NOTES ON THE BEHAVIOR OF BLUE CRABS, CALLINECTES SAPIDUS RATHBUN, 1896 FEEDING ON TWO MORPHOLOGICALLY DISSIMILAR CLAMS

We examined feeding by the blue crab, Callinectes sapidus on the softshell clam, Mya arenaria (their shells gape at both ends) and the Atlantic rangia clam, Rangia cuneata (their shell margins shut tightly). The maximum sizes of both species that could be opened by crabs was determined, along with techniques used by crabs to open and eat them. The largest softshell clam eaten was significantly larger than the largest rangia that could be opened. Provided with five softshell clams in each of five size groups at the start of a 14-day experiment, crabs on Day 1 ate proportionately more intermediate-sized (4-5 cm) clams, then larger clams (5-7 cm), smaller clams (3-4 cm), and the largest clams (7+cm) in that order; this pattern continued throughout the first three days, by which time nearly all clams had been eaten. We provided two or three size classes of softshell clams to four size classes of crabs (small, medium, large, and jumbo) and gave two size classes of rangia to two sizes of blue crabs (small, jumbo) and videotaped the crabs' feeding techniques. Blue crabs used 11 techniques to open softshell clams compared with 5 used to open rangia. When opening softshell clams, small and medium crabs pried, pulled apart, and bored into shells frequently, cracking and crushing next most frequently. Large and jumbo crabs cracked and crushed most frequently, followed by prying and pulling shells apart. Twisting the siphon and clam body was also common. Differences in clam shell morphology and strength imply that softshell clams are more easily fed upon by blue crabs than are rangia clams. Size provides partial protection from blue crab predation in that small blue crabs might not be able to feed easily on large softshell clams and perhaps only the largest blue crabs can open large rangia. L'alimentation du crabe bleu Callinectes sapidus a partir de deux especes de clams, Mya arenaria, clam a coquille molle (a coquille beant aux deux extremites) et le clam rangia Rangia cuneata (a coquille fortement fermee) a ete examinee. Pour les deux especes, la taille maximum qui peut etre ouverte par des crabes a ete determinee, ainsi que des techniques utilisees par les crabes pour les ouvrir et les ingerer. Le plus grand clam a coquille molle mange etait significativement plus grand que le plus grand clam rangia qui a pu etre ouvert. Dans une experience de 14 jours, on a fourni 5 clams a coquille molle a chacun des cinq groupes de taille. Le jour 1, les crabes mangent dans l'ordre les clams de taille intermediaire (4-5 cm), puis les grands clams (5-7 cm), les clams les plus petits (3-4 cm) et les plus grands (7+cm). Ce modele se poursuit pendant les trois premiers jours, temps au bout duquel pratiquement tous les clams ont ete manges. Nous avons donne deux ou trois tailles de clam a coquille molle a quatre classes de taille de crabes (petit, moyen, grand et jumbo) et deux tailles de rangia a deux tailles de crabes (petit et jumbo) et nous avons filme les techniques d'alimentation. Les crabes bleus utilisent 11 techniques pour ouvrir les clams a coquille molle et seulement 5 pour ouvrir les rangia. Pour ouvrir les clams a coquille molle, les crabes petits et moyens forcent, dechiquetent et percent les coquilles frequemment, cassant et ecrasant moins souvent. Les crabes grands et jumbo cassent et ecrasent plus souvent puis forcent et ecartent les coquilles. Tordre le siphon et le corps du clam est aussi assez frequent. Des differences dans la morphologie de la coquille et sa solidite impliquent que les clams a coquille molle sont plus aisement consommes que les clams rangia. La taille procure une protection partielle contre la predation par les crabes bleus, donc les petits crabes bleus ne seraient pas capables de se nourrir facilement de clams a coquille molle et seulement les plus grands crabes peuvent ouvrir les grands clams rangia.