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

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.

MATERIALS AND METHODS

Study Locations

This study was conducted in the southern Middle Atlantic Bight at the mouth of Delaware Bay just north of Cape Henlopen (Fig. 1). Tidal currents at the bay mouth may reach velocities >1 m [s.sup.-1], and tidal amplitude reaches nearly 2 m during spring-tide periods (Garvine 1991). At subtidal frequencies, surface circulation in the adjacent coastal ocean is dominated by buoyancy-driven flow associated with the Delaware coastal current and wind-driven flow characterized by oscillating periods of upwelling and downwelling circulation (Garvine et al. 1997, Epifanio & Garvine 2001).

[FIGURE 1 OMITTED]

Megalopal Collection

Megalopae were collected over 4 cruises that took place during mid to late summer in 2001 and 2002. These periods were chosen to coincide with expected peak density of blue crab and fiddler crab megalopae at the study site (Jones & Epifanio 1995). Each cruise consisted of 3 consecutive nights of sampling during a spring tide period (Table 1). Thus, the total sampling effort consisted of 12 nights of sampling distributed over the 2 y of the investigation. Patches of megalopae were encountered on 5 of those nights.

The duration of each night of sampling varied from 5-6 h depending on weather conditions, and the total number of samples per night varied from 30-37 (see later). Sampling consisted of surface plankton tows from an 8-m vessel. Surface tows provided an adequate picture of megalopal distribution because it has been demonstrated in previous studies that blue crab and fiddler crab megalopae migrate to surface waters during periods of nocturnal flooding tides (Epifanio et al. 1984, Dittel et al. 1991, Tankersley & Forward 1994). Tows were taken every 10 rain with a plankton net (0.5 m diameter; 253-[micro]m mesh) during the flood tide phase. Tow length was always 300 m; distance towed was determined using differential GPS. Because results of previous work indicated that megalopae should be most common in the water column during nocturnal flood tides (Tankersley & Forward 1994, Tankersley et al. 1995, Forward et al. 1997, Welch & Forward 2001), the ideal starting time for each series of plankton tows would have coincided with sunset and low slack water. However, these events rarely occur at exactly the same time, and some compromise was necessary in determining actual starting times. For example, low slack water occurred more than >1 h before sunset on 2 dates when patches of megalopae were observed in the water column. On the first of these dates (September 16, 2001), sampling was begun approximately 1.5 h before sunset and continued until ~0.5 h before slack high water (Table 2). On the second of these dates (September 18, 2001), the initiation of sampling was delayed by inclement weather until ~1.5 h after sunset and ~3 h past low slack water. In this case we decided to continue sampling beyond high slack water to achieve the necessary data points for autocorrelation analysis (see below); this represented the only ebb-tide sampling conducted during the investigation. Sampling never extended beyond sunrise in either of the 2 y of the investigation.

The overall sampling scheme had an Eulerian character in that tows were always taken from the same location, in the same compass direction (toward 138[degrees] true North), and over the same distance during the flooding phase of the tide. Essentially, we were sampling at a fixed location, whereas water and megalopae were transported via tidal currents past the sampling site. Samples were preserved in 4% formaldehyde for later analysis. Water temperature and salinity were measured at hourly intervals. The volume of water filtered for each tow was calculated from flowmeters (General Oceanics Model 2030) fixed in the center of the nets. Samples were split with a Folsom plankton splitter (Dittel & Epifanio 1982). The number of splits was determined by the size of the sample. Subsamples were chosen randomly for further analysis. Megalopae in each subsample were enumerated and identified through standard microscopy, and densities (number of megalopae [m.sup.-3]) were calculated using relevant flowmeter readings.

Megalopae of C. sapidus and Uca spp. represented the most abundant taxa collected throughout the investigation. We did not distinguish between the 2 likely species of Uca (U. pugnax and P. minax) because examination of morphologic characteristics does not allow identification of fiddler crab megalopae beyond the level of genus (Sandifer 1972).

