Flexibility in timing of molting of fiddler crab ... - Inter Research

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Mar Ecol Prog Ser 146: 55-60, 1997E4l-day3-dayIn the laboratory, the presence of muffled marsh sediment~had no effect on timing of molting (Mann-WhitneyU-test, p = 0.43). The median day of molting ofmegalopae in dishes with FSW was 12 0 (mean = 13.8,SD = 6.9), and in dishes with both FSW and muffledsediment it was 12.5 d (mean = 14.6, SD = 10.1). Thissuggests that the muffled sediments did not provide atactile cue for molting in the field.MuffledSediment TypeFreshFig. 2. Uca pugnax. Proportion (mean i SD) of fiddler crabmegalopae molting to first crab stage within marsh (Spartinaalterniflora) habitats during 2 field deployments. Althoughproportion molting during the l-day deployment did not differbetween sediment treatments, the proportion moltingwithin 5 replicate cages during the 3-day deployment wassignificantly greater (ANOVA on arcsine-transformed data,F,,, = 53.95, p < 0.001) for sediment freshly collected fromnearby fiddler crab burrows than similar sediment that hadbeen previously combusted in a muffle furnace (2 to 3 h at550 to 600°C). Survivorship of ~ndividuals was greater than90% during both deploymentsDays Since Experiment BeganFig. 3. Uca pugnax. Cumulative molting frequency of fiddlercrab megalopae maintained in the laboratory without fieldcues. Sixteen 6-day-old megalopae from the same broods utlllzedIn the field expenment were scored daily for numbermoltlng to crab 1, commencing with ~nitiation of the 3-daydeployment. The correspond~ng mean molting percentage Inthe field during the 3-day deployment is indicated by 'F'(fresh sediment) and 'M' (muffled sediment)lopae molted to the first crab stage, which is comparableto the percentage molting during the 3-day periodof exposure to natural cues in the field.Megalopae maintained in the laboratory molted farmore slo~vly than their siblings in the field. At the endof the 3-day field deployment, none of the specimensin the lab had molted to the first crab stage (Fig. 3).Even 1 wk after the field experiment ended, only 19%of the megalopae in the lab had molted.DISCUSSIONOur short-term field experiment provides strong evidencethat natural cues greatly affect metamorphosis(molting) of crab megalopae, by inducing moltingbetween 1 and 3 d after initial exposure. Both naturalseawater overlying a marsh and natural marsh sedimentsdramatically affected timing of molting, comparedto molting of sibling megalopae maintained inthe laboratory. In fact, the magnitude of the response,as indicated by proportion rnolting, was greater thanthat seen in laboratory experiments (O'Connor 1991,O'Connor & Gregg unpubl.). However, Christy (1989)also observed that sediments collected adjacent toadult burrows stimulated substantially earlier moltingof Uca pugilator megalopae in the lab. The dramaticresponse of larvae to cues in the field compared withmolting of siblings in the lab (Fig. 3) strongly suggeststhat larvae in the lab were in fact delaylng metamorphosis(Pechenik 1990), rather than simply developingat a slower rate. Even though daytime air temperaturesin the field became rather high, average water temperaturewas similar to that in which megalopae in thelaboratory were maintained. Therefore, temperaturedifferences between the laboratory and the field werenot great, and probably did not accelerate molting ofrnegalopae in the field.The question arises as to the identity of the cues innature. Two types of cues were important in stimulatingmolting: cues in the water and cues associated withbenthic sediments. Water-soluble cues could havebeen derived from benthic sediments, marsh grass, oradult fiddler crabs inhabiting the marsh. Marsh grass isan unlikely cue, based on testing of leachates fromSpartina in the laboratory (O'Connor & Greggunpubl.). It is likely that soluble cues from either conspecificadults or natural marsh sedirnents, or both,were effective in stimulating molting. In laboratorystudies, cues derived from adult crabs significantlyaffect timing of molting of fiddler crab larvae (Christy1989, O'Connor 1991, O'Connor & Gregg unpubl.). Inaddition, substances associated with natural marshsedirnents, rather than sediment particles themselves,may represent a contact-dependent cue for megalopae.Sediments without natural organisms and mole-
O'Connor & Judge- Molting of f~ddler crab megalopae 59cules (i.e. muffled marsh sediments in FSW) were noteffective at stimulating molting in the laboratory.Blue crab megalopae also molt sooner when exposedto potential habitat cues in the laboratory. Megalopalmolting is advanced by exposure to estuarine seawatercompared with offshore seawater (Wolcott & De Vries1994), and soluble materials leached from seagrassesare particularly effective at stimulating molting (Forwardet a1 1994, 1996). The present study providesadditional evidence that crab megalopae molt inresponse to chemical cues derived from adult habitats.Benthic invertebrate larvae are known to utilize avariety of environmental cues to locate and metamorphosein suitable adult habitat. Hydrodynamic processesinfluence larval delivery and behavior at a varietyof spatial scales (Eckman et al. 