FISHERY BULLETIN: VOL. 73, NO. 4 



Indo-West Pacific subregion. Nematoscelis 

 microps males with paired chitinous plates on the 

 first abdominal segment occur mostly in the 

 tropical regions of the Pacific, Atlantic, and Indian 

 oceans. James (1973) found this form of N. microps 

 occurring in the northeast Atlantic south of lat. 

 20°N. Adult males of N. microps without any 

 photophore enlargement were also found in all 

 oceans, but they occur mostly in the subtropical 

 regions. 



Nematoscelis tenella and A^. atlantica males 

 have two forms of photophore enlargement 

 (Figure llB,c and llB,d): one form has the second 

 and third photophores enlarged along with the 

 presence of saddle-shaped paired plates on the 

 dorsal side of both the first and second abdominal 

 segments; the other form shows photophore 

 enlargement on the fourth abdominal segment 

 along with paired plates on the third abdominal 

 segment. These two species occur together in the 

 northern and southern subtropical provinces of all 

 oceans. They also occur together in the tropical 

 regions of the Atlantic Ocean. When the two 

 species occur in the same geographical area, the 

 males of both species do not show the same pattern 

 of photophore enlargement. In such cases the pat- 

 tern of photophore enlargement is as follows: 



Species 



Nematoscelis tenella 

 Nematoscelis atlantica 



Subtropics Tropics 



First form * Second form * * 



Second form * * First form * 



*Saddle-shaped plates on first and second abdominal segments 

 and photophore enlargements on second and third. 

 '*Saddle-shaped plates on third abdominal segment and pho- 

 tophore enlargement on fourth. 



Nematoscelis atlantica does not occur in the 

 tropical regions of the Pacific and Indian oceans; 

 A^. tenella males in this region have the second 

 form of photophore enlargement. There is no clear 

 evidence of photophore enlargement in N. difficilis 

 and N. megalops. 



Even though a pattern of photophore en- 

 largement appears to be characteristic of each 

 species, not all mature males show this feature. 

 When present, about 60-80% of the males in each 

 sample had specific patterns of photophore en- 

 largement as described above. Structure of the 

 petasma of these forms did not differ from the 

 typical forms. One is tempted to speculate on the 

 evolutionary significance of this feature. The oc- 

 currence of the dorsal chitinous plate and the 

 enlargement of the photophore in adult males may 



have some joint functional importance, probably 

 related to sexual behavior. Evidently, these ab- 

 normal conditions are not random phenomena; 

 patterns are of specific nature and have clear as- 

 sociation with sexual maturity. The fact that N. 

 tenella and A^^. atlantica do not have the same form 

 of photophore enlargement when they occur 

 together in a geographical area suggests the pos- 

 sible role of these specific patterns in enhancing 

 species recognition for mating. 



DISCUSSION 



Among the morphological characters of a 

 species, feeding and reproductive structures af- 

 ford specific features that have diagnostic value. 

 The specificity of the feeding appendages reflects 

 presumed niche specializations. The structural 

 uniqueness in the reproductive system ensures 

 reproductive isolation of the species upon which 

 the biological species concept is founded (Mayr 

 1942). Therefore, a key based on these characters 

 should be the best in distinguishing individual 

 species. The selection of the maxillliped as a diag- 

 nostic character has the advantage that the same 

 key may be used for both sexes. However, it did 

 not prove possible to make such a key for the larva. 



The species of Nematoscelis can be grouped into 

 two subgroups, one with N. difficilis and N. 

 megalops and the other with the rest of the five 

 species. The present study has brought out both 

 ecological and systematic evidences to support 

 such a grouping. The sequential development of 

 larval characteristics also suggests phylogenetic 

 differences between the two subgroups. Hansen 

 (1912), using the structure of adult maxillule, 

 proposed a division of Nematoscelis into these two 

 groups. A similar division was also made by 

 Mauchline (1967) on the basis of structural 

 differences in the adult maxilla. 



When closely related species are partially sym- 

 patric, behavioral mechanisms might operate to 

 insure reproductive isolation of the species (Mayr 

 1966). Usually the presence or absence of in- 

 tergradation between sympatric populations 

 serve as indicators of interbreeding or reproduc- 

 tive isolation. Presumably, the absence of in- 

 tergradation between species of Nematoscelis oc- 

 cupying the same geographical area suggests 

 reproductive isolation. However, the absence of 

 reproductive isolation between the two forms of 

 A^. gracilis is probably shown by the occurrence of 

 sexually mature intermediate forms in the 



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