Ventilation rate and behavioural responses of two species of intertidal goby

(Pisces: Gobiidae) at extremes of environmental temperature

Joanne M.J. Ford1,2, Ian R. Tibbetts1,* & Lee Carseldine11Centre for Marine Studies, The University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia2Lead Discovery, Cerylid Biosciences, 576 Swan Street, Richmond, Victoria 3121, Australia(*Author for correspondence: Tel.: +61-7-3365-4333, Fax: +61-7-3365-4755, E-mail: i.tibbetts@mailbox.uq.edu.au)

Received 25 February 2002; in revised form 15 June 2004; accepted 19 May 2004

Key words: Gobiidae, intertidal, rock pools, gill ventilation rate, behaviour, temperature, adaptation

Abstract

We investigated the behavioural responses of two gobiid fish species to temperature to determine ifdifferences in behaviour and ventilation rate might explain any apparent vertical zonation. A survey of theshore at Manly, Moreton Bay revealed Favonigobius exquisitus to dominate the lower shore and Pseu-dogobius sp.4 the upper shore. These species were exposed to a range of temperatures (15–40 �C) inaquaria for up to 6 h. At 20 �C F. exquisitus exhibited a mean gill ventilation rate of 26 ± 1.4 bpm (beatsper minute) differing significantly from Pseudogobius, which ventilated at a fivefold greater rate of143 ± 6 bpm. The ventilation rate in F. exquisitus underwent a fivefold increase from normal local watertemperature (20 �C) to high temperature (35 �C) conditions, whereas that of Pseudogobius did not evendouble, suggesting that Pseudogobius sp. is a better thermal regulator than F. exquisitus. While bothspecies emerged from the water at high temperatures (>30 �C) the behaviours they exhibited while im-mersed at high temperature were quite different. F. exquisitus undertook vertical displacement movementswe interpret as an avoidance response, whereas Pseudogobius sp. appeared to use a coping strategyinvolving movements that might renew the water mass adjacent to its body. The thermal tolerances andbehaviours of F. exquisitus and Pseudogobius sp. are in broad agreement with their vertical distribution onthe shore.

Introduction

A wide variety of fish species have successfullycolonised intertidal shores and adapted to thefluctuating environmental conditions that arecharacteristic of intertidal habitats (Gibson, 1982;Davis-Jana, 2000). Thomson and Lehner (1976)have shown that the interaction between biologicaland physico-chemical factors regulates speciesrichness and the relative abundance of organismswithin intertidal rock pools. The response by fishto changes in their physico-chemical environmentmay influence which species are intertidal residents(i.e., inhabiting the intertidal zone throughouttheir life cycle) and which are transient (i.e.,

occurring intertidally for a few tidal cycles orparticular stages within their life cycle). In addi-tion, differences in the physico-chemical environ-ment may dictate the vertical levels on the shoreparticular members of the shore assemblage areable to occupy.

With the exception of recent studies in south-eastern New South Wales (Griffiths, 2003; Grif-fiths et al., 2003), there is a marked lack ofinformation concerning the species composition,relative abundance and distribution of Australianintertidal fish communities. Most of our knowl-edge of intertidal pool communities comes from

Hydrobiologia 528: 63–73, 2004.� 2004 Kluwer Academic Publishers. Printed in the Netherlands.

63

South Africa, Europe and along the Pacific coastof North America (Pyefinch, 1943; Chadwick,1976; Thomson & Lehner, 1976; Davis-Jana,2001). To relate the responses of two pool dwell-ing gobiid species to extremes of environmentaltemperature and to their occurrence on the shore,we first determined their vertical distribution. Ti-dal pool temperature ranges experienced by theseshore dwellers were measured and used in labo-ratory experiments to mimic the natural environ-ment; and hence were used to determinecorrelations between observed differences in thevertical distribution of these gobiid species andtheir thermal tolerances, and behavioural re-sponses to various temperatures. These behavio-ural strategies are discussed in terms of theirinfluence on the distribution of shore dwellinggobies in the face of temperature extremes char-acteristic of intertidal rock pools.

