MOSER and AHLSTROM: ROLE OF LARVAE IN SYSTEMATICS 



larvae about 6.0 mm long. Thereafter the pattern 

 diverges as shown in Table 1, but both species 

 gradually add about a dozen pairs during the lar- 

 val period. Specimens of B. pterota from the Per- 

 sian Gulf off India, formed photophores at some- 

 what larger sizes than larvae of B. panamense, 

 but in the same sequence. Transformation occurs 

 at a small size, 10-12 mm in B . panamense-pterota 

 and 11-13 mm inB. fibulatum. 



Photophores appear relatively late in larvae of 

 B. suborbitale and S. glaciale. however, the Bri, 

 Br2, Op2, and PO series appear in late larvae of 

 both species (Table 1). Transformation occurs at 

 about 9-11 mm in both species. The larvae of 5. 

 panamense-pterota and B. fibulatum are close to 

 the larvae of Diogenichthys in several characters 

 including body shape, gut shape, and early ap- 

 pearance of photophores. 



As in Benthosema, the larval characters of 

 Hygophum suggest some divergence within the 

 genus, although all species have a highly charac- 

 teristic series of isthmal melanophores, form the 

 dorsal fin late in the larval period, and develop no 

 photophores other than the Br2, as larvae (Figure 

 4). The genus contains three divergent types of 

 larvae. The most unusual of these are the ex- 

 tremely elongate larvae of//, reinhardti and//. 

 atratum, which have very narrow eyes that are 

 underlain by prominent choroid tissue and are 

 borne on short stalks (Figure 4A). The amount of 

 pigmentation along the gut and tail and on the 

 myosepta and fin fold increases throughout the 

 larval period. 



A second larval type is represented by the 

 largest number of species, H. proximum, H. 

 hygomi, and //. brunni, all illustrated (Figure 

 4B-D), as well as H. benoiti, H. hanseni, and an 

 undescribed form in our collection. These larvae 

 are only moderately slender and have unstalked 

 eyes of moderate width, subtended by prominent 

 choroid tissue. Melanophores are located chiefly 

 on the head and gut, however some species have 

 pigment on the myosepta and fin fold. The trend in 

 this group of species is for the early larval stages 

 to have the heaviest pigment and for melano- 

 phores to be lost as development proceeds. 



A third type of larva is exhibited by H. mac- 

 rochir, H. taaningi, and an undescribed form in 

 our collection (Figure 4E, F). These are relatively 

 deep-bodied, have large, relatively wide eyes with 

 little or no choroid tissue, and lack tail pigment. 

 Also, the gut has a highly distinctive form; the 

 anterior half has a very small diameter and opens 



dorsally into a prominent enlarged posterior sec- 

 tion. In H. macrochir this enlarged section is 

 covered with large melanophores. Larvae of this 

 group occur only in the Atlantic. 



The genus Hygophum affords an excellent ex- 

 ample of the taxonomic utility of larval stages. 

 The juveniles and adults of some species are 

 notoriously difficult to identify. In contrast, the 

 larvae of these species are highly distinct and can 

 be readily identified. We have 11 such distinct 

 larval types, whereas only 9 species are currently 

 known for the adults. Search for adults of the two 

 remaining larval types has led to the discovery of 

 two undescribed species. In addition, characters of 

 the adults of this genus reveal little about the 

 relationships of the member species (Becker, 

 1965). A study of the larvae, however, shows that 

 there are three highly distinct subgeneric groups, 

 each containing from two to six closely related 

 species. Such an independent view of the complete 

 species complement of a genus is an invaluable 

 tool in the formal revision of that genus. 



Larvae of the species of Symbolophorus are 

 perhaps the most cohesive of all myctophine gen- 

 era (Figure 5A). In all species known to us the 

 pectoral fin is large and is supported by an elon- 

 gate aliform base. Also, the pelvic fins are large 

 and develop earlier than in any other lanternfish 

 genus. The narrow eyes have choroid tissue and 

 are borne on short stalks. The amount of pigmen- 

 tation decreases towards the end of the larval 

 period. Most species attain a large size — up to 24 

 mm. The species differ principally in the size at 

 which various larval structures appear. 



The closely related genus, Myctophum, has a 

 diversity of larval form unmatched in the family 

 (Figures 5, 6, 7). Before taking up the bulk of the 

 species in this genus we must first examine the 

 most aberrant of all lanternfish larvae, that of M. 

 aurolaternatum (Figure 5B). In this remarkable 

 larva the eyes are borne on long stalks and the free 

 trailing section of the gut is almost as long as the 

 fish itself. The dorsal fin forms at the margin of the 

 fin fold. These characters are so bizarre that it 

 would seem preposterous to identify it as a 

 lanternfish larva, much less that of M. aurolater- 

 natum. Nonetheless, A. Vedel Taning first sug- 

 gested the true identity of this larva (E. Bertelsen, 

 pers. commun.) which we can now confirm since 

 recently receiving the critical transforming 

 specimens through the courtesy of Warren 

 Freihofer (California Academy of Science). The 

 uniqueness of this larva would certainly suggest 



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