Host-associated morphological convergence in symbiotic pea crabs

Many marine crustaceans form symbiotic relationships, yet there has been little work quantifying morphological adaptations in species specialized on different hosts. Here we examine morphological adaptations of symbiotic pea crabs (Pinnotheridae) to different host phyla. Multiple authors have noted that crabs living with burrowing hosts (burrowing shrimps and annelids) have different carapace shapes than species living with non-burrowing hosts (bivalves, gastropods, echinoderms, and ascidians), but this hypothesis has never been tested. Using digital analyses of taxonomic images, we calculated carapace aspect ratio of 149 pinnotherid species, and used phylogenetic ANOVA to test whether aspect ratio differed among species living with different hosts. Pea crab species living with burrowing hosts had significantly larger carapace aspect ratios (wide carapaces) than species living with non-burrowing hosts (round or square carapaces, or otherwise non-wider than long). Convergent evolution of morphological features in species with specialized host use may be a common—but understudied—pattern in many marine groups.