RESEARCH ARTICLE

Rebecca Fisher Æ Jeffrey M. Leis Æ Domine L. Clark

Shaun K. Wilson

Critical swimming speeds of late-stage coral reef fish larvae:variation within species, among species and between locations

Received: 26 July 2004 / Accepted: 4 April 2005 / Published online: 21 June 2005� Springer-Verlag 2005

Abstract The swimming abilities of larval fishes areimportant for their survival, potentially affecting theirability to avoid predators, obtain food and control dis-persal patterns. Near settlement swimming abilities mayalso influence spatial and temporal patterns of recruit-ment. We examined Critical speed (U-crit) swimmingability in late stage larvae of 89 species of coral reeffishes from the Great Barrier Reef and the Caribbean.Coefficients of variation in U-crit calculated at theindividual level were high (28.4%), and this was notexplained by differences in size or condition factor ofthese same larvae. Among species U-crit ranged from5.5 cm s�1 to 100.8 cm s�1 (mean=37.3 cm s�1), with95% of species able to swim faster than the averagecurrent speed around Lizard Island, suggesting thatmost species should be capable of influencing theirspatial and temporal patterns of settlement. Inter-spe-cific differences in swimming ability (at both the familyand species levels) were significantly correlated with sizeand larval morphology. Correlations were found be-tween swimming performance and propulsive area,fineness ratio and aspect ratio, and these morphologicalparameters may prove useful for predicting swimming

ability in other taxa. Overall, the swimming speeds oflarvae from the same families at the two locations wererelatively similar, although the Lutjanidae and Acan-thuridae from the Caribbean were significantly slowerthan those from the great barrier reef. Differences inswimming speed and body form among late stage larvaesuggests that they will respond differently to factorsinfluencing survival and transport during their pelagicphase, as well as habitat use following settlement.

Introduction

Swimming performance is important to the survival offishes, affecting their ability to avoid predators and findfood (Plaut 2001). This is particularly true for larvalfishes, which have higher relative metabolic rates (Webb1977) and are more susceptible to predation because oftheir small size (Miller et al. 1988). Furthermore, thesustained swimming ability of larval fishes may beimportant for locating reefs and finding suitable settle-ment habitat (Stobutzki and Bellwood 1997; Mont-gomery et al. 2001). Because coral reef fishes often havespatially isolated and specific habitat requirements,swimming abilities at the time of settlement may be veryimportant for survival.

For these reasons, the swimming ability of fishes hasbeen studied extensively for many years, using a varietyof methods. The U-crit method for measuring maximumswimming speed (modified from Brett 1964) is an easymeans of measuring swimming performance and involvesswimming fish at incrementally increasing speeds untilexhaustion (Plaut 2001). The technique has been usedextensively over the last 40 years to examine the swim-ming speeds of fishes in relation to a variety of ecological,biological, and environmental factors (Jones et al. 1974;Hartwell and Otto 1991; Kolok 1991; Hawkins andQuinn 1996; Lowe 1996; Myrick and Cech 2000; Greenand Fisher 2004). U-crit is a measure of maximum aer-obic swimming speed maintainable over sort periods.These abilities may be important in terms of the potential

Communicated by P. W. Sammarco, Chauvin

R. Fisher (&)Department of Marine Biology, James Cook University,Townsville, QLD 4811, AustraliaE-mail: rebecca.fisher@noaa.govTel.: +1-831-4203971Fax: +1-831-4203980

J. M. Leis Æ D. L. ClarkDepartment of Ichthyology, Australian Museum,6 College Street, Sydney, NSW 2010, Australia

S. K. WilsonThe School for Field Studies,Centre for Marine Resource Studies,South Caicos, Turks and Caicos Islands

Present address: R. FisherDepartment of Biology, University of Windsor,401 Sunset Avenue, Windsor, ON N9B3A1, Canada

Marine Biology (2005) 147: 1201–1212DOI 10.1007/s00227-005-0001-x

for larvae to move between locations on a small scale(e.g. within slicks), to move vertically to access differentcurrent or prey regimes, to move away from reefs athatching, or to move among habitats at settlement(Fisher et al. 2000). U-crit values are often correlatedwith other measures of swimming performance that arecritical to larval survival, including routine swimmingactivity (Plaut 2000; Fisher and Bellwood 2003) andsprint swimming speeds (Reidy et al. 2000). Further-more, U-crit swimming ability is closely correlated withswimming speeds able to be maintained for many hours(Fisher and Bellwood 2002; Fisher and Wilson 2004) aswell as in situ swimming speeds (Leis and Fisher 2005).Consequently, U-crit, while simply a measure of short-term maximum speed, may also provide insight intoother ecologically important abilities of larvae.

Within species, swimming performance can varyconsiderably, and is often attributed to variation in sizeand condition (McKenzie et al. 1998; Koumoundouroset al. 2002). High levels of variation have been shown inthe relative condition of coral reef fish juveniles at set-tlement (Kerrigan 1996) that may translate into differ-ences in survival (McCormick 1998). Despite its potentialimportance, no attempt has been made to examineindividual variation in swimming abilities and how thisrelates to body size and condition of settling larvae.Furthermore, species level U-crit measurements for coralreef fish larvae are currently restricted to only a handfulof species from the great barrier reef (Stobutzki andBellwood 1994; Fisher et al. 2000). SomeU-crit data havebeen recently presented at the family level (Fisher 2005),facilitating comparisons of abilities at higher taxonomiclevels. However, large variation in the swimming abilityamong species within families exists (Leis and Carson-Ewart 1997; Stobutzki 1998; Fisher and Wilson 2004).Ultimately, when considering the ecological impacts oflarval behaviour, species level data will be required. Atpresent no studies have examined U-crit swimmingabilities of a wide range of reef fish taxa, or the mor-phological factors potentially influencing these abilities.In addition, no studies have attempted to determine if theU-crit swimming abilities of related taxa are similaramong different locations. If differences occur withintaxa, they are most likely to occur across biogeographicalboundaries such as the Isthmus of Panama, and the EastPacific Barrier. Thus, the Caribbean and Great BarrierReef are ideal locations to examine biogeographicalvariation in the swimming performance of larval fish.

