Crabs of the genus Platypodiella (Brachyura: Xanthidae) are known for their association with zoantharians (Hexacorallia: Zoantharia), predominantly of the genus Palythoa (Den Hartog and Holthuis 1984; Den Hartog and Türkay 1991). They make cavities inside their zoantharian hosts, which they use as shelter.

During the Statia Marine Biodiversity Expedition to St. Eustatius (Lesser Antilles, Dutch Caribbean) in June 2015, small crabs (~5–10 mm wide) of Platypodiella spectabilis (Herbst, 1794) were most commonly observed either inside or in close proximity to encrusting Palythoa caribaeorum (Duchassaing and Michelotti, 1860) (Fig. 1a, b). A larger crab of the same species (~30 mm wide) was found hiding in between sponges and coral (M.A. Faasse and G.W.N.M. van Moorsel, pers. comm.), whereas crevices in dead coral may also be a common habitat (Martin and Zimmerman 2007). All crabs were characteristically colored in variable patterns of orange, yellow, black and white (Fig. 1; Martin and Zimmerman 2007).

Fig. 1
figure 1

Platypodiella spectabilis at St. Eustatius. a Inside its regular zoantharian host, Palythoa caribaeorum. b Digging into a P. caribaeorum. c Inside a Niphates digitalis sponge with the parazoanthid Umimayanthus parasiticus

Unexpectedly, two small P. spectabilis individuals were found in cavities inside Niphates digitalis (Lamarck, 1814) sponges associated with the parazoanthid Umimayanthus parasiticus (Duchassaing and Michelotti, 1860). These crab dwellings were similar to burrows observed in Palythoa (Fig. 1c). Although N. digitalis was one of the most commonly observed sponges of St. Eustatius, no P. spectabilis was found in sponges without a zoantharian associate.