Settlement times of blue crab

Marine Biology(2002) 141:863-875 DOI 10.1007/s00227-002-0896-4

R.A. Tankersley 9 J.M.

W e l c h 9 R . B . F o r w a r d Jr.

Settlement times of blue crab (Callinectes sapidu megalopae during flood-tide transport

Received: 5 July 2001 /Accepted: 18 June 2002 / Published online: 9 August 2002 9 Springer-Verlag 2002 Settlement by blue crab (Callinectes sapidus Rathbun) megalopae on artificial settlement substrates was monitored relative to tidal currents throughout ten nights from July to September 1997 in which the phase relationship between tides and the light:dark cycle differed. Most megalopae were in intermolt, and the total number settling to collectors sampled at hourly intervals was greater than totals on collectors immersed all night. Maximum settlement occurred at slack water before ebb tide (SBE), with a smaller peak at slack water before flood tide (SBF). These results support the hypothesis that during flood-tide transport (FTT) blue crab megalopae remain swimming during flood tide at night in response to water turbulence and settle in response to the decline in turbulence occurring near SBE. Settlement peaks near SBF can be explained by a behavioral response of megalopae to increasing salinity at the beginning of flood tide, which results in an ascent response lasting only a few minutes. Depth maintenance in the water column is not maintained at SBF because of low water turbulence. Since light inhibits swimming and upward movement into the water column, settlement, and, presumably, transport were reduced when SBE occurred near the times of sunrise and sunset. Collectively, these results suggest that the phase relationship Abstract

Communicated by J.P. Grassle, New Brunswick R.A. Tankersley([]) Department of Biological Sciences, Florida Institute of Technology, 150 West University Blvd., Melbourne, FL 32901, USA E-mail: [email protected] Fax: + 1-321-6747238 J.M. Welch Department of Biology,Wittenberg University, P.O. Box 720, Springfield,OH 45501, USA R.B. Forward Jr. Duke UniversityMarine Laboratory, Nicholas School of the Environment. 135 Duke Marine Lab Road, Beaufort. NC 28516, USA

between the tide and light:dark cycles affects FTT, the timing of settlement, and behaviors associated with habitat selection.

Introduction The majority of benthic marine invertebrates possess a planktonic larval stage of development. A critical step in the life cycle is the transition from a planktonic to a benthic existence, which may involve some habitat selection by competent larvae at the time of settlement. Among the possible factors influencing the initial distribution of benthic invertebrates, the roles of hydrodynamic and physical processes in inducing behaviors involved in settlement and habitat selection are largely unknown. Recent evidence suggests that settlement involves both passive and active larval movements (review by Butman 1987). Larval dispersal by water currents occurs over large spatial scales (> km) and is primarily considered a passive process since horizontal water motion typically exceeds the weak swimming capacity of larvae (Mileikovsky 1973; Chia et al. 1984). At smaller spatial scales near the settlement site, behavioral control of larval movement and habitat selection is possible. However, passive transport and active habitat selection are not mutually exclusive and their relative importance depends, at least in part, on spatial and temporal differences in the flow environment (Butman 1987). For example, behavioral responses of larvae to environmental stimuli, including light, pressure, gravity, and salinity change, may alter their vertical position in the water column, placing them at depths allowing passive transport or where fluid motion is reduced and settlement is possible (Mileikovsky 1973; Butman 1987). Although hydrodynamic processes responsible for generating currents often vary with time (e.g., tidal currents), few field studies have explored the influence of temporal changes in water flow on the timing of settlement and the selection of potential habitats.