Physical Data and Determination of Spatial Series

Data from each night of sampling yielded a time series of megalopal density at 10-min intervals over a single flood tide phase. However, the intent of our investigation was to determine the spatial (as opposed to temporal) scale of patches of megalopae at the study site. This required the conversion of each time series to its equivalent spatial series. We did this by using data from an S-4 current meter that was moored ~2 m below the surface at the station. Because our sampling concept was Eulerian, we were able to use current meter data to represent water motion past the station. Because tidal currents are rectilinear at the study site, the direction toward which the current flowed was always 318[degrees] (true North) during the flood tide phase. Thus, we could treat the current as a scalar quantity and calculate the displacement of water during each sampling interval as:

D = S x T

where D is displacement (m), S is average current speed for a given time interval (m [s.sup.-1]), and T is duration of each interval (600 s). The total displacement of water on any given night was calculated as the sum of the 10-min displacement values determined over the entire 5-6-h sampling period. This value was then used to derive a spatial series of megalopal density that was based on the original time series (Fig. 2). Coordinate values in this spatial series consisted of megalopal density along the Y-axis and displacement of water along the X-axis. Because of variation in current speed (S) at different times in the flood phase, the displacement values (D) for the respective sampling intervals were not equal. Thus, the coordinate X-Y data points were not evenly distributed along the X-axis. This required further manipulation of the spatial series before statistical analysis could be undertaken (see later).

[FIGURE 2 OMITTED]

Statistical Analysis of Patch Dimension

Analysis of spatial distribution included determination of some threshold density of megalopae that signified the existence a "patch" (Natunewicz & Epifanio 2001). This process was qualitative and involved initial inspection of the data sets to get a sense of the "signal to noise" (i.e., "patch to non-patch") distribution of the larvae. In this study, we chose a threshold value of 7 megalopae [m.sup.-3], which was approximately 5 times the median density of C. sapidus megalopae over the entire duration of the study and l0 times the median density of Uca megalopae. This threshold is about two orders of magnitude lower than the threshold value used by Natunewicz and Epifanio (2001) for zoea larvae of C. sapidus and three to four orders of magnitude lower than maximum densities observed in previous investigations of patches of zoea larvae (Natunewicz et al. 2001, Petrone 2003). Thus, our threshold value reflects the expected decrease in density of megalopae resulting from mortality of zoea larvae. Moreover, this threshold is coherent with reported values for relative densities of zoeae and megalopae in other investigations (Dittel & Epifanio 1982, Epifanio et al. 1988, Olmi 1995). Time series of megalopal densities that did not include at least one value greater than the threshold were considered "patch-less" and were excluded from further analysis.

The next step in the analysis was entirely quantitative and used autocorrelation techniques to determine the characteristic length scale of the patches. In the context of patches of zooplankton, this value is best visualized as 0.5 x L, where L is one of the linear dimensions of a patch (Natunewicz 2000). Thus, the characteristic length scale of a circular patch would be equivalent to the radius (see below), whereas the length scale of an ellipse could be equivalent to half the dimension of either of the axes. In previous studies of zoea larvae (Natunewicz & Epifanio 2001, Petrone 2003), the shape of patches observed in open water ranged from quasi-circles to fat ellipses (i.e., ratio of major-to-minor axis ranged from 1.1-1.8). As a matter of convenience, we will consider the length scales determined in the present study as the radii of the respective patches.

Autocorrelation demands X-Y coordinates that are distributed at equal intervals along the X-axis, and this was not the case in any of the spatial series of megalopal density generated in our study (see above). Thus, each of the spatial series was subjected to additional processing to meet the requirements of the statistical analysis. This consisted of dividing the entire domain of the X-axis (i.e., the entire displacement of water for a given sampling night) into a number of equal intervals determined by the number of samples collected on a given night. Values for the megalopal density corresponding to each of these intervals were interpolated from the original data using cubic spline techniques (Press et al. 1992). This resulted in a set of evenly spaced X-Y values that were used in subsequent autocorrelation analysis. Depending on the number of intervals constituting a series, autocorrelation coefficients were calculated for 10 or 15 lags per series.