This study represents the first comprehensive species-level study of the U-crit swimming capabilities of latestage larval coral reef fishes, and includes data collectedfrom Great Barrier Reef (Lizard Island) and Caribbean(Turks and Caicos Islands). U-crit swimming data arecompared within species, among species, among familiesand among locations. Several morphological variablesare also examined in order to determine the effect of sizeand condition factor on intra-specific variation as wellthe relationship between overall morphology andswimming ability.

Materials and methods

Collection of specimens

Larvae were captured using light traps (similar in designto Stobutzki and Bellwood 1997) from both Lizard Is-land (Great Barrier Reef, Australia) and South CaicosIsland (Turks and Caicos Islands, Caribbean). At LizardIsland, sampling took place in November 2000–January2001, November–December in 2001 and December 2003.At South Caicos, sampling took place in October–December in 2003. At both locations light traps wereplaced 100–500 m off the leeward side of the island, andfished all night. The catch was retained in opaque bucketsin fresh seawater and immediately returned to a flow-through aquarium facility where larvae were placed inshaded aquaria for 1-2 h to allow recovery from capture.Experiments were conducted using the aquarium facili-ties at the Lizard Island Research Station and theDepartment of Environment and Coastal Resources atSouth Caicos. Where possible, larvae from Lizard Islandwere identified to species level with the aid of Leis andCarson-Ewart (2000), comparison to voucher specimensat the Australian Museum, and in some cases by growingout larvae to juvenile stages. Those that could not beidentified to species were identified into clearly distin-guishable types and allocated a type number. At SouthCaicos larvae were identified to species by comparison tonewly settled juveniles and online identification guides(http://www.cookman.edu/noaa/ichthyoplankton)Unfortunately, this site has been closed in anticipation ofpublication of the hard copy book. An additional eightlarvae of Acanthochromis polyacanthus, a pomacentridspecies, which does not have a pelagic phase, were cap-tured with nets on the Lizard Island reefs.

Experimental protocol

All swimming experiments were conducted at ambientseawater temperatures, which ranged between 28�C and30�C at both locations. The majority of larvae wereswum within 6 h of capture and all were swum within12 h, using swimming flumes based on the design byStobutzki and Bellwood (1997). U-crit was measured byplacing larvae in the swimming flume and increasing thespeed incrementally over time until the larva could nolonger maintain its position for the full time interval.For larvae measured at Lizard Island in November2000–January 2001, November–December 2001 and forall measurements at South Caicos Island, the experi-mental protocol followed Bellwood and Fisher (2001).Speed increments were equivalent to approximatelythree body lengths per second (bl s�1) with a timeinterval of 2 min. For larvae measured at Lizard Islandduring December 2003, speed increments used were1.6 cm s�1 with a time interval of 5 min. The criticalswimming speed (U-crit) of larvae was calculated as: U-

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crit=U+(t/ti·Ui), where U is the penultimate speedexpressed in cm s�1, Ui is the velocity increment ex-pressed in cm s�1 (3 bl s�1 or 1.6 cm s�1), t is the timeswum in the final velocity increment and ti is the set timeinterval for each velocity increment (2 or 5 min). Someindividuals of five species could swim faster than themaximum speed of the swimming flume used at LizardIsland in 2003/2004. In such cases, we used the maxi-mum speed obtained as the nominal U-crit value for theindividual. Calculated means may slightly under-repre-sent actual U-crit for these species, and SE will beminima.

After each trial larvae were digitally photographedand preserved in either 70% ethanol or 10% bufferedformalin. From digital images measurements were madeof total length (from the outer edge of the caudal fin tothe tip of the upper jaw), caudal fin length (from the tipof the caudal fin to the caudle peduncle), body depth(the height of the fish measured at the widest region),body area (the area of the fish in lateral view excludingthe fins but including the head and gut region), pro-pulsive area (the area of the fish including the fins butexcluding the head and gut region), muscle area (thearea of the fish excluding the fins and the head and gutregion), caudal fin depth, caudal peduncle depth andcaudal fin area. All measurements were taken to thenearest 0.1 mm. Body width (at the widest region, usu-ally the head) was also measured to the nearest 0.1 mmusing vernier callipers.

Data analysis

U-crit methodology

Because two slightly different methods were used tomeasure U-crit in this study (1.6 cm s�1 increments with5-min intervals versus 3 bl s�1 increments with 2-minintervals), we first examined if these produced signifi-cantly different estimates of U-crit, using 11 commonspecies from Lizard Island (two species of Apogonidae,eight species of Pomacentridae, one species of Nemip-teridae and the clupeid Spratelloides). Correlation wasused to examine the coherence of swimming abilityestimates obtained using the two methods and regressionwas used to determine if this relationship was isometric.A two-way ANOVA incorporating method and specieswas used to determine if there was a significant differ-ence in swimming ability between the two methods forany species. Assumptions of ANOVA were examinedgraphically. As both methods produced similar results,pooled data was used for all subsequent analyses.

Intra-specific comparison

Eight species (five from the Turks and Caicos Islandsand three from Lizard Island) were used to examine theeffect of total length and condition factor on swimming

capabilities within species. Wet weight for some specieswas determined from frozen specimens, whereas forothers this was from preserved specimens. Althoughthis will influence the weight measurements, becauseonly one type of preservation method was used for anyone species, this would not affect the results of intra-specific comparisons. Condition factor was calculatedas the residual weight values obtained from regressionagainst total length. MANCOVA was used to examinethe effect of both total length and condition factor(entered as independent variables) on U-crit (dependentvariable).

