Acanthocephalans in Sub-Antarctic and Antarctic

Abstract

Acanthocephalans (spiny head worms) are a medium-sized phylum (about 1000 species have been described) of usually small (few mm to over 1m) vertebrate intestinal parasites. They are pseudocoelomates with bilateral symmetry and usually cylindrical bodies. The sexes are separate, with females usually larger than males. The body consists of a proboscis, neck, and trunk. The proboscis, neck, and internal organs connected with them (proboscis receptacle and lemnisci) form the fore-body. In some cases, the trunk may be divided into two parts of different shape: fore-trunk and hind-trunk. The proboscis is armed with recurved hooks. The hooks consist of two parts: blade (thorn) and root, both usually directed posteriorly. Hooks situated at the base of the proboscis (basal hooks) are usually rootless. The proboscis (usually retractable) may be invaginated into the proboscis receptacle. The latter contains a cerebral ganglion. Two lemnisci lie parallel to the proboscis receptacle. The trunk may be unarmed, or armed with spines. This armament is usually restricted to the anterior part of the trunk, but sometimes reaches the posterior end of the body. The genital pore may be subterminal or terminal. Spines surrounding the genital pore are often separated from the other ones by a bare zone. In such cases, the armament of the trunk is divided into somatic and genital spines. Ligaments (one or two) run along the trunk, and sexual organs are attached to them. The male reproductive system consists of 2 testes, cement glands (4–8 in number in Antarctic species), seminal ducts, cement ducts and reservoirs, Safftigen’s pouch, penis and the copulatory (retracted or everted). The female reproductive system consists of ovarian balls, a uterine bell (an organ for selection of immature and mature eggs), a uterus, and a vagina, with a single or a double sphincter. Ovarian balls are enclosed in ligament sacs in juvenile females and are liberated during maturation. Eggs mature in the pseudocoelom of a female. In fact, in mature females these are not eggs, but the first larval stage (acanthors) enclosed in 3–4 envelopes. More correct terms are “shelled acanthors” and “embryophores”, but these are rarely used. Acanthocephalans have reduced the muscular, nervous, circulatory, and excretory systems and complete loss of the digestive system. Absorption and excretion take place through the tegument. The latter contains a system of canals known as the lacular system. The number and arrangement of main lacular canals are of fundamental value in the classification of higher taxa (classes). Excretion is by diffusion except in Oligacanthorhynchidae (with two protonephridial organs). The life cycles involve an arthropod (intermediate host) and a vertebrate definitive or paratenic host. Eggs are shed with the host’s faeces, when the definitive host, the appropriate intermediate host ingests them, and the acanthor is liberated and pierces the gut wall. In the arthropod body cavity, the acanthor develops into an acanthella and then into an infective cystacanth, which matures to adulthood in the gut of the definitive host, following ingestion of the infected arthropod (Amin 1987; Zdzitowiecki 1991).