as turbulence, hydrostatic pressure, temperature, 

 and salinity are not responsible for synchronizing the 

 semilunar rhythm. 



Hypotheses concerning the ecological consequen- 

 ces and adaptive significance of semilunar cycles of 

 larval release in Uca spp. and other estuarine crabs 

 have been discussed recently by Christy (1982). His 

 analysis indicates that convergence among estuarine 

 crab species in the semilunar timing of larval release 

 results in rapid seaward transport of larvae, which 

 minimizes the effects of lethal combinations of high 

 temperature and low salinities or intense predation 

 in the upper estuary. However, Rhithropanopeus 

 harrisii completes its larval development within the 

 upper reaches of estuaries and shows a variable cycle 

 of hatching, depending on tidal predictability, which 

 may be advantageous for retention of larvae within 

 the estuary (Forward et al. 1982). Lunar spawning 

 cycles of insular coral reef fish may have evolved to 

 solve similar problems of larval dispersal away from 

 predators while assuring return of the recruits to the 

 adult habitat (Johannes 1978; Ross 1983). Semilunar 

 spawning cycles of Fundulus heteroditus, Menidia 

 menidia, Leuresthes tenuis, and other fish which 

 spawn in the upper intertidal zone (Taylor and 

 DiMichele 1983; Middaugh 1981; Clark 1925) may 

 have the adaptive advantage of removing spawning 

 adults and eggs from subtidal predators. Although 

 the eggs of F. heteroditus are tolerant of desiccation 

 (Able and Castagna 1975; Taylor et al. 1977), the 

 major advantage of the spawning site appears to be 

 higher oxygen levels and reduced sedimentation 

 than in the creek (Taylor and DiMichele 1983). How- 

 ever, spawned eggs of F. heteroditus and M. 

 menidia are usually found in areas inundated daily 

 by high tides (Middaugh 1981). In the present study, 

 62% of the days had tides which flooded the marsh 

 (Fig. 1), where eggs are probably deposited in the 

 bases of leaves of Typha angustifolia plants (see 

 Taylor and DiMichele 1983). Although there may be 

 additional advantages in areas with predictable tidal 

 fluctuations with lunar periodicity, the major advan- 

 tage of semilunar spawning rhythms in F. heterodi- 

 tus appears to be improved fertilization success af- 

 forded by synchronized spawning. 



Acknowledgments 



We are grateful for the assistance of L. Wiechert, 

 S. Hodgkins, C. Trowbridge, and K. Comtois. R. 

 Cory and P. Dresler of the U.S. Geological Survey 

 provided the data for measured tidal levels. R. Ross 

 and M. Taylor provided helpful discussion. D. Cor- 

 rell, T. Jordan, and two anonymous reviewers pro- 



vided suggestions for improvements in earlier drafts 

 of the manuscript. This work was supported in part 

 by the Smithsonian Work/Learn Internship Pro- 

 gram, Maryland Department of Natural Resources, 

 and the Smithsonian Environmental Sciences Pro- 

 gram. 



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