Materials and methods

Study site

The rocky shore at Manly (27� 27¢ S; 153� 11¢ E)in Moreton Bay, southeast Queensland supportsnumerous, shallow, irregularly shaped tidal poolsamongst which occur areas of bare rock and mud.The upper (‡mean high water neap tide level),mid- (mean high water neap tide level)mean lowwater neap tide level) and lower (mean low waterneap tide level‡) shore zones measured 30, 100 and80 m, respectively in depth as measured perpen-dicular to the water line. In each of these threecontiguous zones, five pools of intermediate size,similar depth and lacking submerged aquaticvegetation were sampled to obtain fish speciescomposition, abundance and distribution data inMarch 1995. To determine the natural range ofwater temperature experienced by fishes inhabitingsuch pools during low tide, measurements of watertemperature within each of the 15 pools sampledwere taken over a diurnal tidal cycle during bothsummer and winter when temperatures are ex-pected to be at their most and least extreme,respectively. Temperature, salinity (Stansens YSIModel 33 S-C-T Meter) and pH (pHep3 licrop-Hep waterproof pH tester) were measured duringhigh tide for comparison.

Species composition, abundance and distribution

The ichthyocide rotenone (7 g l)1) was used tosample the entire fish assemblage of pools. After30 min, total collection of all fish present was as-sumed. Captured fish were fixed in 10% formalin/seawater, identified to the lowest taxonomic levelpossible with the aid of an Olympus dissectionmicroscope.

Collection and acclimation

Thirty-six live individuals of the most abundantgoby in upper and lower shore pools were col-lected for use in experiments on the effect of tem-perature on opercular ventilation rate andbehaviour. CO2 was bubbled through an air-stoneinto exposed rock pools during low tide at a reg-ulated pressure of 80–100 kPa, rendering the fishunconscious (Kwik & Tibbetts, 1999). Theunconscious fish were carefully collected with dip-nets, revived in fresh aerated seawater and trans-ported to the laboratory. Fish were held for 7 daysin aerated saltwater aquaria maintained at20 ± 1 �C (mean ± SE), salinity 31 ± 1 ppt, pH8.2 ± 0.1 (to mimic natural flood tide conditions)and fed a mixture of flake fish food and frozenprawn each morning. Nitrite, nitrate and ammo-nium concentrations were monitored using Aqua-sonic test kits.

Experimental procedure

Six temperatures (15, 20, 25, 30, 35 and 40 �C)were established in 2‘ glass aquaria, each con-taining 500 ml of aerated seawater in separateconstant temperature rooms. Six individuals ofeach goby species were placed one to an aquariumand allowed to acclimate for 1 h at each experi-mental temperature. Visual recordings of opercu-lar ventilation rate (beats min)1) were made for1 min every hour for 6 h. Oxygen concentrationwas maintained at saturation and stratificationavoided by continuous aeration. Individuals wereremoved from experiments on becoming uncon-scious and revived by placing them in aquariamaintained under control (20 �C) conditions.Water temperature in the experimental aquariawas constantly monitored with a Brannan 76 mmimmersion thermometer, while control salinity and

64

pH conditions were monitored with an opticalrefractometer (Atago S-10) and pHep3 licropHepwaterproof pH tester, respectively.

To quantify the effect of temperature on oper-cular ventilation rate, the Q10, which is a change inrate per 10 �C, was calculated. A doubling inventilation rate with a 10 �C rise in temperaturewould give a Q10 of 2, for example. Most fisheshave a Q10 of between 2 and 3 (Schmidt-Nielsen,1997).

Statistical analysis

Data to be analysed using analysis of variance(ANOVA) were checked for homogeneity of var-iance using Levene’s test. Heteroscedastic datawere transformed (Ln (Y)) prior to analysis. Theeffect of time (hour 1–6) on the response variableopercular ventilation rate was investigated sepa-rately for each species using one-way ANOVAwith repeated measures. A two-way factorialANOVA (Statistica V6, Statsoft Inc.) was em-ployed to determine the influence of the fixedfactors temperature and species on the responsevariable, Ln opercular ventilation rate (beatsmin)1) in the first hour of the experiment, and thesignificance of any interactions between the twofactors. Where the F-value indicated significance, aTukey–Kramer HSD test was applied to determinetreatment differences (p < 0.05).

