Locomotion in the Ocean: How does swimming mode affect body shape evolution in marine fishes?

Marine teleost fishes are renowned for their diverse body shapes, ranging from the pancake-shaped ocean sunfish to the elongated noodle-shape of the snipe eel. But how did fishes become so morphologically diverse? What contributes to the astounding range of body shapes we often see in aquariums and nature documentaries?

One factor considered to be closely linked to fish body shape is swimming mode. In general, fishes get around their aquatic environments using one of two locomotor strategies: oscillation of the body and caudal fin (called BCF locomotion, this is common in tunas and jacks) or via movement of the median and paired fins (MPF locomotion, common in butterflyfishes and surfperches).

Over the past century, a large body of observational and experimental research has suggested a strong relationship between swimming mode and fish body shape exists, although empirical validation of this idea had yet to occur. This changed recently when a team of scientists led by Dr. Sarah Friedman took a deep dive into the fish locomotion pool. In a study published in Integrative and Organismal Biology, Friedman and colleagues tested long-standing expectations of swimming mode’s relationship to body size using a large dataset consisting of 2295 marine fish species found in demersal (on or near the bottom) and pelagic (open water) habitats. The authors measured eight body traits for each species, with the average value of three adult specimens calculated for each species (see Figure 1).

Figure 1 (Figure 1 from Friedman et al. 2021). Graph shows the distribution of all eight traits measured and color-coded by locomotion mode. Asterisks mark traits showing statistical significance from analyses. Fish outlines show median body shape of fishes for each swimming mode: MPF swimming represented by *Neoglyphidodon polyacanthus*, BCF represented by *Erythrocles monodi*.

Following historical expectations, the authors hypothesized that BCF swimmers would have slenderer bodies, intermediate body depths and narrower caudal peduncles (built like a torpedo). On the other hand, they hypothesized that MPF swimmers would have shorter body lengths, with deeper and more laterally compressed bodies (built like a saucer pan). They also predicted that BCF swimmers would show less variation in body shape compared to MPF swimmers. Considering the body is not used for MPF swimming, they reasoned that MPF swimmers would have less constraints for body shape evolution, allowing for higher trait variation and greater body shape diversity to evolve.

Results showed strong support for their hypotheses on locomotion and body shape diversity. MPF swimmers were generally found to possess deeper body shapes while BCF swimmers possessed more elongated body shapes (see Figure 2). MPF swimmers were also found to show more variation in body width, which suggests that different swimming modes allow fishes to diversify in unique ways from each other. Surprisingly, they found that BCF swimmers actually showed 1.8 times more body shape diversity and higher trait variation than MPF swimmers, a striking contrast to their prediction that BCF swimmers would show less body shape variation.

Figure 2 (Figure 2 from Friedman et al. 2021). Morphospace showing a combination of the eight traits measuring fish body shape. Inset box with arrows shows the loadings on each PC axis. Each dot represents a species colored by locomotion mode (BCF = yellow, MPF = blue), while fish silhouettes demonstrate the extreme-most shapes on each axis.

This study provides the first large-scale quantitative assessment of swimming mode’s influence on body shape in marine teleost fishes. Overall, the authors found strong support for some of the expectations of locomotion’s relation to body shape established by previous generations of research. However, not all expectations were met as they also found cases of elongate MPF swimmers, and some of the most deep-bodied species were actually BCF swimmers. The authors inferred that a substantial overlap of categories suggests the influence of swimming mode on fish body shape acts as a significant trend with numerous exceptions, rather than a strict rule that all fish bodies follow.

Empirical validation is a crucial component of the scientific process, as it can both confirm long-held ideas within a field in addition to providing new avenues of inquiry to investigate. In this study, the surprising result that BCF swimmers have greater morphological variability than MPF swimmers lends further support to the idea that traits with more than one function exhibit faster rates of evolution. In the future, closer examination of multi-functional traits like body shape in other organismal systems will be a promising line of research sure to make a splash!

Kieffer WIlliams is … For more information see his website at http://