BIOLOGY AND TAXONOMY OF THE GENUS NEMATOSCELIS 



(CRUSTACEA, EUPHAUSIACEA) 



K. GOPALAKRISHNAN' 



ABSTRACT 



The seven species of Nemafoscelis, N. difficilis, N. megalops, N. gracilis, N. microps, N. tenella, N. 

 atlantica, and A^. lobata, are described in a comparative manner, and keys for their identifications are 

 provided. The key to the larvae is based on structural differences in the carapace and rostrum of f urciUa 

 stages, whereas the key to adults is mostly based on diagnostic features of the first thoracic leg 

 (maxilliped) and a male secondary sexual structure, the petasma. Nematosceiis gracilis is represented 

 by two distinct forms; they are considered ecophenotypes, since their patterns of geographical dis- 

 tribution appear correlated with differences in environmental characteristics, particularly the dis- 

 tribution of dissolved oxygen in the water column. Diagnostic features of these forms are pointed out. 

 The antennule and carapace are sexually dimorphic in adults of all Nematosceiis. Abdominal pho- 

 tophores in the males show species-specific patterns of enlargement. 



The genus Nematosceiis consists of seven species. 

 It was described by G. 0. Sars (1883, 1885) as con- 

 sisting of A'', megalops, N. tenella, N. microps, and 

 A^. rostrata. Hansen synonymized A'', rostrata with 

 A^. microps and added four species: A'^. gracilis and 

 A^. atlantica in 1910, A^. difficilis in 1911, and A^. 

 lobata in 1916. Nematosceiis lobata was not found 

 by subsequent workers but the other species were 

 discussed by Ruud (1936), Boden (1954), Boden et 

 al. (1955) and Mauchline and Fisher (1969). 

 Taxonomically Nematosceiis has been a difficult 

 genus. 



Like other species of euphausiids, Nematosceiis 

 species have been identified mainly on the basis of 

 differences in the male copulatory organ, the pe- 

 tasma. Since the petasma is an adult character, it 

 has been difficult to identify immature specimens 

 and mature females. Characters such as the shape 

 of the eyes and structure of the second thoracic leg 

 have been used to discriminate species. Einarsson 

 (1942) showed structural differences in sper- 

 mathecae (thelyca) of females, but such 

 differences appear slight and are difficult to 

 examine. Mauchline and Fisher (1969) pointed out 

 difficulties encountered in the identification of 

 species of this genus. In the present study an at- 

 tempt is made to point out the diagnostic value of 

 the first thoracic leg (maxilliped) in discriminating 

 all species of Nematosceiis. This appendage 



'Scripps Institution of Oceanography, La Jolla, CA 92037; 

 present address: Hawaii Institute of Marine Biology, University 

 of Hawaii, P.O. Box 1346, Kaneohe, HI 96744. 



Manuscript accepted October 1974. 

 FISHERY BULLETIN; VOL. 73, NO. 4, 1975. 



usually remains attached to the animal caught by 

 nets (as compared with the elongate second leg 

 which is usually lost). It can be used as a diagnostic 

 character in both sexes. The structural differences 

 among the petasmae are also reexamined. The 

 morphology of individual species will not be given 

 separately, but species differences will be pointed 

 out in a comparative manner. Since all species of 

 this genus are sexually dimorphic, it is necessary 

 to describe both sexes. 



Another aspect to which little attention has 

 been paid is the significance of developmental 

 features in determining phylogenetic associa- 

 tions. Larvae of Nematosceiis are often difficult to 

 separate as to species. The taxonomy of the larvae 

 is yet to be worked out because the recognized 

 adult characteristics are of no use in the larval 

 identification. Gopalakrishnan (1973) summarized 

 the available information on the sequential 

 morphological development of an individual 

 species. As Gordon (1955) and others have pointed 

 out, larval characteristics may be more useful than 

 those of adults in recognizing phylogenetic in- 

 terrelationships of species. Adults show a greater 

 degree of differentiation than the larvae, and 

 their characters are more useful in the identifica- 

 tion of the species than determining phylogeny. 

 Moreover, part of the morphological variability 

 observed in adults is sometimes ascribed to non- 

 genetic modification that probably has no 

 phylogenetic significance. Usually phylogenetic 

 interrelationships are summarized in a classifica- 

 tion. In this connection, larval characters are used 



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