48 



Fishery Bulletin 88(1), 1990 



of the first and second maxillipeds of C magister, setal 

 numbers for the maxillipedal epipods overlap among 

 all four local species. 



In his description of the larval development of Cancer 

 magister, Poole stated that the megalopa possessed five 

 abdominal segments and a telson, and this number was 

 repeated by both Ingle (1981) and Iwata and Konishi 

 (1981) in their reviews of megalopal characters of 

 Cancer species. As all other described larvae possess 

 a six-segmented abdomen, this single character would 

 be expected to provide a very simple and easy means 

 for recognizing megalopae of C. magister. Poole's 

 (1966) description of the megalopae was based on two 

 specimens collected from Drakes Bay, California; how- 

 ever, Poole indicated that he had compared these indi- 

 viduals with laboratory-reared specimens. He noted no 

 significant differences. His figure (1966: fig 6A) indi- 

 cates no suture between the sixth abdominal somite and 

 the telson, and he refers to the uropods as the "pleo- 

 pods of the telson." Unless a major, and evolutionar- 

 ily significant, variation occurs in the postlarvae of this 

 species over its range, Poole's description is incorrect. 

 We have examined a substantial number of C. magister 

 megalopae from the Puget Sound Basin, and in all cases 

 the sixth abdominal somite is clearly separated from 

 the telson by a well marked suture; the uropods arise, 

 as in other megalopae, from the distal margin of the 

 sixth somite. 



Megalopae of Cancer have been broadly grouped by 

 Orensanz and Gallucci (1988) into the size categories 

 small, medium, and large. In the species of local in- 

 terest, C. gracilis is grouped among those species with 

 small megalopae: C. oregonensis and C. productus in 

 the medium-sized group and C. magister as the single 

 representative in the large category. However, Oren- 

 sanz and Gallucci have reported a bimodal recruitment 

 of C. magister, with the late-summer megalopae being 

 of considerably smaller size. Thus the use of size in dis- 

 tinguishing C. ynagister from other local species may 

 be less reliable during certain periods of the year than 

 previously assumed. The megalopae of C. oregonensis 

 and C. productus are morphologically very close and 

 no definitive means of distinguishing between the two 

 species at this stage has been available. During the 

 course of this study an apparently constant and easily 

 recognizable character has been found that will distin- 

 guish C. productus from C. oregonensis, i.e., the pres- 

 ence in the former species of an acute process on the 

 ventral surface of the ischium of the cheliped (Fig. 4D) 

 that is absent in the latter species (Fig. 2D). In fact, 

 the absence of a spine or process on the ischium of the 

 cheliped distinguishes C. oregonensis megalopae from 

 all three of the other local species. Poole (1966) de- 

 scribed a spine only on the "basi-ischipoidite of the first 

 walking leg" of C. magister; however, in our northern 



populations we have found that a strong, acute spine 

 is present on the ventrodistal margin on the ischium 

 of the cheliped and on the ventrodistal margin of the 

 coxa of the 2nd and 3rd pereopods. Ally (1975) reports 

 a spine ("hook") on the ventral surface of the ischium 

 of the cheliped in C. gracilis. We have not been able 

 to examine local megalopae of this species; therefore, 

 in preparing the following key to the local species, we 

 have relied on the completeness and accuracy of Ally's 

 description. 



Key to the megalopae of northern populations of 

 Ca ncer 



1 No spine or process on ventrodistal surface/ 

 margin of ischium of cheliped . . . . C oregonensis 

 Acute spine or process on ventrodistal 

 surface/margin of ischium of cheliped 2 



2 Megalopae of small size (<3.0 mm); 2nd and 

 3rd pereopods lacking coxa) spine or process 



on ventrodistal surface C. gracilis 



Megalopae of medium to large size (<4.0 



mm); 2nd pereopod with coxal spine or 



process on ventrodistal surface 3 



3 Megalopae with acute spine on ventrodistal 



surface of coxa of 3rd pereopod C magister 



Megalopae usually without process, or rarely 



with very small process on ventrodistal sur- 

 face of coxa of 3rd pereopod C. productus 



Acknowledgments 



The senior autlior expresses his deep appreciation to 

 Dr. G. Jamieson and to the staff at the Pacific Bio- 

 logical Station, Nanaimo, B.C., for providing the oppor- 

 tunity to participate in the station's sampling progi-am. 

 This research was supported in part by Washington 

 Sea Grant Program #R/F-73-pd and by a National 

 Science Undergraduate Research Grant to Dr. S. 

 Sulkin, Western Washington University. Particular 

 thanks are due Dr. R.B. Manning, National Museum 

 of Natural History, and R. Van Syoc, California 

 Academy of Sciences, for making specimens of Cancer 

 tirannerl and C. oregonensis available to us. We also 

 wish to gratefully acknowledge the assistance and sup- 

 port provided by the staff of the Shannon Point Marine 

 Center. The helpful suggestions of two anonymous 

 reviews substantially improved the manuscrijit. 



