The five stages of the eel

Forest and Bird, Volume 23, Issue 1, 1 February 1992, Page 12

The five stages of the eel

Eels

Leptocephalus of a shortfinned eel This larval stage of the eel hatches in the breeding grounds thought to be near Samoa. It is about 60 mm, transparent and gelatinous with strange backward pointing teeth, and spends 12 to 15 months drifting on the ocean currents towards New Zealand. No leptocephalus of the longfinned eel has been found.

Glass eel When approaching the coast the leptocephalus becomes much more elongated although it retains its transparency. It is in this stage, known as glass eels, that they enter our river systems.

Elver Within days of entering the lower reaches of rivers glass eels develop a sandy-brown colouring, their stomachs form and they become known as elvers.

Adult longfinned eel As eels grow the head becomes more bulbous with a prominent dome behind the eyes. Some eels will live to a great age and a great size before heading towards the ocean.

Migrating longfinned eel Before setting off on their ocean journey, maturing eels take on a "breeding livery". The bulbous snout of female longfins becomes much more slender and streamlined and the fleshy lips become thinner. The eyes enlarge — to almost twice the size in females — and the pectoral fins become longer. Similar changes occur in shortfinned eels.

Adult female shortfinned (top) and longfinned eels, giving an indication of their relative sizes. Since eels vary widely in size depending on sex and the stage of the life cycle, the most useful distinction is the length of the dorsal fin. In longfinned eels it extends much further forward than the anal fin. Longfinned eels also have a larger mouth and are more likely to be dark grey to black while shortfinned eels tend to be olive-green.