Ecological lifestyle and gill slit height across sharks

Abstract

Metabolic morphology-the morphological features related to metabolic rate-offers broad comparative insights into the physiological performance and ecological function of species. However, some metabolic morphological traits, such as gill surface area, require costly and lethal sampling. Measurements of gill slit height from anatomically accurate drawings, such as those in field guides, offer the opportunity to understand physiological and ecological function without the need for lethal sampling. Here, we examine the relationship between gill slit height and each of the three traits that comprise ecological lifestyle: activity, maximum body size, and depth across nearly all sharks (n = 455). We find that gill slit heights are positively related to activity (measured by the aspect ratio of the caudal fin) and maximum size but negatively related to depth. Overall, gill slit height is best explained by the suite of ecological lifestyle traits rather than any single trait. These results suggest that more active, larger and shallower species (and endothermic species) have higher metabolic throughput as indexed by gill slit height (oxygen uptake) and ecological lifestyle (oxygen expenditure). We show that meaningful ecophysiological relationships can be revealed through measurable metabolic morphological traits from anatomically accurate drawings, which offers the opportunity to estimate class-wide traits for analyses of life history theory and the relationship between biodiversity and ecological function.

Keywords: Chondrichthyes; elasmobranch; fast–slow life history; metabolism.

Figure 1.

Schematic of how CFAR (A) is calculated (h ²/s). Black box represents the height of the caudal fin (h); the double-sided arrow in (a) squared (h ²) and blue tail shading are the surface areas (s). Panels show examples of the variation observed across shark species with (a) high (longfin mako Isurus paucus), (b) medium (prickly dogfish Oxynotus bruniensis) and (c) low (longfin catshark Apristurus herklotsi) levels of activity.

Figure 2.

Relationship between activity level (CFAR) and mean gill slit height. Points are coloured by median depth and sized by maximum body size (total length, cm). Green lines are 500 random draws of conditional fit from the posterior distribution of the global model with blue lines indicating intercept differences at varying depths; blue is mean depth (90 m), dark blue and light blue are 1 s.d. in either direction (492 and 17 m). Inset silhouettes are of the longfin mako shark (Isurus paucus; top-right) and the longfin catshark (Apristurus herklotsi; bottom-left).

Figure 3.

Model coefficient estimates for each of the four models evaluated. Ecological lifestyle (global) model estimates are in black (circles) while univariate model estimates are in grey (activity—triangles, maximum body size—squares and median depth—diamonds). Lines for each coefficient indicate the 95% credible interval.