Inter-specific comparisons

U-crit swimming speeds were compared using a one-wayANOVA for species with n>5, followed by a TukeyHSD test to compare individual species. Mean U-crit forall families with replicate species were obtained byaveraging across species and then compared using a one-way ANCOVA.

The influence of morphology on swimming abilitywas examined at the species level using the two mostspecious families (Apogonidae and Pomacentridae) andat the family level by averaging across species.Regression was used to examine the effect of totallength (size) and residuals from the regression weresaved and used to compare U-crit among species andfamilies after removal of the effect of length. Becauseall of the raw morphological variables measured werealso highly correlated with length (they all effectivelymeasure an increase in size), we used the residualssaved after regressing each against length for analysis.Correlations were performed between residual U-critand residual caudal fin length, residual body depth,residual propulsive area and residual body width.Correlations were also performed between residual U-crit and several morphometric ratios important toswimming, including: muscle ratio (the ratio of themuscle area of the fish relative to the total body area),propulsive ratio (the ratio of the propulsive area tototal body area), fineness ratio (the average between thebody width and body depth, divided by length, Bain-bridge 1960), aspect ratio (caudal fin height divided bycaudal fin area, Sambilay 1990) and caudal peduncledepth factor (caudal peduncle depth divided by bodydepth, Webb and Weihs 1986). A principle componentsanalysis incorporating the residual morphologicalvariables as well as the morphometric ratios was usedto describe the general morphology among species andfamilies.

Between locations

Frequency histograms summarizing the swimmingspeeds across species were constructed for Lizard Islandand the Turks and Caicos Islands and compared using av2 goodness of fit test (Zar 1999). Because no species we

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studied were common to both areas a direct comparisonof U-crit of larvae at Lizard Island and South Caicoswas done at the family level using an ANCOVA withtotal length as a covariate, to account for potential dif-ferences in size.

Results

We measured U-crit in 643 larvae of 89 different speciesfrom 22 families (Table 1). Swimming ability differedsubstantially among individuals, species and families.Coefficients of variation calculated at the individual levelsuggested that, on average, variation in U-crit was28.4% of the mean. At the species level (within families),variation was considerably lower (14.2%), but acrossfamilies variation was very high (41.8%).

Intra-specific comparison

The two U-crit protocols adopted for measuringswimming speeds of larvae at Lizard Island producedvery similar results (R2=0.73, F1,10=24.75, P=0.0007)with a slope and intercept that did not differ signifi-cantly from 1 to 0, respectively (Fig. 1). Only one of 11species showed a significant difference (Pomacentrusnagasakiensis, F1,133=5.4, P=0.02), suggesting that atthe scale examined U-crit method has little effect onestimates of swimming speed for the larvae of thesespecies.

On average length explained 14.3% of the variationin swimming ability across individuals, with only oneof eight species (Plectropomus leopardus) showing asignificant R2 (Table 2). There was also no effect ofresidual weight (condition factor) on U-crit (F1,7 =2.44, P=0.12), with the only species exhibiting asignificant correlation (Acanthurus coerulus) actuallyshowing a slight decrease in U-crit with increasingresidual weight (Table 2). On average, residual weightexplained only 10% of the variation in U-critacross individuals (based on R2 values presented inTable 2).

Inter-specific comparisons

For species with >5 individuals available, U-crit dif-fered significantly among species (F1,553=19.3,P<0.001). The slowest swimmers were the clupeidSpratelloides, averaging 12.7±1.9 cm s�1 (Table 1). Theslowest demersal species was an unidentified apogonid(Apogonid sp. G), averaging 13.5±2.1 cm s�1 (Table 1).The fastest individual fish was a holocentrid measured atLizard Island, swimming up to 100.8 cm s�1 (Table 1).The fastest species for which >5 individuals wereexamined was Sargocentrum coruscrum, a holocentridfrom the Turks and Caicos Islands, averaging72.3±1.5 cm s�1 (Table 1). The two families with the

most U-crit values were the Apogonidae and Pomacen-tridae, and for both, U-crit varied significantly amongspecies (Tukey’s test P<0.05). Within the family Po-macentridae, U-crit ranged from 14.4±4.3 cm s�1 to54.9±3.5 cm s�1 (Table 1). The slowest pomacentrid(by 7 cm s�1) was Acanthochromis polyacanthus, a spe-cies lacking a pelagic stage. The family Apogonidaeshowed less variation, with U-crit ranging from13.2±1.2 cm s�1 to 29.2±2.8 cm s�1 (Table 1). Onlyfour other families had >2 species for which >5 indi-viduals were measured, including the Acanthuridae,Chaetodontidae, Lutjanidae and Monacanthidae and innone of these families did U-crit differ significantlyamong species.

Total length (size) explained 59 and 55% of the var-iation in U-crit across species for the families Apogoni-dae and Pomacentridae, respectively (Fig. 2). Within thefamily Apogonidae, particularly poor swimmers relativeto length included the Floweria sp., Apogon cyansomaand Apogon exostigma (White circles, Fig. 3a). Goodswimmers included Apogon cf doederlini and Nemia oc-tospina (Black circles, Fig. 3a). The poorest swimmingspecies relative to length in the Family Pomacentridaewas Acanthochromis polyacanthus (white circles,Fig. 3b). Particularly good swimmers included Poma-centrus wardi and Pomacentrus nagasakiensis (Blackcircles, Fig. 3b).

