C. japonicus (Rathbun) and C. opilio elongatus 

 Rathbun. 



Since C. bairdi has become commercially impor- 

 tant, its biology and distribution are receiving 

 more attention. Descriptions of the larvae for C. 

 bairdi and C. opilio are important because both 

 are taken commercially and their distribution 

 overlaps. Haynes (1973) described prezoeae and 

 stage I zoeae of C. bairdi (and C. opilio), but stage 

 II zoeae and megalopa have not been described. 



In this paper we describe megalopa of C. bairdi 

 and compare them with megalopa of C. opilio 

 (Motoh 1973) and C. opilio elongatus (Kurata 

 1963b) — the only other Chionoecetes species for 

 which the megalopal stages have been described. 



There seems to be some lack of consistency in 

 the literature concerning the singular and plural 

 of the megalopal stage. The original singular was 

 called megalops, because of the large and promi- 

 nent eyes. Many authors (e.g., Kurata 1963a, b; 

 Makarov 1967; Motoh 1973) have changed this to 

 megalopa for both singular and plural. Others 

 (e.g., Hart 1960; Poole 1966) have latinized 

 megalopa in the plural to megalopae. In this man- 

 uscript both singular and plural of the megalopal 

 stage will be referred to as megalopa since this is 

 more widely accepted. 



Methods and Materials 



About 50 larvae 1 of C. bairdi were taken from 

 Fish Bay near Sitka, Alaska, at lat. 57°22'N, long. 

 135°33'W on 14 April 1971. They were caught 

 with 70-cm-diameter nylon bongo nets towed 8 to 

 9 m below the surface; mesh sizes of the nets were 

 0.505 and 0.333 mm. The larvae were held in a 

 3-liter aquarium supplied with continuous- 

 flowing filtered seawater. The aquarium was 

 transferred from the research vessel to the 

 laboratory on 19 April. The water temperature 

 fluctuated between 8° and 10°C on the vessel and 

 6.3° and 6.9°C in the laboratory. The C. bairdi 

 larvae fed upon other zooplankton caught during 

 the same tow until that food was gone. By then, it 

 appeared all the larvae were at the megalopal 

 stage, and we began feeding them finely chopped 

 herring. Some megalopa were preserved on 19 

 April in 59c formaldehyde and seawater. Their 



'The specimens preserved 14 April were lost and could not be 

 examined to determine their stage of development. We believe 

 that they were stage II zoeae or megalopa or a combination of 

 both. 



identification as C. bairdi was confirmed by rais- 

 ing the remaining megalopa to the juvenile stage 

 (maximum carapace width 13.9 mm) and compar- 

 ing them with the juvenile morphology described 

 by Garth (1958). 



Megalopal larvae identical morphologically to 

 those we had raised were collected in a vertical 

 plankton haul on 21 May 1973, at the entrance to 

 Resurrection Bay south of Seward, Alaska, at lat. 

 59°48'N, long. 149°30'W. These specimens were 

 dissected and used as the basis for our illustrations 

 of morphology, appendage setation, and other 

 characteristics. 



Illustrations (Figure 1) were prepared with the 

 aid of a camera lucida. An ocular micrometer was 

 used to measure body dimensions of nine of the 

 preserved specimens. The measurements were 1) 

 carapace length (two measurements had to be 

 taken because the rostral tip was often 

 damaged — straight-line distance from rostral tip 

 to posterior median margin of carapace and 

 straight-line distance from the notch between 

 rostral and preorbital spine to posterior median 

 margin of carapace); and 2) carapace width 

 (straight-line distance between widest part of 

 carapace). 



To compare our description of megalopal larvae 

 of C. bairdi with descriptions of megalopa of other 

 species in the genus, we used our collections from 

 the Chukchi Sea and descriptions by Motoh ( 1973) 

 for C. opilio and descriptions by Kurata (1963b) 

 for C. opilio elongatus. 



Description of Megalopa 



Carapace length 3.12 to 3.48 mm (mean 3.30 

 mm) inclusive of rostrum and 2.60 to 2.80 mm 

 (mean 2.73 mm) from rostral notch. Carapace 

 width 1.80 to 2.12 mm (mean 1.97 mm). 



Carapace triangular shaped and bears seven 

 major processes (Figure la-c). Anterior rostral re- 

 gion bears three sharp spines, two preorbital and 

 one rostral. Rostral spine three times length of 

 preorbital spines (measuring from rostral notch) 

 and points ventrally. Frontal and rostral region 

 slightly depressed. Pair of anterolateral spines 

 separated by thin median ridge. Pair of cardiac 

 dorsolateral spines sweep slightly posteriorly. 

 Minute but conspicuous lateral spines occur in 

 region of pterygostomial-branchial ridge. Small 

 ridge along posterolateral margin of carapace 

 bears a wartlike protuberance medially, directly 

 above proximal end of abdomen. Eyes stalked. 



460 



