Substrate selection by blue crab Callinectes sapidus megalopae and first juvenile instars

Abstract

Various marine and estuarine species utilize chemical cues during settlement. We inves- tigated responses by megalopae and first juvenile (J1) blue crabs to common Chesapeake Bay sub- strates in mesocosm and field experiments. Mesocosm trials examined responses of megalopae or J1 crabs to sand, marsh mud, live oysters Crassostrea virginica, sun-bleached oyster shell, eel grass Zos- tera marina and artificial seagrass in replicate 160 l tanks. Either 10 megalopae or J1 crabs isolated in each of 6 substrates were allowed total access after acclimation to test the null hypothesis of equal distribution among substrates after 13 h. Thirty-five percent of megalopae were recovered from Z. marina, with the remaining substrates containing fewer than half that many. In contrast, 30 % of J1 crabs (with only 17% recovered from Z. marina) were found in live C. virginica. A field experiment quantified responses of ingressing megalopae to Z. marina, marsh mud, and C. virginica. Overnight settlement was significantly higher in Z. marina (x = 3.3 ind.; 60% of total) when compared to mud (x = 0.9; 16 %) or C. virginica (x = 1.3; 24 %). Likewise, J1 crabs were significantly more numerous in Z. marina (x = 3.7 ind.; 55% of total) than in C. virginica (x = 1.8; 27 %) and mud (x = 1.2; 18 %). J1 crab distribution in field plots likely reflected habitat selection by megalopae; laboratory results were equivocal and probably due to artifacts associated with density-dependent agonism. The initial non-random distribution of blue crabs in Chesapeake Bay may be deterministic and due to habitat- selection behavior by megalopae. Selection for seagrass assures the greatest likelihood of maximal survival and accelerated growth. Similar relationships may also exist in estuarine-dependent species with comparable habitat requirements and life-history characteristics.