Estuaries Vol. 15, No.2, p. 186-192 June 1992

S'vvimming Velocities and Behavior of Blue Crab11"",..lIi ""+,,,.. ,..,.. i,J",.. 0,.. \ I\. "" 1 : ....\ vallll IC::;vlC::;\) \)afJ'UU\) nClLlIUUllJ Ivn::;YCllutJClC::; III

Still and Flowing \tVater1

MARK W. LUCKENBACH

Virginia Institute ofMarine ScienceThe College of William and MaryWachapreague, Virginia 23480

ROBERT J. ORTH

Virginia Institute ofMarine ScienceThe College of William and MaryGloucester Point, Virginia 23062

ABSTRACT: Habitat selection capabilities of the recruiting larval stages of marine invertebrates are limited, inpart, by their ability to maneuver in flowing water. Distributional and experimental evidence suggest that blue crab(Callinectes sapidus) megalopae may preferentially settle into vegetated habitats. However, the behavior and swim­ming capabilities of megalopae in flowing water have not previously been investigated. Laboratory experimentswere conducted'in a small, recirculating seawater flume to determine the swimming response of megalopae tovarying flow velocities. Nighttime trials were conducted at six flow velocities: 0, 1.9, 3.6, 4.8, 6.3, and 9.3 cm S-I.

Behavior and swimming velocities of field-collected C. sapidus megalopae were video recorded. Megalopae exhibitednegative phototaxis and were found in the water column at all flows in the dark. The maximum sustained swimmingspeed observed was 12.6 cm S-1 and the mean swimming speed in still water was 5.0 cm S-I, with short bursts inexcess of 20 cm S-I. Megalopae frequently oriented into the current and were capable of swimming upstream againstthe current at flow speeds <4.8 cm S-I; at greater velocities they were not able to do so. The results suggest that atlow to moderate current velocities C. sapidus megalopae have the ability to actively move in search of settlementsites and to maintain their positions in desirable sites rather than relying strictly on passive movements by currents.

IntroductionClassic substrate selection experiments with lar­

vae of infaunal and epifaunal invertebrates havebeen performed in still water and over very smallspatial scales (see reviews by Meadows and Camp­bell 1972; Scheltema 1974; Crisp 1976). Recentwork has questioned the ability of several taxa tomove actively between potential settlement sitesbecause swimming speeds are slow relative toboundary-layer flow velocities (polychaetes: But­man 1986; Butman et al.1988a; barnacles: Wetheyet al. 1988; bivalves: Butman et al. 1988b). Thus,the encounter rates of recruiting larval stages withparticular habitats are presumed to be set by hy­drodynamic processes coupled with vertical swim­ming with little or no effect of directed horizontalswimming. Megalopae of the blue crab, Callinectessapidus Rathbun, are involved in estuarine rein­vasion and benthic habitat selection. Distributionsof early juvenile stages suggest that some shallow

1 Contribution no. 1744 from the Virginia Institute ofMarineScience.

© 1992 Estuarine Research Federation 186

water habitats, particularly vegetated sites, may beselected as settlement sites (Tagatz 1968; Heck andOrth 1980; Heck and Thoman 1984; Zimmermanand Minello 1984; Orth and van Montfrans 1987).Laboratory experiments recently have shown thatgiven still water conditions, C. sapidus megalopaeselect vegetated over barren substrate (J. vanMontfrans personal communication). Vegetatedhabitats are limited to shallow water in estuariesand are often separated by expanses of unvegetat­ed substrate. How C. sapidus megalopae within anestuary encounter these sites-whether by passivetransport or directed swimming to shallow water­and how (or if) once within vegetated areas theyare able to maintain their positions until meta­morphosis is unknown. Determining the ability ofblue crab megalopae to maneuver and to swim un­der flow conditions is required before evaluatingtheir capability for actively encountering and re­maining in desirable recruitment habitats.

Reported swimming speeds for marine inverte­brate larvae range from 0.003 cm S-1 for amphi­blastula larvae ofa sponge to 8.3 cm S-1 for xanthidcrab zoeae, with few swimming speeds reported

0160-8347/92/020186-07$01.50/0

above 3 cm S-,1 (reviewed by Chia et al. 1984). Avertical swimming speed of 4.2 cm S-1 has beenreported for Cancer magister megalopae (Jacoby1982) and a horizontal swimming speed of 3.3 cmS-1 has been measured for Cyciax(wihops novemdan­tatus megalopae (Knudsen 1960).

