Phenotypic Accommodation in Sea Urchins Grown under Geometric Constraint

Sea urchins and their relatives are grouped together taxonomically in the class Echinoidea (phylum Echinodermata). Echinoids exhibit remarkable variation in their pentaradially symmetric body shape phenotypes, from spheroid tests (skeletons) in sea urchins to discoid tests in sand dollars. Researchers have speculated on the morphological evolutionary transitions among these phenotypes without reaching consensus. We designed and performed an experiment in which sea urchins grew under a geometric constraint manifested through restricted vertical spatial dimension physically limiting growth in height. Individuals in this treatment group became flatter relative to individuals in a control group after 8 weeks. These findings may be considered as instantiating phenotypic accommodation, an underappreciated phenomenon that may constitute the first step in novel phenotype evolution. We present preliminary speculations relating phenotypically accommodated changes in shape and morphological evolutionary relationships among echinoids. By demonstrating a plausible initiating mechanism in a morphological transition, we hope to contribute to theoretical discussions about phenotypic evolution, emphasizing the role that physical factors play in changing complex biological systems.