As with species level comparisons, ANOVA alsoindicated significant differences in the swimming per-formance among families (F1,64= 847, P<0.001). Thefastest swimming family was the Holocentridae, fol-lowed by the Carangidae, Siganidae, and Acanthuridae(Table 1). The slowest families were the Clupeidae,Apogonidae and Pomacanthidae (Table 1). In contrastto the within family comparisons, only 18% of thevariation in U-crit among families was explained bylength, and this was only marginally significant(P=0.05, Fig. 2c). Poor swimmers relative to lengthincluded Clupeidae, Sphyraenidae, Mullidae and Tetra-odontidae (White circles, Fig. 3c). Good swimmers in-cluded Carangidae, Holocentridae and Siganidae (Blackcircles, Fig. 3c).

There were significant positive correlations betweenresidual U-crit and residual body depth for both thefamilies Pomacentridae and Apogonidae, as well asacross all families combined (Table 3). This was con-firmed by PCA, which indicated that for all threegroups, good swimmers were associated with greaterrelative body depth (Fig. 3). Significant negative corre-lations occurred between residual U-crit and finenessratio (the average between body depth and body width,divided by length) for the families Pomacentridae andApogonidae, as well as across all families combined(Table 3). This was also confirmed by PCA, whichindicated that for all three groups, poor swimmers wereassociated with greater fineness ratios (Fig. 3). Finenessratios ranged from 3.0 for the Chaetodontidae, up to10.8 for the family Sphyraenidae. Not surprisingly, therewas a close negative correlation between fineness ratio

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Table 1 U-crit for 22 families and 89 species of light trap caught fishes from Lizard Island (LI) and the Turks and Caicos Islands (TCI)

Family species Locaton n U-crit (cm s�1) TL (mm)

Average ± SE Range Average ± SE

Acanthuridae 50.5±1.4 35.1–78.0 35.4±0.5Acanthurus bahianus TCI 20 50.1±1.9 42.4–78.0 36.4±0.5Acanthurus chirurgus TCI 6 47.8±1.8 40.5–52.9 35.9±0.7Acanthurus coeruleus TCI 8 48.3±3.3 35.1–63.5 35.3±0.4Acanthurus sp. LI 2 61.5±2.6 58.9–64.1 26.9±1.8Naso brevirostris LI 2 61.2±5.7 55.5–66.8 31.9±0.4Apogonidae 20.5±0.8 5.5–45.0 13.5±0.3Apogon exostigmab LI 4 30.0±7.2 16.2–43.0 21.4±1.3Apogon cf doederlini LI 7 29.2±2.8 24.6–45.0 15.6±0.5Apogon cyanosoma LI 14 15.1±1.7 5.5–26.3 13.3±0.5Apogon trimaculatus LI 9 24.3±3.0 10.0–39.8 15.5±0.8Apogonid sp. A LI 9 19.8±1.2 12.0–22.7 12.1±0.5Apogonid sp. B LI 8 18.2±2.2 8.5–27.4 9.7±0.8Apogonid sp. C LI 12 22.2±2.9 11.8–39.5 11.6±0.5Apogonid sp. D LI 14 19.9±1.5 10.0–29.1 15.3±0.8Apogonid sp. E LI 7 16.2±2.1 5.9–22.5 12.7±0.5Apogonid sp. F LI 6 25.6±3.9 9.1–34.3 14.2±0.4Apogonid sp. G LI 6 13.7±2.1 8.5–21.5 9.5±0.4Fowleriasp. LI 4 13.2±1.2 10.9–16.5 11.1±0.6Nemia octospina LI 3 26.7±2.2 22.4–29.2 14.6±0.3Phaeoptyx pigmentaria TCI 3 23.6±4.9 14.4–31.1 15.8±0.3Balistidae 51.5 51.5–51.5 24.3Balistid sp. LI 1 51.5 51.5–51.5 24.3Blennidae 18.4±5.9 0.3–45.0 19.5±0.7Exsenius stictus LI 5 5.5±2.2 0.3–11.1 19.0±0.4Petroscirtes lupus LI 4 34.7±6.8 14.9–45.0 20.0±1.6Carangidae 71.2±4.3 50.3–92.7 29.7±1.0Gnathanodon speciosus LI 9 71.2±4.3 50.3–92.7 29.7±1.0Chaetodontidae 48.8±2.3 9.9–62.5 15.5±0.7Chaetodon aureofasciatusa LI 4 51.5±4.2 43.8–62.5 16.8±3.0Chaetodon auriga LI 3 41.6±7.8 26.7–52.6 21.2±1.7Chaetodon melanotus LI 1 9.9 9.9–9.9 21.6Chaetodon plebiusa LI 11 53.9±3.4 28.4–62.5 13.4±0.3Chaetodon rainfordia LI 9 47.2±3.1 34.7–62.5 14.4±0.2Chaetodon trifascialisa LI 1 62.5 62.5–62.5 13.6Chaetodontid sp. LI 1 41.4 41.4–41.4 21.1Clupeidae 18.7±2.4 4.3–41.2 32.6±1.7Jenkinsia spp. TCI 7 32.4±2.5 23.4–41.2 37.7±2.4Spratelloides LI 16 12.7±1.9 4.7–28.9 30.8±2.0Holocentridae 75.0±1.8 54.7–100.8 36.8±0.5Holocentrid sp. LI 1 100.8 100.8–100.8 34.8Sargocentron coruscuma TCI 21 72.3±1.5 54.7–81.0 36.7±0.5Sargocentron vexillariuma TCI 4 83.0±0.9 80.2–83.9 37.6±2.1Lethrinidae 38.4±3.3 15.8–79.7 21.8±0.9Lethrinus spp. LI 19 38.4±3.3 15.8–79.7 21.8±0.9Lutjanidae 49.4±2.0 27.6–68.3 26.2±0.8Caesio cuninga LI 7 53.3±4.6 34.2–62.5 21.9±1.8Lutjanus analis TCI 4 40.2±0.9 37.9–41.7 30.7±2.4Lutjanus apodus TCI 2 42.1±1.7 40.4–43.8 21.3±1.9Lutjanus carponotatus LI 8 52.0±4.0 35.2–67.0 29.1±0.2Lutjanus cyanopterus TCI 1 45.3 45.3–45.3 32.3Lutjanus quinquelineatus LI 7 52.4±5.4 27.6–68.3 26.1±0.8Ocyurus chrysurus TCI 2 42.3±0.3 42.0–42.6 23.4±0.5Monacanthidae 27.1±3.2 4.6–79.4 24.2±1.0Oxymonacanthus longirostris LI 1 31.1 31.1–31.1 23.3Paramonacanthussp. LI 13 27.8±5.2 4.6–79.4 23.1±1.2Pseudomonacanthus peroni LI 6 24.2±1.6 18.1–29.9 27.6±1.2Stephanolepis setifer TCI 1 31.3 31.3–31.3 19.4Mullidae 47.0 47.0–47.0 75.4Upeneus tragula LI 1 47.0 47.0–47.0 75.4Nemipteridae 34.3±2.3 15.6–60.0 16.5±0.7Scolopsis bilineatus LI 15 36.6±2.3 26.9–60.0 16.2±0.9Scolopsissp. LI 4 25.8±5.1 15.6–39.9 17.5±0.2Nomeidae 28.0±6.3 5.9–42.9 20.0±1.4Psenessp. LI 5 28.0±6.3 5.9–42.9 20.0±1.4

