AHLSTROM ET AL.: ARGENTINOIDEI 



161 



Table 40. Comparative Morphometry of Aroentinoid Larvae. Mean values (%) of body proportions for three ontogenetic stages (preflexion- 



flexion-postflexion) are listed. 



other species which have relatively smaller, more elliptical eyes. 

 Eyes are sessile in B. milleri and in the B. wesetht group but are 

 stalked to some degree in all other species known. In B. slilbius 

 and relatives (B. urotranus, and B. schmidli') the stalks are short 

 and found only in early larvae. Stalks are longer and persist into 

 later larval stages in other species, reaching a ma.ximum of 65% 

 of the head length in B. bericoides. 



In opisthoproctids the head is moderate in size in the slender 

 forms, Bathylychnops and Dolichopleryx, and longer and more 

 massive, with a pronounced hump or bend at the nape, in the 

 deep-bodied genera. All genera have an elongate snout and Bath- 

 ylychnops has a unique triangular flap at its tip. Bathylychnops 

 has round eyes that are rotated slightly dorsoanteriad. In the 

 other genera, the eyes are tubular and directed dorsally, even 

 in the smallest larvae available. Eye diverticulae with associated 

 accessory retinae, characteristic of opisthoproctid adults, begin 

 to form at the end of the larval period. 



Fins —A major feature of all argentinoid larvae is the devel- 

 opment of a prominent median finfold in which the dorsal and 

 anal fins develop, connected to the trunk by a series of hyaline 

 strands (Figs. 83-87). The first fins to form are the pectorals. In 

 argentinids and bathylagids they are relatively small and de- 

 velop rays late in the larval period. Microstomatid and opis- 

 thoproctid pectoral fins are generally larger; however, there is a 

 wide size range, from relatively small fins in Microstoma to 

 large, fan-like fins in some species of Nansenia (e.g., N. groen- 

 landica) to very elongate pectorals in Dolichopleryx binocularis. 



Ossification of rays begins earlier in these groups, usually before 

 notochord flexion. 



After the pectorals, the caudal fin is usually the next to form. 

 In argentinids notochord flexion and development of principal 

 caudal rays occurs at a size about midway in larval growth 

 whereas in opisthoproctids this occurs earlier in the larval pe- 

 riod. In bathylagids the process is somewhat delayed and in 

 some species (e.g., B. euryops. B. milleri) notochord flexion may 

 not be completed until near the end of the larval period. 



The dorsal and anal fins begin to form at about the stage of 

 notochord flexion in all argentinoids except opisthoproctids, 

 where notochord flexion slightly precedes the appearance of 

 dorsal and anal fins. The anal fin begins forming far posteriad 

 in argentinoids, just posterior to the anus or the point of de- 

 flection of the free terminal gut section. In B. milleri and in the 

 deep-bodied opisthoproctids with coiled guts there is a space 

 between the anus and the anal fin origin. 



The position of the dorsal fin is varied among argentinoids 

 and forms in the larvae in approximately the same position that 

 it will occupy in the adult. The fin has its most anteriad location 

 in Argentina where its origin is well forward of the midpoint of 

 the body (Fig. 83). The extreme case is found in A. silus where 

 snout to dorsal origin is about 38% of the body length in larvae 

 and about 43% in adults. In most bathylagids the dorsal origin 

 is slightly anterior to mid-body. The exceptions are B. slilbius 

 and relatives, where the dorsal origin is slightly posterior to 

 mid-body, and B. wesethi and relatives where it is located still 

 further posteriad. 



Fig. 84. Larvae of Microstomatidae. (A) Microstoma microstoma. 1 1.0 mm, from Schmidt (1918); (B) Microstoma sp., 12.0 mm, CalCOFl 

 5 104 Sta. 90.52; (C) Nansenia Candida. 8.4 mm, CalCOFl 5007 Sta. 1 00.70; (D) N. crassa. 8.5 mm, CalCGR 5 103 Sta. 1 37.50; (E) N. groenlandica, 

 10.0 mm, from Schmidt (1918); (F) N. oblita, 9.0 mm, ibid; (G) Xenopthalmichthys danae. 16.5 mm, from Bertelsen (1958). 



