FISHERY BULLETIN: VOL. 73, NO. 4 



150 



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0.90 - 



0.70 



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 A A 



• •• 



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Nematoscelis gracilis 



A "new form" 



• "intermediate" 



o "old form"- Indian Ocean 



• "old form"- Pacific Ocean 



o 



oo 



l°° 8 



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 ^o ° o 





20 3.0 



Carapace Length (mm) 



40 



Figure 10.— Nematoscelis gracilis forms: ratio of proximal 

 process to median lobe of petasma plotted against carapace. 



propodus of first thoracic leg; number of marginal 

 spine on lacina externa of maxillule; number of 

 setae on second segment of mandibular palp. 



Sexual Dimorphism in Nematoscelis 



Sexual dimorphism in euphausiids was best 

 documented by Hansen (1910, 1912). Einarsson 

 (1942) and Nemoto (1966) provided further details. 

 The most sexually dimorphic characters are: the 

 lateral denticle, keel, and rostrum of carapace; 

 antennule; eyes; sixth and seventh thoracic legs; 

 first and second abdominal pleopods; preanal 

 spine. 



The states of the lateral denticle, keel, and ros- 

 trum in both sexes of Nematoscelis are illustrated 

 in Figure 5. The carapace and rostrum are shorter 

 in males than in females. The rostrum in the male 

 is rarely variable; in the female it is always long 

 and slender, except in A'^. gracilis and A^. tenella. 

 The lateral denticle is absent in all species except 

 in males of A'^. microps and both sexes of A^. 

 atlantica. McLaughlin (1965) reported the oc- 



currence of this denticle in both subadult and adult 

 stages of A^. difficilis caught from the northeast- 

 ern Pacific Ocean. In the present study, the lateral 

 denticle on the carapace was found only in 

 immature specimens of this species, but not in 

 adults. These individuals were collected from the 

 North Pacific Drift and California Current areas. 

 No sexual dimorphism was observed in the shape 

 of eyes in Nematoscelis. The antennular peduncle 

 in males has the two distal segments much thicker 

 than in females; the second segment somewhat 

 shorter and the third segment much shorter than 

 in females (Figure llA,a,b). The lower flagellum 

 of the antennule has the basal segment much 

 thickened in males and furnished with tufts of 

 sensory setae. In A^. gracilis males, the proximal 

 part of this flagellum bends downward so as to 

 accommodate the enlarged basal segment (Figure 

 llA,a). 



Sexual dimorphism in abdominal photophores is 

 characteristic of Nematoscelis. Einarsson (1942) 

 pointed out a few examples of enlargements of 

 abdominal photophores in this genus. James (1973) 

 reported the existence of this feature in the North 

 Atlantic species of N. tenella, N. atlantica, and N. 

 microps. In the course of examining the material 

 from all oceans, certain interspecific differences of 

 photophore enlargement have become evident. In 

 females of all species of Nematoscelis, the pho- 

 tophores on each of the first four abdominal seg- 

 ments are more or less alike in size and shape 

 (Figure lb). However, in males, one or more of 

 these photophores often show considerable en- 

 largement. The patterns of this enlargement ap- 

 pear to be consistent, species specific, and 

 therefore of diagnostic value. 



Associated with photophore enlargement is the 

 occurrence of paired chitinous saddle-shaped 

 plates on the dorsal side of the abdominal segment 

 anterior to that in which the photophore is 

 enlarged (Figure IIB). Types of photophore 

 enlargement in species of Nematoscelis are shown 

 in this figure. In the Indian Ocean, A'^. gracilis 

 males have the first abdominal photophore 

 enlarged and lack chitinous plates on the dorsal 

 side of the abdomen. Nematoscelis microps males 

 have the second photophore enlarged and either 

 with a dorsal hump (Figure llB,b) or paired chi- 

 tinous plates on the dorsal side of the first ab- 

 dominal segment. Taniguchi (1966) reported oc- 

 currence of this hump on N. microps collected from 

 the northeastern Indian Ocean. Humped males of 

 this species are frequently found in the tropical 



810 



