Pentagonal Urchin Crab

Echinoecus pentagonus

Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Family: Pilumnidae
Genus: Echinoecus
Species: Echinoecus pentagonus

The pentagonal urchin crab (Echinoecus pentagonus) is a small symbiotic crab that lives on sea urchins, sheltering among their spines or near their mouth. Its rounded, five-sided carapace gives it its name, and its colour usually blends with its host.

Only around 1 cm wide, it is most often associated with Echinothrix urchins. Adults can live close to the urchin's mouth or anal area, feeding on the host's tissue and on food particles, while the spines protect the crab from predators.

These crabs are found on urchins on sand and rubble slopes in the Philippines. Spotting one means carefully checking urchins with a torch, without touching the spines.

Photo: Jonathan Walz, CC BY 4.0, via iNaturalist