On the Metamorphosis and Burrowing Habits of the Snake Eel, Muraenichthys gymnotus

Abstract

The leptocephali of the snake eel, Muraenichthys gymnotus Bleeker, were collected from the inshore waters of Tosa Bay, Kochi Pref., Japan, from October through December in 1976 and 1977.The premetamorphic larvae are small, ultimately reaching about 70 mm in total length just before the onset of metamorphosis.The number of larval teeth is expressed by 0-1 (grasping tooth) +0-V (anterior teeth) +0-8 (posterior teeth) in the upper jaw, and 0-1+I-VI+0-4 in the lower.
The pectoral fin is feeble and short, about 5 times in head length.The origin of the dorsal fin is situated slightly in advance of the anus.The total myomeres are 131 to 140, preanal myomeres 66 to 70, and postanal myomeres 64 to 71.The anterior margin of the gall-bladder is at level of myomeres 18 to 23, the 1st vertical blood vessel at myomeres 16 to 20, the last vertical blood vessel at myomeres 58 to 65.The alimentary canal is long and narrow, looped slightly in 6 to 9 places.The pigmentation of the body is conspicuous and characteristic.
From the results of our rearing experiment, it can be concluded that the metamorphosis of the larvae is accomplished rapidly in about one week.The larval teeth on both jaws are lost entirely on the 2nd day after the onset of metamorphosis, and the conical teeth begin to appear on the 5th day.During the metamorphosing stage the position of the anus slowly moves forward, then the dorsal fin originates above or slightly behind the anus on the 5th day.Small melanophores are scattered on the snout, lower lip and dorso-lateral surface of the body on the 4th day.The total length reaches its minimum size, 51.2 mm in average, on the 5th day.
The juvenile stage lasts for about 20 days in the rearing experiment.The body is well elongate, its depth is lower than 30 times in total length.The pectoral fin is scarcely visible or entirely absent.In the last juvenile stage the dorsal fin originates behind the anus at a distance about equal to the length of the snout, and the bodys are colored with light brown.
The premetamorphic larvae cease to be planktonic and burrow themselves into the sand bottom just before metamorphosis.The snake eel is a typical head burrower throughout metamorphosing and juvenile stages.