FISHERY BULLETIN: VOL. 80, NO. 2 



rarely endopod of pleopod 2 with 1 or 2 small 

 setae; protopods of pleopods 1-3 with 2 setae; 

 those of pleopods 4 and 5 with or without 1 seta on 

 protopod. 



DISCUSSION 



Yaldwyn (1957) defined two subgenera, 

 Sergestes s.s. and Sergia, within what he termed 

 the rather unwieldy genus Segestes s.l. Recently, 

 the subgenera were raised to full genera by 

 Omori (1974). The species of Sergestes have 

 specialized luminescent modifications of the 

 gastrohepatic gland called organs of Pesta and 

 are without cuticular pigmentation and dermal 

 photophores, while species of Sergia are without 

 organs of Pesta and, with some exceptions, have 

 uniform cuticular pigmentation and often 

 dermal photophores. The two genera are 

 themselves divided into species groups, six in 

 Sergestes and three in Sergia, which appear to be 

 easily distinguished and are considered to be 

 natural phyletic units (Judkins 1978). 



The arcticus group includes only two species, 

 Sergestes arcticus and S. similis, and is 

 characterized by the morphology of third 

 maxilliped, fifth pereiopod, antennular pedun- 

 cle, and petasma (Yaldwyn 1957). Sergestes 

 similis differs from S. arcticus in having a more 

 slender and fragile body and antennular 

 peduncle, in a longer and more upwardly 

 directed rostrum, in proportions of posterior 

 arthrobranchs above the third and fourth 

 pereiopods, and in some proportions and 

 armature of petasma and thelycum (Milne 1968). 



The close relationship of S. similis and S. 

 arcticus which has been inferred from adult 

 morphology may also be seen in their larval 

 morphology, especially in the shape of eye, in the 

 ornate armature of protozoeal carapace, and in 

 the armature of carapace, abdomen, and telson 

 in the zoeal phase. Gurney and Lebour (1940) 

 described larvae now known to be representative 

 of all of the species groups within Sergestes s.l. 

 and noted that the protozoea II and III of S. 

 arcticus were very distinct in form of eye and 

 peculiarly branched spines. They described 

 some features of protozoea II and III, zoea I and 

 II, and postlarva I of S. arcticus and gave figures 

 of the second protozoea and zoea, with telson of 

 postlarva I. They stated that the "brushlike 

 endings" of the long spines on rostral, lateral, 

 and posterior processes of protozoea II and on 

 supraorbital, lateral, and posterior processes of 



protozoea III were most characteristic of the 

 species. The protozoea II and III of S. similis, 

 identified in this study, have the same distinctive 

 armature of carapace spines. 



The larval stages of S. arcticus discussed by 

 Gurney and Lebour (1940) resemble the compar- 

 able stages of S. similis in the details they 

 described and figured. Gurney and Lebour, 

 however, do not deal with the structure of mouth- 

 parts and thoracic appendages; rather, they note 

 that these appendages seem to be uniform 

 throughout the genus and refer the reader to the 

 earlier descriptions of S. arcticus by Wasserloos 

 (1908) and Hansen (1922). Gurney, in a later 

 work (1942), does figure the appendages of 

 protozoea III of S. cornutus, an atlanticus group 

 species, and they appear very similar to those of 

 the same stage of S. similis. 



The protozoeal stages of S. arcticus described 

 by Wasserloos (1908), on the other hand, differ 

 from those of S. similis in setation and/or 

 segmentation of antennule, antenna, and 

 mouthparts, but appendages are not figured; the 

 armature of carapace differs in protozoea II and 

 III on lateral and supraorbital processes, re- 

 spectively. The species appear similar in 

 described and figured features of the zoeal 

 phase. 



Hansen (1922) offered a brief summary of 

 Wasserloos' description of the protozoeal phase 

 and added both generic and specific comments, 

 with figures, on the zoea and postlarva of S. 

 arcticus from his own observations. He noted 

 that the mouthparts of the protozoea are like 

 those of the zoeal stages which he described in 

 some detail but which do not always agree with 

 details of the protozoeal phase described by 

 Wasserloos. Hansen also noted that the rostrum 

 in protozoea III is little modified from stage II, 

 yet conspicuous secondary spines are lost in this 

 molt. In the zoeal phase, S. similis larvae differ 

 from those of S. arcticus, as described by Hansen, 

 in segmentation of maxillule and first maxil- 

 liped. 



Unfortunately, because the descriptions of S. 

 arcticus by Wasserloos (1908) and Hansen (1922) 

 were found to be inconsistent with each other and 

 with that of Gurney and Lebour (1940), and they 

 could not be interpreted with confidence, a 

 detailed comparison with S. similis was not 

 possible. A reexamination of the larval stages of 

 S. arcticus is needed to detect specific differences 

 that may exist between the apparently very 

 similar arcticus group species. 



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