FISHERY BULLETIN: VOL. 71, NO. 1 



temperature. In contrast, larvae in my study 

 passed through fewer instars at the moderate 

 temperature (25°C) than at higher or lower tem- 

 peratures, and at each temperature larvae re~ 

 quired fewer molts to reach the postlarval stage 

 in my study than in Knowlton's (1970). Simi- 

 larly, Ewald (1969) found that Tozeuma ca^'ol- 

 inense larvae passed through fewer instars at 

 25°C than at 15° and 20°C. He also reported 

 that there were marked differences in the num- 

 bers of instars among T. carolinense larvae from 

 different populations. Perhaps a similar effect 

 was partially responsible for differences between 

 the numbers of P. vulgaris larval instars ob- 

 served by Knowlton (1970) and by me. 



The final zoeal instar was of greater duration 

 than the other instars in both Knowlton's (1970) 

 study and in mine, but the reason for the delay 

 of this molt is not known. However, Hubschman 

 (1963) reported that the X organ-sinus gland 

 complex does not become functional as the pri- 

 mary molt regulator in Palaemonetes until after 

 metamorphosis. He suggested that perhaps the 

 rapid larval molting cycle was under the hor- 

 monal control of some type of larval molting 

 gland, the existence of which remains specu- 

 lative. The longer duration of the final zoeal 

 instar thus may reflect transfer of control over 

 molting from some unknown larval molt-regu- 

 lating mechanism to the X organ-sinus gland 

 complex, or breakdown of the larval regulatory 

 mechanism prior to assumption of molt-regu- 

 lating function by the X organ-sinus gland com- 

 plex, or other internal reorganization prior to 

 metamorphosis. 



Because of the characteristic variability of 

 temperature and salinity in estuaries, success of 

 a particular decapod species may depend on the 

 ability of the larvae to survive frequent expo- 

 sure to suboptimal temperature-salinity condi- 

 tions, to settle and/or metamorphose only under 

 those conditions which are suitable for survival 

 of the adult form, and to remain within, be car- 

 ried into, or return to a given area to replenish 

 the parental population. The number of larval 

 instars may also be important, since ecdyses are 

 critical periods in larval life, and highest mor- 

 tality of cultured decapod larvae often occurs 

 then (Ong, 1966; Knowlton, 1970; Roberts, 



1971). Reduction of the number of premeta- 

 morphic molts thus may increase larval survival. 

 So, considering survival, rate of development, 

 and number of instars, it appears that optimal 

 conditions for larval development of P. vulgaris 

 occur at a moderate temperature of about 25°C 

 in salinities of 10 to 30^/f. Knowlton (1970) 

 also concluded that a temperature of 25 °C was 

 optimal over the salinity range tested (15 to 

 35'/w) in his experiment. 



ACKNOWLEDGMENTS 



I would like to thank my graduate committee 

 (Drs. G. C. Grant, W. G. Maclntyre, W. C. Pin- 

 schmidt, Jr., and M. L. Wass, and especially Mr. 

 W. A. Van Engel, Chairman) and my wife, Betty, 

 for constant help and encouragement, and Drs. 

 M. E. Chittenden and J. Loesch for advice re- 

 garding the design and analysis of the exper- 

 iment and for critical review of the manuscript. 

 I was the recipient of a National Defense Edu- 

 cation Act Title IV Graduate Fellowship during 

 the study. 



LITERATURE CITED 



Bowler, M. W., and A. J. Seidenberg. 



1971. Salinity tolerance of the prawns, Palaemone- 

 tes vulgaris and P. pugio, and its relationship 

 to the distribution of these species in nature. 

 Va. J. Sci. 22:94. 



Broad, A. C, and J. H. Hubschman. 



1962. A comparison of larvae and larval develop- 

 ment of species of Eastern U.S. Palaemonetes 

 with special reference to the development of 

 Palaemonetes intermedius Holthuis. Am. Zool. 

 2:394-395. 



EWALD, J. J. 



1969. Observations on the biology of Tozeuma 

 carolinense (Decapoda, Hippolytidae) from Flor- 

 ida, with special reference to larval development. 

 Bull. Mar. Sci. 19:510-549. 

 Hubschman, J. H. 



1963. Development and function of neurosecretory 

 sites in the eyestalks of larval Palaemonetes 

 (Decapoda: Natantia). Biol. Bull. (Woods 

 Hole) 125:96-113. 



Knowlton, R. E. 



1965. Effects of some environmental factors on 

 larval development of Palaemonetes vulgaris 

 (Say). J. Elisha Mitchell Sci. Soc. 81:87. 



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