequate adult territory lies next to those of other adults,is proportional in area to the size of the adult, and con-tains a refuge tunnel whose entrance is sufficiently large.Compared with solitary juveniles, those <4 cm totallength inhabiting adult colonies experienced reduced het-erospecific competition for algal food and consequentlybenefited from a greater density of algae. A cost of re-cruiting into an adult colony, however, was increased at-tacks by adults. Juveniles that settled in adult coloniesavoided attacks by retreating into small holes inaccessi-ble to adults. As juveniles in adult colonies grew, theywere chased less often by adults, whereas they them-selves chased adults and heterospecific fish more often.Because territory size correlated with fish size in adultcolonies, its area had to expand as the young fish grew,and that expansion was done at the expense of neighbors.Obtaining the space needed by an adult may be possibleonly when the juvenile settles directly into an adult col-ony. Juveniles that first settle down solitarily, or in juve-nile colonies, may later attempt to enter adult colonies.However, because they do so as larger juveniles, theywould have difficulty insinuating themselves into smallrefuges, which is essential for retreat from the adults.

Keywords Pomacentridae · Territory · Settlement ·Feeding · Moorea

Introduction

Studies of survival to adulthood of coral-reef fishes havetraditionally focused on recruitment of planktonic larvae(Doherty 1982; Victor 1986; Williams et al. 1994). Somestudies, however, have documented the importance ofsurvival of the post-settlement juvenile fish (Jones 1991;Tupper and Boutilier 1995). Successful post-settlementsurvival can vary with the distribution of suitable habi-tats (Levin 1993, 1994), habitat complexity and theavailability of food (Tupper and Boutilier 1995), andabundance of predators and shelters (Shulman 1985;Miranda and Hubbard 1994).

Sale et al. (1980) proposed three strategies by whichjuveniles of the damselfish Pomacentrus wardi settle andgain space. These are (1) “flank attack” – recruits settleon the borders of adult territories; (2) “topological de-ception” – juveniles settle in adult territories but avoidadult aggression by taking cover in the broken-up sub-stratum; and (3) “I’ll take almost anything” – recruitssettle in areas previously and currently unoccupied byadults. Assuming that adults occupied the most favorablesites, and noting that they defended their sites from juve-niles, Sale et al. (1980) argued that the first two strate-gies are used by juveniles to gain access to favorablespace. Sale (1974) had earlier noted flank attack and to-pological deception in juvenile Abudefduf lachrymatus,P. apicalis, P. flavicauda, and P. wardi and documentedtheir ability to then expand their territories at the expenseof neighboring adults. Juvenile P. wardi that utilized thethird strategy settled for inferior sites, probably waitingfor vacancies in favorable space (Sale et al. 1980).

The damselfish we studied, Stegastes nigricans, iswidely distributed in the Indo-Pacific Ocean (Allen1975). Adults defend permanent individual territoriesthat provide algal food, sites for reproduction, and shel-ter. They differ from many territorial damselfishes in that

Communicated by R.F. Oliveira

J. Sau-Fung Lee1 (✉ ) · G.W. BarlowDepartment of Integrative Biology and Museumof Vertebrate Zoology, University of California,Berkeley, CA 94720-3140, USAe-mail: JL275@cornell.edu

Present address:1 Department of Neurobiology and Behavior, Cornell University,Ithaca, NY 14853-2702, USA

acta ethol (2001) 4:23–29DOI 10.1007/s102110100040

O R I G I N A L A RT I C L E

Jonathan Sau-Fung Lee · George W. Barlow

Recruiting juvenile damselfish: the process of recruiting into adult colonies in the damselfish Stegastes nigricans

Received: 23 August 2000 / Received in revised form: 4 January 2001 / Accepted: 11 January 2001 / Published online: 16 March 2001© Springer-Verlag and ISPA 2001

their territories are densely clustered, producing distinctcolonies. Preliminary observations revealed juveniles in-habiting adult colonies, solitary juveniles, and juvenilecolonies. Those in juvenile colonies were too infrequentto study.