Data used in all statistical procedures weretransformed (Ln) to normalise variances. The ef-fect of time on opercular ventilation rate for thetwo species under investigation could only be sta-tistically analysed for 15, 20 and 25 �C due to thenecessary termination of experiments at tempera-tures under which individuals lost equilibrium andconsciousness.

Results

Pool communities

A total of 338 juvenile fish belonging to 11 specieswere captured by rotenone sampling (Table 1), ofwhich 229 were gobiids.

Favonigobius exquisitus was the most abundantspecies close to low- and mid-tide level (54.2 and58.3%, respectively). Pseudogobius sp.4 dominated

the numerical abundance in the upper shore zone(42.9%). Pandaka lidwilli was also found in higherabundance on the upper shore (13.6%) than themid- (2.4%) and lower shore (0%) zones, but wasless common than Pseudogobius sp.4. Accordingly,the two species chosen for aquarium studies oftheir behavioural responses to extremes of tem-perature were Favonigobius exquisitus and Pseu-dogobius sp.4.

In order to frame the experimental regimes,measurements of physical variables were made atthe collection site. At high tide the seawater at theManly shore had a pH of 8.0, salinity 31 ppt,temperature 20 �C. Pools in the low shore zonehad a temperature range of 14–32 �C, a pH rangeof 8.1–8.9 and a salinity range of 25–31 ppt. Poolson the upper shore had a temperature range ofrange of 18–38 �C, pH range of 6.3–8.2, and asalinity range of 20–41 ppt.

Effect of temperature on ventilation rate

Under controlled experimental conditions(approximately 20 �C, salinity 31 ppt and pH 8.2),Pseudogobius sp.4 exhibited a mean opercularventilation rate (143 ± 6 beats min)1) 5.6 timesthat of F. exquisitus (26 ± 1.4 beats min)1)(Fig. 1). Under moderate experimental tempera-ture conditions, F. exquisitus moved its operculumin clearly observable, even strokes as opposed toPseudogobius sp.4, which exhibited very shallow,rapid movements of its operculum. At 30 �C, afivefold increase in opercular ventilation rateabove the basal rate (under optimal experimentalconditions) was observed for F. exquisitus, abehavioural response that may be indicative ofrespiratory distress. At 35 �C, mean opercularventilation rate at the end of the first hour reachedthe maximum observed for F. exquisitus(162 ± 16 beats min)1), while the maximummeanopercular ventilation rate of 248 ± 16 beatsmin)1 was exhibited by Pseudogobius sp.4 at theend of the first hour at 40 �C at which time theexperiment was terminated (Fig. 1).

F. exquisitus displayed a dramatic increase inQ10 from 0.90 to 6.85 as the temperature rangerose from 15–20 �C to 20–25 �C (Fig. 2). As watertemperature increased further, a progressive de-cline in Q10 was evident. Pseudogobius sp.4, on the

65

Table

1.Fishspeciescompositionandnumericalabundance

(total,meanandpercentage)

offiverock

poolsin

each

ofthreeshore

zones

atManly,south-east

Queensland,

duringlate

summer

(March,1995)

Species

Lower-shore

zone

Mid-shore

zone

Upper-shore

zone

Totalno.

Mean(±

SD)

%Totalno.

Mean(±

SD)

%Totalno.

Mean(±

SD)

%

Favonigobiusexquisitus

90

18.0

±54.2

54.0

49

9.8

±9.26

58.33

13

2.6

±2.30

14.77

Sillagomaculata

52

10.4

±19.5

31.0

10.2

±0.45

1.19

51.0

±1.22

5.68

Omobranchusrotundiceps

91.8

±2.95

5.4

12

2.4

±2.70

14.29

71.4

±1.67

7.96

Amoyafrenatus

40.8

±0.84

2.4

Amoyasp.4

30.6

±1.34

1.8

71.4

±2.61

8.33

10.2

±0.45

1.14

Gerressubfasciatus

20.4

±0.55

1.2

10.2

±0.45

1.19

10.2

±0.45

1.14

Platycephalusindicus

20.4

±0.55

1.2

51.0

±1.0

5.68

Pseudogobiussp.4

10.2

±0.45

0.6

40.8

±0.84

4.76

39

7.4

±10.2

42.86

Mugilogobiuscf

platystoma

10.2

±0.45

0.6

10.2

±0.45

1.14

Omobranchuspunctatus

20.4

±0.55

1.2

81.6

±3.58

9.52

20.4

±0.55

2.27

Pandakalidwilli

20.4

±0.55

2.38

14

2.4

±2.70

13.64

TotalAbundance

166

100

84

100

91

100

66

other hand, exhibited relatively constant Q10 val-ues between 0.91 and 1.45 across all the tempera-ture ranges. The mean opercular ventilationrates of Pseudogobius sp.4 rose by 153% in re-sponse to increased water temperature, whereasthat of F. exquisitus rose by >500%.