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and residual body depth. The only other significantmorphological variable common to more than onegroup was caudal peduncle depth factor, which wasnegatively correlated with U-crit in both the Apogonidaeand across families (Table 3, Fig. 3). Residual U-crit inthe Pomacentridae was positively correlated with resid-ual head width (Table 3, Fig. 3). For the analysis acrossall families combined, residual U-crit was also positivelycorrelated with residual propulsive area and aspect ratio(Table 3), trends that were also confirmed by PCA(Fig. 3). Aspect ratios at the family level ranged from as

low as 1.0 for the Clupeidae up to 2.6 for the Acan-thuridae.

Between locations

There was no significant difference in the distribution ofswimming speed across species between Lizard Islandand South Caicos (v2=7.1, df=4, P>0.1, Fig. 4) and atboth locations more than 91% of the species examinedwere able to swim faster than the average current speed

Table 1 (Contd.)

Family species Locaton n U-crit (cm s�1) TL (mm)

Average ± SE Range Average ± SE

Pomacanthidae 20.7±3.1 5.6–26.9 16.4±0.2Pomacanthus sextriatus LI 6 20.7±3.1 5.6–26.9 16.4±0.2Pomacentridae 37.6±0.9 0.2–88.0 15.7±0.3Pomacentrid sp. A LI 5 21.1±1.6 17.5–26.7 11.1±0.2Pomacentrid sp. B LI 7 22.1±1.4 16.5–27.3 12.5±0.3Abudefduf vagiensis LI 11 46.3±5.7 26.7–73.0 17.4±0.6Acanthochromis polyacanthusc LI 8 14.4±4.3 0.2–35.8 12.0±0.3Amblypomacentrus breviceps LI 6 33.1±3.5 21.2–44.6 12.6±0.3Amphiprion clarkii LI 1 34.7 34.7–34.7 10.4Chromis atripectoralis LI 12 35.6±3.2 21.8–61.6 10.8±0.3Chrysiptera rollandi LI 18 25.4±0.7 16.9–44.4 12.2±0.1Dacyllus reticulatus LI 2 32.4±1.4 31.0–33.8 14.1±3.1Dascyllus aruanus LI 10 24.0±0.8 19.7–27.1 9.4±0.2Dischistodus prosopotaenia LI 12 28.5±2.8 11.6–42.6 11.7±0.1Neoglyphidodon nigoris LI 1 38.2 38.2–38.2 11.9Neopomacentrus azysron LI 11 35.3±0.7 31.9–39.0 17.2±0.1Neopomacentrus cyanomos LI 15 37.9±1.4 30.7–45.8 16.4±0.2Neopomacentrus sp. LI 6 41.0±5.5 25.9–58.7 16.8±1.1Pomacentrus amboinensis LI 11 35.6±4.2 22.0–68.6 14.9±0.2Pomacentrus brachialis LI 9 37.8±1.0 33.5–45.4 16.1±0.1Pomacentrus chrysurus LI 3 33.7±10.7 19.5–54.7 16.4±0.7Pomacentrus coelestisa LI 6 43.9±3.9 35.5–62.5 19.8±0.3Pomacentrus lepidogenys LI 18 41.6±3.4 26.3–88.0 16.9±0.4Pomacentrus moluccensis LI 14 35.6±3.5 18.1–59.1 14.5±0.4Pomacentrus nagasakiensis LI 14 49.1±4.9 25.5–83.6 16.9±0.2Pomacentrus wardia LI 8 50.2±2.9 42.6–62.5 16.6±0.5Pristotis obtusirostrisa LI 16 54.9±3.4 41.8–88.0 28.3±0.9Stegastes diencaeus TCI 6 38.7±1.9 33.0–47.3 15.6±0.5Stegastes leucostictus TCI 8 37.6±3.9 16.7–54.7 13.3±0.4Stegastes partitus TCI 12 42.6±2.2 34.5–63.3 17.2±0.2Stegastes planifrons TCI 4 40.2±3.7 35.2–50.8 13.7±0.4Pseudochromidae 27.2±2.2 8.2–42.0 17.5±0.3Pseudochromid sp. A LI 2 28.9±2.6 26.3–31.5 17.6±0.1Pseudochromid sp. B LI 7 29.9±3.0 21.3–42.0 17.8±0.3Pseudochromid sp. C LI 5 22.7±4.0 8.2–31.5 17.1±0.6Serranidae 31.5±3.2 13.2–43.4 21.4±0.6Plectropomus leopardus LI 12 31.5±3.2 13.2–43.4 21.4±0.6Siganidae 67.1±8.9 34.2–87.1 29.5±1.2Siganus spp. LI 6 67.1±8.9 34.2–87.1 29.5±1.2Sphyraenidae 29.8±8.4 21.5–38.2 23.0±3.8Sphyraena sp. LI 2 29.8±8.4 21.5–38.2 23.0±3.8Tetraodontidae 34.6±8.9 1.5–68.3 32.6±1.0Canthigaster bennetti LI 6 34.6±8.9 1.5–68.3 32.6±1.0Terapontidae 51.0±7.8 25.8–72.5 19.2±1.6Terapon theraps LI 6 51.0±7.8 25.8–72.5 19.2±1.6