In a related colonial species, S. planifrons, the closerto the center of the colony, the fewer were the chasesagainst heterospecific fishes and the greater were the number of chases against conspecific individuals (Meadows 1995). Surrounding territories buffered indi-viduals near the center from heterospecific intruders butalso exposed them to more hostile behavior from conspe-cific neighbors. In S. dorsopunicans the algal biomass interritories increased roughly proportionately with thesize of the territory holder (Foster 1985). These studiesalerted us to examine the tradeoffs for recruits ofS. nigricans between settling initially into adult coloniesas opposed to solitarily. This, in turn, motivated us toquantify the frequency and effectiveness of excludingothers and some of the consequences.

We predicted that, compared with solitary juveniles,juveniles with large neighbors should be more bufferedfrom intrusions by heterospecific competitors for algalfood. Consequently, they should have less need to chaseheterospecific intruders and should do so less often thando solitary juveniles. As another consequence, juvenilesin adult colonies should have access to more food.

Because adult neighbors also defend territoriesagainst conspecific individuals, they might try to driveaway juveniles settling nearby. We expected that com-pared with solitary juveniles, juveniles in adult colonieswould be more frequently chased by conspecific colonialadults as the juveniles entered and attempted to maintaintheir positions in the colony.

Methods

Study site and general observations

Data were collected by snorkeling and breath-hold diving duringOctober and November 1997, at the reef in front of the UC Berke-ley Gump Biological Research Station, located on the west side ofPaoPao Bay in Moorea, French Polynesia. Visibility ranged from1 to 5 m, depending on weather. The depth of the reef varied from0.5 to 3 m before it dropped off to 30 m. The damselfish inhabitedstands of dead Acropora on the reef and were seen elsewhere in-habiting partially live Porites. Fish total length (TL) was esti-mated by observing the fish until it swam in front of a referenceobject, which was then measured (McKaye 1986). Fish <6 cmwere arbitrarily designated as juveniles; subsequent analysis indi-cated that their home-range behavior was close to, but not yet, sta-bilized. Hereafter, all reference to fish length refers to total length.

Suitable space

Size of fish, for instance, should be directly proportional to thesize of the entrance to the tunnel it occupies. Tunnels of adequatesize may be scarce and hence limit the presence of adults. To as-sess this, we also estimated the general availability of tunnelslarge enough for mature adults.

Distribution

We laid a 200-m transect (transect A) along the bottom of the reef(Brock 1982) to investigate the distribution of fish relative to eachother and to the physical environment. Every fish within 1 m toeach side of the line was observed for 5 min. We recorded size ofhome range and location; dimensions of tunnel entrances, poten-tial or occupied; whether in a colony or solitary; and TL. The areaof the home range was calculated from the length and breadth ofthe space occupied by a fish. For each tunnel entrance, we mea-sured its width and depth (sounded by inserting a ruler into thehole until the rear wall was hit). Tunnels, however, were typicallyextensive networks; fish were observed entering from one openingand exiting from another.

Feeding

Each juvenile, 21 in adult colonies and 21 solitary, was observedonce for 10 min. Access to food was estimated by counting bitesat algae, which were then converted to bites/minute and plottedagainst TL. We also measured home-range size and recorded chas-ing of herbivorous heterospecific intruders. We then tested for dif-ferences between solitary juveniles and those in adult colonies.

Preliminary observations revealed an obvious difference in thedensity and length of tufts of algae growing within and outside ofcolonies. Consequently, algal density was estimated in and aroundthe home ranges of ten juveniles in adult colonies by using chiseland hammer to remove a portion of the dead coral. Samples weretaken from three places: inside the juvenile’s territory, inside anadjacent adult’s territory, and outside the colony. Algal densitywas also estimated for ten solitary juveniles by sampling in thesame manner from two places, inside and outside of the juvenile’sterritory.

Because algal cover was relatively uniform within each loca-tion, each sample is considered representative of the sampled area.The extracted bit of coral was dried for 24 h in a standard plantdesiccator. After drying, all algae was removed from 1 cm2 of sur-face area under a dissecting microscope and weighed to the near-est ten-thousandth of a gram.