1983, Butman 1987).Surface characteristics may also serve as importanttactile cues for larvae (Rittschof et al. 1984, Wethey1986). There is ample evidence that larvae activelyexplore such surfaces after contact (Mullineaux & Butman1991, Walters 1992). In addition to their welldocumentedrole in crustacean b~ology, chemlcal cueshave been shown important in many benthlc groups,including polychaetes (Pawlik et al. 1991) and bivalves(Turner et al. 1994), even under simulated natural flowregimes. While we are not able to rank the relativeimportance of these cues in the present study, thestrong differential response under field cond~tions toour manipulations suggests that chemical cues are criticalto the settling larvae of benthic invertebrates.The present study represents a new approach to thestudy of cues stimulating metamorphosis of invertebratelarvae. The caging technique allows control oflarval age at exposure to cues, and manipulation of thesource of the cues to which they are exposed. Our technlquealso ensures that equal numbers of larvae areexposed to different potential habitat cues. Other fieldexperiments, primarily dealing with larval choice of settlementsite, rely on chance transport of planktonic larvaeto experimental sites, and must assume that larvaehave full access to different experimental substrata.Even in studies in which larvae are pelaglc for short periodsand can be tracked directly by human observers,e.g. relatively large ascidian larvae (Olson 1985, Stoner1990), researchers are unable to control larval abundanceor delivery to f~eld substrata The success of thepresent caging study should open new avenues of fieldexper~mentation on larval metamorphosis.Acknowledgenlents We are very grateful to our studentsAmanda Gregg, Kerry Cudmore McCarthy David Cnno, andDanlelle Cigliano for theii assistance in the laboratory and thefield We thank Richard B Foiward Jr, Jan Pechenik, and ananonymous reviewer for their comments on the manuscriptWe also thank the staff of Demarest Lloyd State Park forallow~ng us to set up our odd-looking expeliment in the parkLlTERATURE CITEDButman CA (1987) Larval settlement of soft-sediment invertebrates,the spat~al scales of pattern explained by actlvehabitat selection and the emerglng role of hydrodynamicalprocesses. Oceanogr Mar Biol A Rev 25:113-165Chr~sty JH (1989) Rapld development of megalopae of the fiddlercrab Uca pugilatorreared over sediment: implicationsfor models of larval recruitment. Mar Ecol Prog Ser 57.259-265Eckman JE (1983) Hydrodynamic processes affecting benthicrecruitment. Limnol Oceanogr 28:241-257Eggleston DB. Armstrong DA (1995) Pre- and post-settlementdc,tei-minants of estuarine Dungeness crab recruitmentEcol Monogr 65:193-216Fernandez M, Inbarne 0, Armstrong D (1993) Habitat selectionby young-of-the-year Dungeness crab Cancer maglsterand predation risk In intertidal habitats Mar Ecol ProgSer 92:171-177Forward RB Jr, De Vries MC, Rittschof D, Frankel DAZ,Bischoff JP, Fisher CM, Welch JM (1996) Effects of environmentalcues on metamorphosis of the blue crab Calllnectessapidus Mar Ecol Prog Ser 131:165-177Forward RB Jr, Frankel DAZ, Rittschof D (1994) Molting ofmegalopae from the blue crab Callinectes sapldus: effectsof offshore and estuarine cues Mar Ecol Prog Ser 113:55-59Heri-nkind WF, Butler MJ IV (1986) Factors regulating postlarvalsettlement and juvenile microhab~tat use by spinylobsters Panullrus argus. Mar Ecol Prog Ser 34:23-30hlense DJ, Wenner EL (1989) Distribution and abundance ofearly life h~story stages of the blue crab, Calllnectessapidus, in tidal marsh creeks near Charleston, South Carol~na.Estuanes 12 157-168Mull~neaux LS, Butman CA (1991) lnitial contact, explorationand attachment of barnacle (Balanus amphltrite) cypndssettling in flow. Mar Biol 110.93-103NOAA (1995) T~de tables, high and low water predictions,east coast of North and South Amerlca, includ~ng Greenland.US Dept of Commerce. Washington, DCO'Connor NJ (1991) Flexib~lity in timlng of the metamorphicmolt by fiddler crab megalopae Uca pugilator. Mar EcolProg Ser 68:243-247O'Connor NJ (1993) Settlement and recruitment of the f~ddlercrabs Uca pugnax and U pug~lator In a North Carolina,USA, salt marsh Mar Ecol Prog Ser 93-227-234Olson RR (1985) The consequences of short-distance larvaldispersal in a sesslle marine invertebrate Ecology 66-30-39Orth RJ, vdn Montfrans J (1987) Ut~lization of a seagrassmeadow and t~dal marsh creek by blue crabs Callinectessapidus. I. Seasonal and annual vanations in abundancewith emphas~s on post-settlementjuveniles. Mar Ecol ProgSer 41 283-294Pabvhk JR. Butman CA, Starczak VAR (1991) Hydrodynamicfacilitation of gregarious settlement of a reef-budding tubeworm Science 251 421-424Pechenik JA (1990) Delayed metamorphosis by larvae of benthicmarine invertebrates: Does it occur? Is there a price topay? Ophelia 32.63-94R~ttschof D, Branscomb ES, Costlo\v JD (1984) Settlement andbehavior in relation to flow and surface in larval barnacles,Balanus arnphltrite Darwin J Exp Mar Biol Ecol 82.131-146Stoner DS (1990) Recruitment of a troplcal colonial ascidian:relatlve ~mportance of pre-settlement vs post-settlementprocesses. Ecology 71.1682-1690
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