Figures in bold are the averages and standard errors (SE) for eachfamily, n number of fish swum at each location, TL total lengthaindicates species with at least one individual that outswam thechambers,

bmay also be Apogon kalloperus or A. frenatus,cAcanthochromis polyacanthus do not have a pelagic phase andwere therefore captured on scuba directly from reefs around LizardIsland

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around Lizard Island (Fig. 4). The modal speed acrossspecies at both locations (32.5%) was 27–40.5 cm s�1.At the family level, there was strong coherence in U-critfrom both Lizard Island and South Caicos, with theorder of swimming performance from slowest to fastestsimilar at both locations (Fig. 5). However, ANCOVA(correcting for the potential influence of length) indi-cated a significant difference within the families Lut-janidae and Acanthuridae (F1,408=9.5, P=0.002), withboth swimming slower in the Turks and Caicos than atLizard Island (Fig. 5).

Discussion

The late stage larvae of coral reef fishes are excep-tionally strong swimmers, with critical speeds of up to100 cm s�1. At both locations more than 90% of spe-cies could swim faster than 13.5 cm s�1, the averagecurrent speed around Lizard Island (Frith et al. 1986),and the speed previously used in endurance swimmingexperiments (Stobutzki and Bellwood 1997). Indeed,

the most commonly occurring swimming speed was twoto three times faster than 13.5 cm s�1. There is nodoubt that swimming speed is an important factorwhen considering the potential for behaviour to influ-ence dispersal distances, recruitment success and thepotential for self recruitment in reef fish larvae.Swimming speeds of the late stage larval coral reeffishes differed among individuals, species and families,suggesting considerable diversity in their response tothe ecological factors important during their time in thepelagic environment, as well as during and after set-tlement.

U-crit methodology

The two methods of measuring U-crit produced verysimilar estimates of swimming speed, despite consider-able differences in the overall time swum during theexperiments (12.8 min on average for increments of 3 bls�1 and 2-min intervals versus 77 min for increments of1.6 cm s�1 and 5-min intervals). This suggests that,within limits, the duration of time increments may havelittle effect on the final U-crit estimates. These findingsare consistent with several studies reporting only minordifferences between time increments of considerablyvarying duration (2–64 min, Hartwell and Otto 1978,1991; 2–10 min, Peake and McKinley 1998). Althoughreduced time intervals may slightly overestimate U-critabilities in some cases, this effect does not appear to bemore than about 7–9% (calculated from Farlinger andBeamish 1977; Williams and Brett 1987) suggesting thatU-crit estimates of swimming speed are relativelyinsensitive to the details of the methodology employed.

Although originally established to measure aerobic orsustained swimming capacity of fishes (Brett 1964), it isgenerally agreed that U-crit is a measure of prolongedswimming that may incorporate both aerobic andanaerobic muscle activity (Wilson and Egginton 1994;Reidy et al. 2000). In coral reef fish larvae, U-crit can beused to predict routine (Fisher and Bellwood 2003), in-situ (Leis and Fisher 2005) and sustainable swimming

Table 2 Summary statistics for regressions carried out between U-crit swimming ability and length and residual weight (condition factor)for eight species of late stage larvae

Species Location Total length Condition factor

R2 Slope P R2 Slope P

Acanthurus bahianus TCI 0.06 0.61 0.30 0.04 0.62 0.30Acanthurus coerulus TCI 0.02 �0.16 0.96 0.27 �5.48 0.04Sargocentrum coruscum TCI 0.01 �0.25 0.74 0.06 �1.14 0.59Pristotis obtusirostris LI 0.15 �1.56 0.35 0.02 �1.16 0.24Neopomacentrus cyanomos LI 0.05 1.86 0.54 0.02 1.22 0.72Stegastes leucosticus TCI 0.42 �6.34 0.08 0.14 �4.27 0.38Stegastes partitus TCI 0.04 2.14 0.54 0.08 3.59 0.42Plectropomus leopardus LI 0.41 8.36 0.05 0.22 8.25 0.57

Residuals were obtained from linear regression between total length and weight for each species. Bolded P values indicate significantregressions

Fig. 1 U-crit measured using 3 bl s�1 increments and 2-min timeintervals versus U-crit measured using 1.6 cm s�1 increments and5-min time intervals.White circle Spratelloides, light gray circlesApgononidae, dark grey circle Scolopsis bilineatis and black circlesPomacentridae. The solid line indicates the fitted regression, thedotted lines 95% confidence limits and the gray line is the isometricrelationship between the two methods

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speeds (up to 24 h; Fisher and Bellwood 2002; Fisherand Wilson 2004), although it remains to be seen if theserelationships hold for fishes with different swimminggaits (gait transition speed may be a more appropriatemeasure of relative speed than U-crit, Drucker 1996).Overall, it appears that U-crit is a useful measure ofmaximum swimming speed that is comparable acrosstaxa, affected to only a small extent by methodology andclosely correlated with other, more time intensive butecologically relevant, measures of swimming perfor-mance.