Two different studies furnished data on the size of home range.One set of data came from the transect described above, which in-volved 5 min of observation per datum. The other study here wasdone directly but entailed 10 min of observation. Because the esti-mates of size of home range in the two studies did not differ sig-nificantly, we pooled them.

We recorded chases against herbivorous heterospecific intrud-ers. This was done both for fish in adult colonies and for solitaryjuveniles.

Interactions with conspecifics

Chases by and against conspecific adults were counted during thesame 10-min periods described above. Adults did not always re-spond to chases by juveniles, so chases against adults are betterdescribed merely as hostile interactions directed toward adults byjuveniles. Again, we compared chases by solitary juveniles withthose by juveniles in adult colonies.

Size and availability of refuge tunnels

Adult colonies were always situated on relatively tall patches ofdead coral that provided adequately large tunnel entrances. To de-termine the size of the fish that could fit into tunnel entrances, 112specimens of Stegastes nigricans from the California Academy ofSciences were measured for total length, greatest body depth, andgreatest body width. Depth and width were then regressed on TL.The largest fish that could physically conform to the entrance wasestablished from regressions and then plotted against the size ofthe fish occupying the home range that contained the tunnel en-trance.

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We laid a 200-m transect line (transect B) across the reef to es-timate the distribution of different-sized tunnel entrances and hab-itat saturation. At each meter, the region within 50 cm to eitherside of the transect line was observed for 5 min. We noted home-range size, the number and size of tunnel entrances within thehome range, size of fish, whether in a colony or solitary, and ter-rain (sand or dead coral). If no fish was present, only terrain andtunnel-entrance sizes were recorded.

Number of colony members in relation to size of coral patch

Patch size was calculated from its length and breadth. Total num-ber of fish was estimated from the number sighted in a 2-min peri-od, using a hand counter; we used the median of three separatecounts. The fish seldom remained out of view for more than half aminute, so a 2-min observation was deemed adequate.

Assessing territorial defense

The effect of intruder size on territorial fish was evaluated bymeans of the model-bottle test (Myrberg and Thresher 1974). Werecorded for 5 min the reactions to a glass jar containing a 3.4-cmjuvenile inserted into the territories of a total of eight fish of thefollowing lengths: 1.9, 3.2, 4.3, 4.7, 4.9, 6.2, and 10.6 cm. Aggres-sive responses were given names and divided into five categories.

1. Bite. Biting at the bottle, the culmination of a forward motionof less than one body length.

2. Charge and bite. Same as “bite,” except the forward motion isan accelerating swim of more than one body length.

3. Charge without bite. An accelerating forward motion of morethan one body length toward the bottle that stops short of mak-ing contact.

4. Display. The attacking fish presents one side to the facingmodel-bottle fish while extending its median fins. The attack-ing fish then often rotates on its long axis so as to present itsdorsal spines to the model-bottle fish.

5. Tail beat. The territory holder presents one side to the facingmodel-bottle fish and beats its tail toward it.

Statistical tests

To assess relationships, we calculated regression coefficients andr values and tested slopes against the null hypothesis of zero. Dif-ferences in algal biomass were evaluated with a Kruskal–Wallisone-way analysis of variance (ANOVA) on ranks and the Stu-dent–Newman–Keuls method. In general, differences were evalu-ated using the Mann–Whitney rank sum test.

Results

Distribution

Juveniles in adult colonies always occurred at the bound-aries of adult territories and often at the junction of sev-eral of them. As the term implies, solitary juveniles werewidely spaced. Juvenile colonies were situated outside ofadult colonies and consisted of two to ten juveniles incontiguous territories; their territories were defendedagainst both heterospecific and conspecific fish. No ju-veniles >6 cm were found solitarily or in juvenile colo-nies. Transect B gave similar results.

Costs and benefits – food

Bites taken by solitary juveniles did not differ signifi-cantly from those by juveniles in adult colonies(P>0.05). When home-range size was plotted separatelyfor solitary juveniles and juveniles and adults in adultcolonies, it was positively correlated with fish size forboth situations (r=0.62 and r=0.87, respectively; Fig. 1).Comparing juveniles in adult colonies with solitary ones,those in adult colonies had the smaller home ranges(P<0.01; Fig. 1).

