226 



ONTOGENY AND SYSTEMATICS OF FISHES-AHLSTROM SYMPOSIUM 



Table 62. Sequence of Formation of Photophores which Appear in Fourteen Genera of Myctophidae. The Bfj appear first in all genera 



listed. Parentheses indicate photophores appear late in larval period. 



PO, 



PO, 



PVO, PLO 



VO, AOa, AOa, 



Benthosema 

 suborbitale 

 glaciate 

 pterola 

 fibulalum 



Diogenichthys 

 lalernalus 

 atlanticus 



Myclophum 

 spinosum 

 lychiwbium 

 asperum 

 brachygnalhum 

 obtusirosire 

 selenops 



Lobianchia 

 Diaphus 



theta 



pacificus 



Gymnoscopelus 



Lampanyctodes 



Scopelopsis 



Lainpichthys 



Notoscopelus 



Lampadena 



Ceratoscopelus 



Lepidophanes 



Bolinichlhys 



22--2 1 1 333------33 



- - - - (1) (1) (1) (1) (1) (1) ------- - 



--1-4 6---2 3 5--5-6- 



--1--3 5--2---6--46 



_________ 1 ________ 



(5) 



1 

 2 

 2 

 2 



3 



1 



1 



(1) 



(4) 

 (3) 



(7) (8) 

 (5) - 



(4) 



(1) 



- (9) (3) (6) 



- - (6) - 



3 

 3 

 1 



2 



1 



(1) 



ordinal relationships. One set is the size at various develop- 

 mental milestones. Myctophid larvae hatch at about 2 mm length 

 with a yolk-sac remnant. Notochord flexion occurs in a narrow 

 size interval (0.5-2.0 mm) and the size at mid-flexion is typically 

 about half the maximum larval size. Size at transformation also 

 occurs within a short length interval, usually not exceeding 2 

 mm. Most myctophid species transform in the length range of 

 12-19 mm, although some (e.g., Electrona rissoi, Notolychnus 

 valdiviae) are as small as 9-10 mm at transformation and some 

 species of Symbolophorus reach about 23 mm before transfor- 

 mation. Gymnoscopelus nicholsi has the largest larvae recorded, 

 up to 28 mm. 



Head, body, and gut shape are distinctive for most species 

 and within most genera there is a similarity of shape (Figs. 1 1 5- 

 124). While most myctophid larvae are moderately slender, 

 body shape can range from highly attenuate (e.g., Hygophum 

 reinhardti) to markedly robust (e.g., some Myctophum and 

 Lampanyctus species). Some are deep-bodied but laterally com- 

 pressed (e.g., Gonichthyini). Robust larvae and deep-bodied, 

 laterally compressed forms tend to have large heads and jaws, 

 while attenuate forms have flat heads. 



The eye is varied in size and shape and provides numerous 



characters. In the Myctophinae the eyes are elliptical in outline 

 in contrast to most Lampanyctinae which have rounded eyes. 

 Further specializations in Myctophinae are the presence of var- 

 iously shaped choroid tissue on the ventral surface of the eye 

 in most genera and eye stalks in several genera. Among 1am- 

 panyctine genera eyes are sessile and only Lobianchia doflcini 

 and species of Triphoturus have markedly narrowed eyes with 

 choroid tissue. 



The gut has distinctive transverse rugae and ranges from short, 

 to elongate, to trailing free from the body. In most myctophids 

 it extends to about the midpoint of the body and is slightly S- 

 shaped. The curvature tends to be more pronounced in taxa 

 with short guts. In two myctophine genera (Metelectrona and 

 some Hygophum species) the anterior section of the gut is small 

 in diameter and opens dorsally into the relatively larger pos- 

 terior section. 



In most myctophids, ray formation and ossification of fins 

 proceeds in the following sequence: caudal, pectoral, anal, dor- 

 sal, and pelvic. However, in some Symbolophorus species the 

 pelvic fin forms early and ossification of rays precedes that of 

 the anal and dorsal fins. In most species the pectoral fin is 

 relatively small, but deep-bodied and robust forms in both 



Fig. 1 16. Larvae of Myctophini. (A) Benthosema glaciale. 10.5 mm; (B) B. suborbitale. 9.2 mm; (C) B. pterola. 8.5 mm; (D) B. fibulatum. 

 8.7 mm; (E) Diogenichthys lalernalus. 1.1 mm; (F) D. atlanticus. 8.8 mm. A-D from Moser and Ahlstrom (1974); E and F from Moser and 

 Ahlstrom(1970). 