Intra-specific comparison

Intra-specific (among individuals) variation in swim-ming ability was large (on average 28% of the meanswimming ability of a species), and this did not appearto be due to length or condition (as measured by residualweight). Although a strong relationship between bodysize and locomotor performance has been observed in

lizards and amphibians, this is often not the case for fish,which frequently show no significant relationship,probably due to the small size range of individualsexamined (Kolok 1999). This was also the case in ourstudy, with variation in total length representing onaverage only 10% of the mean. A wide array of physi-ological and biological characteristics have a significantimpact on intra-specific variation in swimming ability,including size (Koumoundouros et al. 2002), bodymorphology (Boily and Magnan 2002), dietary fatty acidcomposition (McKenzie et al. 1998), growth rate (Kolokand Oris 1995) and muscle enzymatic activity (Farrell etal. 1991). Variability in swimming abilities within speciessuggests this is a heritable trait, with the potential toinfluence individual survival during the pelagic phaseand at settlement.

Inter-specific comparison

At the species level, length was significantly related toswimming ability, supporting previous studies on inter-specific variation in swimming performance (Miller et al.1988; Wardle 1975; Stobutzki 1998; Bellwood andFisher 2001). However, at the family level, only 16% ofthe variation in swimming performance was explainedby length. This also supports previous studies at highertaxonomic levels (e.g. Bainbridge 1960) and may beattributed to a multitude of both morphological andphysiological factors. Although a variety of physiologi-cal parameters influence swimming performance (e.g theproportion of red muscle, the circulatory system and gillarea; Wardle 1977) these are beyond the scope of thepresent study, and here we concentrate only on the grossexternal morphology of the larvae.

Differences in relative muscle mass (Webb 1984),drag co-efficient (Sagnes et al. 2000) and changes inbody form (Webb 1984; Boily and Magnan 2002) sig-nificantly influence the swimming capability of fishes.Both residual body depth (BD) and fineness ratio (FR)(which are closely correlated with each other) were ableto explain some of the inter-specific and inter-familialdifferences in swimming speed. Maximizing volumewhile minimising overall drag should optimise swim-ming performance, because this would allow thegreatest relative muscle mass. Fineness ratio estimatesthe combined total drag of the body due to frictionalresistance and form drag, and the optimal ratio is 2.5,with deviations from this resulting in decreased swim-ming stamina (Bainbridge 1960). We found a strongnegative correlation between swimming ability andfineness ratio, which supports these theoretical consid-erations. Among families, fineness ratios ranged fromaround 3.0 for the Chaetodontidae, which were rela-tively fast swimmers for their size, up to 10.8 for theSphyraenidae, which were particularly slow swimmers.Relative propulsive area (PA) was also correlated withswimming ability across families, supporting previousstudies on larval coral reef fishes (Fisher et al. 2000).

Fig. 2 U-crit versus total length averaged at the species-level forthe families Apogonidae (a) and Pomacentridae (b), and averagedacross families (c). Solid line indicates the fitted least squares linearregression

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Despite a significant relationship at the family level,this was not the case within families; probably due tolow variability in PA at this level for late stage larvae.Fisher et al. (2000) examined swimming abilitythroughout development, during which time larvae

undergo substantial changes in their relative propulsivearea. Along with fineness ratio and propulsive area,aspect ratio was also significantly related to swimmingspeed across families, confirming the importance of thisvariable to swimming ability in fishes (e.g. Sambilay1990; Webb and Weihs 1986). A high aspect ratio isthought to be characteristic of pelagic marine fishthat have enhanced cruising speeds, enabling them totravel over wide areas in search of food and breedinggrounds (Webb 1994). On the other hand, a deepcaudle peduncle is believed to be associated with‘‘accelerators’’, which hover in the water column andambush prey, and have poor sustained swimmingspeeds (Webb 1994). This is also consistent with ourlate stage larvae, for which there was a negative cor-relation between caudle peduncle depth factor (CPDF)and swimming ability.

The diversity of forms seen among the late stagelarval fishes examined here highlights the tradeoffsthat exist between sustained, burst and manoeuvra-bility performance (Webb 1994). While most larvaestart out their pelagic life with a relatively similarform (Webb and Weis 1986), by the end of their pe-lagic life, larvae of coral reef fishes exhibit a widearray of body morphologies and fin types comparableto the diversity seen among adults (Leis and

Fig. 3 Principal components analysis of nine morphologicalvariables, at the species-level for the families Apogonidae (a) andPomacentridae (b), and at the family level (c). Cumulatively,principle components 1 and 2 explain 68.3, 62.7 and 77.1% of thevariation in morphology among the Apogonidae, Pomacentridaeand families, respectively. Bubble plots on the left indicateproportional residual U-crit (residuals are obtained from theregressions presented in Fig. 2. Open bubbles indicate negativevalues (poor swimmers relative to length) and filled bubbles indicatepositive values (good swimmers relative to length). CFL—residualcaudal fin length, BD—residual body depth, PA—residual propul-sive area, HW—residual head width, MR—muscle area to bodyarea ration, PR—propulsive area to body area ration, FR—finenessratio, AR—aspect ratio and CPDF—caudal peduncle depth factor.Species codes are the first letter of the genus combined with the firstthree letters of the species name (see Table 1). Family codes areac—Acanthuridae, ap—Apogonidae, ba—Balistidae, ca—Carang-idae, ch—Cheatodontidae, cl—Clupeidae, ho—Holocentridae,le—Lethrinidae, lu—Lutjanidae, mo—Monacanthidae, mu—Mul-lidae, ne—Nemipteridae, no—Nomeidae, pa—Pomacanthidae,pe—Pomacentridae, ps—Pseudochromidae, se—Serranidae,Si—Siganidae, sp—Sphyraenidae, tt—Tetraodontidae, tr—Tera-pontidae

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Carson-Ewart 2000). Because we have measuredswimming abilities at the very latest stages of pelagiclife, immediately prior to settlement, overall abilitieslikely reflect a compromise between the swimmingdemands placed on benthic juveniles and those ofpelagic larvae.