Effect of time on opercular ventilation rate

One-way analysis of variance with repeated mea-sures indicated that the effect of Time on LnOpercular Ventilation Rate was significant forboth species at each of the three temperatures( p < 0.05) (Table 2). The Tukey–Kramer HSDtest revealed no easily identifiable pattern(s) in therelationship between opercular ventilation rateover the 6 h period at each of the three tempera-

tures, however, careful examination of the dataseems to suggest lower mean pair variation (andhence greater significant differences) for Pseu-dogobius sp.4 than that observed for Favonigobiusexquisitus (Table 3).

In the two-way factorial analysis of variance onLn Opercular Ventilation Rates between temper-ature (n ¼ 5) and Species (n ¼ 2) during the firsthour of the experiment, the degree of interactionwas significant in both species (p < 0.0001) (Ta-ble 4). In addition, the Tukey–Kramer HSD testrevealed that opercular ventilation rates of mostspecies-temperature combinations were signifi-cantly different (p < 0.005) (Table 5), which to-gether with the significant effect on opercularventilation rate observed between temperature andspecies in the first hour of the experiment, confirmthe distributions of Favonigobius exquisitus andPseudogobius sp.4 as lower and upper shoredwellers, respectively.

Table 2. Summary of a one-way analysis of variance with repeated measures showing the significant effect of Time (hours 1–6) on Ln

Opercular Ventilation Rate at three temperatures (15, 20, 25 �C) for two species (Favonigobius = F. exquisitus; Pseudogobius =

Pseudogobius sp.4)

Species Temp. (�C) SS DF MS F p

Favonigobius 15 0.2553 5 0.0511 7.04 0.0003

20 0.2408 5 0.0482 2.81 0.0381

25 0.3070 5 0.0614 6.10 0.0008

Pseudogobius 15 0.8926 5 0.1785 75.7 <0.0001

20 0.2357 5 0.0471 12.6 <0.0001

25 0.3746 5 0.0749 48 <0.0001

Values in bold indicate significant differences (p < 0.05).

0

50

100

150

200

250

300

15 20 25 30 35 40

Temperature (oC)

Ope

rcul

ar v

entil

atio

n ra

te (

bpm

)

Favonigobius exquisitus

Pseudogobius

Figure 1. Mean opercular ventilation rate (beats min)1), aver-

aged over a 6 h period (n ¼ 6 at each temperature) for

Favonigobius exquisitus and Pseudogobius sp.4 exposed to six

experimental temperatures at normal salinity, pH and 100%aeration. Error bars ¼ ±1SD.

Figure 2. Changes in Q10 calculated at 5 �C intervals for six

individuals of Favonigobius exquisitus and Pseudogobius sp.4

when exposed in aquaria to five experimental temperatures (15,

20, 25, 30, 35 �C). Error bars ¼ ±1SD.

67

Table3.Tukey–Kramer

HSD

resultsfrom

theone-wayanalysisofvariance

fortheeff

ectofTim

e(hours

1–6)onLnOpercularVentilationRate

atthreetemperatures(15,20,

25�C

)fortw

ospecies(Favoni=

Favonigobiusexquisitus;Pseudo=

Pseudogobiussp.4)

