The Reproduction of the Entozoic Nemertean Gononemertes australiensis Gibson (Nemertea: Hoplonemertea : Monostylifera)–Gonads, Gametes, Embryonic Development and Larval Development

Abstract

G. australiensis is an entozoic nemertean found in the atrium of the ascidian Pyura pachydermatina (Herdman) var. intermedia Herdman. The host is common in the low intertidal and shallow sublittoral waters of the Sydney region of New South Wales.

The nemertean is dioecious and possesses numerous gonads. In mature female worms, oocytes at different stages of maturity are always found throughout the body. The more mature oocytes lie close to the gut. Each temporary gonad produces several oocytes. The mature male worms contain ripe testes close packed within the parenchymal tissue along the body.

The spermatozoon of G. australiensis has a head 6.5 µm long and a tail 38 µm in length. The head is rod- shaped with a blunt simple acrosome. The base of the head is surrounded by a mitochondria1 bulge which also encloses the connecting piece. A close apposition between the nucleus and the mitochondrion occurs in this area. The sperm is intermediate in type between primitive and modified nemertean spermatozoa.

The egg of G. australiensis is spherical, yolky, 0.18mm in diameter and surrounded by a glutinous membrane. Fertilization is external. Cleavage, gastrulation and later embryonic development are similar to those of other hoplonemerteans and the resulting larva is of the hoplonemertean type. The larva is free- swimming for 3-5 days before invading a host. Some features of larval structure and behaviour are specializations associated with the entozoic habit of the nemertean. The development of the larval ocelli, lost soon after host invasion, is one of these. The young larva is photopositive, but after the ocelli have formed it becomes photonegative. Older larvae respond to a water current by an attachment response. No stomodaeum is formed during embryonic or larval development and completion of the differentiation of the proboscis is delayed until host invasion takes place. The larva exists on yolk until this time. The resources of the larva are concentrated on host location rather than on the feeding, growth and continued development typical of the larvae of other free-living hoplonemerteans.