The allometry of spiracles and gill slits: a window into the respiratory capacity of rays

Metabolic rate governs the pace of life through the allocation and expenditure of energy, yet metabolic rate data are unavailable for most species. External morphological traits, such as gill slits, have been used to infer respiratory physiology and metabolism in a handful of species, however, whether that pattern is generalizable across taxa is unknown. In rays, spiracle length may be adaptive to oxygen uptake capacity, environmental oxygen availability, and activity level, but previous work is limited to a small number of species. Thus, we evaluate the allometry of external respiratory morphology (spiracles and gill slits) in rays using 550 museum specimens representing 88 species. We test whether spiracle length and gill slit height grow isometrically with body size (total length or disc width) within and across species. Overall, we find spiracles and gill slits are morphologically diverse, varying widely in shape within and between families. Despite high morphological variability, spiracle length and gill slit height scale isometrically and are a consistent proportion of body size. Our findings are consistent with literature hypothesizing external respiratory morphology (spiracles and gill slits) is tightly related to metabolic rate and activity level, and demonstrate the value of natural history collections in ecological research. Our results also provide the basis for expanding our understanding of metabolic morphology, enabling future work on life history, extinction risk, and ecological function.