Callinectes sapidus megalopae have weii-devel­oped swimming appendages and are larger by near­ly an order of magnitude than most of the settle­ment stages for species whose swimming speedshave been reported. Thus. l!Teater swimminl! speedand habitat ~election cap~bilitiesare expected forthis species than for settlement stages of other ben­thic invertebrate taxa (especially polychaetes andbivalves). Requisite for predicting the active com­ponent of habitat encounter by blue crab megalo­pae is an understanding of (1) the behavioral rep­ertoire in response to physical factors (and how itchanges ontogenetically) and (2) the swimmingability ofmegalopae under natural flow conditions.We report here on the behavior and the swimmingspeeds of C. sapidus megalopae under still and flow­ing water conditions, and discuss the importanceof horizontal swimming to habitat selection.

Materials and MethodsBlue crab megalopae used in all flume experi­

ments were collected from nighttime plankton towsin the lower York River (37°15'02"N, 76°29'44"W),a tributary of the lower Chesapeake Bay. Replicateplankton nets were deployed from a pier on eachof two consecutive nights. Megalopae were col­lected from net samples each morning and thenheld in aquaria under ambient light and temper­ature conditions for 1-2 d before being placed intothe flume. During the holding stage and flume ex­periments megalopae were kept in York River wa­ter (230/00) taken from the same place the megalopaewere collected. Surface water temperature at thetime nets vvere retrieved averaged 14°C, but tem­perature was not controlled during the experi­ment. Water in the 60-1 holdin!! aquarium was re­placed daily with unfiltered York River water,providing some food (zooplankton) for megalopaeduring this period, but ration was not controlled.

Experiments were conducted in a small, recir­culating seawater flume similar to that describedby Vogel (1981). The flow channel of the flumewas constructed of Plexiglas and measured 2 mlong x 0.5 m wide; for all trials the flume was filledwith I-JLm-filtered York River water to a depth of15 cm. Flow was generated in the flume by a plasticpropellor attached to a variable speed motor. Ob­servations ofdye movements within the flume haverevealed that water flow along the channel is ap­proximately 2-dimensional, ,vith discernable ve...

Swimming Velocities of Megalopae 187

locity reductions occurring about 1 cm above thebed.

Experiments were run at six flow velocities. Ve­locitv mag-nitudes were determined bv the timerequ'ired fur a neutrally buoyant drogu~at the sur­face to traverse 20 cm. The drogue was releasedupstream of the center test section in the centerlineof the flume. Its passage across the test section wasvideo recorded (RCA Color Video CameraCLC025). Three to five drogue releases per flowsetting were performed and drogue passage be­tween fixed distance markers (20 cm) was timedwith a hand-held stopwatch from the video record.Each drogue passage was timed 3 times to provideseparate estimates of the variance associated withthe timing measurement and individual drogue re­leases. Variance associated with the measurementof the passage of a single drogue ,Alas <4% of themean; variance between drogue passages within aflow setting was < 10% of the mean. Thus, for thetemporal and spatial resolution with which we mea­sured centerline velocities, substantial velocity fluc­tuations were not evident. However, since the pro­pellor drive mechanism in the flume generatesturbulence, velocity fluctuations necessarily oc­curred on other scales. Mean centerline flow ve­locities used in these experiments were 0, 1.9, 3.6,4.8,6.3, and 9.3 cm S-I.

Several hundred megalopae were placed in theflume 24 h before the experiment. All experimen­tal trials were run at night because there may bean endogenous component to the nighttime swim­ming activities. Lighting was with fluorescent bulbspositioned above and lateral to the flume channelto provide visually even illumination of the testsection; however, lighting intensity was not mea­sured and is not presumed to reflect natural light­ing conditions. At the time of the trials the flowvelocity was set to the desired level, the room lightsextinguished, and the system allowed to run for 15,...,....~ ....... Th.o l~noh~~ 'UT..o.T".o .. 1-...0. ......... ,. .............. .0,.1 _ ...... ,.. ...... ....1 .......... .£:101"1",..1_.1.1.1.1.1.1 • ..I.. .1.1'-' .1.I5.1u.. i3 \'V '-'.I '-' L.I.I'-'.I.1 LU.I.I.I'-'U V.l1 allu .1.I1'-'5alV-

pae swimming in the center of the flume were tapedfor 2 min. Li!!hts were then turned off. the flowspeed adjusted to the next level, and the'sequencerepeated. Flow speeds were first varied in ascend­ing order from 0 cm S-1 to 9.3 cm S-I, then indescending order.

