of a presunrise emergence of zooplankton and has considerable implications for 

 reef bioenergetics and planktivore feeding behavior. 



As in the previous work where plankton were collected over Caribbean reef 

 substrata at different time intervals (Ohlhorst 1982), Oithona colcarva , various 

 calanoids and harpacticoids , copepod nauplii, amphipods, and polychaetes were 

 important components of the fauna. The relative abundances of these organisms 

 from the two studies are correlated (p <^ 0.05, SRC). There are, however, 

 differences in the behavior of certain taxa between these studies. In Jamaica, 

 for example, the harpacticoid Microsetel la spp. migrated in greater numbers 

 during the day than at night, while at St. Croix the peak migration was during 

 the first and second hour after sunset. Also, isopods were an important 

 component of the Jamaican fauna but were relatively rare at St. Croix. Oithona 

 colcarva was less abundant at St. Croix than in Jamaica, and its activity 

 pattern differed. At Jamaica 0. colcarva was captured in the greatest numbers 

 during the second hour after sunset, while at St. Croix the capture rate of 

 this copepod was highest from midnight to just prior to sunrise. The sample 

 size needs to be increased at both reef locations to determine if these 

 differences are real . 



The differences between this study and those over primarily sand substrata 

 in the Caribbean (Robichaux, et al . , 1981; Youngbluth, 1982) may be related to 

 habitat differences and/or trap design. The relative abundance of nine common 

 taxa from this study and the unskirted traps of Robichaux, et al . (1981) were 

 positively correlated (p _< 0.05, SRC). There was no correlation, however, 

 between the relative abundance of taxa in this study and that from any of the 

 treatments of Youngbluth (1982). While harpacticoids were a very important 

 component of the reef zooplankton in St. Croix, they did not dominate to 

 the degree reported by Robichaux, et al . (1981) and Youngbluth (1982) in the 

 Bahama Islands. The St. Croix samples usually were dominated by the cyclopoid 

 Oithona colcarva . This is a swarming meroplanktonic species unlikely to be 

 captured in traps sealed over sand. Cumaceans and calanoids were important 

 over St. Croix reefs, as was found with certain trap designs over sand by 

 Youngbluth (1982) but not by Robichaux, et aj_;_ (1981). 



This preliminary analysis of data collected from St. Croix is consistent 

 with earlier di el studies (Walter, et al . , 1981; Ohlhorst 1982) which indicated 

 that zooplankton move up into the water column throughout the night with a pulse 

 in activity following sunset. Although zooplankton are therefore available to 

 planktivores throughout the night, the indication that there are predawn (this 

 study) and postdusk peaks of migration is consistent with the hypothesis that 

 fish predation is an important selective factor upon zooplankton behavior since 

 these dawn and dusk peaks of emergence coincide with periods when there are few 

 fish predators (Hobson, 1975). 



ACKNOWLEDGMENTS 



This study was made possible by logistical support and a grant (NA 82-AAA- 

 01345) provided by N0AA. Additional support and funding were provided by the 

 West Indies Marine Laboratory of Fairleigh Dickinson University, St. Croix; the 

 Ecology Center of Utah State University; and the Lerner-Gray Fund for Marine 



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