Temporal Trends of Two Spider Crabs (brachyura, Majoidea) in Nearshore Kelp Habitats in Alaska, U.s.a

Pugettia gracilis and Oregonia gracilis are among the most abundant crab species in Alaskan kelp beds and were surveyed in two different kelp habitats in Kachemak Bay, Alaska, U.S.A., from June 2005 to September 2006, in order to better understand their temporal distribution. Habitats included kelp beds with understory species only and kelp beds with both understory and canopy species, which were surveyed monthly using SCUBA to quantify crab abundance and kelp density. Substrate complexity (rugosity and dominant substrate size) was assessed for each site at the beginning of the study. Pugettia gracilis abundance was highest in late summer and in habitats containing canopy kelp species, while O. gracilis had highest abundance in understory habitats in late summer. Largescale migrations are likely not the cause of seasonal variation in abundances. Microhabitat resource utilization may account for any differences in temporal variation between P. gracilis and O. gracilis. Pugettia gracilis may rely more heavily on structural complexity from algal cover for refuge with abundances correlating with seasonal changes in kelp structure. Oregonia gracilis may rely on kelp more for decoration and less for protection provided by complex structure. Kelp associated crab species have seasonal variation in habitat use that may be correlated with kelp density.