Ontogenetic changes in habitat use by postlarvae and young juveniles of the blue crab

Changing habitat requirements are evident during the developmental cycles o f many species. In this field investigation, we attempted to distinguish be tween depth (shallow vs deep), habitat structure (seagrass species), and st udy site as factors influencing the distribution and abundance of postlarva e and juvenile blue crabs Callinectes sapidus in the Chesapeake Bay. Deep ( greater than or equal to 70 cm mean low water [MLW]) and shallow (less than or equal to 50 cm MLW) suction samples in monospecific Zostera marina and Ruppia maritima beds were taken in the York River, a tributary of the Chesa peake Bay. Our studies revealed ontogenetic changes in habitat use, which s uggested that blue crabs are influenced differently by physical and biologi cal factors even during the earliest life stages. Postlarvae through 3rd in star distributions were not related to seagrass species, but their densitie s increased with distance upriver (regression, p < 0.004, n = 36 to 38, pos tlarvae: r(2) = 0.173, 1st instars: r(2) = 0.308, 2nd-3rd instars: r(2) = 0 .231). This suggests that the smallest instar distributions are related to larval supply and physical forces, such as currents and winds, which determ ine water-column transport. In contrast, 4th and greater instars were signi ficantly more abundant in Ruppia than in Zostera (ANOVA, df = 1, p < 0.05), possibly because of the high shoot density of Ruppia beds. Habitat use by 4th and greater instars may be related to seasonal changes in seagrass shoo t density. Water depth did not influence the distribution of any crab stage . We suggest that habitat selection and differential mortality among habita ts influence larger instar distributions more strongly than they influence the distribution of postlarvae and the earliest instars of C. sapidus.