than P. simpsoni, with relatively less massive chelae 

 than the latter. Statistically, there is a significant (P = 

 0.05) difference between the two crabs (independent 

 of size) in length and width of carapace, length and 

 depth of body, length of merus and ischium of the 

 third maxilliped, and length and height of the 

 major palm. 



Habitat 



There is a relatively effective isolation of the two 

 mud crabs by habitat. Panopeus simpsoni occurs 

 intertidally or subtidally in association with the 

 American oyster, but not usually in the marsh bank 

 environment. Panopeus obesus occurs in marsh 

 banks, but not subtidally. Both species inhabit inter- 

 tidal rubble areas. 



Feeding 



Though food type for both species is similar, P. 

 obesus is much more aggressive in capturing and con- 

 suming prey. 



These findings reinforce the conclusion of Williams 

 (1983) thatP. obesus andP. simpsoni are specifically 

 distinct. 



Acknowledgments 



We thank R. W. Heard, J. F. Fitzpatrick, T. S. Hop- 

 kins, and G. Crozier for assistance and guidance to 

 the senior author. W. Martz, J. Martin, G. Goeke, and 

 D. Anderson helped in field sampling, and Faye P. 

 Reames gave technical assistance. J. L. Russo gave 

 statistical advice. B. B. Collette, D. L. Felder, R. B. 

 Manning, B. Sullivan, and anonymous reviewers 

 offered critical comments on the manuscript. 



Literature Cited 



Benedict, J. E., and M. J. Rathbun. 



1891. The genus Panopeus. Proc. U.S. Natl. Mus. 14:355- 

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1982. Guide to common tidal marsh invertebrates of the 

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 Jarvis, G. T. 



1974. MUSIC ST ATPAK reference manual. 1st ed. Com- 

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1974. The distribution of mud crabs (Xanthidae) in Alabama 

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 Rathbun, M. J. 



1930. The cancroid crabs of America of the families of 

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Smith, S. I. 



1869. Notes on new or little known species of American can- 

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 Williams, A. B. 



1965. Marine decapod crustaceans of the Carolinas. U.S. 



Fish WildL Serv., Fish. Bull. 65:1-298. 

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Robert C. Reames 



Austin B. Williams 



Department of Biological Sciences 

 University of South Alabama 

 Mobile, Ala. 



Present address: 5313 11th Ct South 

 Birmingham, AL 35212 



Systematics Laboratory 

 National Marine Fisheries Service, NOAA 

 National Museum of Natural History 

 Washington, DC 20560 



EFFECT OF TEMPERATURE ON RATE OF 



EMBRYONIC DEVELOPMENT OF 



WALLEYE POLLOCK, 



THERAGRA CHALCOGRAMMA 



Recent studies by the Northwest and Alaska 

 Fisheries Center Auke Bay Laboratory of the 

 National Marine Fisheries Service, Auke Bay, Alas- 

 ka, have focused on causes underlying mortality of 

 eggs and larvae of walleye pollock, Theragra 

 chalcogramma, a species of considerable economic 

 importance in Alaskan waters. One aspect of these 

 studies is to predict age of walleye pollock embryos in 

 samples from surveys at sea. Knowledge of age of em- 

 bryos is necessaiy for estimating peak spawning time 

 and daily production of eggs, and for predicting 

 abundance and distribution of spawning fish. 

 Because length of the incubation period is dependent 

 on temperature of the water mass in which the eggs 

 are developing (Hamai et al. 1971), embryo age 

 (hours since fertilization) can be estimated provided 

 water temperature is known. 



In this study, we determined the relation between 

 temperature and rate of development of walleye 

 pollock embryos at constant incubation tem- 

 peratures and at fluctuating temperatures (simu- 

 lated). We then derived equations and a contour plot 

 for estimating the age of an embryo (time from fer- 

 tilization, in hours) at a given incubation temperature 

 and stage of development. We also derived an equa- 



890 



FISHERY BULLETIN: VOL. 81. NO. 4. 1983. 



