Sea Biscuit

• The five-petalled pattern on its test marks out areas of tube feet specialised for gas exchange: these are its gills.
• It covers itself with shell fragments and plant debris held in place by its spines, both to camouflage itself and to weigh itself down against the current.
• Sea biscuits are urchins that have flattened out to live buried, and have lost the long defensive spines of their relatives.
• It is a thick, domed sea biscuit, not flat like the sand dollars of Atlantic beaches, and has a characteristic oval outline.
• It camouflages itself by covering its back with sand grains, shell fragments and scraps of seagrass held in place by its tube feet.
• Its larvae do something exceptional: they can clone themselves by splitting in two, multiplying the number of individuals at no cost in fertilisation. It was discovered in this species and in other sand dollars.
• It swallows sediment continuously and sifts it with a set of internal jaws, Aristotle's lantern, which in the sand dollars is flattened.

Skeleton type

Invertebrate - endoskeleton of calcareous plates (echinoderm)

Habitat and environment

Seagrass meadows and sandy bottoms of the Caribbean, from 0 to 50 m

Sources consulted: Linneo (1758) · Eaves y Palmer (2003), Nature · Smithsonian Marine Station