Reduction of body segmentation in the skeleton shrimp Caprella scaura (Amphipoda: Caprellidae) inferred from comparative internal anatomy of Caprelloidea

In bilaterians, particularly arthropods, body segments are often functionally differentiated along the anterior-posterior axis, resulting in lineage-specific morphologies. Although the acquisition of novel traits in body segments or appendages is considered a key driver of animal evolution, the loss or reduction of these structures has also contributed to the adaptation to new environments and emergence of novel body plans. Members of the family Caprellidae (Caprelloidea: Amphipoda: Crustacea), commonly known as skeleton shrimps, exhibit an unusual body plan characterized by highly elongated thoracic segments (pereonites) and a markedly reduced abdomen derived from the pleon (pleonites and urosomites). Caprellids also lack most appendages that are retained on the pleon in non-caprellid amphipods. Despite these distinctive features, the internal organization of organs within the reduced body segments of caprellids remains poorly understood. In this study, we conducted histological observations of Caprella scaura and found that organs essential for survival and reproduction, particularly the digestive and reproductive systems, are entirely located within the pereonites. Comparative observations of a related non-caprellid amphipod Podocerus sp., revealed a similar distribution of reproductive organs in the pereonites, whereas the pleon contained only extensions of the digestive organs. These results suggest that, in the common ancestor of Caprelloidea, essential organs were already positioned within the pereonites. Furthermore, previous studies indicate that pleonal appendages became functionally reduced in caprellids in association with specialization for an epiphytic lifestyle. The anatomical organization, together with functional changes in appendages, may have facilitated the evolution of the highly degenerated body plan characteristic of caprellids.