No statistical difference in algal density cm–2 existedbetween the territories of adults and the areas occupiedby juveniles in the adult colonies (P>0.05; Fig. 2). Fur-ther, the algae within the adult colonies, both for adultsand juveniles, was denser than that outside the colony orwithin the home range of solitary juveniles (P<0.05;Fig. 2). Density of algae within the home range of soli-tary juveniles did not differ from adjacent unoccupiedareas (P>0.05; Fig. 2).

In adult colonies, the smallest juveniles, those <4 cm,chased heterospecific intruders less often than did soli-tary juveniles (P<0.05; Fig. 3). But the number of chasesagainst heterospecific fish by the larger juveniles(>4 cm) within adult colonies did not differ statisticallyfrom that of solitary juveniles (P>0.05; Fig. 3).

Costs and benefits – interactions with conspecific fish

The larger the juvenile in the colony, the less the adultschased it (r=–0.52, P<0.05; Fig. 4). The locations of thechases also differed between smaller and larger juve-niles. In almost every formal and casual observation,smaller individuals were chased regardless of their loca-tion. Larger ones, in contrast, were left alone as long asthey stayed near the centers of their own home ranges.Furthermore, in almost every observation, larger individ-

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Fig. 1 Size of home range/territory of solitary juveniles (n=36,dark squares) and fish of all sizes in adult colonies (n=63, shadeddisks). Solid lines represent the regressions and the dashed linesshow the 95% confidence intervals

uals were chased after venturing outside of their ownhome ranges and feeding on the territories of neighbor-ing adults.

Juveniles <4 cm in adult colonies never initiated hos-tile interactions with conspecific adults, although adultsdid enter their areas. Six of seven juveniles >4 cm inadult colonies initiated such interactions. Consequently,juveniles >4 cm in adult colonies had more agonisticinteractions with adults than did solitary juveniles andjuveniles <4 cm in adult colonies (P<0.01).

Suitable space

The depth of those tunnel entrances whose entrancesmeasured >5 cm was positively correlated with the TL of

its fish occupant (r=0.79; P<0.01). Fish >8 cm TLalways held territories that contained at least one, andoften more than one, tunnel entrance measuring >5 cmwide and >10 cm deep. Fish size was positively corre-lated with the maximum size of fish that could take shel-ter in the largest tunnel entrance in the home range(r=0.81; P<0.01; Fig. 5).

Saturation of habitats

In the region of the reef traversed by transect A, 50%was covered with sand and 8% with dead coral contain-ing no tunnel entrances. No Stegastes nigricans of anysize was found in those areas. In 28% of the area, how-ever, we observed tunnel entrances of a size that juve-niles were seen to inhabit. Of those cavities, 30% wereoccupied by juveniles. Moreover, 14% of that area had

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Fig. 2 Algal richness in adult territories (in ad terr), in juvenileterritories within the colonies (in juv terr), in territories of solitaryjuveniles (in sol juv), outside the territories of solitary juveniles(out sol juv), and outside the adult colony (out ad col). The means(bars) connected by horizontal lines do not differ from chance.Standard errors are shown

Fig. 3 Chases 10 min–1 by juveniles in adult colonies (n=21, tri-angles) and by solitary juveniles (n=21, squares) against hetero-specific intruders

Fig. 4 Chases directed by adults at juveniles in their colonies(n=21). Solid line represents the regression and the dashed linesshow the 95% confidence interval

Fig. 5 Maximum size of fish that can fit into the largest hole plot-ted against the size of the fish occupying the area containing theholes (n=35). Solid line represents the regression and the dashedlines show the 95% confidence interval

cavities of a size that adults were seen to inhabit. Ofthose tunnel entrances, 93% were inhabited by adults.The parts of the reef that featured tunnel entrances largeenough for, but not inhabited by, adults were located>5 m from any other tunnel entrances of an equivalentsize.

Relationship between sizes of coral patch and fish colony

Not surprisingly, the number of fish in a colony rosewith increasing size of the coral patch (r=0.94; P<0.01;Fig. 6).