The length scale of a given patch was defined as the point in space at which the distribution of megalopal densities in space first becomes de-correlated (Rowe & Epifanio 1994). For purposes of the present analysis, de-correlation was defined as the point at which the correlation coefficient first assumed a value [greater than or equal to] 0 (Richerson et al. 1978). The length scale was determined by summing the number of lags required to achieve de-correlation and multiplying by the distance represented by each lag. Because autocorrelation analysis can be conducted by lagging the respective data sets in either direction, the point of de-correlation represents only half the overall dimension of the patch. In the case of a circular patch, this is represented by the radius.

RESULTS

Physical Data

Current speeds over the duration of the study ranged from slack water near the times of low and high tides to >50 cm [s.sup.-1] at maximum flood. For each night of sampling, the flooding tide was characterized by a progression of decreasing temperatures and increasing salinities. Temperatures over the course of the study ranged from 21.6[degrees]C to 24.9[degrees]C, and salinities ranged from 28.3 [per thousand] to 32.0 [per thousand].

Characterization of Patches

We used results of autocorrelation analysis to confirm the existence of patches during the various cruises and to determine their spatial scale. A total of 6 patches of megalopae (3 C. sapidus and 3 Uca) were observed over the course of the investigation (Table 1). As expected, the density of megalopae (see below) was considerably less than reported for zoea larvae ([10.sup.2] - [0.sup.4] [m.sup.-3]) in previous investigations (Natunewicz & Epifanio 2001, Jancaitis 2003, Petrone 2003). In our study, the maximum density of Uca megalopae within a patch ranged from approximately 10-70 [m.sup.-3]. Density of C. sapidus megalopae never exceeded 16 individuals [m.sup.-3].

Patches arrived at the sampling site throughout the flood tidal phase, but megalopae were rare in the water column near times of slack water (Table 2, Fig. 3, Fig. 4), before sunset (Fig. 3a), or during the ebb phase (Fig. 3b). The radii of patches of C. sapidus megalopae ranged from approximately 500-2000 m, whereas the radii for Uca patches was typically smaller with values around 500 m (Table 1, Fig. 3, Fig. 4). The ranges for C. sapidus and Uca patches overlapped those reported for zoeae of C. sapidus in earlier work near Delaware Bay (Natunewicz & Epifanio 2001, Natunewicz et al. 2001).

[FIGURES 3-4 OMITTED]

DISCUSSION

Results of this study show that blue crab and fiddler crab megalopae are distributed in discrete patches as they are transported into the estuary from the adjacent shelf. As expected, maximum density of megalopae within patches was less than that observed for zoea larvae in previous investigations of patch dynamics (Natunewicz & Epifanio 2001, Natunewicz et al. 2001, Petrone 2003), but was similar to the highest densities reported in earlier, more general studies of megalopal distribution (Dittel & Epifanio 1982, Epifanio et al. 1988, Little & Epifanio 1991, DeVries et al. 1994, Olmi 1995). Because our sampling plan restricted measurements to a single dimension, earlier work has shown consistently that patches of crab larvae assume a quasi-circular to elliptical shape when found in open water (Natunewicz & Epifanio 2001, Natunewicz et al. 2001, Jancaitis 2003, Petrone 2003). Thus, the length scale in the present investigation serves as a surrogate for the approximate radius of a patch. Autocorrelation analysis indicated that the radius of patches of C. sapidus megalopae in our investigation ranged from around 550-2000 m. This implies that typical patches of blue crab megalopae have greater linear dimensions than those reported for conspecific patches of zoea larvae, which have radii on the order of 250-1,200 m (Natunewicz & Epifanio 2001). In contrast, patches of Uca megalopae in our study had radii (~500 m), well within the range reported for zoea larvae. The reasons for these different results are unclear.

