MORGAN: ASPECTS OF LARVAL ECOLOGY OF SQUILLA EMPUSA 



Larvae reared in the laboratory survived 

 longest and molted the greatest number of times 

 at 20° C-25%0 and 25° C-25%o which corresponds to 

 the temperatures and salinities found in the bay 

 where the larvae were most abundant. The mean 

 temperature of the bay from July through Sep- 

 tember, the season of larval occurrence of S. em- 

 pusa, ranged from 19.7 ° to 25.2° C while the mean 

 salinity was recorded from 21.5 to 23.1%o in 1971 

 and 1973. 



The greater abundance of larvae in the eastern 

 and channel subareas of the bay is likely a result of 

 the higher salinities. In Chesapeake Bay 

 salinities are higher on the eastern side than on 

 the western side due to the earth's rotation 

 (Coriolis force) and the differences are enhanced 

 by the larger inflow of freshwater from rivers on 

 the western side (Pritchard 1952). The lower 

 salinities of the upper reaches of the sampling 

 area are also probably responsible for the lesser 

 larval abundances in subareas G and H. 



In 1972, Tropical Storm Agnes produced the 

 most extensive flooding and greatest freshwater 

 runoff in Chesapeake Bay in many decades, if not 

 centuries, causing the distribution and abundance 

 of most estuarine organisms to be seriously dis- 

 rupted (Andrews 1973). The mean salinity of the 

 lower Chesapeake was reduced to 16.5%o in July 

 only to increase to 19.4%o in October. Although the 

 larvae were more abundant in 1972 than in 1971, 

 the reduced salinity resulted in a distribution 

 compressed into the more southern subareas 

 where the salinity was greater. Few larvae were 

 captured in subareas D, G, and H where salinities 

 ranged from 15.3 to 17.3%o, which would be ex- 

 pected considering the poor development of larvae 

 reared at 15%o during the temperature and salin- 

 ity experiment. Grant et al. (1976) found other 

 zooplankters in the lower Chesapeake Bay to be as 

 abundant in 1972 as in 1971 and their distributions 

 were also compressed in 1972. 



Of the S. empusa larvae reared at the most 

 favorable temperature and salinity combinations 

 for survival and growth, 3% of the larvae were 

 reared through eight of the nine larval stages in 6 

 wk, indicating that the length of the pelagic larval 

 development would be slightly longer than 6 wk. 

 However, the appearance of the postlarvae in the 

 bay 1 mo after the initial appearance of the larvae 

 indicates a substantially briefer period of larval 

 development, provided that all larvae originated 

 within the bay. The development of the larvae 

 reared in the laboratory may have been extended 



as a result of dietary insufficiencies and an overall 

 more stressful environment. Furthermore, the 

 few specimens of stages V and VIII collected early 

 in the 1976 larval season may have drifted into the 

 bay from more southerly populations where eggs 

 may have hatched earlier and been transported by 

 currents into Chesapeake Bay. Nevertheless, since 

 all larval stages and the postlarva were collected 

 in Chesapeake Bay throughout their seasonal oc- 

 currence, it appears that the populations of S. 

 empusa in the bay is self-sustaining. In addition, 

 the temperature and salinity tolerances of the 

 larvae correspond to those of the adults, which 

 may occur in salinities as low as 16%o, but are most 

 abundant in waters >25%o (Cowles 1930; Gunter 

 1950; Parker 1956; Lee and McFarland 1962). 



ACKNOWLEDGMENTS 



I am indebted to George C. Grant of the Virginia 

 Institute of Marine Science who made available 

 plankton samples collected during the RANN sur- 

 vey of the lower Chesapeake Bay. He also kindly 

 provided the map of the survey area and com- 

 mented on the manuscript. 



Thanks are also due Anthony J. Provenzano, Jr. 

 for his guidance during the investigation and his 

 comments on the manuscript. 



This work was supported in part by the National 

 Science Foundation Grant DEB-76-11716. 



LITERATURE CITED 



Andrews, J. D. 



1973. Effects of Tropical Storm Agnes on epifaunal inver- 

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 234. 

 BROOKS, W. K. 



1878. The larval stages of Squilla empusa Say. Johns 

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 Caldwell, R. L., and H. Dingle. 



1976. Stomatopods. Sci. Am. 234(l):80-89. 

 COWLES, R. P. 



1930. A biological study of the offshore waters of 

 Chesapeake Bay Bull. U.S. Bur. Fish. 46:276-381. 

 DRAGOVICH, A. 



1970. The food of skipjack and yellowfin tunas in the Atlan- 

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Fish, C. J. 



1925. Seasonal distribution of the plankton of the Woods 

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 FLEMINGER, A„ and R. I. CLUTTER. 



1965. Avoidance of towed nets by zooplankton. Limnol. 

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Grant, G. C, B. B. Bryan, F Jacobs, and J. E. Olney. 



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