864

Accordingly, the present study considers the relationship between the timing of postlarval settlement relative to current flow in a tidal estuary. Larval release in the blue crab Callinectes sapidus occurs during the summer and early fall near the mouths of estuaries (Tankersley et al. 1998; R. Tankersley, personal observation). Hatching is synchronized to occur during ebb tides, and larvae are advected seaward to undergo development in coastal areas (e.g. Smyth 1980; McConaugha et al. 1983; Epifanio et al. 1989). After passing through seven or eight zoeal stages, blue crabs molt to the megalopa stage (Costlow and Bookhout 1959). Onshore transport of megalopae from the shelf to the entrances of estuaries is mediated by wind-driven Ekman circulation associated with downwelling-favorable wind events (reviewed by Epifanio 1995; Epifanio and Garvine 2001). Once near the mouth of the estuary, megalopae undergo flood-tide transport (FTT) resulting in up-estuary movement to nursery areas (reviewed by Forward et al. 2002b). Flood-tide transport is defined as movement into the water column during flood tide and residence on or near the bottom during ebb tide (reviewed by Forward and Tankersley 2001). Numerous field studies in estuaries of the western North Atlantic indicate blue crab megalopae are abundant in the water column during nocturnal flood tide, but not at other times (Dittel and Epifanio 1982; Brookins and Epifanio 1985; Mense and Wenner 1989; Little and Epifanio 1991; De Vries et al. 1994; Olmi 1994). Megalopae are not present in the water column during daytime flood tides because light inhibits swimming (Forward and Rittschof 1994; Tankersley et al. 1995). For a megalopa undergoing FTT there is a behavioral sequence consisting of: (1) an ascent from the bottom during flood tide, (2) position maintenance in Fig. 1 Callinectes sapidus. Conceptual model of flood-tide transport and settlement of megalopae in estuaries (modified from Forward and Tankersley 2001; Forward et al. 2002b)

the water column during transport, (3) a descent back to the bottom toward the end of flood tide, and (4) position maintenance by either attaching to substrate or remaining near the bottom during ebb tide (Forward and Tankersley 2001). This sequence could result from a biological rhythm in activity or vertical migration and/ or behavioral responses to environmental factors (Forward and Tankersley 2001). Since blue crab megalopae have a circadian rhythm in activity that is inappropriate for FTT (Tankersley and Forward 1994; Forward et al. 1997a), the underlying behavior results from a series of responses to environmental cues associated with different phases of the tide. The current conceptual model (Fig. 1) for FTT is that blue crab megatopae ascend from the bottom during flood tide in response to a relative rate of salinity increase (+AS) (phase 2; De Vries et al. 1994; Tankersley et al. 1995). Water turbulence due to flood currents stimulates sustained swimming (Welch 1998; Welch et al. 1999; Welch and Forward 2001), and megalopae are transported passively by strong horizontal currents (phase 3). The decline in turbulence during slack water at the end of flood tide (i.e., slack before ebb; SBE) cues them to stop swimming and attach to substrate (phase 4). The sequence of cues during ebb tide is inappropriate to stimulate an ascent from the substrate, because salinity typically declines (-AS) in estuaries during falling tides. One main prediction of this model is that megalopae are cued to settle out of the water column and attach to substrate at the end of flood tide by the decrease in water turbulence (Welch et al. 1999; Welch and Forward 2001). The present study was designed to test this hypothesis under field conditions. Settlement patterns of blue crab megalopae were measured and compared to current speeds and water turbulence levels during nocturnal flood and ebb tides. The prediction of the model was verified, but settlement was modified by the timing of SBE relative to the time of sunset and sunrise.

Phase 3

Hydrodynamic Transport Megalopae swim in response to high turbulence to maintain their position in the water column and are transported upestual y by strong flood-tide currents.

Phase 2

Vertical Migration~Ascent During nocturnal./lood tides, megalopae ascend into the water column in response to rising salinity

Phase 4

Settlement~Habitat Selection Phase 1

Position Maintenance During the day and nocturnal ebb tides, megalopae maintain their position near the bottom