The video camera was mounted on a tripod ap­proximately 1.5 m from the flume and focusedthrough the sidewall approximately midway alongthe channel length. The system provided for gooddepth of field, and we achieved workable focus fora r-egion approximately one-half of the total flumewidth. In preliminary trials we mounted the cam­era above the flume to provide a pian view of thechannel. From this perspective it was clear thatmegalopae did very little cross-channel s,vimming,

188 M. W. Luckenbach and R. J. Orth

but we could not distinguish larvae swimmingstrictlv alonQ" channel from those with vertical aswell a~ horiz~ntal velocity components, so we optedfor the lateral view.

Swimming speeds and directions were measuredfrom the videotape records for at least 20 megalo­pae in each trial for the period between 10 sand30 s after the lights were turned on. The speedswere estimated by measuring the time to traversea known distance interval marked on the flume wallnear the middle of its length. A 10-cm interval wasused for all flow velocities except for the highestspeed (9.3 em S-I) for which a 20-cm interval wasused to reduce the proportional error associatedwith measuring the rapid transit times at the short­er distance. rVfegalopae within 3 em of the bottomwere not included in these measurements (conser­vativelv avoidinQ" the reQ"ion of aooarent velocitvchang~ with distance ab~ve the b~d). /

From the video records we identified the firstmegalopa within the specified time period whichmet the criteria of (1) traversing the test intervalwithout any discernable vertical velocity compo­nent (horizontal movement only), (2) being withinthe focal depth of the video camera (approximatelyone-half of the channel width), and (3) being atleast 3 em above the flume bed (well above theregion of obvious velocity shear). Five separatemeasurements of the time for the megalopa to tra­verse the marked distance (10 em or 20 em) werethen made with a hand-held stopwatch. These rep­licate measurements served only to assess the vari­ance component associated with our measurementtechnique « 7% of the mean in all cases) and toprovide a reliable mean observed velocity for eachmegalopa measured; the measurements were notused as replicates in subsequent analyses of swim­ming speeds. In turn each subsequent megalopawithin the 20 s time interval which met the threecriteria above was timed in this manner until 25animals or all meeting the specifications were mea­sured.

Megalopal swimming velocities were computedas

Net SwimminQ" Velocitv= ObservedUVelocity - Water Velocity (1)

HT~t'h t'h~ nru:~ltl'7,::::1t. clCTn ;n thp rlirprt;nn nf thp uT~tprl'Y J.LJ.J. '-.1..1."-" Y'-'LJ.l.L.I." '-' ...".I.5.&..1. .I. ...... L ...... "" '-4 ... .1. '-'''''' ....... '-'.1..1. '-'.I. .......... '" •• _ ...."" ...

flow. Mean net swimming speeds are reported asscalar quantities without regard to direction. Wechose these forms of data r-epresentation for thefollowing reasons. Net values (for velocity andspeed) are used because they represent the activecontribution of the megalopa to transport. Netswimming velocities show the relationship betweenflow direction and swimming, while net swimmingspeeds provide descriptors of megalopal swimmingcapabilities regardless of the direction of flow.

Differences between mean net swimming speeds(without concern for direction) were tested usinQ"~ne-way ANOVA followed by a'SNK multiple com'::parisons test. Compliance with assumptions of nor­mality and homoscedasticity were assessed withstandard normal plots and Cochran's C test (SPSSX1986); We tested for differences in the frequencydistributions of swimming velocities between as­cending and descending trials within a flow treat­ment using Kolmogorov-Smirnov tests. No differ­ences were observed between the runs in any ofthe six comparisons (p > 0.20 in every case), sodata from both runs ofa flow treatment were pooledfor aH subsequent analyses. Swimming velocitieswithin a flow treatment were grouped into postl.~~ ~~'~~~_:~r ~~ C) ~~ r-I :~.~_.,~lr ~~rl .l.~ l..,JIV\... La.l.L5V11\...,;) Ul "" L.Ul., .l.l.lL\....l va.l., a.l.lU l...I.I'-- .1.1 y-pothesis of equal frequency distributions among allflow treatments was tested with a G-test for inde­pendence. We compared frequency distributionsof swimming velocities among individual flowtreatments following the method ofSokal and Rohlf(1981, p. 744) for unplanned tests of significance.