Ecological implications

Larvae differ significantly in their U-crit abilities, indi-cating high variability among taxa in sustained swim-ming speeds (Fisher and Wilson 2004). This findingadds weight to previous studies on swimming endur-ance (Stobutzki and Bellwood 1997; Stobutzki 1998)and suggests substantial variability in their ability toalter their dispersal patterns, overall dispersal distances,and control their temporal and spatial patterns of set-tlement. For example, the settlement dynamics of reeffish families with good swimming abilities (e.g. Holo-centridae, Siganidae, Acanthuridae and Lutjanidae)might be expected to show a poor relationship withhydrodynamic features compared to slower swimminggroups. They may also be less likely to show dispersalpatterns similar to that of passive particles predicted byhydrodynamic models. The diversity of swimmingabilities among taxa has important implications formarine reserve design, which is greatly influenced bylarval dispersal distances (Botsford et al. 2001; Moraand Sale 2002). If variability in swimming performancetranslates into large differences in dispersal distancesand/or levels of local retention, it seems unlikely thatdifferent reef fish families (and in some cases differentspecies within families) will be adequately protected bya single design.

Research has found a strong relationship betweenswimming capability and among-habitat and micro-habitat use by adult labrids in both tropical (Fulton etal. 2001) and temperate (Fulton and Bellwood 2004)locations, suggesting that swimming capabilities may beof general importance in shaping habitat use by fishes ata variety of spatial scales. The large variation in swim-ming abilities of settlement stage larvae, at all taxonomiclevels observed here, suggests that the swimming abilitycould be an important factor influencing both survivaland choice of settlement habitats for a wide range of reeffishes. For fishes that are relatively site attached and donot move among habitats following settlement, it may bethat swimming ability at this early juvenile stage iscritically important in driving habitat selection or dif-ferential post-settlement mortality among habitats thatmay ultimately shape adult distributions. Detailedanalysis of settlement preferences and habitats of juve-niles relative to their swimming abilities is required todetermine the extent to which swimming ability plays arole in structuring settlement patterns of coral reeffishes.

This study is the first to present swimming data oflarval fishes from both the Great Barrier Reef and the

Fig. 4 Frequency histogram of U-crit across species for LizardIsland and South Caicos Island. Bins are divided in to multiples of13.5 cm s�1, the average current speed around Lizard Island (Frithet al. 1986) and the speed used in most previous sustainedswimming experiments (Stobutzki and Bellwood 1997)

Fig. 5 A comparison of U-crit swimming speed across families atLizard Island and South Caicos Island. An * indicates a significantdifference between the two locations within each family. Familiesfor which only one individual was examined at either location havebeen excluded from the analysis but are plotted on the graph forcomparison (Holocentridae and Monacanthidae)

Table 3 Correlations between residual U-crit and nine morpho-logical variables at the species and family level of analysis

Variable Apogonidae Pomacentridae Family level

CFL 0.30 0.36 0.07BD 0.56 0.37 0.39HW 0.38 0.45 0.27PA 0.35 0.14 0.42MR �0.45 0.18 �0.17PR 0.15 0.02 �0.02FR �0.53 �0.43 �0.42AR �0.12 0.19 0.43CPDF �0.55 �0.07 �0.59

Variables included are CFL—residual caudal fin length,BD—residual body depth, HW—residual head width, PA—resid-ual propulsive area, MR—muscle area to body area ration,PR—propulsive area to body area ratio, FR—fineness ratio,AR—aspect ratio and CPDF—caudal peduncle depth factor.Correlations significant at alpha=0.05 are shown in bold

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Caribbean. Despite the fact that these locations are bothgeographic and phylogenetically distinct, there was stillstrong coherence in the swimming ability of the few reeffish families we were able to examine. Although therelative swimming ability of the different families wassimilar, swimming speeds of some of the faster taxaappeared to be reduced in the Caribbean relative to theGreat Barrier Reef (e.g. Acanthuridae and Lutjanidae).Although we can only speculate on the reasons for whyswimming abilities seem to be lower in Caribbean speciesof these faster swimming families, the ecological conse-quences may include reduced impact of swimmingbehaviour on dispersal distances and settlement patternsof late stage larvae of these species. However, a largerscale comparison of a greater number of species atmultiple locations is required to properly evaluate rela-tive swimming ability between these two oceans. Overall,the majority of species from both locations appear to becapable of swimming at speeds considerably faster thanaverage currents and can potentially have a substantialinfluence on recruitment processes using swimmingbehaviour.

Acknowledgements We thank I.C. Stobutzki, D.R. Bellwood andM. McCormick for use of experimental equipment. Valuable fieldassistance was provided by S. Street, P. Hansen, H. Parks, K.Hutson, S. Golding, D. Fisher and R. Ferris. We also gratefullyacknowledge field logistical support by the Lizard Island ResearchStation (Australian Museum) as well as the Centre for MarineResource Studies (The School for Field Studies), South CaicosIsland. The Department of Environment and Coastal Resources(South Caicos Island) generously provided access to aquariumfacilities. Some experimental specimens were provided by S.Simpson and O. Haine. We also thank D. Wilson for valuablecomments on the manuscript and assistance with larval identifica-tion of Caribbean species. This work was funded by a Lizard IslandDoctoral Fellowship (Australian Museum) (RF), the AustralianCoral Reef Society (RF), an ARC Discovery Grant (DP0345876)(JML) and a DST International Science Linkages Programme(ISL-CG03-0043) (JML). Portions of this work were carried outunder Australian Museum Animal Care and Ethics Approval 01/01(JML) and James Cook Ethics Approval A202, 402 (RF).

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