RAJU: THE GENUS MONOGNATHUS 



METAMORPHIC FORMS OF 

 MONOGNATHUS SP. 



Figure lA, B, C, G, H 



SIO 60-241, central North Pacific, 7"25.5'N- 

 7^55.0'N, 144^29.0'W-144=35.0'W, depth of cap- 

 ture 0-2,100 m, IKMT, 1(42.2 mm), 7 July 1960. 

 SIO 60-276, central North Pacific, 24=28. 9'N- 

 24=36.9'N, 147=55. 5'W-147=27.0'W, 0-3,000 m, 

 IKMT, 1(48 mm), 7-8 Aug. 1960. SIO 60-275, 

 central North Pacific, 23=23. 4'N-23=40.0'N, 

 15r04.0'W-150=38.8'W, 0-3,000 m, IKMT, 1(58 

 mm), 6 Aug. 1960. SIO 60-283, eastern North 

 Pacific, 28=13. 0'N-28=19.1'N, 135=21. 8'W- 

 134=54.1'W, 0-3,000 m, IKMT, 1(50 mm), 12 Aug. 

 1960. 



Description (Figure IB, G): Total length 48 

 mm, body elongate, compressed, and transparent. 

 Depth 12.0 in total length, maximum depth at 

 middle of body. Head long. Cranium weak, 

 triangular, 10.7 in total length. Snout slightly 

 blunt. Olfactory organ rudimentary, a small 

 curved tube open at both ends. Eyes lost due to 

 damage. Upper jaw membranous, without teeth. 

 Ethmoid tooth long, projecting into mouth. Eth- 

 moid gland paired, well developed. Lower jaw 8.6 

 in total length. Suspensorium 24.0 in total length. 

 Adductor mandibulae well developed. Gills, liver, 

 and part of gut damaged and lost. Posterior 

 region of gut projects out of body outline. Opis- 

 thonephros a coiled tube extending behind vent 

 (not shown in figure). Ovaries tubular, elongate, 

 with few ova (250-300). Dorsal originates on myo- , 

 tome 32 and anal on myotome 30. Myotomes W- 

 shaped, 30 +83 = 113. Midlateral brown, 

 chromatophore patches conspicuous, one on left 

 and four on right side. Brown chromatophores on 

 tip of upper and lower jaws, at base of dorsal fin 

 rays on myotomes 50-60, and anal fin rays on 

 myotomes 31-49. Juvenile pigmentation appears 

 on body as minute, uniformly scattered brown 

 chromatophores (not shown in figure). 



Changes during metamorphosis: Most of the 

 morphological changes undergone are similar to 

 those observed in the metamorphosis of the eels. 

 In the smallest specimen (42 mm. Figure lA) 

 the body is deeper, and the lower jaw and 

 suspensorium are relatively shorter. A median 

 ethmoid tooth is not yet formed. In later stages 



(48 mm, 50 mm, 42.2 mm; A, B, C of Figure 

 1, respectively) there is a slight decrease in the 

 length and depth of the body, and increase in the 

 length of the head, snout, lower jaw, and 

 suspensorium. A median ethmoid tooth is formed 

 with its associated gland. Larval midlateral pig- 

 mentation begins to fade with the gradual 

 development of juvenile pigmentation. The dorsal 

 and anal fins move slightly forward. Two speci- 

 mens have developed a pair of tubular ovaries 

 containing about 250-300 spherical ova 0.06 mm 

 in diameter (not shown in figure). An interesting 

 aspect of the metamorphosis of monognathids is 

 the degenerative changes that take place in the 

 head, olfactory organ, and eye. The bones of the 

 head become very weak and membranous. The 

 eye and the olfactory organs are reduced to minute 

 structures. A median ethmoid tooth with a pair 

 of glands develops. The wide W-shaped myotomes 

 become narrower with the decrease in the depth 

 of the body, and in the larval condition they may 

 approach a V-shape as in Cyema and L. latis- 

 simus and L. pseudolatissimus. 



Remarks: The four metamorphic stages de- 

 scribed above are assigned to Monognathus 

 sp. on the basis of the characteristic median 

 ethmoid tooth and the absence of an upper jaw. 

 These stages share some features with M. isaasci 

 such as general shape of the body, large head, 

 projecting ethmoid tooth, myotome number, and 

 structure of the tail. ButM. isaacsi has a pectoral 

 fin which is not seen in the metamorphic forms. 

 Assignment to the species is not possible at this 

 time. 



The leptocephali of monognathids are not 

 identified as yet. But the features of metamorphic 

 forms (Table 2) indicate that they are small 

 (40-60 mm), elongate larvae with a series of five 

 splanchnic, unequally placed, midlateral melano- 

 phores with pigmentation on the gut, and with 

 about 100-120 wide V-shaped myotomes. 



GENERAL REMARKS 



Upper jaw: The name Monognathidae was 

 given to these fishes by Bertin (1937a), who 

 assumed that only one jaw (lower jaw) was pres- 

 ent. But Tchernavin (1947b) pointed out that 

 there is no evidence that a palatopterygoid 

 cartilage is absent in the Monognathidae. As all 



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