Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Linda C Schaffner

Committee Member

Mary C. Fabrizio

Committee Member

Carl T. Friedrichs

Committee Member

Debrorah K. Steinberg

Committee Member

Jacques van Montfrans


All estuarine crabs have two larval stages, the zoea and the megalopa. Zoeae are entirely planktonic, whereas megalopae begin as plankton before beginning to search for settlement substrates as late stage megalopae. At both stages, crab larvae are subject to environmental conditions of the estuary. With changing environmental conditions due to anthropogenic activities and climate change, an understanding of how these planktonic larvae respond to environmental conditions is necessary for understanding subsequent larval supply to benthic populations and implication for fisheries management and habitat restoration. For this dissertation, I: 1) analyzed long-term timeseries in the lower Chesapeake Bay for spatial and temporal change in zoeal abundance and to identify the environmental drivers of these changes; 2) examined effects of climate and environmental conditions on long-term change of dominant crab zoeae in the lower Chesapeake Bay; 3) investigated how warming temperatures in the Bay affect phenology of dominant crab zoeae; and 4) conducted a field experiment in the lower Bay to characterize settlement pattern of megalopae and determine environmental predictors of settlement. Zoeal assemblages of the lower Chesapeake Bay were characterized by distinct assemblages in the tributaries versus the mainstem Bay. Zoeae of the estuarine mud crab (Rithropanopeus harrisii) and fiddler crabs (Uca minax and Uca spp.) dominated in the tributaries, whereas a more diverse assemblage of crab zoeae, including larvae of the mud crab Hexapanopeus angustifrons, pea crab Pinnixa sayana, blue crab Callinectes sapidus, squatter pea crab Tumidotheres maculatus, and fiddler crabs Uca spp. were dominant taxa in the mainstem. Salinity was the single most important factor explaining patterns in the composition of zoeal assemblages in space and time. Variability in total zoeal abundance and richness in both regions was also significantly related to salinity. Abundance of four dominant species, Callinectes sapidus, Hexapanopeus angustifrons, Dyspanopeus sayi, and Pinnixa chaetopterana, in the mainstem significantly decreased from 1985-2002, while no significant trend was observed for dominant species in the tributaries. Variability in abundance of dominant zoeae was significantly related to the North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO), which likely influenced zoeal abundance via streamflow into the Bay. From 1968 to 2002, I found no significant effect of warming on phenology of the zoeae of dominant species, except Rhithropanopeus harrisii, which had an earlier start of season (date of 10% of highest peak of abundance) as water temperature increased. in a field experiment, settlement of C. sapidus megalopae was episodic, beginning in mid-July and extending into November. in contrast, the settlement periods of R. harrisii and D. sayi were shorter than C. sapidus, begining in early-July and ending in early October. Local conditions, especially salinity and water temperature, were as important as factors associated with transport processes (e.g., tidal range, wind direction, lunar phases) in determining settlement of crab megalopae. This dissertation illustrated that zoeal assemblages and successful settlement of megalopal stage of crabs in the lower Chesapeake Bay were sensitive to changes in environment conditions. These changes in zoeal abundance and settlement success have important implications for adult populations of estuarine crabs.




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