ResuitsBEHAVIORAL OBSERVATIONS

Megalopae exhibited negative phototaxis, mov­ing up in the water column when lights were offand down to the bottom when lights were on. Wedid not specifically quantify this because megalopaern1l1r1 nnt h,::::lo nhc,::::Io"t""u,::::Iorl llnrl,::::lo"t'" nn_l;crht rnnr1;t;nnfi:...... ....., "'" .....,l, ....., .....,.....,.:1 ,. O l. ""....., ....., .

This behavior was obvious, however, from obser­vations made immediately after lights were turnedon, when virtually alllar~ae werein the water col­umn, and several minutes later when all megalopaehad moved to the flume bed. Movement to thebottom of the flume was via both sinking and down­ward swimming. Some megalopae moved to thebottom immediately after the lights were turnedon. others continued swimminQ" horizontallv forse~eral minutes before moving down. Unfortu­nately, we are unable to estimate percentages oflarvae moving to the bottom versus remaining inthe water column at anyone time. The generalpattern within the population was one of gradualm~)Vement to the bottom over a period of severalmInutes.

Orientation by horizontally swimming megalo­pae was strictly parallel to the flow. Cross-channelswimming was"not observed in any of the trials withflowing water. However, even in the no-flow treat­ment there was a tendency for megalopae to swimalong the axis of the flume channel. Thus, it maybe that the lack of swimming perpendicular to theflow is an artifact of the system.

None of the flow soeeds evoked a downwardmovement ofmegalop~e(independent of light). At

all flow speeds, megalopae were observed through­out the water column immediately after the lightswere turned on; megalopae always moved to thebottom after several minutes of lighted conditions.Interesting, this was true even at higher flow ve­locities for which megalopae were unable to effec­tively manuever horizontally (see below). We hadanticipated that at flow velocities exceeding theirhorizontal swimming capabilities mega!opae wouldmove to the bottom of the flume even in darkness.

SWIMMING VELOCITIES

Megalopae were capable of short bursts of swim­ming « 10 cm distance) at speeds in excess of 20cm S-I. Though our systematic measurements ofmegalopae swimming were made over distances of10 cm or 20 cm as described above, we occasionallynoted megalopae moving more rapidly over short­er distances. Speeds of some of these swimmingbursts were estimated over shorter distances; themost rapid recorded swim speed was 24.1 cm S-1

over a one-cm distance. Our measurements of theseshort swimming bursts were quite selective, how­ever, and all values reported below are for themore systematically measured swimming speedsover distances of either 10 cm or 20 cm. The max­imum swimming speed maintained for distancesgreater than 10 cm was 12.6 cm S-I. Mean netswimming speeds differed between flow treat­ments, with megalopae in the highest flow (9.3 cmS-I) exhibiting the lowest observed swimming speed;but in only the 6.3 cm S-1 flow speed did mean netswimming speed differ from other treatments (Ta­ble 1).

The frequency distribution of swimming veloc­ities (Fig. 1) also varied with flow treatment (G testof independence: G = 138, df = 5, P < 0.0005).In the no-flow treatment megalopae swam equallyin either direction along the axis ofthe flume, witha maximum speed of 12 cm S-I and a mean speedof 5.0 cm S-I (Fig. lA; Table 1). With a 1.9 cm S-1

flow, megalopae again oriented in either direction,with the most active swimmers facing into the wa­ter flow (Fig. IB). Note that with this data repre­sentation on animal moving passively downstreamwith the flow would have a net swimming velocityof 0, and, in the case of Fig. IB, an animal swim­ming into the flow but making no headway wouldhave a net swimming velocity of - 2 cm S-I. There­fore the maximum rate at which megalopae wereobserved to move (observed velocity in eq. 1) down­stream in this treatment was 10 cm S-I (8 cm S-1

swimming + 2 cm S-I flow) and the maximum up­stream rate was -8 cm S-I (-10 cm S-I swimming+ 2 cm S-1 flow).

At flow velocities of 3.6 cm S-1 and 4.8 cm S-1

(Figs. lC and ID), a proportion of the megalopae

Swimming Velocities of Megalopae 189

in the flume continued to orient into the flow andmake headway upstream. Approximately 18% ofthe animals in both flow conditions moved up­stream against the flow. Mean net swimming ve­locities in each of these flow treatments did notdiffer from zero (two-tailed t-tests, p < 0.05); thatis, on average, megalopae swam upstream anddownstream with equal net speeds, but there wasa tendency in both flows for more animals to movewith the current than against it.

