THE LARVAL DEVELOPMENT OF SERGESTES SIMILIS HANSEN 

 (CRUSTACEA, DECAPODA, SERGESTIDAE) REARED IN 



THE LABORATORY 



Margaret Knight 1 and Makoto Omori 2 



ABSTRACT 



The larval development of Sergestes similis Hansen reared in the laboratory includes the following 

 stages: nauplius I-IV, protozoea I-III, and zoea I-II. These forms together with the first two 

 postlarval stages are described and illustrated. 



Sergestes sim His and S. arcticus, closely related species which comprise the arcticus species group, 

 are very similar in larval as well as adult morphology especially in the ornate armature of 

 protozoeal carapace apparently specific to the group. In contrast, the two species of the atlanticus 

 group, S. atlanticus and S. comutus, differ distinctly from each other in carapace armature of the 

 protozeal stages. The difference between these two species groups in variation within each group 

 indicates that larval morphology may be of value in the study of interspecific relationships within 

 Sergestes. Sergestes similis and Sergia lucens, species of closely related genera, differ in number of 

 naupliar stages, in armature of body in protozoeal and zoeal phases, and in development of some 

 appendages. 



The pelagic shrimp Sergestes similis is abundant 

 in the North Pacific Drift ranging from Japan to 

 North America between 40° and 50°N, and is a 

 prominent constituent of the plankton in the 

 cooler waters of the California Current. 



Within the genus Sergestes (Omori 1974), S. 

 similis is located in the arcticus species group, as 

 defined by Yaldwyn (1957), which includes only 

 the two species 5. arcticus Kroyer and S. similis 

 Hansen. Sergestes arcticus is widely distributed, 

 occurring in the North Atlantic, the Mediter- 

 ranean, and all sectors of the Southern Ocean, 

 while S. similis is restricted to the subarctic and 

 transitional zones of the North Pacific; available 

 data indicates that the species are geographi- 

 cally isolated from one another (Judkins 1972). 

 The life history and distribution of S. similis and 

 its importance in oceanic ecosystems of the 

 Pacific have been discussed by Pearcy and Forss 

 (1969), Omori et al. (1972), and Omori and Gluck 

 (1979). 



The purpose of this paper is to describe and 

 illustrate the larval development of S. similis 

 and to compare the larvae with those of the 

 closely related species S. arcticus described by 

 Wasserloos (1908), Hansen (1922), and Gurney 



•Scripps Institution of Oceanography, University of Cali- 

 fornia, La Jolla, CA 92093. 



2 Research Laboratory of fisheries resources. Tokyo 

 University of Fisheries, Konan, Minato-ku, Tokyo 108, Japan. 



Manuscript accedpted October 1981. 

 FISHERY BULLETIN: VOL. 80, NO. 2, 1982. 



and Lebour (1940). The larvae of S. similis are 

 also compared with the early stages of S. 

 atlanticus and S. comutus (Gurney and Lebour 

 1940), which comprise the atlanticus group, to 

 note the difference in variation within species 

 groups in protozoeal morphology, and with the 

 larvae of Sergia lucens (Omori 1969) to note the 

 differences between species of closely related 

 genera. The description of Sergestes similis is 

 based on both individuals reared in the 

 laboratory by Omori (1979) during his study of 

 the growth, feeding, and mortality of larval and 

 postlarval stages of the species off southern 

 California, and on specimens from preserved 

 plankton samples. 



Gurney and Lebour (1940), in the major work 

 on larvae of the genus, remarked that "perhaps 

 the most interesting feature of the development 

 of Sergestes is the striking difference which 

 exists between the larvae of the different species, 

 while the adults are often separable with diffi- 

 culty," and suggested that knowledge of the 

 larvae, when complete, may give a better indica- 

 tion of the relationships of species than adult 

 morphology. 



METHODS 



Omori (1979) described the procedures used 

 for rearing the larvae of S. similis in the lab- 

 oratory. Larvae from the population of the 



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