Patches of crab megalopae in the mouth of Delaware Bay—an analysis of spatial scales

Journal of Shellfisheries Research, Jan, 2005 by M. Brandon Jones, Charles E. Epifanio

ABSTRACT Recent studies have shown that zoea larvae of brachyuran crabs often occur in patches that are formed at spawning and persist on time scales ranging from days to at least a week. In the present work we investigated the spatial distribution of the megalopal stage of 2 taxa of brachyurans (Callinectes sapidus and Uca spp.) in Delaware Bay on the east coast of North America (ca. 38.6[degrees]N, 75.2[degrees]W). We used a combination of high-frequency plankton sampling and data from a moored current meter to characterize the length scale of patches of megalopae at a single station in the mouth of the bay. The study consisted of 12 separate sampling periods during the late summers of 2001 and 2002. During each period, plankton were collected every 10 rain over a single flood-tidal phase. Current-velocity data were collected simultaneously via an S-4 current meter moored at the station. Subsequent analysis demonstrated the transport of patches of megalopae past the station during five of the sampling periods. The radius of the patches was determined through autocorrelation analysis. The radii of patches of C. sapidus megalopae ranged from approximately 500-2,000 m, whereas the radii for Uca patches was typically smaller with values around 500 m. This patchy distribution has important consequences for transport and settlement of the megalopae in juvenile habitat.

KEY WORDS: Callinectes sapidus, Uca, patches, megalopa, zoea, crab

INTRODUCTION

Larvae of some species of estuarine crabs are retained within the estuary until settlement, whereas others are flushed to the continental shelf where they may develop through several zoeal stages before returning to the estuary as megalopae (Epifanio 1988). For example, a number of species of mud crabs (family: Xanthidae) produce zoea larvae that are retained in the estuary. These larvae exhibit a pattern of vertical migration that allows them to take advantage of vertical shear in tidal currents, ultimately resulting in retention (Cronin & Forward 1979, Cronin 1982, Dittel & Epifanio 1982, Forward & Tankersley 2001). Conversely, crabs in the families Ocypodidae (e.g., fiddler crabs Uca spp.) and Portunidae (e.g., blue crabs Callinectes sapidus) often have zoea larvae that are passively exported from estuarine systems.

Once in the waters of the continental shell early zoeal stages of Uca are typically distributed near the surface, whereas later stages assume deeper positions in the water column. This allows the larvae to take advantage of gravitational circulation for eventual transport back toward the estuary (Epifanio et al. 1988, Dittel et al. 1991). In contrast, blue crab zoeae remain in surface waters of the continental shelf where they are subject to buoyancy-driven and wind-driven dispersal (for review see Epifanio & Garvine 2001). Eventual delivery of blue crab megalopae back to the estuary is provided by across-shelL downwelling circulation, which along the east coast of North America is coincident with southward wind events (Jones & Epifanio 1995, Garvine et al. 1997). On entering the estuary, the megalopae of both taxa undergo a well-documented change in behavior (Epifanio et al. 1984, DeVries et al. 1994). This consists of upward migration during nocturnal flood tides, which allows megalopae to take advantage of circulation at tidal frequency to facilitate further transport up the estuary (Tankersley & Forward 1994, Tankersley et al. 1995, Forward et al. 1997, Welch & Forward 2001).

Regardless of differences in their respective transport agents, the settlement of megalopae of blue crabs and fiddler crabs occurs as discrete temporal pulses. This has been interpreted as circumstantial evidence that megalopae are distributed as well defined aggregations in space (Jones & Epifanio 1995, Epifanio 1995, Natunewicz 2000). Indeed, recent work in Delaware Bay and on the adjacent continental shelf has documented the aggregated distribution of zoeal stages of blue crabs and fiddler crabs in patches ranging from fat ellipses to quasi-circles and with dimensions ranging from 500-2,400 m in the along-shelf direction and 1,000-2,200 m in the across-shelf dimension (Natunewicz 2000, Natunewicz & Epifanio 2001, Jancaitis 2003, Petrone 2003). Moreover, there is credible evidence that individual patches of early-stage zoeae maintain their integrity for at least a week post hatching and that these patches respond in predictable ways to relevant transport processes (Garvine et al. 1997, Natunewicz et al. 2001). There is also evidence that advanced zoeal stages are often distributed in discrete aggregations, which implies that individual patches of larvae may maintain their integrity throughout the duration of zoeal development (Natunewicz 2000, Jancaitis 2003, Petrone 2003). Thus, a characterization of the fine-scale distribution of the megalopa stage remains the missing piece of the puzzle and is the subject of this study.

In this study, we describe a 2-y investigation of the distribution of C. sapidus and Uca spp. megalopae at a single station in the mouth of Delaware Bay. The study is unique in its application of continuous, high frequency plankton sampling conducted coincident with high-frequency determinations of current velocity. The overall sampling scheme had an Eulerian character that allowed determination of the horizontal distribution of megalopae as they were transported past the station.