In the two higher flows, 6.3 cm S-I and 9.3 cmS-I, megalopae were unable to move against thecurrent (Figs. IE and IF). In the 6.3 cm S-1 flowtreatment ca. 17% of the animals oriented into theflow and were able to maintain their positions withrespect to the bottom, but they were not able tomake headway upstream. Approximately 10% ofthe megalopae in the 9.3 cm S-1 flow oriented intothe current, but they were swept downstream atabout 7 cm S-1 (-2 cm S-1 net swimming velocity).Most megalopae in the 6.3 cm S-1 flow activelyswam in the direction of the flow (Fig. IE) suchthat the observed velocities (swimming + flow) inthe downstream direction ranged from 10 cm S-1

to 18 cm S-I. At the highest flow (Fig. IF), megalo­pae had difficulty maintaining their orientation inany direction and most managed to swim onlyweakly in the direction of the current. Again, meannet swimming velocities were not significantly dif­ferent from zero in either of these treatments (t­tests, p < 0.05).

DiscussionCallinectes sapidus megalopae are strong swim­

mers relative to the recruiting stages of most ben­thic invertebrates. The maximum and average hor­izontal swimming speeds for these megalopae (12.6cm S-1 and 4.3 cm S-I, respectively) exceeded thosereported for other marine invertebrate larvae (Chiaet al. 1984). This difference is expected since thepost-larval megalopa is larger by several orders ofmagnitude than those of the larvae for which swim­ming speeds have been reported. Nevertheless thesefindings provide the first data on the swimmingrates of C. sapidus in flowing water and they estab­lish the megalopa's capacity for active explorationand retention within settlement sites via horizontalswimming.

Behavioral observations indicating that Calli­nectes sapidus megalopae are negatively phototacticare consistent with field studies that have foundgreater abundances of megalopae in near surfacewaters in nighttime vs. daytime collections (Epi­fanio et al. 1984; Mense and Wenner 1989). Thisdiffers from Sulkin (1984), who found C. sapidusmegalopae exhibit positive phototaxis in responseto light intensities exceeding 75 W m-2 • We did

190 M. W. Luckenbach and R. J. Orth

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20

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

I'1J 40 30.....,tI C.A. D.A.~ 30.",

~ 20~

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'+->~ 10Q)C,,)

~Q)

Q..,-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

30 60

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F+20 40

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10 20

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0-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

Net Swimming Velocity (cm S-I)Fig. 1. Distribution of Callinectes sapidus net swimming velocities in flume trials. Positive values indicate movement in the direction

ofcurrent, negative values movement against the current. (See text for computation ofnet swimming velocities for observed velocities.)Mean centerline flow velocity: A) 0 cm s-'; B) 1.9 cm s-'; C) 3.6 cm s-'; D) 4.8 cm s-'; E) 6.3 cm s-'; F) 9.3 cm s-'. Sample sizes asin Table 1. Treatments with the same symbols are not significantly different at an experimentwise error rate of 0.05.

not measure light intensity during our experi­ments, but the levels were adequate to induce anegative phototactic response. Timing of the neg­ative phototactic response by megalopae in our ex­periments was graded within the population; someindividuals moved to the bottom within a few sec­onds after lights were turned on and others re­mained in the water column for several minutes.We did not attempt to estimate the mean time todescent, but most megalopae were still swimmingduring the 10-30 s post-illumination period whenmeasurements were made. Lighting conditionswere largelydictated by our video recording needsand we did not attempt to model lighting condi­tions in the field. Yet, our observation that megalo­pal response to the lights was gradual (i.e., they did

not all stop swimming and drop to the bottom) istaken as evidence that the lighting conditions didnot severely inhibit horizontal swimming. The pos­sibility that a startle response to light enhanced thehorizontal swimming speeds cannot be dismissed,in which case the values reported here would re­flect possible rather than necessarily actual swim­ming speeds in the field. ,

The exclusively along-channel swimming ofmegalopae in treatments with flow would appearto indicate that they orient in relation to watermovement. However, since little cross-channelswimming was observed in the no-flow treatment, .this may be an artifact of our experimental system.Lighting over the test section of the flume was notobviously nonuniform, and the source of a direc-

TABLE 1. Callinectes sapidus megalopae net swimming speedsby flow velocity treatment. Mean values (x) and standard de­viations (SD) are in em S-I; n is the number of megalopae mea­sured at each flow speed. Similar symbols indicate a lack ofsignificant differences between means (ANOVA: F = 5.09, d.£.= 5, P = 0.0002 SNK: experimentwise error rate = 0.05).