In the current investigation, megalopae were collected at different times during the flood tide phase but were rare in samples collected near the times of slack water, before sunset, or during the ebb phase. This temporal pattern has been observed in earlier studies of megalopal distribution (Epifanio 1988, Little & Epifanio 1991, DeVries et al. 1994) and generally supports the contention that blue crab and fiddler crab megalopae use flood tide transport to facilitate upstream movement within the estuary (Tankersley & Forward 1994, Welch & Forward 2001). Results of previous laboratory studies of these taxa suggest that flood tide transport is based on rhythmic vertical migration of the megalopae (Forward & Tankersley 2001). For example, blue crab megalopae in open waters of the continental shelf have an endogenous circadian rhythm in which they are active in the day and inactive at night (Tankersley & Forward 1994). However, in estuarine waters, megalopae also respond to exogenous cues and exhibit a negative photoresponse that dominates their endogenous circadian rhythm (Forward & Tankersley 2001). Additional exogenous cues (i.e., increases in salinity and turbulence) act to override the endogenous behavior, resulting in C. sapidus megalopae only being present in the water column during nocturnal, flooding tides (Welch & Forward 2001).

In contrast, fiddler crab megalopae have an endogenous circatidal rhythm where swimming activity is increased during times of flood tides; this, combined with negative geotaxis, results in upward swimming in the water column (Tankersley & Forward 1994). However, Uca megalopae also exhibit negative phototaxis (Tankersley et al. 1995), which has been invoked to explain the general absence of fiddler crab megalopae in the water column during daylight hours, regardless of tidal stage (Dittel et al. 1991, Little & Epifanio 1991, DeVries et al. 1994).

Results from previous investigations of patch dynamics indicate that aggregations of crab larvae are probably formed at the time of hatching and are the result of simultaneous spawning by assemblages of female crabs (Petrone 2003). Moreover, these patches of zoea larvae appear to remain aggregated on a scale of days to weeks, and careful field measurements have documented development through several zoeal stages within a given patch (Natunewicz & Epifanio 2001). Because aggregative physical processes (e.g., Langmuir circulation, frontal boundaries, gyres) are generally considered important in controlling the horizontal distribution of zooplantonic organisms (Epifanio 1987, Clancy & Epifanio 1989, Eggleston et al. 1998), recent observations demonstrate that patches of blue crab larvae maintain a well defined structure, even when advected to areas of the continental shelf where circulation patterns favor dissipation (Natunewicz et al. 2001). The mechanisms for maintaining patch structure under these conditions are not clear and are not inferable from the present investigation.

That aside, the results of our study demonstrate unequivocally that blue crab and fiddler crab megalopae are distributed in well-defined patches as they enter the estuary. However, our results do not answer the question of whether a typical patch of larvae maintains its spatial integrity through the entire sequence of development. For example, an alternative explanation of our results might postulate a scenario where several consecutive episodes of nocturnal migration to surface waters would serve to meld a number of small patches of megalopae into one large patch or at least into a nearly contiguous series of small patches that might be interpreted statistically as one large patch. This could occur if the nightly ascent of a number of patches of megalopae from the bottom were slightly out of phase. In this case it would be possible for a late-ascending patch to migrate upward into an early-ascending patch that was already in surface waters passing above.

Another explanation for the observed patchy distribution might invoke the concentration of otherwise dispersed megalopae in tidal fronts as they enter the estuary (Epifanio 1987, Clancy & Epifanio 1989, Eggleston et al. 1998). However, our sampling program was not designed to investigate frontal dynamics and casts no light on this possibility. Yet another scenario would result in aggregation at preferred locations on the bottom when megalopae sink out of the water column near the time of slack high water (Welch & Forward 2001), (i.e., megalopae could be attracted to areas of structured bottom such as seagrass or algal beds). The upward migration of these bottom-aggregated megalopae on the next nocturnal flood tide (Tankersley & Forward 1994, Tankersley et al. 1995) would result in a newly formed patch at the surface. However, the present conceptual model for megalopal transport within the estuary (Forward & Tankersley 2001, Epifanio & Garvine 2001) calls for megalopae to leave the water column near the time of high slack tide when turbulent kinetic energy has fallen below some threshold value (Welch & Forward 2001). Therefore, the location of the eventual landing point on the bottom would seem purely dependent on chance. Thus, it is not at all clear how this mechanism would allow megalopae to aggregate in a particular area of bottom--especially in the deep coastal water (>20 m) immediately offshore of our sampling station where any effects of settlement cues would appear moot.

