MOSER and AHLSTROM: ROLE OF LARVAE IN SYSTEM ATICS 



larval period. None of the species forms photo- 

 phores during the larval period other than the Br2 

 pair. 



The characters that most clearly separate the 

 three developmental lines in Electrona are eye 

 shape and the amount of choroid tissue developed 

 under the eye. Electrona antarctica has an elon- 

 gate choroid mass uniquely divided into two nar- 

 row eyes (Figure 2A). Also, E. antarctica larvae 

 attain a large size (20 mm), are the deepest-bodied 

 oiaWElectrona larvae, and have the heaviest pig- 

 mentation. The two species in the second de- 

 velopmental line transform at a small size (ca. 10 

 mm in E. rissoi and 12-13 mm in £■. carlsbergi), 

 have a small choroid mass under a moderately 

 narrow eye, and develop scant pigment (Figure 

 2B). In the third line, consisting of £. subaspera 

 and E. paucirastra, the eye is the least narrow, 

 has no choroid tissue, and the larvae attain a large 

 size (20 mm) (Figure 2C). 



The larva of the species described as Metelec- 

 trona ahlstromi (Wisner, 1963) is illustrated in 

 Figure 2D. It is more laterally compressed than 

 any species of Electrona and has no interspace 

 between the anus and origin of the anal fin. In 

 some features it resembles the larvae of 

 Hygophum; it has a late-forming dorsal fin and the 

 gut is shaped very similarly to that inH. taaningi 

 and H. macrochir. Its pigment is unique and the 

 eye is distinct, with the ventral edge of the scleral 

 envelope characteristically squared off. Also, in 

 late-stage larvae, in addition to the Br2, a second 

 pair of photophores (PO5) develops, a feature 

 found in neither Hygophum nor Electrona. Paxton 

 (1972) synonymized Metelectrona with Electrona 

 and suggested that M. ahlstromi andE. ventralis 



are synonyms, however, the uniqueness of the 

 larva strongly suggests the resurrection of 

 Metelectrona as a valid genus. 



The genus Benthosema is the least cohesive of 

 any genus in the subfamily Myctophinae, from the 

 viewpoint of larval structure (Figure 3). We can 

 find only four types of larvae in the world ocean, 

 although Nafpaktitis (1973) recognizes five 

 species on adult characters. We cannot distin- 

 guish larvae of B. pterota and B. panamense 

 although Nafpaktitis has listed a number of con- 

 vincing characters that distinguish the adults of 

 the two species. We find two highly divergent 

 species pairs. One is composed of B. glaciale and 

 B. suborbitale with a narrow eye subtended by a 

 lunate choroid mass and with a pronounced inter- 

 space between the anus and the anal fin origin, 

 reminiscent of Protomyctophum and Electrona 

 (Figure 3A-C). In the other pair, consisting of B. 

 panamense-pterota and B. fibulatum, the eye is 

 wider, is subtended by a mere sliver of choroid 

 tissue and the gut, of moderate length, lacks a 

 postanal interspace (Figure 3D, E). 



The one feature held in common by the four 

 species is the development of some photophores in 

 addition to the Bra during the larval period. The 

 only other myctophine genera that develop photo- 

 phores in addition to the ubiquitous Br2 during the 

 larval period are Diogenichthys, Myctophum, 

 and Metelectrona. This feature is much more pre- 

 valent among genera of the Lampanyctinae and is 

 helpful in defining groups of related genera there 

 (Moser and Ahlstrom, 1972). 



In B. panamense-pterota and B. fibulatum the 

 Dn pair is formed soon after the Br2 at about 

 5.0-6.0 mm. The PO5 pair is the third to appear in 



Table 1. — Sequence of photophore formation in larvae of three species oi Benthosema . 



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