865

Materials and methods Field studies Field studies were conducted from July to October 1997 from a bridge platform located ~1.5 km inside the entrance to the Newport River estuary (Beaufort, North Carolina, USA: 34~ 76~ see De Vries et al. 1994 for a map of the area). Water depth at the platform was about 4 m at mean low low water (MLLW). This area experiences semi-diurnal tides with a maximum amplitude of < 1 m during neap tides and 1-1.4 m during spring tides. Blue crab megalopae in the estuary near the bridge platform have previously been shown to undergo FTT (De Vries et al. 1994). Settlement was monitored by sampling the number of megalopae that attached to artificial settlement substrates (passive collectors) consisting of "hog's hair" filter material (sleeves) surrounding an inner plastic cylinder (Metcalf et al. 1995). A total of eight collectors were suspended from the edge of the bridge platform and positioned so that their tops were ,-~1 m below the surface at MLLW. For each sampling period, collectors were divided into two groups. Those in the first group ( N = 4 ) were removed from the water at 1-h intervals and replaced with collectors having clean hog's hair sleeves. The second group (N = 4) remained in the water at the same depth as the above collectors during the entire sampling period (i.e., sunset to 2 h after sunrise). This procedure permitted a comparison between the sums of megalopal abundances at hourly intervals and cumulative settlement over the entire night. Upon removal from the water, the filter material was rinsed with fresh water to remove megalopae attached to the sleeves using the standard protocol of Metcalf et al. (1995). The rinse water was collected in buckets and sieved to remove megalopae, which were placed back in seawater in collection bottles. All plankton and collector samples were examined within 12 h using a dissecting microscope. Blue crab megalopae were identified using the description of Costlow and Bookhout (1959) and classified according to molt stage as intermolt or premolt (Aiken 1973; Anger 1983; Stevenson 1985). In addition, the number of first stage juvenile (J I) crabs was counted. Since premolt megalopae could molt to the Jl stage between the time of collection and counting, the total number of megalopae and crabs was used to estimate megalopal settlement on each collector. During megalopal sampling, simultaneous measurements were made of water level, current speed, and water turbulence. Tide height relative to MLLW was obtained from a tide station [NOAA National Water Level Observation Network (NWLON)] located near the sampling platform. Current speed measurements were made with an acoustic doppler velocimeter (ADV; Sontek, San Diego, Calif.). The ADV was attached to a 5 cm diameter steel pole mounted on the bridge platform and was positioned so that its sensor was located ~1.0 m below the surface of the water at MLLW. Thus, current measurements were obtained at approximately the same depth as the settlement collectors. Mean current vectors were calculated based upon the velocities along the x-, yand z-axes sampled at 25 Hz over a l-rain period. Sampling was repeated every 15 rain. Current speeds (CS) recorded during ebb and flood tides (based on direction of flow) were considered negative and positive, respectively. Turbulence was calculated as turbulent kinetic energy (TKE)(Marras6 et al. 1990; Peters and Gross 1994) using the formula: TKE = ~ ( ~ +s2v +s~)

(1)

2 2 where sx, s;, and s.2 represent the variances of the current velocities (cm s -~) along the'x-, y-, and z-axes, respectively. Sampling was conducted on ten nights throughout the summer that were divided into five periods in which samples were collected on two consecutive nights. Since the phase relationship between the tide and light:dark cycles differed throughout the lunar cycle and was hypothesized to influence the time of settlement, the sampling

periods were classified into three groups based on the timing of SBE relative to sunrise and sunset. During the period of the first group (16-18 September 1997), SBE occurred 1-2 h after sunset. Thus, only the last few hours of the flood tide occurred in darkness. In the second series (11-13 July, 10-12 August, 1997) SBE was in the middle of the night. Finally, in the third series (16-t8 July, 1416 August, 1997) SBE was near sunrise. For the last two series, most of the flood-tide period occurred during the dark phase. Minor peaks in settlement occurred near the time of slack water before flood (SBF; see "Results"). To identify the environmental variables that may be responsible for stimulating megalopae to enter the water column at SBF, simultaneous measurements of bottom and surface currents and salinity near the bottom were conducted at the sampling platform during two complete tidal cycles on 1%18 October 1997. Since Callinectessapidusmegalopae are known to ascend in response to an increase in salinity (+AS) (De Vries et al. 1994; Tankersley et al. 1995), the intent of these measurements was to determine whether there was a transitory increase in salinity near the bottom soon after SBF (i.e. beginning of flood tide). Near-surface currents were measured with the ADV as described above, and bottom currents were measured with an Inter Ocean S-4 current meter (Inter Ocean Systems, San Diego, Calif.) positioned ~0.5 m above the substratum. Salinity was measured with a Sea-Bird S B E 25-03 sea logger (Sea-Bird Electronics) having the conductivity sensor positioned ,~6 cm above the bottom. Conductivity was measured at a sampling rate of 8 Hz, and the average value recorded at 1-s intervals. Conductivity values were converted to salinity and used to calculate the rate of change in salinity (AS; psu s-~). Although these physical measurements were made 1 month after the last measurements of megalopal settlement, measurements of conductivity in the Newport River estuary (Blanton et al. 1999) and near the sampling site (De Vries et al. 1994) at different times of the year and lunar cycle indicate that the pattern for rates of change in salinity are consistent. In fact, the basis for suspecting that salinity increases occurred near SBF was close inspection of the data of De Vries et al. (1994) and Blanton et al. (1999), which were collected from 8 July to 5 September 1992 and 4 March to 18 April 1996, respectively. Data analysis To determine the synchrony and phase relationships between water level (WL), current speed (CS), and water turbulence (TKE) values recorded during each sampling period, tidal periods and amplitudes were estimated using periodic regression analysis (Batschelet 1981). Water-level measurements were fitted to the following sinusoidal model using nonlinear least-squares regression analysis (SPSS 1998): WLi=Y0+A.cosy.(tt+t0),