Tem

p.15�C

Tem

p.20�C

Tem

p.25�C

H1

H2

H3

H4

H5

H6

H1

H2

H3

H4

H5

H6

H1

H2

H3

H4

H5

H6

Favoni

H1

0.0365

0.0003

0.0009

0.0635

0.0574

0.7700

0.9822

0.9793

0.9920

0.3737

1.0000

0.9668

0.9468

0.0193

0.0774

H2

Y0.2928

0.6275

0.9998

0.9999

N0.9869

0.9889

0.4274

0.0298

N0.9774

0.9285

0.0226

0.0890

H3

YN

0.9908

0.1909

0.2076

NN

1.0000

0.8043

0.1157

NN

0.5602

0.1074

0.3257

H4

YN

N0.4720

0.5004

NN

N0.7923

0.1105

NN

N0.0023

0.0106

H5

NN

NN

1.0000

NN

NN

0.7151

YY

NY

0.9878

H6

NN

NN

NN

YN

NN

NN

NY

N

Pseudo

H1

0.3749

0.0001

0.0002

0.0001

0.0119

0.0014

1.0000

0.1948

0.6373

0.3854

N0.001

0.002

0.003

0.0012

0.0050

H2

N0.0001

0.0001

0.0001

0.0002

Y0.0009

0.2702

0.0002

0.0001

Y0.2481

0.0865

0.0001

0.8292

H3

YY

0.0001

0.9920

0.0001

NY

0.1371

0.7503

0.4951

YN

0.9925

0.0001

0.0212

H4

YY

Y0.0001

0.1582

NN

N0.0067

0.0024

YY

N0.0001

0.0055

H5

YY

NY

0.0001

NY

NY

0.9981

YY

YY

0.0001

H6

YY

YN

YN

YN

YN

YN

YY

Y

Numericalvalues

inbold

andtheletter

Yin

bold

ineach

matrix

indicate

significantdifferences(p

<0.05)betweentimes.

68

Rate of consciousness

Consciousness in individuals of the two fish speciesunder investigation can be defined for the purposeof this experiment as the ability of the fish tomaintain normal movement, equilibrium and co-ordination. Conversely, a state of unconsciousnesswas assumed when experimental individualsexhibited noticeable loss of movement, equilib-rium and co-ordination. The rate of consciousnessin F. exquisitus declined after the first 2 h at 30 �C,with all individuals exhibiting a state of uncon-sciousness before the end of the third hour at35 �C at which point the experiment was termi-nated. In comparison, the rate of consciousness inPseudogobius sp.4 only began to decline after 2 hat 35 �C with one unconscious individual being

removed from the experiment. At 40 �C, all indi-viduals of Pseudogobius sp.4 were removed fromthe experiment by the end of the second hour dueto clear signs of loss of equilibrium.

Behavioural response to temperature

Activity levels of individual F. exquisitus increasedquite markedly at higher temperatures. At 30 �Cand above, an emersion response was repeatedlyexhibited by F. exquisitus with the majority of fisheither occupying the upper part of the water col-umn or the aerial space just above the surface of thewater. Emergent fish attached themselves to theside of the glass jar by means of their fused, sucker-like pelvic fins. During emersion, no gill movementwas observed and the skin always appeared moist

Table 4. Summary of a two-way factorial analysis of variance showing that the interaction between Temperature (15, 20, 25, 30, 35 �C)and species in the first hour of the experiment had a significant effect on Ln Opercular Ventilation Rates of Favonigobius exquisitus and

Pseudogobius sp.4. collected from the lower and upper shore, respectively

SS DF MS F p

Intercept 1253.848 1 1253.848 76727.78 <0.0001

Temp 12.336 4 3.084 188.72 <0.0001

Species 17.150 1 17.150 1049.50 <0.0001

Temp * Species 4.506 4 1.126 68.93 <0.0001

Error 0.801 49 0.016

Table 5. Tukey–Kramer HSD results from the two-way analysis of variance on the interaction between Temperature (15, 20, 25, 30

35 �C) and species (Fl = Favonigobius exquisitus; P4 = Pseudogobius sp.4) on Ln Opercular Ventilation Rate

