A review of the ecomorphology of pinnotherine pea crabs (Brachyura: <i>Pinnotheridae</i>), with an updated list of symbiont-host associations

Almost all pea crab species in the subfamily Pinnotherinae (Decapoda: Brachyura: Pinnotheridae) are considered obligatory endo- or ectosymbionts, living in a mutualistic or parasitic relationship with a wide variety of invertebrate hosts, including bivalves, gastropods, echinoids, holothurians, and ascidians. While the subfamily is regarded as one of the most morphologically adapted groups of symbiotic crabs, the functionality of these adaptations in relation to their lifestyles has not been reviewed before. Available information on the ecomorphological adaptations of various pinnotherine crab species and their functionality was compiled in order to clarify their ecological diversity. These include the size, shape, and ornamentations of the carapace, the frontal appendages and mouthparts, the cheliped morphology, the ambulatory legs, and the reproductive anatomy and larval characters. The phylogenetic relevance of the adaptations is also reviewed and suggestions for future studies are made. Based on an updated list of all known pinnotherine symbiont–host associations and the available phylogenetic reconstructions, it is concluded that, due to convergent evolution, unrelated species with a similar host interaction might display the same morphological adaptations