It seems to us that a more parsimonious explanation, and one more consistent with the present conceptual model, would have megalopae already distributed in patches as they approach the estuary. Once the patch enters the estuary, the megalopae within the patch would begin to display a pattern of diel/tidal vertical migration wherein a rise in salinity during nocturnal flood phase would elicit upward swimming, high turbulent kinetic energy during flood would maintain upward swimming, and a decline in turbulence near slack water would cue a cessation of swimming with subsequent sinking to the bottom--all of this tempered by the interactive effect of light (Tankersley & Forward 1994, Tankersley et al. 1995). That all the megalopae in the patch would exhibit these respective behaviors more or less simultaneously seems entirely reasonable. So in this scenario, the expected pattern of vertical migration is not the process that forms the observed patches, but rather the process that controls the diel/tidal periodicity of the occurrence of a previously formed patch in surface waters of the estuary.

But these various possibilities aside, our findings provide the first clear evidence that crab megalopae are often distributed in well defined patches and that these patches have radii on the order of hundreds to a few thousand m. Moreover, our results provide support for the hypothesis that the observed pattern of episodic megalopal settlement within many estuaries (e.g., Jones & Epifanio 1995, vanMontfrans et al. 1995) is at least partly a function of this patchy distribution.

TABLE 1.
Characteristics of megalopal patches for blue
crabs Callinectes sapidus and fiddler crabs Uca spp.

                      C. Sapidus            Uca spp.

Date             Density    Radius    Density    Radius
2001 Sept. 5       --         --        --         --
2001 Sept. 6       --         --       67.5       500
2001 Sept. 7       --         --        --         --
2001 Sept. 16      8.7       1500       --         --
2001 Sept. 17      --         --        --         --
2001 Sept. 18      8.0       2000       --         --
2002 July 28      16.4       550       10.1       600
2002 July 29       --         --        --         --
2002 July 31       --         --        9.1       600
2002 Aug. 12       --         --        --         --
2002 Aug. 13       --         --        --         --
2002 Aug. 14       --         --        --         --

Density = maximum concentration of megalopae within a patch
([m.sup.-3]). Radius = characteristic length scale of patch (m) as
determined by auto-correlation. Dashes indicate that maximum
megalopal density did not exceed the threshold value on a
particular sampling date (see text).

TABLE 2.
Details of sampling schedule on dates when patches of megalopae
were observed in the water column at the mouth of Delaware Bay.
Predicted times of sunset (SS), low slack water (LSW), and high
slack water were (HSW) taken from US Naval Observatory tables and
NOAA tide tables, respectively. Note that low slack water occurred
approximately 3.5 h before sunset on September 16, 2001 and
approximately 1.5 h before sunset on September 18. Sampling began
approximately 1.5 h before sunset on September 16 and approximately
1.5 h after sunset on September 18 (see text for discussion). All
times are EDT.

    Date          SS       LSW      Start       Stop        HSW
                                   Sampling    Sampling

2001 Sept. 06    19:25    19:06     19:35       02:05      01:12
2001 Sept. 16    19:09    15:44     17:35       01:35      22:05
2001 Sept. 18    19:06    17:38     20:35       03:35      23:48
2002 July 28     20:17    19:28     19:30       01:00      01:48
2002 July 31     20:14    21:49     21:50       02:30      04:15

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M. BRANDON JONES AND CHARLES E. EPIFANIO*

Graduate College of Marine Studies, University of Delaware, 700 Pilottown Road, Lewes, Delaware 19958

* Corresponding author. E-mail: [e-mail]