40 



Fishery Bulletin 88(1), 1990 



stages may hold the chie to periodic changes in the 

 strength of year-classes. Cancer species pass through 

 five larval (zoeal) and one postlarval (megalopal) stage. 

 The megalopal stage represents the transition between 

 the planktonic zoeae and the benthic adult. As the zoeae 

 provide a major food source for several fish species 

 (Garth and Abbott 1980), it is probable that the assess- 

 ment of abundance of megalopae will provide the most 

 reliable information on annual recruitment for commer- 

 cial species. Therefore, it is essential that megalopae 

 of the four species can be distinguished in mixed plank- 

 ton tows. Information on the megalopal development 

 of C. magister, C. productus, and C. gracilis are avail- 

 able; this is the first description of the megalopae of 

 C. oregonensis. 



Iwata and Konishi (1981) suggested that the setal for- 

 mulae for the antenna and the endopod of the third 

 maxilliped would permit specific identifications of the 

 megalopae of several Cancer species, including C. 

 magister and C. productus; however, their evaluation 

 was made only from the published data available. Quin- 

 tana and Saelzer (1986), in their summary of megalopal 

 characters oi Cancer species, remarked that endopodal 

 setation of the third maxilliped was variable among 

 species and very difficult to determine. They instead 

 placed emphasis upon the setation of the epipod of the 

 second maxilliped. Unfortunately, there often are con- 

 siderable discrepancies between published descriptions 

 of the same species. For example, Anderson (1978) em- 

 phasized the significance of the antennal setation in 

 distinguishing megalopae of C. anthonyi Rathbun from 

 C. productus. However, his setal count for C. anthonyi 

 differed appreciably from that illustrated, but not 

 specified, by Trask (1974) in his description of this 

 species. 



At least some morphological variation in larval char- 

 acters may be attributed to differing geographic areas 

 (Wencker et al. 1983, Shirley et al. 1987). We initially 

 compared megalopae of C. oregonensis with megalopae 

 of C. productus collected from Puget Sound. However, 

 when we then compared the local C. jyroductus mega- 

 lopae with the description of megalopae from Califor- 

 nia (Trask 1970), we found substantive differences. 

 Therefore, we have compared laboratory-reared and 

 locally captured population of C oregonensis megalopae 

 with northern populations of C. productus megalopae 

 and have redescribed the latter. From these descrip- 

 tions, together with the descriptions provided by Poole 

 (1966) for the megalopae of C. magister and Ally (1975) 

 for C. gracilis magalopae, we have prepared a key to 

 the megalopae of the northern populations of the four 

 Cancer species. 



Materials and methods 



Samples from naturally occurring populations of mega- 

 lopae of Cancer oregonensis and C. productus were col- 

 lected from two broad regions within the Puget Sound 

 Basin. The first region included 16 sites in the Strait 

 of Georgia, British Columbia; the second consisted of 

 17 sites in the Strait of Juan de Fuca, Washington. Sup- 

 plemental samples were also collected near Anacortes, 

 Washington. Monthly sampling took place over a period 

 of 5 months. May through September, 1987. Each sam- 

 pling cruise consisted of nightly trawling with an otter 

 neuston sampler (Mason and Phillips 1986) over a 3-day 

 period. Megalopae were transferred to the laboratory 

 and isolated in compartmented trays and maintained 

 in a constant temperature unit (CTU) at 15°C in 



filtered seawater of 31"/ The megalopae were 



changed daily into fresh seawater, and newly-hatched 

 Ar-temia salina (Linnaeus) nauplii were provided as 

 nourishment. Exuviae were preserved in 70% ethyl 

 alcohol, thirst crab instars were used to confirm species 

 identifications. Twelve exuviae of each species were 

 stained using 1% chorazol black in equal parts phenol 

 and lactic acid; mouthparts and appendages were 

 dissected and mounted in polyvinyl alcohol lactophenol. 

 Morphological characters were then described and il- 

 lustrations were made with the aid of a camera lucida 

 mounted on a Wild M-5 stereomicroscope. An addi- 

 tional 42 exuviae of C. oregonensis and 48 exuviae of 

 C. productus were examined for significant mor- 

 phological characters not requiring dissection. Labor- 

 atory-reared megalopae of C. oregonensis were obtain- 

 ed from rearing studies. Six of the laboratory-reared 

 megalopae were stained and dissected following the 

 above procedure and an additional ten specimens were 

 examined for gross morphology. Setal counts for all ap- 

 pendages are cited from proximal to distal. 



Results 



The megalopal stage of Cancer oregonensis is described 

 from specimens collected from naturally occurring 

 populations, and its morphological characters are com- 

 pared with those of laboratory-reared individuals. 

 Megalopae of C. productus from naturally occurring 

 populations were similarly examined, and their mor- 

 phological characters are compared with those de- 

 scribed by Trask (1970) for a California population of 

 this species (Table 1). Because a number of significant 

 character differerences exist between the populations, 

 a redescription of C. productus megalopae, based on 

 the northern populations, is included. 



Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NCAA. 



