EFFECTS OF SUBSTRATE SELECTION AND POSTSETTLEMENT SURVIVAL ON RECRUITMENT SUCCESS OF THE THALASSINIDEAN SHRIMP NEOTRYPAEA-CALIFORNIENSIS TOINTERTIDAL SHELL AND MUD HABITATS

We quantified recruitment of young-of-the-year (YOY) burrowing thalass inidean shrimp Neotrypaea californiensis to 2 habitats of differing st ructural complexity-epibenthic bivalve shell and bare mudflat-and exam ined how differential settlement and post-settlement predation influen ce patterns of YOY abundance. Local densities of shrimp were quantifie d prior to construction of shell habitat in Grays Harbor estuary, Wash ington (USA). Subsequent recruitment of YOY shrimp to epibenthic shell and bare mudflat was measured during a peak settlement pulse and 10 m o post-settlement. In addition, patches of sediment overlying shell wi thin the shell plot ('subsurface shell') were sampled 10 mo post-settl ement. Differential settlement in shell and mud habitats was quantifie d in field and laboratory experiments. We also examined predator-prey interactions between YOY Dungeness crabs Cancer magister and newly set tled shrimp in shell habitat in a laboratory experiment in which prey consumption crab(-1) was quantified as a function of shrimp density. R esults of our studies indicate that dense coverage of epibenthic shell applied to the intertidal site reduced recruitment of ghost shrimp. E pibenthic shell habitat had significantly fewer YOY shrimp than bare m udflat at peak settlement and 10 mo post-settlement, and significantly fewer shrimp than 'subsurface shell' at 10 mo post-settlement. Succes sful colonization of 'subsurface shell' suggests that shrimp postlarva e settled preferentially in areas of the shell plot covered with mud o r possibly were exposed to lower levels of predation than in contiguou s epibenthic shell areas. Results of the field experiment revealed tha t 2 to 5 times fewer shrimp postlarvae settled in shell than mud treat ments; a similar but non-significant trend of lower settlement in shel l than mud substrate was observed in the laboratory habitat-choice exp eriment. YOY Dungeness crabs preyed on shrimp in the laboratory experi ment; prey density had a significant effect on consumption rates but n ot on proportional shrimp mortalities. In sum, although other processe s most likely contributed to patterns of YOY shrimp distribution, post larval habitat selection for mud substrate was a key determinant of re cruitment success. Recruitment patterns may be further modified by pos t settlement mortality of YOY shrimp in shell due to YOY Dungeness cra b predation.