Net Swimming Speed

Flow Speed it (SD) n

0.0 5.0 (2.6) 50 a1.9 4.5 (3.2) 50 a3.6 4.3 (3.3) 50 a4.8 3.7 (2.6) 50 a6.3 7.6 (3.0) 40 b9.3 2.4 (1.5) 41 a

tional bias along-channel is not apparent. A ten­tative explanation at this point may be that thePlexiglas side walls reflected some light resultingin a gradient with distance from the walls. Thematter remains unresolved.

Megalopae were able to swim upstream at flowvelocities less than approximately 5 cm S-I. At thetwo greater flow rates, megalopae were unable toswim upstream, but in the trials with 6.3 cm S-I

mean flow, some individuals maintained their po­sition relative to the flume bottom by swimminginto the current. Megalopae were unable to main"tain a consistent orientation at the highest flowsetting (9.3 cm S-I) and managed only weak swim­ming.

While our findings provide useful informationfor establishing the role horizontal swimming mayplay in habitat selection by blue crab megalopae,scaling considerations suggest that caution shouldbe exercised in extrapolating to natural environ­ments. Light intensity and frequency spectrum werenot adjusted to simulate natural conditions. Depthand width constraints of the flume severely re­stricted the scale of both water and crab move­ment. The spatial scales over which we made swim­ming speed measurements (10-20 em) is at thelower end of the range of scales of interest in fieldenvironments. The ability of megalopae to main­tain these swimming speeds over greater distancesis not known. Short, narrow flumes, such as theone we used here, have inherent problems withrespect to dynamic scaling (Nowell and Jumars1987). Conseauentlv, while this svstem is verv ame­nable to making b~havioral obs~rvations, the re­sults with respect to the flow dynamics are bestviewed as first-order estimates. Finally, we stressthat these results derive from a single experiment.While replicaiion at the level of individual megalo­pae is appropriate for the tests performed here,repeating the experiment with different batches oflarvae and similar design would permit estimatesof other sources of variation.

Swimming Velocities of Megalopae 191

Despite these caveats, our findings do reveal sev­eral ecologically important aspects regardingswimming of Callinectes sapidus megalopae. First,megalopae are present throughout the water col­umn over the range of flow rates used here. S~c­

ond, these animals are strong swimmers that canorient into the current and swim upstream at lowto moderate flow velocities. Third, at moderate tohigh flow rates C. sapidus megalopae are unable tomaintain their horizontal orientation and aretransported largely at the mercy of water currents.Given that a correlation has been reported be­tween molt stage of C. sapidus megalopae and set­tlement intensity (Lipcius et al. 1990) and that theremay be ontogenetic changes in phototaxis (Epifa­nio et al. 1984; Sulkin 1984), interactions betweenphysical factors (light and current speed) and de­velopmental changes in swimming behavior andability need to be examined.

In shallow-water estuarine habitats, tidally-driv­en currents can be expected to fall below the max­imum speed against which megalopae can swim fora significant portion of most tidal cycles. This maybe particularly true in vegetated habitats in whichflow velocities are significantly reduced (Fonseq.et al. 1982; Eckman 1987). Our data suggest thatduring those times megalopae possess the abilityto maneuver and actively affect their encounterrates with suitable settlement sites.

ACKNOWLEDGMENTS

\A"Te are grateful for the comments of R. Lipcius, R. ~1:ann,

E. Olmi, and D. Sved on earlier drafts of the manuscript as wellas those provided by two anonymous reviewers. S. Mauger aidedin the coBection of megalopae. Support was provided by a pri­vate grant from the Allied-Signal Foundation and by grant no.NA86AA-D-SG042 from the National Sea Grant College Pro­gram of the National Oceanic and Atmospheric Administra­tion, U.S. Department of Commerce, to the Virginia GraduateMarine Science Consortium and the Virginia Sea Grant CollegeProgram.

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Received for consideration. Abril 22. 1990Accepterffor publication, March 28; 1991