Non-lethal measurement of pectoral fin aspect ratio in coral-reef fishes

Fish swimming mode and body morphology affect the energetics of swimming in a wave-surge water flow

Fish swimming modes and the shape of both the fins and body are expected to affect their swimming ability under different flow conditions. These swimming strategies and body morphologies often correspond to distributional patterns of distinct functional groups exposed to natural and variable water flows. In this study, we used a swimming-respirometer to measure energetic costs during prolonged, steady swimming and while station holding in a range of simulated oscillatory wave-surge water flows, within the natural range of flow speeds and wave frequencies on coral reefs. We quantified the net cost of swimming (NCOS, metabolic costs above resting) for four reef fish species with differences in swimming mode and morphologies of the fin and body: a body and caudal fin (BCF) swimmer, the Hawaiian flagtail, Kuhlia xenura, and three pectoral fin swimmers, the kole tang, Ctenochaetus strigosus, the saddle wrasse, Thalassoma duperrey, and the Indo-Pacific sergeant major, Abudefduf vaigiensis. We found that the BCF swimmer had the highest rates of increase in NCOS with increasing wave frequency (i.e. increased turning frequency) compared with the pectoral fin swimmers. The wrasse, with a more streamlined, higher body fineness, had lower rates of increase in NCOS with increasing swimming speeds than the low body fineness species, but overall had the highest swimming NCOS, which may be a result of a higher aerobic swimming capacity. The deep-bodied (low fineness) pectoral fin swimmers (A. vaigiensis and C. strigosus) were the most efficient at station holding in oscillating, wave-surge water flows.

Comparative morphology of shark pectoral fins

Sharks vary greatly in morphology, physiology, and ecology. Differences in whole body shape, swimming style, and physiological parameters have previously been linked to varied habitat uses. Pectoral fin morphology has been used to taxonomically classify species and hypotheses on the functional differences in shape are noted throughout the literature; however, there are limited comparative datasets that quantify external and skeletal morphology. Further, fins were previously categorized into two discrete groups based on the amount of skeletal support present: (a) aplesodic, where less than half of the fin is supported and (b) plesodic where greater than half of the fin is supported. These discrete classifications have been used to phylogenetically place species, though the methodology of classification is infrequently described. In this study, we sampled fins from 18 species, 6 families, and 3 orders, which were also grouped into five ecomorphotype classifications. We examined the external morphology, extent of skeletal support, and cross-sectional shape of individual cartilaginous elements. Using phylogenetic comparative methods, we show that fin shape does not differ significantly between ecomorphotypes, suggesting there may be some mechanical constraint. However, we find that the internal anatomy of the fin does vary significantly between ecomorphotypes, especially the extent and distribution of calcification of skeletal support, suggesting that the superficial similarity of fin shapes across ecomorphotypes may belie differences in function. Finally, we find that a number of morphological variables such as number of radials, radial calcification and shape, and fin taper all correlate with the extent of skeletal support. Within these morphospaces, we also describe that some orders/families tend to occupy certain areas with limited overlap. While we demonstrate that there is some mechanical constraint limiting external variations in shark pectoral fin morphology, there are compounding differences in skeletal anatomy that occur within ecomorphotypes which we propose may affect function.

Water flow and fin shape polymorphism in coral reef fishes

Water flow gradients have been linked to phenotypic differences and swimming performance across a variety of fish assemblages. However, the extent to which water motion shapes patterns of phenotypic divergence within species remains unknown. We tested the generality of the functional relationship between swimming morphology and water flow by exploring the extent of fin and body shape polymorphism in 12 widespread species from three families (Acanthuridae, Labridae, Pomacentridae) of pectoral-fin swimming (labriform) fishes living across localized wave exposure gradients. The pectoral fin shape of Labridae and Acanthuridae species was strongly related to wave exposure: individuals with more tapered, higher aspect ratio (AR) fins were found on windward reef crests, whereas individuals with rounder, lower AR fins were found on leeward, sheltered reefs. Three of seven Pomacentridae species showed similar trends, and pectoral fin shape was also strongly related to wave exposure in pomacentrids when fin aspect ratios of three species were compared across flow habitats at very small spatial scales (<100 m) along a reef profile (reef slope, crest, and back lagoon). Unlike fin shape, there were no intraspecific differences in fish body fineless ratio across habitats or depths. Contrary to our predictions, there was no pattern relating species' abundances to polymorphism across habitats (i.e., abundance was not higher at sites where morphology is better adapted to the environment). This suggests that there are behavioral and/or physiological mechanisms enabling some species to persist across flow habitats in the absence of morphological differences. We suggest that functional relationships between swimming morphology and water flow not only structure species assemblages, but are yet another important variable contributing to phenotypic differences within species. The close links between fin shape polymorphism and local water flow conditions appear to be important for understanding species' distributions as well as patterns of diversification across environmental gradients.

Physiological plasticity to water flow habitat in the damselfish, Acanthochromis polyacanthus: linking phenotype to performance

The relationships among animal form, function and performance are complex, and vary across environments. Therefore, it can be difficult to identify morphological and/or physiological traits responsible for enhancing performance in a given habitat. In fishes, differences in swimming performance across water flow gradients are related to morphological variation among and within species. However, physiological traits related to performance have been less well studied. We experimentally reared juvenile damselfish, Acanthochromis polyacanthus, under different water flow regimes to test 1) whether aspects of swimming physiology and morphology show plastic responses to water flow, 2) whether trait divergence correlates with swimming performance and 3) whether flow environment relates to performance differences observed in wild fish. We found that maximum metabolic rate, aerobic scope and blood haematocrit were higher in wave-reared fish compared to fish reared in low water flow. However, pectoral fin shape, which tends to correlate with sustained swimming performance, did not differ between rearing treatments or collection sites. Maximum metabolic rate was the best overall predictor of individual swimming performance; fin shape and fish total length were 3.3 and 3.7 times less likely than maximum metabolic rate to explain differences in critical swimming speed. Performance differences induced in fish reared in different flow environments were less pronounced than in wild fish but similar in direction. Our results suggest that exposure to water motion induces plastic physiological changes which enhance swimming performance in A. polyacanthus. Thus, functional relationships between fish morphology and performance across flow habitats should also consider differences in physiology.

Localised intraspecific variation in the swimming phenotype of a coral reef fish across different wave exposures

Wave-driven water flow is a major force structuring marine communities. Species distributions are partly determined by the ability to cope with variation in water flow, such as differences in the assemblage of fish species found in a given water flow environment being linked to swimming ability (based on fin shape and mode of locomotion). It remains unclear, however, whether similar assembly rules apply within a species. Here we show phenotypic variation among sites in traits functionally linked to swimming ability in the damselfish Acanthochromis polyacanthus. These sites differ in wave energy and the observed patterns of phenotypic differences within A. polyacanthus closely mirrored those seen at the interspecific level. Fish from high-exposure sites had more tapered fins and higher maximum metabolic rates than conspecifics from sheltered sites. This translates to a 36% larger aerobic scope and 33% faster critical swimming speed for fish from exposed sites. Our results suggest that functional relationships among swimming phenotypes and water flow not only structure species assemblages, but can also shape patterns of phenotypic divergence within species. Close links between locomotor phenotype and local water flow conditions appear to be important for species distributions as well as phenotypic divergence across environmental gradients.