Development and morphology of podocopan ostracod limbs (Crustacea) – A review

Ostracods are tiny bivalved crustaceans, which have colonised almost all aquatic ecosystems. Their extensive fossil record, stretching back to the Ordovician, attests to their remarkable success, in part due to their calcitic carapace - a hard bivalved shell that can enclose the rest of the body for protection against unfavourable environmental conditions. However, the carapace, and the requirement for the limbs to fit within it, has resulted in a reduced number of limbs, which in turn show evidence of reduction from a biramous crustacean limb. Consequently, ostracod limbs are characterized by limited features and homoeomorphy, hindering our understanding of their evolution. Studies of ontogenetic development can offer additional insights into how ostracod limbs have evolved. For instance, there are at least four developmental pathways to a seven-segmented antennule in adults, which is significant for taxonomic classifications and phylogenetic analyses. Ontogenetic data can also identify possible plesiomorphic and apomorphic characters for the group, thereby testing phylogenetic and taxonomic frameworks. This review focuses on the Podocopa, the largest of the two extant subclasses, and explores how studying limb development during ontogeny can provide insights into the evolution of the group.