A Mating Aggregation of the Spider Crab (Libinia emarginata)

Observations made while SCUBA diving report for the first time in the spider crab (Libinia emarginata) the occurrence of a mating aggregation in Fishers Island Sound, Connecticut, USA. Observations between 30 April and 20 May 1986 showed the aggregation covered a depth range of 5-8 m and an area of approximately 5 000 m2 at a density of approximately 1 crab/m2. There was no significant difference between the sizes of mature females and ovigerous females. Paired males were significantly larger than unpaired males in the overall population, while paired females were not significantly larger than unpaired females in the overall population. There was no correlation between male and female size in paired crabs. Observations of agonistic bouts between males, and male-female pairs exhibiting "obstetrical" behavior, supplement previously published laboratory observations. However, the observations of "obstetrical" behavior showed it had a wider context that involved mature, nonovigerous females as well as females with newly-extruded eggs, and suggested that this behavior should instead be called "protective" behavior. J. Northw. Atl. Fish. Sci., Vol. 13: 77–82 78 J. Northw. Atl. Fish. Sci., Vol. 13, 1992 Representative samples of crabs were collected during dives on 1, 5 and 8 May, 1986 while only the number of ovigerous females was determined during dives on 14 and 20 May 1986. Paired crabs were collected during all dives in 1986 and kept separate from the others. All collected crabs were sexed and carapace length (CL), from the notch of the rostrum to the posterior margin of the carapace, was measured to the nearest 0.1 mm. The abdominal flap of female crabs and egg clutches were examined to differentiate between immature and mature individuals (Hinsch, 1972). Shell condition was also noted (e.g. intermolt, postmolt). Collected crabs were not returned to the study site.