A Shrimp that Speaks Crab-Ese

Abstract

Pairs of the crab Trapezia ferruginea and the snapping shrimp Alpheus lottini live in very stable communities on Indo-Pacific pocilloporid corals. Both crabs and shrimps were observed to defend the coral against intruders of both species in the field. In the laboratory it has been shown that various appeasement patterns must be performed by an intruder shrimp towards a crab before the shrimp is accepted on the coral by the crab. These patterns appear to be borrowed from the crab's appeasement repertoire and are never performed between shrimps among themselves. The shrimp, as the weaker of the two, has to learn how to inhibit the stronger crab's aggression by using the crab's own language. Many crustaceans are associated with living reef corals (Garth, 1964; Knudsen, 1967; Castro, 1971, 1976; Patton, 1976). Along the eastern coast of Somalia the crab Trapeziaferruginea Latreille and the snapping shrimp Alpheus lottini Guerin are, respectively, the most common xanthid crab and alpheid shrimp (Vannini et al., 1981). Both adult T. ferruginea and A. lottini are usually found in pairs, but they can coexist with a certain number of conspecifics if these are much smaller in size (Garth, 1964; Castro, 1971, 1976, 1978; Barry, 1965). On the coral both species feed on coral mucus and on detritus and organic matter trapped by the mucus (Knudsen, 1967; Patton, 1974, 1976)., In Australia, A. lottini and Trapezia cymodoce (Herbst) were observed, in the field, to coexist in close proximity with no detectable aggression between them; a few observed interactions were assumed to be cleaning activities by the shrimp toward the crab (Lassig, 1977). On the contrary, it was observed both in nature (Lassig, 1977) and in the laboratory (personal observation) that both the crustaceans were aggressive towards intruders of both or other related species. A study of the behavior of A. lottini and T. ferruginea was attempted in Somalia in the laboratory of the Centro per la Faunistica ed Ecologia Tropicali of the Consiglio Nazionale delle Ricerche in Mogadishu. MATERIALS AND METHODS Live corals of the genus Pocillopora were collected during July-August 1979 and August-September 1981, from the reef of Gesira (20 km south of Mogadishu, Somali Democratic Republic) and kept in small perspex containers (10 x 10 x 8 cm, with perforated bases) which were installed in 75-1 aquariums provided with running sea water. A fragment of the coral originally inhabited by the caged animal(s) was placed in each container. Single individuals were caged for 24 h. Then an intruder of the same or different species and sex was introduced. The behavior of the two animals was recorded for 10 min and a further inspection followed 12-15 h later. Each animal was used only once.