Obtaining suitable adult space – model-bottle experiment

Aggressive behavior toward the model-bottle fish wasexhibited by all six fish >1.9 cm. Such behavior con-sisted mainly of charges and bites and just bites, al-though the number of times these aggressive acts wereperformed varied. The two smallest juveniles (both1.9 cm) exhibited the seldom-seen behavior in this spe-cies of floating slowly upward or laterally with the headhigher than the rest of the body and the body perpendicu-lar to the reef floor. Their coloration was paler than nor-mal, becoming light tan, and the forehead became darker.On a few occasions during casual observations of naturalinteractions, juveniles <1.9 cm (as small as 0.9 cm)directed aggressive behavior toward other juveniles, butnever against appreciably larger juveniles (>1 cm larger)than themselves.

Discussion

Formation of a colony of adults in shallow water de-pends on the size and distribution of tunnel refuges.

When refuges are large enough and sufficiently closelyspaced, adults hold territories next to one another, form-ing a colony. Clusters of large refuges were always occu-pied. This suggests competition is intense for entry intoadult colonies and that suitable habitats are limited (Sale1972). This poses a severe problem for newly settlingjuveniles.

As Sale et al. (1980) observed for juvenile Pomacen-trus wardi, juvenile Stegastes nigricans that managed toinsert themselves into an adult colony did so by shelter-ing in cavities too small for adults to enter and chasethem out. With growth, a juvenile must expand its smallhome range into a territory that provides shelter and suf-ficient algae for a fish of its size. To do so the young fishmust expand its territory at the expense of neighboringadults. This is what Sale (1974) reported for four speciesof damselfishes, Abudefduf lachrymatus, P. apicalis,P. flavicauda, and P. wardi.

The same pattern was followed by juvenile S. nigri-cans. As they became larger they began to fight backagainst the adults and expanded their territories to in-clude more algae at the expense of their neighbors. Butwhy should the adults attack juveniles progressively lessas the juveniles grow and become more of a threat to theadults’ food resource?

Sale et al. (1980) reasoned that adults eventually ha-bituate to the juveniles. If a juvenile can persist longenough by taking cover in small holes, the adult mayeventually accept the juvenile and reduce aggression to-ward it. This is consonant with the negative correlationwe found between fish TL and chases by adults.

The negative correlation, however, also supports analternative hypothesis. Adults may just be responding tothe increasingly greater risk presented to it by the largerjuvenile. Whatever the behavioral mechanism, if a juve-nile can persevere despite the pressure by adults, it expe-riences ever less pressure from those adults.

Settling within a colony – costs and benefits

In our study, a juvenile in an adult colony had access to adense supply of algal food, almost 4 times as much perunit area as was available to solitary juveniles. The algaewas luxurious because the large adults attacked anddrove away heterospecific herbivorous fishes. Becausethe small juveniles (<4 cm) did not need to engage insuch defense, they benefited from adult territorial behav-ior. However, this was acquired at a cost because theadults also tried to attack the small juveniles and in sodoing drove them into hiding.

As the juveniles grew larger than 4 cm, they starteddefending their territories against conspecific adults andalso shared in chasing out heterospecific competitors.Thus they expanded their territories and gained access tomore algal food, but this required a higher investment interritorial defense.

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Fig. 6 The number of fish in a colony, shown as a function of thesize of the patch of coral (n=14). Solid line represents the regres-sion and the dashed lines show the 95% confidence interval

Territorial defense and expansion

Although preliminary and few in numbers, model-bottletests supported the idea of selective defense. The model-bottle fish was 3.4 cm, and fish smaller than that de-fended their territories against larger bottled fish, demon-strating that a territory holder will defend against aslightly larger fish. The tiny (1.9 cm) juveniles, however,were not aggressive to the 3.4-cm model-bottle intruder,although fish smaller than 1.9 cm were seen a few timesto defend against other small juveniles during other ob-servations.