i=l,n

(2)

where WL is water level relative to mean low low water (MLLW; dependent variable), Y0 is the y-intercept (in m), A is the tidal amplitude (in m), 2 is the period length (in h), tg is the time when the sample was recorded, and to is the peak phase or acrophase (i.e. time of high tide; in h). No attempt was made to adjust the model or add additional terms to account for diel differences or asymmetries in tidal amplitude. For CS and TKE, the nonlinear regression analysis was repeated after substituting each variable for the dependent variable (WL/) in the sinusoidal model (Eq. 2) and replacing 2 with either the calculated 2 for the WL data collected during the same sampling period (for CS) or 1/2 this value (for TKE). The coefficient of determination (R 2) for each regression equation provided an estimate of the proportion of the variation in the dependent variable accounted for by the sinusoidal model. Only results of regression analyses in which iteration resulted in stable parameter estimates (convergence) are reported. Temporal relationships among abundances of megalopae in collector samples and tidal current speeds during each sampling period were determined using cross-correlation analysis (CCA) (Chatfield 1996). To ease interpretation, absolute values rather than direction-coded (i.e. + flood, - ebb) current speeds were used.

866 Since CS and TKE values were highly correlated and synchronized (see "'Results"), separate analyses comparing the phase relationship between TKE and megalopal settlement were not necessary. All variables were transformed using first-order autoregressive models to remove autocorrelation prior to analysis (Chatfield 1996). Cumulative settlement of rnegalopae on collectors sampled at hourly intervals was compared to settlement on collectors that remained in the water during the entire sampling period (i.e. all-night collectors) using linear regression analysis. All values were logtransformed prior to analysis in order to satisfy the assumptions (i.e. normality and homoscedasticity) of the test.

sampling periods and accounted for 21-61% of the variability in the data (Table 1). In general, turbulence was highest during m a x i m u m current speeds and declined to their lowest values during slack water (i.e. current speeds = 0), as indicated by their similar phase relationships (to) relative to water level (Table 1; see Figs. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11).

Developmental stage of megalopae

Results Hydrological variables As expected, periodic regression analysis of the water level data yielded period length (2) estimates between 11.10 and 12.81 h (Table 1), which are consistent with a semi-diurnal tidal regime. Sinusoidal models were also successfully fit to the current speed measurements for all sample dates. Tidal currents were out o f phase with changes in water level and peaked 1.81-3.29 h before high tide (Table 1; see Figs. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11). Turbulent kinetic energy measurements were more variable (Table 1), making it difficult to obtain stable parameter estimates for the periodic regression. One reason for the high variability was that the mounting for the A D V was not rigid enough to prevent unwanted oscillations during periods of high current speeds and wave activity. Although these oscillations had little effect on the mean current values recorded for each sampling interval, calculations of T K E were more sensitive to high frequency noise. Therefore, water turbulence values based upon current measurements that exceeded the signal-to-noise ratio of the A D V were excluded from the analysis. Nevertheless, sinusoidal models were successfully fit to T K E measurements for eight of the ten Table 1 Results of periodic regression analysis on hydrologic variables (water level, current speed, and turbulence) measured during each sampling period in 1997. Wavelength (2; in h) estimates for water level values were used to fit the remaining sinusoidal model parameters (Yo y-intercept; A amplitude; to acrophase) for current speed and turbulence measurements collected during the Sampling period

Water level

Megalopal abundance and settlement The cumulative abundance o f megalopae settling on collectors depended upon the timing of SBE relative to the day:night cycle (Table 3). Mean total numbers of megalopae settling on both hourly sampled and all-night collectors were an order of magnitude higher when SBE occurred in the middle of the night as c o m p a r e d to the other times o f SBE (Table 3). Although differences could be partially the result o f nightly variations in same sampling period. Acrophase values are expressed relative to water level and therefore represent the time (in h) of maximum flood currents and water turbulence relative to high slack water. Stable model parameters could not be estimated for turbulence values calculated using current speed data collected on 17-18 July and 14-15 August, 1997

