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Population dynamics of the sheep crab Loxorhynchus grandis (Majidae) Stimpson 1857 at La Jolla California

  • Author(s): Hobday, Alistair J
  • Rumsey, Scott M
  • et al.
Abstract

An unexploited population of the sheep crab Loxorhynchus grandis (Majidae, Brachyura) was studied for three years in the nearshore region adjacent to the Scripps Institution of Oceanography. Over 1000 crabs were collected in monthly surveys at three sites in water of between 10 and 20 meters depth. Size, sex, reproductive condition, carapace condition, and leg damage were recorded before crabs were individually tagged and released. Four crab aggregations were observed at one of the sites (Sand Plain). The function of these aggregations seems to be for reproduction, as a lower percentage of brooding females, and a higher percentage of males was observed than when crabs were not aggregated. A total of 564 crabs was tagged at the Pier site and the overall recapture rate was 39%. At the Pier adult females were more abundant than males, were recaptured more often (46% vs. 20%), and were also more sedentary than males. The Jolly-Seber mark-recapture technique was used to estimate the regional L. grandis population size, which ranged between zero and 300 crabs. Juvenile crabs were only found at the Dike Rock site and exhibited a peak in abundance between January and May. Adults were found occasionally at Dike Rock, but were most abundant at the Pier. Peak crab abundance at the Pier was in July for two years, and in January for one year. Adult males had significantly more damage to the claws and first pair of walking legs, and likely lose them in aggressive interactions. Females had a high frequency of damage to the first pair, perhaps sustained in mating behavior, while juveniles had radially symmetric patterns of leg damage, suggesting they were more passive and did not react to threats of leg damage. The speed that these crabs can move (0.40 km hr-1 ) was estimated by following tagged crabs for short periods of time. The results of this study also suggest that L. grandis has both alongshore and offshore separation of ontogenetic stages.

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