The two 1.9-cm juveniles exhibited classic conflictbehavior (Bastock et al. 1953; Lorenz 1964), indicatingthe tiny juveniles were motivated to attack but were atthe same time inhibited by the risk presented by the largeintruder. This reinforces the proposition that territoriesare defended only if the benefit of excluding the intruderexceeds the cost of defense.

The location of chases by adults at juveniles changedwith size. Smaller juveniles (<4 cm) were attackedthroughout their home ranges. However, larger juveniles(>4 cm) were not attacked if they stayed in the center oftheir home ranges. This suggests that adults came to re-spect the larger juveniles’ home ranges as territories.

Aggressive chases occurred exclusively at the edgesof the larger juveniles’ territories, and usually only whenthe juveniles ventured outside of their territories and intoa neighboring adult’s territory to feed. This indicates thatjuveniles were expanding their territories to obtain morefood. Further evidence of this expansion is provided bythe correlation between fish size and territory size of ju-veniles in adult colonies.

Continued growth by the juvenile and the concomi-tant increase in competitive abilities leads to a continuedexpansion of its territory. This may be how S. nigricansjuveniles that originally settle in adult colonies make thetransition to adulthood in adult colonies. Obtaining suit-able adult space is a long process and may require settle-ment into the colony as a small juvenile.

Settling as a solitary juvenile – costs and benefits

A solitary juvenile, in our observations, had a much largerarea from which to feed, compared with a colony dweller,but the algal mat was much thinner, not differing fromcontiguous undefended areas. Even when adjusted forthe greater area, solitary juveniles had less algae avail-able to them than did colony-dwelling ones. In addition,they had to expend more energy defending their territo-ries. Juveniles in colonies were harassed by adults,though this harassment decreased as the juveniles grew.Solitary juveniles appear to lead a precarious existence.

We saw no solitary juveniles greater than 6 cm. Fromthis, we deduce that by the time solitary juveniles reachthat size they must enter an adult colony, join a floaterpopulation, or succumb to predation. Observing a paral-lel situation in P. wardi, Sale et al. (1980) reasoned that a

solitary juvenile might later move into a space vacatedby the death of an adult.

Comparing delayed and immediate recruitment to a colony

Entrance into adult colonies at 6 cm, however, is diffi-cult. Juveniles that large would have to establish a terri-tory immediately, and a relatively large one of about500 cm2 (Fig. 1) in the presence of extremely hostile ter-ritorial adults. Juveniles that settle as new recruits in col-onies, on the other hand, can take cover in small holesand need to maintain only small home ranges. These ju-veniles in adult colonies can then gradually expand theirterritories as they grow.

To test the effect of vacancies in the adult colony,Williams (1978) removed adult individuals of the dam-selfish S. planifrons, a fish similar to S. nigricans inecology and evolutionary history. The vacated spaceswere occupied by “floater” fish, nonterritory-holding,nonreproductive S. planifrons. The juveniles already inthe colony fared poorly against these larger floaters. Iffloaters exist in S. nigricans, some individuals may occa-sionally successfully join a colony when an adult disap-pears. However, if natural adult mortality in S. nigricansis low, then removal experiments are potentially mislead-ing because vacant spaces rarely appear in nature. There-fore, members of floater populations may only rarelyhave the opportunity to enter adult colonies. These hy-potheses about the limitations placed on obtaining adultspace in a colony need to be tested by marking both soli-tary and colony-dwelling juveniles at the time of settle-ment and following their survival and success thereafter.

Acknowledgements Steve Strand, director of the Richard B.Gump South Pacific Biological Research Station, helped developideas central to this investigation, and René Galzin generouslyshared with us his experience with the local fishes and made avail-able the resources at the Centre de Recherches Insulaires et Obser-vatoire de l’Environment. We gratefully acknowledge the GumpStation and the Department of Integrative Biology for financialand logistic support. Dave Borslien, Kelly Boyle, and Jung Lee as-sisted with data collection. The research would not have been pos-sible without the advice of Sarah Boyer, John Herr, and KateSchaefer. Tracy Benning, Carole Hickman, Jere Lipps, and DavidStoddart had the difficult jobs of guiding the participants in thefield course. Dave Catania of the California Academy of Scienceskindly made available specimens for the growth curves.

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