Temp/

species

{1} {2} {3} {4} {5} {6} {7} {8} {9} {10}

1 Fl-15 �C 0.0002 1.0000 0.0002 0.0002 0.00015 0.00015 0.00015 0.00015 0.00015

2 P4-15 �C 0.0002 0.0002 0.9387 0.0002 0.88603 0.00343 0.03525 0.99705 0.00016

3 Fl-20 �C 1.0000 0.0002 0.0002 0.0002 0.0002 0.00015 0.00015 0.00015 0.00015

4 P4-20 �C 0.0002 0.9387 0.0002 0.0002 0.1518 0.12687 0.00074 0.51139 0.00015

5 Fl-25 �C 0.0002 0.0002 0.0002 0.0002 0.0002 0.00015 0.00015 0.00015 0.00015

6 P4-25 �C 0.0002 0.8860 0.0002 0.1518 0.0002 0.00017 0.63656 0.99989 0.00216

7 Fl-30 �C 0.0002 0.0034 0.0002 0.1269 0.0002 0.0002 0.00015 0.00052 0.00015

8 P4-30 �C 0.0002 0.0353 0.0002 0.0007 0.0002 0.6366 0.00015 0.33194 0.31018

9 Fl-35 �C 0.0002 0.9971 0.0002 0.5115 0.0002 0.9999 0.00052 0.33194 0.00075

10 P4-35 �C 0.0002 0.0002 0.0002 0.0002 0.0002 0.0022 0.00015 0.31018 0.00075

Values in bold indicate temp/species combinations (1–10) that are statistically different (p < 0.05).

69

due to the periodic return of individuals to thewater column during the emersion response.

Behavioural responses of Pseudogobius sp.4to increasing temperature included rapid side-ways flicking movements of the body, tail andpectoral fins. This was also accompanied by in-creased vertical movements within the water col-umn, but not to the same extent as F. exquisitus.At 40 �C total emergence occurred in some ani-mals.

Due to the effect of aeration with relatively coolpumped air, constant temperature rooms had to beregulated at a temperature several degrees higherthan the experimental temperature to achieve thedesired water temperature. Thus, individuals ofboth species emerging from the water column wereexposed to higher temperatures than that of thewater.

Discussion

Species composition, abundance and distribution

In terms of numerical abundance, gobiid fishesdominated the intertidal rock pool assemblage.Griffiths (2003) studying shores in southeasternNew South Wales also found gobies to dominate.The two species of goby under investigation,namely, Favonigobius exquisitus and Pseudogobiussp.4, clearly dominate the lower- and upper-shorezones respectively, while species such as Amoya(Arenigobius) frenatus, Amoya sp.4 and Mugilobiuscf platystoma occur in relatively low numbers be-tween the lower- and mid-shore zones. The phys-ical environment of intertidal rock pool habitats isvery important in the organisation of the intertidalcommunity (Griffiths et al., 2003), with speciesdiversity increasing when stressful conditions suchas high temperatures were minimised throughshade (Takada, 1999). Interestingly, the responsesof mobile species such as fish to this environmentalstress were species-specific, as found in the presentstudy. F. exquisitus and Pseudogobius sp.4, inaddition to revealing vertical distributions limitedto the lower- and upper-shore zones, respectively,also exhibited marked differences in their thermaltolerances and behavioural responses to tempera-ture extremes.

Changes in opercular ventilation rate ofFavonigobius exquisitus and Pseudogobius sp.4with temperature

The responses in aquaria of these two gobies totemperature regimes they are likely to experiencein intertidal rock pools was in broad agreementwith their observed vertical distribution on theshore. Environmental fluctuations in temperaturehave repeatedly been shown to significantly influ-ence the distribution of intertidal organisms(Southward, 1958; Thomson & Lehner, 1976;Davis-Jana, 2000; Hernandez et al., 2002) and assuch, lower-shore zone inhabitants are likely to beexposed to less fluctuating temperature extremesthan their upper-shore zone counterparts due tothe cyclic diurnal nature of tides. Southward et al.(1995) in their 70 year study on the marine com-munity of the western English Channel found thatsignificant shifts in the abundance and distributionof zooplankton and intertidal organisms occurredin response to rising sea temperatures, withnumerical abundance of warm and cold waterspecies increasing and decreasing respectively inresponse to elevated water temperatures. Williamsand Suryanarayanan (1997) also reported a dis-tinct correlation between an organism’s position inthe intertidal zone and their salinity and temper-ature tolerance, with mature organisms showinggreater tolerance than their immature counter-parts. Over a time period corresponding to thatduring which tidal pools are exposed during lowtide, opercular ventilation rate in F. exquisitus andPseudogobius sp.4 increases significantly withtemperature ( p < 0.0005). Heath & Hughes(1973) examined respiratory changes during heatstress in rainbow trout and also found ventilationrate to increase with temperature. Increases in O2

consumption with a rise in temperature are to beexpected since the p[O2] of water controls venti-latory activity in aquatic animals (Dejours et al.,1977; Wilkes et al., 1981; Bridges et al., 1984). Thisphysiological response occurs due to low O2 ten-sions in the warm water and the increased demandfor O2 (Smith, 1991), which we indexed usingchanges in ventilation rate.

