Cannibalism: food or neighbours?

MENDEZ CASARIEGO, M.A.; MONTEMAYOR, D.; VALIÑAS, M.S.; LUPPI, T.A.

Atenas

Encuentro; The Crustacean Society Summer Meeting and the 10th Colloquium Crustacea Decapoda Mediterranea; 2012

Crustacean Society

Cannibalism is a common feature widespread among several species. The rational explanations for this behaviour are basically resumed to: a) the cannibal is under conditions of crowding of conspecifics, and/or b) low availability or low quality food that could lead to starvation. To achieve a good meal, the cannibal has to fight with a conspecific that possess similar abilities. For that reason, cannibalism often occurs in size-structured populations between cohorts. In this work, we performed a simple geographical sampling in order to detect differences in cannibalistic behaviour using an intertidal burrowing crab, Neohelice granulata. The broad geographic distribution of this crabs allowed us to compare sites with different crab densities and food quality. We selected intertidals with differences in sediment grain size and organic matter content in sediment. In these intertidals (San Antonio Oeste, San Blas, Villa del Mar, Maldonado, Bahia Blanca, Mar Chiquita) we performed the following procedures: a) sediment granulometry, b) organic matter content of sediment, c) density of crabs, d) % of crab pieces in stomachs of crabs, e) estimation of juvenile cannibalism by tethering crabs. We performed correlations with these variables to describe which of these explains more of the cannibalism observed in this species. As expected, tethered crabs and the % of crabs found inside crab stomachs were correlated (r2= 65%). The highest value of r2 was found between the cannibalized crabs tethered and the organic matter (OM) sediment content (r2=0.97). The cannibalized tethered crabs seem to be highly explained also by grain size (r2= 0.84), which is associated with OM. Density showed no correlation with cannibalized crabs (r2= 0.02, p>0.05). Across the geographical distribution, N. granulta inhabits different types of sediment, from fine grain size to cobble beaches which affect the OM content and food availability. But in these estuaries density varies as well. This allows us to confront these areas to explore in the field what triggers cannibalism in this crab. The results found in this work, even no experiment was performed, suggests that food is the more important variable explaining 97% of the cannibalism showed in this species and population density had no effect in juvenile mortality by cannibalism. The association with OM and grain size, then, suggests that estuaries with fine grain size will show higher OM content and less cannibalism that estuaries with large grain size.