Shelldwellers

Shelldwellers are a class of marine crustaceans first described in the late twentieth century by marine biologists studying deep-sea sponges. The group is distinguished by its hard, calcified exoskeleton that forms a protective shell, which has led to the common name. Shelldwellers belong to the infraorder Crustacea and are closely related to the Decapoda. They are typically found in the mesophotic to bathyal zones of temperate and tropical oceans, where they inhabit crevices of coral reefs or the upper layers of soft sediment. Their shells resemble a miniature pearl oyster, with a flattened underside that allows them to cling to substrates and resist water currents.

The anatomy of shelldwellers consists of a segmented body divided into a head, thorax, and abdomen. The carapace fully envelops the dorsal side, while a distinct ventral plate covers the abdomen. Their limbs are adapted for walking and feeding; the front pair ends in pincers used to gather detritus and small organisms, while the remaining pairs are more elongate and aid in locomotion. Shelldwellers possess gills located beneath the thoracic segments, allowing efficient respiration in low-oxygen environments.

Behaviourally, shelldwellers are primarily nocturnal. During daylight hours they retreat into crevices within the shell, and come out at night to feed on plankton and biofilm. They are hermaphroditic, with reproductive strategies that involve the release of sperm and eggs into the surrounding water. While not apex predators, they play an important ecological role by controlling algal growth and serving as prey for larger fish.

Research into shelldwellers has expanded our understanding of deep‑sea ecosystems, particularly in relation to biogeographic distribution and shell formation. Their shells have been used in paleoclimate studies because of their calcite composition, which records temperature and acidity of ancient waters. The reclassification of several species within this group in 2018 helped clarify evolutionary relationships among crustaceans, highlighting convergent evolution in shell development.