Deconstructing bathymetric body size patterns in deep-sea gastropods

Attempts to understand body size as an adaptation to the deep sea have relied primarily on measuring and interpreting size-depth relationships. The numerous studies documenting bathymetric size trends during the last 30 yr report widely divergent results. Body size has been found to significantly increase or decrease with depth in different taxa, but often shows no statistical pattern. Some of this variation is attributable to methodological inconsistencies and/or differences in how taxa or functional groups respond to environmental gradients associated with depth. Here, we show that the shape of the size-depth relationship also depends on the depth zone inhabited. We measured shell size for gastropods (81 species, 3423 individuals) collected in the western North Atlantic from depths of 200 to 5000 m. This database makes it possible to assess size within species and among taxa, and to document how body size changes across the continental margin and abyssal plain. Quantile regression shows that maximum size attained among all gastropods increases with depth in the bathyal zone, and then decreases in the abyss. This overall unimodal pattern appears to be generated partly by within-species clines. Size tends to increase with depth in species with upper-to mid-bathyal distributions, and to decrease with depth in species with lower-bathyal to abyssal distributions, independent of feeding type and mode of larval development. The shift from predominantly positive to negative specific size-depth clines at the bathyal-abyssal transition and the prevalence of smaller individuals in the abyss may reflect a fundamental change in ecological opportunity.