Wave energy and the role of swimming in reef fish ecology

Although functional explanations for the influence of wave energy on marine communities have been well reported for sessile taxa, the underlying mechanisms in highly mobile organisms such as reef fishes have remained largely unexplored. The present study compared levels of wave-induced water motion among reef habitats with the swimming mode and performance of resident reef fishes. Encompassing ten families (Acanthuridae, Chaetodontidae, Labridae, Lutjanidae, Nemipteridae, Pomacanthidae, Pomacentridae, Serranidae, Siganidae and Zanclidae), the generality of this functional relationship was examined both among and within swimming modes, and across tropical and temperate reef fish assemblages. Wave-induced water motion was quantified using Lagrangian and Euler measures of flow across five habitat zones and four exposure regimes commonly found on coral reefs. Significant spatial variations in net flow velocity (7.4 - 43.2 cm s-1) and rates of flow direction change (0.06 - 0.66 Hz) were found among habitats of different depth. Water motion within the crest and flat habitats was largely wave-driven, as rates of flow direction change (0.63 - 0.66 Hz) corresponding closely with surface wave periodicity, with relatively little contribution (14 - 16%) from drift (tidal) flow velocities. Similar spatial variation in wave-induced water motion was found amongst reefs of different exposure (6.2 - 59.5 cm s-1), with exposed and oblique reef crests displaying greater temporal variation in wave height and water motion compared to the relatively static sheltered and lagoonal sites. Overall, average levels of wave energy produced substantial variations in water motion over spatial scales relevant to the distribution and abundance of reef taxa. Fin use censuses of 5,230 individuals from 117 species revealed three primary modes of swimming: pectoral (labriform, 70 spp.), pectoral-caudal (chaetodontiform, 29 spp.), and caudal (subcarangiform, 18 spp.). Experimental trials of critical swimming speed (Ucrit) indicated that labriform taxa displayed the highest average speeds in absolute terms (82.4 cm s-1), although considerable overlap was found in the range of experimental speeds displayed among the three modes (9.2 - 82.4 cm s-1). Whilst body shape varied substantially among taxa within the three modes, variations in body depth and cross-sectional profile were not significantly correlated to swimming speed performance. However, direct comparisons between experimental and field speeds revealed a striking separation in performance amongst swimming modes: field speeds in labriform taxa were 83 ± 4 % (mean ± 1 SE) of their experimental speed, whereas field speeds in chaetodontiform and subcarangiform species were 44 ± 2 % and 49 ± 4 % of their experimental speeds, respectively. Ecological arrangement of these functional attributes in relation to wave-induced water motion was similarly striking among swimming modes. Labriform fishes were the most abundant group, predominating in areas with high levels of water motion, whereas pectoral-caudal and caudal-swimming fishes displayed the opposite trend. These differences reflected the relative swimming performance of the three modes, with most labriform fishes displaying field speeds that matched or exceeded the water flows recorded in wave-swept habitats, compared to the considerably slower speed performances in chaetodontiform and subcarangiform fishes. Further examination of the extensive within-mode variation in labriform taxa revealed an ecomorphological basis in pectoral fin shape. Pectoral fin aspect-ratio (AR) was strongly correlated with swimming speed performance in a similar linear relationship across all three labriform families (Acanthuridae, Labridae and Pomacentridae): species with higher AR fins attained faster size-specific swimming speeds using lift-based thrust. Congruent ecological relationships of increasing pectoral fin AR with increasing water motion in all three families indicated that wave energy has shaped labriform fishes from at least two distinct evolutionary lineages into a common ecomorphological form. Comparisons between tropical (Great Barrier Reef) and temperate (Port Stephens, New South Wales) labrid assemblages revealed two alternate strategies for occupying wave-swept habitats. Variation in pectoral fin shape in temperate labrids was strongly correlated with swimming speed in a relationship similar to tropical labrids. However, the reduced diversity of pectoral fin AR of temperate labrids (aspect ratios of 0.52 - 1.43) compared to the tropical labrids (0.90 - 2.08) indicated a lack of lift-based swimming taxa in the temperate assemblage. Although fin shape provided some explanation for the distribution of temperate labrids in relation to wave energy, increased swimming speeds through increased body size appeared to be the dominant mechanism by which temperate species occupied wave-swept habitats. In contrast, coral reef labrids dominate wave-swept habitats using high AR pectoral fins and efficient liftbased kinematics, with no discernable difference in size among habitat zones. Overall, this study found a consistent relationship between the swimming abilities of reef fishes and their distribution in relation to wave energy, with the fastest swimmers occupying the most wave-swept habitats. Biomechanical evidence suggests reef fishes that dominate these wave-swept habitats display high mechanical and energetic efficiency of locomotion. Encompassing fishes from several divergent perciform lineages, these links between swimming performance and wave energy were irrespective of phylogenetic or trophic status, suggesting a strong, overriding influence of wave energy on reef fish distributions. Ultimately, wave energy appears to have interacted in both ecological and evolutionary terms with the locomotor abilities of fishes to provide an underlying theme in reef ecosystems: biophysical interactions between wave energy and swimming performance shape reef fish assemblages.

Name dc:type.qualificationname

phd

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

James Cook University

Year dc:date.issued

2005

Author dc:creator

• Fulton, Christopher John

Language dc:language

en