Current speed

Turbulence Y0 (cm2 s-I)

A (cm2 s-t)

Rz

-2.17 -1.90

1.94 4.15

1.60 3.65

0.21 0.47

0.97 0.91 0.97 0.92

-2.63 -2.39 -1.29 -1.86

0.96 0.54 0.65 0.70

0.31 0.16 0.20 0.25

0.6 l 0.23 0.23 0.29

0.96 0.96 0.96 0.96

-2.11

0.88

0.53

0.53

-2.98

1.64

0.57

0.34

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A (m)

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to

Y0 (cm s-J)

A (cm s-I)

R2

12.22 12. I0

0.73 0.76

0.65 0.65

0.99 0.99

-2.38 -1.97

-3.06 -4.32

69.09 68.53

0.93 0.96

12.37 12.81 12.41 I 1.83

0.67 0.55 0.59 0.64

0.36 0.38 0.35 0.32

0.99 0.99 0.99 0.99

-2.41 -2.19 -l.81 -2.09

-3.55 -9.00 -1.83 -6.11

41.69 37.46 33.62 36.90

11.10 11.95 11.29 12.03

0.50 0.55 0.57 0.60

0.38 0.41 0.39 0.43

0.99 0.96 0.99 0.97

-2.05 -3.29 -2.26 -2.62

-2.35 -7.19 -2.90 -5.99

41.27 49.38 45.00 51.95

2 Series 1 16- 17 Sep 17-18 Sep Series 2 11-12 Jul 12-13 Jul 10-11 Aug 11-12 Aug Series 3 16-17 Jul 17-18 Jul 14-15 Aug 15-16 Aug

The majority (97-98%) of Callinectes sapidus megalopae that settled on passive collectors were in intermolt (Table 2). Moreover, the relative abundances of both premolt and intermolt megalopae were similar for collectors that were sampled at hourly intervals and those that remained in the water all night. Although very few first crabs were present in samples, the highest percentage (1.1%) occurred on the all-night collectors, which p r o b a b l y resulted from premolt megalopae settling and molting during the night. F o r this reason, megalopae and first crabs were pooled to estimate megalopal abundance. These results indicate that the two sampling conditions were not selecting for different molt stage megalopae or first crabs.

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Fig. 2 Callinectes sapidus. Settlement of megalopae when the time of slack water before ebb tide (SBE) was shortly after sunset on the night of 16--17 September 1997. Water level (relative to mean low low water), current speed, water turbulence (TKE), and megalopae abundance on passive collectors (megalopae collector-t) plotted as a function of time of day. Night is indicated by the shaded bar at the top. Solid lines in the water level, current speed, and turbulence plots are the results of fitting a sinusoidal model (Eq. 2) to the data using nonlinear regression analysis (see Table 1). Points where the regression line for current speed intersects the origin represent slack water megalopal abundance, the trends probably result from the time of SBE relative to sunrise and sunset.

Series 1: time o f SBE shortly after sunset Settlement when SBE occurred shortly after sunset was studied on 16-17 September (Fig. 2), when high tide occurred ,-,1.3 h after sunset, and on 17-18 September (Fig. 3), when the time was ,-,2.15 h after sunset. Both sampling dates coincided with spring tides (i.e. full moon). Although the absolute numbers of megalopae settling on collectors were relatively low and varied between the two sampling periods, the general pattern

0:00

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Fig. 3 Callinectes sapidus. Settlement o f megalopae during spring tide when slack water before ebb tide was shortly after sunset on the night o f 17-18 September 1997 (details as in Fig. 2)

was similar. Two distinct peaks in megalopal settlement were apparent in the time series. The first peak occurred at SBE just after sunset (Figs. 2, 3), Since sampling was conducted on consecutive nights, the timing of the settlement peak advanced by 1 h from 16-17 September to 17-18 September and coincided with the shift in the time of SBE (Figs. 2, 3). The second peak occurred later in the night, near the time o f SBF. These peaks in settlement at slack water resulted in large negative crosscorrelation coefficients (maxCC) at l a g = 0 (Table 4). Settlement quickly declined during the first few hours of flood tide (Figs. 2, 3).