At the temperature range of 15–20 �C, F. ex-quisitus displayed a Q10 of 0.90. At 20–25 �C, Q10

increased dramatically to 6.85. At higher temper-atures, Q10 steadily declined. This decrease in Q10

70

with increasing temperature appears to be quitecommon (Dejours, 1973). Q10 and therefore therate of O2 consumption and thereby metabolicactivity of F. exquisitus, is at its highest when theambient temperature range is 20–25 �C. This is inagreement with the preference of F. exquisitus forthat part of the shore near low-water mark, whichis more frequently inundated by seawater at atemperature of about 20 �C. At the lethal limit, theability to take up and transport O2 at low O2

tensions cannot keep pace with the increased de-mand (Davenport & Woolmington, 1981). More-over, increased ventilation rates will contribute tothe O2 deficit, which is possibly reflected by therapid decline in consciousness rate of individualsafter 2 h near the upper tolerable temperaturelimit (30 �C) for F. exquisitus. The maximum wa-ter temperature found at the study site inMarch 1995 was 38 �C. Accordingly, it is likelythat the upper tolerable temperature limit foundfor F. exquisitus in the current study would seem-ingly preclude its occupation of upper shore pools.While this might explain why this species mightavoid upper shore pools, the upper thermal limitin terms of consciousness for Pseudogobius sp.4is not greatly dissimilar (2 h at 35 �C) from that ofF. exquisitus. The metabolic rate of Pseudogobiussp.4 seems to exhibit remarkable temperatureindependence for an intertidal resident that wouldlikely experience large fluctuations in ambienttemperature. The metabolic rate of Pseudogobiussp.4 over all the experimental temperature rangesshows a Q10 of approximately 1.0, which indicatesphysiological accommodation. However, physio-logical accommodation alone cannot be used toexplain occupation by Pseudogobius sp.4 of uppershore pools, nor the presence of some F. exquisitusat that level.

Newell & Branch (1980) state that a ‘zone oftolerance’, influenced by a wide variety of bio-logical and environmental factors and defined byinchoate upper and lower lethal temperatures,exists for intertidal marine organisms, whereas inother less extreme environments, marine organ-isms live well within their limits of survival. Poolsin the upper intertidal area at the study site reachtemperatures that approach or exceed the lethallimits for both F. exquisitus and Pseudogobiussp.4. Thus, it is likely that other characteristicscombine with their physiological tolerance to

enable their occupation of this inhospitableenvironment.

The nature of gill ventilation differs betweenthe two species. Pseudogobius sp.4 fibrillates itsoperculum, whereas F. exquisitus uses distinctopercular strokes to ventilate its gills. It is difficultto make relevant conclusions about this markeddifference in basal ventilation rate without com-parisons of gill surface, cardiac rhythms, O2

transport and cutaneous uptake. Nonetheless, theobservation that the ventilation rate in F. exquisitusundergoes a fivefold increase from normal tohigh temperature conditions, whereas that ofPseudogobius sp.4 does not even double over thesame temperature range, is interesting. Becausethere was little difference in the maximum tolera-ble temperature limits for the two species, wesuggest that this observation is not evidence thatPseudogobius sp.4 is more resilient to increasedtemperature. It may merely be indicative of anupper physical limit to ventilation rate. An expla-nation for the phenomenon of opercular fibrilla-tion in Pseudogobius sp.4 will probably alsorequire investigation of its effect on ventilationvolume and the pattern of water flow over the gillmembranes.

