The lower threshold for phototaxis of dark-adapted larvae of the crab Rhithropanopeus harrisii is lo-' W *m-2 at 500-nm light. This threshold does not change throughout development. In the Newport River estuary (North Carolina), larvae show a pattern of nocturnal vertical migration in which the distribution of later-stage zoeal larvae centers on the lo-' W .m-2 isolume during the day. The behavioral responses underlying this relationship include a negative geotaxis in darkness which changes to a sinking response in light with a natural underwater distribution. The minimum increase in intensity above the lower threshold which evokes sinking decreases progressively with age. The intensity is well above the phototactic response threshold for stage I zoeae, but within one log unit of threshold for the other three zoeal stages. Thus, during the day larvae are associated with a preferred light level. This association results from a negative geotaxis in darkness that changes to a sinking response upon an increase in light intensity.Light is the dominant environmental factor controlling diel vertical migration of zooplankton (Forward 1976a). There are two main hypotheses concerning the manner in which light acts during vertical migration. Early workers hypothesized that animals were following a particular optimum or preferred light intensity level (preferendum hypothesis: Ewald 19 10; Rose 1925; Russell 1927). The other popular hypothesis is that the cue for initiating vertical movements is the rate and direction of change in light intensity from the ambient level (adaptation intensity), which itself can change over a day (Clarke 1930;Ringelberg 1964;Daan and Ringelberg 1969). The latter hypothesis is now widely accepted (see Kerfoot 1980) and is clearly supported by recent studies of arctic zooplankton (Buchanan and Haney 1980). Nevertheless, there are cases in which zooplankton appear to follow a particular isolume over a day (e.g. Boden and Kampa 1967), which would support the preferendum hypothesis. The main objection to this hypothesis is that behavioral responses which would allow an animal to remain at a particular intensity Cronin ( 1982) determined the vertical distribution of larvae of the estuarine crab Rhithropanopeus harrisii over several days and found that vertical migrations were associated with three environmental factors: currents, salinity, and light. We have measured the diel vertical migration of R. harrisii larvae in relation to light in the field and photoresponses in the laboratory. Our results indicate that the mean depth at which larvae aggregate frequently occurs at a particular light level during the day and that aggregation results from a photoresponse. Materials and methodsDiel vertical distributions of larvae of the crab R. harrisii (Gould) were measured in the Newport River estuary (North Carolina). On four occasions, larval distribution was sampled hourly at four depths: just beneath the surface, 1 m, 2 m, and just above the bottom (2.5-3.5 m). Collections were made with a gasoli...

A grazlng zooplankter may minimize its suscept~bil~ty to v~s u a l predat~on by r e d u c~n g its gut contents or ~t s f e e d~n g movements Such behav~oral rnechan~sms require (1) that the prey can detect the predator and (2) that ~t reduces its grazing activity when l~g h t c o n d~t~o n s favor visual predation In this work we have e x a m~n e d the grazing act~vity of 2 estuarine copepods Acart~a tonsa and Acartla hudsonlca in the chemical presence or absence of ~t s natural visual predator the Atlantic sllverside Meludia menidia Our results ind~cate that in the presence of chemical exudates of M rnenldla A tonsa and A hudsonica reduced their gut fullness T h~s response was observed only when l~g h t levels wele high enough for visual predation Reduct~on in gut fullness resulted from decreased ingestion rates, not from changes in gut evacuation rates Light intensity was found to affect the grazlng response to the fish-exuded chemical(s) Such behavior may be advantageous to these copepods in estuaries which contain large numbers of v~s u a l predators KEY WORDS: Acartia tonsa. Acartia hudsonica. Menjdia menidla. Visual predation. Nocturnal feeding. Copepod grazlng. Copepod behavior. Fish exudates O Inter-Research 1999 Resale of full artlcle not permitted

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