Series 2: time of SBE in the middle of the night Settlement times in the lunar m o n t h when SBE occurred near the middle of the night were measured on four nights (11-13 July, 10-12 August; Figs. 4, 5, 6, 7). Neap tides (first quarter m o o n phase) occurred on these nights, and the time of nocturnal high tide varied from

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tide when SBE was near the middle of the night on 11-12 July 1997 (details as in Fig. 2) 0136 hours (t2 July; Fig. 4) to 0239 hours (12 August; Fig. 7). Settlement of megalopae on collectors followed a consistent pattern (Table 4; Figs. 4, 5, 6, 7). Small peaks in settlement occurred at the beginning of flood tide near the time of SBF on all dates except 11-12 July (Fig. 4). Settlement levels then declined to near zero as flood tide currents increased to maximum values. Settlement increased slowly during the latter half of the flood tide as currents declined and then increased dramatically during the sampling interval near the time of SBE (i.e. negative maxCC values at 0 and 1 lag; Table 4). Subsequently, megalopal settlement declined rapidly to near zero at the beginning of the ebb tide.

Series 3: time of SBE near sunrise For these four sampling periods (Figs. 8, 9, 10, 11), the time of SBE ranged from 0.75 h before sunrise on 15

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Fig. 5 Callinectes sapidus. Settlement of megalopae during neap tide when SBE was near the middle of the night on 12-13 July 1997 (details as in Fig. 2) August (Fig. 10) to about I h after sunrise on 18 July (Fig. 9). During all four sampling periods, megalopal settlement on the collectors was relatively low, yet transient peaks were observed at the beginning of flood tide (Figs. 8, 9, 10, 11). Unlike the previous two sampling series, settlement levels were consistently higher near the time of SBF than at SBE. However, the absolute number of megalopae per collector at SBF was very similar on nearly all sampling dates (Figs. 2, 3, 5, 6, 7, 8, 9, 10, 11). During sampling periods when SBE occurred at or slightly after sunrise (Figs. 8, 9, 11), relatively few megalopae settled during flood tide and settlement levels did not increase at the time of SBE. The only exception to this pattern occurred on 14-15 August (Fig. 10) when SBE occurred ~45 min before sunrise. During this period, settlement was relatively low (30 megalopae collector-]; Fig. 10), but levels were similar during the two slack water periods. Thus, sunlight did not cue settlement on collectors located in the upper meter of the water column.

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Fig. 7 Callinectes sapidus. Settlement o f m e g a l o p a e d u r i n g neap

Temporal changes in salinity and current speed

ascend into the water column [threshold rate o f salinity increase ( + A S ) = 0 . 5 • -3 psu s -I (Tankersley et al. 1995)]. To test if a transitory +AS occurred with the onset of flood tide currents, changes in salinity were monitored at the study platform over a complete tidal cycle. Currents were measured at the surface and near the bottom: salinity was measured only near the bottom, since megalopae are predicted to be on or near the bottom during ebb tide. Data are presented for only one period, since results were similar for both sampling dates (Fig. 12). On both sampling dates (17, 18 October), surface and bottom currents were closely synchronized, with the greatest cross-correlation coefficients (maxCC) occurring at 0 lag (17 October: maxCC -- 0.99; 18 October: maxCC = 0.99). Ebb currents were generally faster than flood currents, and currents near the bottom were slightly slower than surface currents (Fig. 12). At the beginning of flood tide, salinity increased rapidly, resulting in several brief positive spikes in the rate of

During most sampling periods, a brief increase in megalopal settlement on collectors was observed at the beginning of flood tide near the time of SBF (Figs. 2, 3, 5, 6, 7, 8, 9, 10, 11). In some cases, settlement numbers subsequently declined and remained low throughout most of the flood tide before increasing dramatically at SBE (Figs. 5, 6, 7, 10). Peaks in megalopal settlement near SBF were inconsistent with FTT, because megalopae are predicted to be near the bottom at the end of ebb tide (i.e. SBF) and enter and remain in the water column during flood tide, when currents are favorable for onshore transport (Forward et al. 2002b; Fig. 1). These results suggested that megalopae entered the water column, but were not cued to sustain swimming, and quickly settled. We hypothesized that these minor peaks were the result of a rapid increase in bottom salinity at the beginning of flood tide that was sufficient to cue megalopae to

tide w h e n SBE was n e a r the midd',e o f the night on 11-12 A u g u s t 1997 (details as in Fig. 2)

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Fig. 8 Callinectes sapidus. Settlement of megalopae during spring tide when SBE was near sunrise on 16-17 July 1997 (details as in Fig. 2)

salinity change (AS) ranging from 0.5• -3 to 1.0xI0 -3 psu s-~, which exceeded the threshold. During ebb tides, salinity decreased, and AS values were generally negative. Although some brief positive spikes in AS occurred during falling tides (e.g. Fig. 12), rates rarely exceeded 0.5x10 -3 psu s-1.

