Characteristics and controlling mechanisms in associations involving the southern oyster drill Thais haemostoma linne with the striped hermit crab Clibanarius vittatus (BOSC) and the Blue Crab Callinectes sapidus (Rathbun).

Field and laboratory studies were conducted to determine characteristics and controlling mechanisms involved in attachment of the southern oyster drill Thais haemastoma to the shells of the striped hermit crab Clibanarius vittatus and the carapace of the blue crab Callinectes sapidus. Field studies were conducted at 3 stations on Horn Island, Mississippi, from 11 to 18 August 1984. The incidence of oyster drill attachment to the blue crab was 2.3% on one occasion at the west end of the island. Eight out of the nine blue crabs that were collected with attached drills also had associated barnacles. The percentage of barnacle-infested hermit crabs with attached drills was 73.3% compared to 41.1% of barnacle-infested hermit crabs without drills. A comparison of the three stations showed a percentage of 9.5% at station 1 on the west end, 6.7% at station 2, 5.5 km east, and 2.0% at station 3, 8 km east of the west end. A comparison of habitats within a lagoonal area (station 2) showed 6.7% on sand bars at the inlet, 3.3% on peat bars also at the inlet, 4.0% on sand substrate within the lagoon, 1.6% on Spartina, and .65% on an oyster reef. All field samples were taken only once and replicates need to be taken to determine variations within different parameters before any strong conclusions can be drawn. In a preliminary investigation of distant chemoattraction of T. haemastoma to prey effluents, the fragile barnacle Chthamalus fragilis was shown to initiate a significant positive rheotropic response in these drills compared to the controls (p less than or = 0.005). The effluents from the spat of the American oyster Crassostrea virginica also initiated a significant positive rheotropic response (p less than or = 0.05), however, to a lesser extent than barnacles. The hooked mussel Ischadium recurvum initiated no reponse from these oyster drills (p less than or = 0.05). It is postulated that a major controlling factor in the drill/crab symbiosis is that of distant chemoattraction from barnacles and other prey species associated with these crabs. It is also inferred that blue crabs and hermit crabs play a role in transporting these oyster drills from the ends of the island to the interior, and then into oyster reefs and detrital beds where the drills drop off to feed. This drill/crab association is described as having commensalistic and phoretic properties.