Behavioural responses to temperature

At higher temperatures, while both species exhibitincreased activity and vertical movement, bothresponses are more marked in F. exquisitus. In astudy of the shanny Lipophrys pholis and the seascorpion Cottus (Taurulus) bubalis, Davenport &Woolmington (1981) found that as temperatureincreased, heightened activity directed towards thewater surface occurred in both species. Suchheightened activity may assist with the location ofcooler water that may be available in such strati-fied environments. However if the pool environ-ment were actually stratified, vertical movementswould result in these fish moving to warmer notcolder water in the summer. The intermittent, ra-pid sideways movement of the body, tail andpectoral fins, observed only in Pseudogobius sp.4,may act to change the water in their immediatevicinity, where warm, poorly mixed water is likelyto be subject to rapid oxygen depletion. Thestrategies employed by the two gobies in responseto elevated temperature seem to be fundamentally

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different. F. exquisitus seeks part of the water bodyin which conditions may be less severe, whilePseudogobius sp.4 uses a coping strategy, renewingthe water mass adjacent to its body.

At high temperatures, some fishes ventilatenear the surface film or even leave the water.Congleton (1980) reported that Californian tidepool fish ventilated with water from the surfacefilm when the O2 tension of the water droppedbelow a critical level. Davenport & Woolmington(1981) showed that the shanny Blennius pholis (L.)and C. bubalis, will emerge into air when the O2

tension of the water falls to low levels. This wasalso shown to be the case in the intertidal sculpinClinocottus recalvus (Wright & Raymond, 1978).Both gobiid species in the present study exhibitedan emergence response, although the extent andfrequency differed between them. Pseudogobiussp.4 emerged into the air from the water column at40 �C, while F. exquisitus emerged much morefrequently and at lower temperatures (30 �C andabove). Individuals of F. exquisitus either occupiedthe upper part of the water column or the aerialspace just above, on emergence, whereas Pseu-dogobius sp.4, when it did emerge, was found quitea distance above the water surface.

While the blenniid B. pholis moves its opercu-lum at a constant rate of approximately 120 bpmwhen emersed (Davenport & Woolmington, 1981),in the present study neither goby ventilated whileemerged. Xiphister atropurpureus (Kittlitz) cansurvive exposure to air by utilising atmospheric O2

directly through trans-cutaneous gas exchange(Daxboeck & Heming, 1982), which according toCongleton (1980) occurs chiefly by molecular dif-fusion through a uniform surface film less than100 lm thick. In order for O2 exchange to con-tinue, Gibson (1969) observed that on emersion ofintertidal fish, the body (particularly the respira-tory surfaces) appeared to be kept moist. EmergentF. exquisitus and Pseudogobius sp.4 were also ob-served to have moist skin. The necessity for aboundary layer of water over the skin to facilitatecutaneous gas exchange might explain why F. ex-quisitus periodically returned to the water. It ispossible that Pseudogobius sp.4 may be more effi-cient at aerial respiration than F. exquisitus asPseudogobius sp.4 left the water for longer periods.Furthermore, it could be inferred, due to Pseu-dogobius sp.4 emerging at higher temperatures for

longer periods and at greater distances from thewater, that the behaviour is commensurate with astrategy aimed at locating alternative habitats.Active emergence of intertidal fishes such as twoNew Zealand species, namely, Acanthoclinus fuscusand Forsterygion sp. has also been shown by Hillet al. (1996) to be an oxykinetic response due tohypoxic conditions in the water column as opposedto a behavioural response. Indeed even some coralreef gobies seem remarkably well adapted to doingwithout water. Nilsson et al. (2004) found the coraldwelling goby Gobiodon histrio able to withstand3 h of air exposure. Marsden (1999) demonstratedthat the cirolanid isopod, Natatolana rossi, iscapable of extended periods of aerial exposure andsuggested that this adaptation is a possible re-sponse to inadvertent displacement into intertidalhabitats. An examination of either their relativeability to find water of better quality or amphibiousactivity may be informative, as it seems that ther-mal tolerance alone is insufficient to explain thevertical zonation patterns exhibited by theseintertidal fishes.

Acknowledgements

We thank Dr Helen Larson of the Museum andArt Gallery of the Northern Territory for theidentification of gobioid fishes; Brendan Crowley,Tri Pham, Lisa LeStrange, David Lee, GenevieveQuirk and Regina Magierowski for assistance inthe field; and Drs Craig Franklin and PeterFrappell for comments on the manuscript.

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