Settlement comparisons The cumulative number of megalopae settling on collectors sampled at hourly intervals was significantly higher than the number settling on collectors that remained in the water during the entire sampling period for most of the ten sampling nights [Fig. 13; sum of hourly samples = (all-night collectors)l144; t = 20.1, df-= 18, P 12 h. Second, megalopae that settled on collectors that remained in the water all night may have been more susceptible to predation, since the chance of predation likely increases with increased immersion time. Finally collectors that remained in the water all night were sampled in the early morning. Thus, light at sunrise may have induced some of the megalopae to leave the collectors and descend to the bottom, thereby reducing the apparent number of settlers. Settlement on artificial substrates is frequently used to estimate decapod larval supply and assess temporal and spatial patterns of recruitment in estuaries (reviewed by Moksnes and Wennhage 2001). Since onshore transport 6f blue crab megalopae to the entrance to estuaries is mediated by wind events (reviewed by Epifanio 1995; Epifanio and Garvine 2001), the number of megalopae available for transport into estuaries and settling on collectors should vary with wind direction and velocity, not lunar phase. Nevertheless, several studies employing passive collectors indicate that settlement has a lunar or semi-lunar periodicity that is related to the neap/spring tide cycle (Van Montfrans et al. 1990; Boylan and Wenner 1993; Mense et al. 1995; Metcalfet al. 1995). An underlying hypothesis tested in these studies was that maximum settlement should coincide with peaks in the magnitude of tidal currents (i.e. during spring tides). This hypothesis was confirmed by Mense et al. (1995), who reported greater numbers of megalopae collected during new and full moons at a site near Masonboro Inlet (Wilmington, North Carolina, USA). However, Boylan and Wenner (1993) found that peak settlement in Charleston Harbor (Charleston, South Carolina, USA) occurred during neap tides, when the time of SBE was several hours after midnight. This result is similar to the increase in settlement at neap tides in the present study (series 2; Figs. 4, 5, 6, 7). Van Montfrans et al. (1990)

studied settlement in the York River (Virginia, USA) and reported increased settlement during spring tides, with peak settlement observed one to four nights later. The time of SBE at new moon was several hours after sunset during the sampling period. A later study at the same location by Metcalf et al. (1995) obtained similar results and concluded that the greatest settlement occurred 4 days after the new and full moon, when the highest proportion of each flood tide occurred at night, yet SBE occurred before sunrise. Assuming settlement near the surface in the present study reflects benthic settlement, the results suggest that apparent semi-lunar patterns in settlement may result from the phase relationship between flood tide and the light:dark cycle. If the supply of megalopae entering an estuary is relatively constant, then the greatest number settling on collectors should occur when all of the flood tide and SBE occur at night. The time in the lunar month for this relationship will vary among locations because of differences in tidal times relative to the day:night cycle. Megalopae undergoing FTT in estuaries are transported to nursery areas. Since the present study used collectors lacking odor associated with nursery habitat, the settlement pattern reflected the transport sequence as megalopae move up an estuary. What is not known is the change in behavior as megalopae are transported into nursery areas and during settlement. There are two obvious possibilities. First, during FTT megalopae are transported passively during flood tide and active habitat selection and settlement only occur at the times of slack water at the end of flood tide. Second, if megalopae encounter a settlement area (e.g. seagrass bed) during FTT, they descend in the water column to settle. Studies of the behavior of megalopae in flow and of chemical cues should clarify the behavior involved in settlement in nursery areas. Acknowledgements This material is based in part on research supported by the National Science Foundation grants no. OCE9819355 and no. OCE-9901146/0096205. We thank T.P. Fitzgerald, B. Gresser, K. Kachurak, M.A. Sigala, and M.G. Wieber for their help with collecting and processing samples.

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