Effects of macroalgal structural complexity on nearshore larval and post-larval crab composition

Larval and post-larval crab distribution was surveyed in three different habitats in Kachemak Bay, Alaska to determine temporal and spatial variability. Distribution varied temporally and spatially from June 2005 to September 2006. Nine sites of varying habitat complexity were surveyed monthly using scuba surveys and light traps to measure habitat variables and quantify crab zoeae and megalopae. A total of 10,016 crabs belonging to seven families were identified. Four species comprised the majority (97%) of the total crab assemblages and included Cancer oregonensis, Fabia subquadrata, Telmessus cheiragonus, and Pugettia gracilis. Peak abundances occurred in summer but varied on small temporal scales with species. No single bay-wide variable determined the appearance of all species. Depending on species, appearance may be influenced by seasonality of environmental variables. Spatially, highest abundances occurred in habitats with less structural complexity. Spatial differences in crab abundance may have resulted from variability on large scale physical transport mechanisms and not kelp-mediated flow alterations. Introduction Crabs may have significant ecological importance in marine food web dynamics as key consumers and a food source for other organisms (Hines 1982; Polis and Strong 1996). Seasonality of larval crab recruitment to nearshore habitats may be important in shaping the spatial distribution of adult populations (Quijon and Snelgrove 2005). Documenting larval crab distribution in time and space is valuable in determining the natal origin of post-larval cohorts and may aid in understanding dispersal patterns and population connectivity. Species with lengthy planktonic larval periods are capable of transport over large distances and may recruit into spatially separate communities (Queiroga and Blanton 2005; Park et al. 2007). Some larval species remain nearshore, while others are carried offshore until they return as megalopae to settle in suitable coastal environments (Lough 1974; Paula et al. 2001; Mace and Morgan 2006) where they live as juveniles and adults. Because most crab larvae and post-larvae are unable to swim against horizontal currents because of high energetic costs or physical limitations, the return to nearshore habitats is likely aided by shoreward currents (Paula et al. 2001; Johnson and Shanks 2002). Transport mechanisms such as tides, internal waves, wind patterns, upwelling events, and density-driven currents are thought to transport invertebrate larvae (Shanks 1995a, 1995b; Paula et al. 2001; Johnson and Shanks 2002; Miller and Shanks 2004; see Queiroga and Blanton 2005 for review). The effects of large scale biotic variables such as biogenic structure, predation, food availability, and behavior on temporal and spatial variation of nearshore larval and post-larval abundance are relatively less understood (see Roughgarden et al. 1988; Wolanski and Hamner 1988; Duggins et al. 1990). Communicated by R. Cattaneo-Vietti. B. Daly (&) School of Fisheries and Ocean Sciences, Seward Marine Center, University of Alaska Fairbanks, PO Box 730, Seward, AK 99664, USA e-mail: [email protected]; [email protected] B. Konar School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, PO Box 757220, Fairbanks, AK 99775, USA e-mail: [email protected] 123 Mar Biol (2008) 153:1055–1064 DOI 10.1007/s00227-007-0878-7