MOSER and AHl STROM: ROLE OF LARVAE IN SYSTEMATICS 



behavior. Finally, some myctophids have small 

 patches of soft whitish, apparently luminous, tis- 

 sue located at various regions of the body. 



The most popular speculation as to the possible 

 function of the patterns of photophores and lumin- 

 ous scales is that they function in species recogni- 

 tion (see McAllister, 1967). An explanation for the 

 universality of the two ventral rows was postu- 

 lated by Clarke (1963). His suggestion that these 

 downward directed rows emit a continuous light of 

 ambient wavelength, which conceals the fish from 

 deeper-living predators by countershading, has 

 much appeal. 



We have long been interested in the mechanism 

 by which such patterns of photophores could have 

 evolved and believe we have gained some insight 

 into this mechanism through our studies of the 

 larval stages. Our proposal, as expressed in an 

 earlier paper (Moser and Ahlstrom, 1972), is that 

 ancestral myctophids had a generalized arrange- 

 ment of unspecialized photophores, one at the 

 posterior margin of each scale pocket, and a group 

 of similar photophores on the head. We further 

 proposed that the specific photophore patterns of 

 contemporary myctophids evolved through pro- 

 gressive enlargement and specialization of certain 

 photophores of the generalized pattern and con- 

 current diminution or loss of the unspecialized 

 photophores. This idea came to us upon discover- 

 ing the remarkable transforming specimens of 

 Scopelopsis multipunctatus, the adults of which 

 have a small photophore at each scale pocket and a 

 group of photophores on the head. In the adults, 

 the "primary" organs can be distinguished only by 

 their modified lens-bearing scales, but in the 

 transforming specimens (Figure IID) the primary 

 photophores stand out clearly as enlarged mem- 

 bers of the meristic series of light organs. It struck 

 us that a similar arrangement of photophores 

 might have existed in the adults of an ancestral 

 species, and led to the development of our ideas on 

 the evolution of photophore pattern. Our theory 

 was further strengthened by neurological findings 

 and by what we feel are inherent weaknesses in 

 Bolin's ( 1939) and Fraser-Brunner's ( 1949) theory 

 that photophore patterns evolved by the upward 

 migration of organs from ventral rows of photo- 

 phores. 



Viewed from the standpoint of our theory the 

 subfamily Myctophinae would be considered 

 highly specialized, since it is here that diminution 

 of secondary photophores has reached its highest 

 degree; they are totally lacking in the subfamily. 



The individual "primary" photophores are typi- 

 cally highly developed and concentrated ventrally 

 on the body. The ventral location of photophores in 

 Myctophinae is probably related to their habitat. 

 That is, they are generally shallow-living active 

 fishes that have well-developed gas bladders and 

 it is plausible that concentration of photophores 

 ventrally on the body evolved as an adaptation for 

 countershading and protection from deeper-living 

 predators. This view of the Myctophinae is com- 

 pletely contrary to those previously held for this 

 subfamily. On the basis of the "upward migration" 

 theory of photophore evolution, myctophines were 

 thought to be primitive unspecialized forms. For- 

 merly, we too subscribed to this view, and con- 

 trasting the then supposed primitive features 

 such as low photophore position and short jaws of 

 the adults with the highly specialized and diverse 

 features of the larvae, we proposed that the 

 evolutionary pace had differed in the larval and 

 adult stages of the subfamily. Our altered opinion 

 would view both larvae and adults of the Myc- 

 tophinae as highly advanced and would interpret 

 the low photophores, prominent gas bladders, 

 short jaws, and often narrow caudal peduncles as 

 specialized adaptations of active surface-dwelling 

 fishes. 



Our view of the Lampanyctinae must also be 

 revamped. Formerly we considered the diverse 

 and often dorsally oriented pattern of photophores 

 and accessary luminous tissue to be highly 

 specialized features. Possibly, the luminous 

 scalelike patches and luminous glands are 

 specialized adaptations, but we feel that the pres- 

 ence of small secondary photophores and the dor- 

 sal positioning of primary photophores in many of 

 the genera, indicate a retention of the ancestral 

 condition. The Lampanyctinae are generally 

 deeper-living than the Myctophinae and many 

 genera are lethargic fishes that rest vertically in 

 the water column (Barham, 1970). In deeper- 

 living fishes with such a behavior pattern there 

 would be little evolutionary advantage in having 

 ventrally concentrated photophores, and the fat- 

 invested swim bladders and long jaws typical of 

 many genera could have evolved in relation to 

 habitat and activity pattern. It is interesting that 

 the most obvious exception in the subfamily, the 

 Diaphini, are active, often surface-dwelling fishes 

 with relatively short jaws and ventrally concen- 

 trated photophores. It is obvious from the present 

 paper that the larvae of Lampanyctinae exhibit 

 much less diversity than the Myctophinae, but we 



411 



