Defense mechanism: "Calappa, when seized, often draws up its legs under the broad shields formed by the overlapping edges of the carapace, thus seeming to sham death. No doubt it is more difficult to dismember in this attitude, but it is quite possible that the action has no such special significance in this case, being instinctively adopted whenever the animal is moved against its will. The colouring of many Calappidae is another feature in which they are adapted to their environment, being sand-like, and clearly protective."
Calappa are usually more active than other sand crabs. (Borradaile, 1903)
"The specimens differ from the type in that: (1) the shields at the sides of the carapace are better developed, resembling those of C. depressa Miers; (2) the endostomial septum is complete in its hinder half and hollowed out in its anterior half only; (3) the whole antero- lateral edge is toothed (as in Alcock's figure. In his description he says that the anterior half is smooth). The name clypeata would be a suitable one to apply to this variety." (Borradaile, 1903)
"Dredged in Haddumati and Mulaku Atolls, in 30 — 39 fathoms." (Borradaile, 1903)
Inhabit sandy areas. (Borradaile, 1903)
Respiration: From their concealment in the sand, they create breathing channels to the surface of the sand. "The Calappidae draw in water, like many other crabs, down a passage formed, so to speak, by 'holding the hands against the breast,' and so into the gill-chamber through an opening in front of the chelipeds, guarded by the wide base of the epipodites of the third pair of maxillipeds. But their hands (chelae) are peculiarly shaped for this purpose, being very broad and flat, and usually having a toothed crest, which is held against the forepart of the body. Through the notches between the teeth water can enter, while at least the coarser sand-grains are kept out." (Borradaile, 1903)
Basic mode of nutrient uptake is heterotrophy.
"Judging from the contents of their stomachs, de Haan came to the conclusion that Calappa... feed on other crabs." (Borradaile, 1903)
