Acantocephala

The acanthocephalans (gr. Acanthus - thorn Kephale - head) commonly named thorny-headed worms are small bilaterians. They are highly adapted to parasitize arthropods or molluscs in their early life stage and vertebrates as adults. Approximately 1150 species are currently known.

The most remarkable feature of acanthocephalans is the presence of an evertable proboscis in the anterior part of the body, armed with several rows of chitinous spines, which they use to hold the gut wall of their host. The adaptation to a parasitic life style resulted in a drastic simplification of their morphology. This makes it difficult to establish relationships with other taxa by using traditional morphological characters. The body is cylindrical, without segmentation. The general cavity is a pseudocœlom. The muscular, excretory and nervous systems are greatly reduced. The central ganglion (brain) lies in the proboscis receptacle and consists of a low number of cells. It innervates the proboscis sensory receptors, which are presumed to be tactile. It also projects a pair of lateral nerves into the trunk to form a nerve-muscle complex (called a retinaculum) and to connect the genital organs which contain a genital ganglion. Acanthocephalans not only lack respiratory and circulatory systems but also an alimentary canal. Thus nutrient uptake occurs directly through their body wall.

Phylogenetic relationships and Protostome evolution: Are Acanthocephala Lophotrochozoa?

Based on morphological data, the phylum was traditionally considered to be related to a number of other pseudocœlomates, including Rotifera, Nematoda, Nematomorpha, Gastrotricha, Kinorhyncha, and Priapulida. More recently, Platyzoa was diagnosed as ciliated non-segmented acoelomates or pseudocoelomates consisting of Acanthocephala, Rotifera, Platyhelminthes, Gastrotricha, and Gnathostomulida. The first phylogenetic analyses using 18S rRNA and protein-coding gene sequences provided support that pseudocœlomates are polyphyletic and that acanthocephalans are closely related to the rotifers (Syndermata), or may even belong to that phylum. The complete mitochondrial genome of the acanthocephalan Leptorhynchoides thecatus was also sequenced [1]. However, this last analysis failed to clarify acanthocephalan’s relationships and did not support Platyzoa hypothesis.

Phenotypic variation and speciation process in Acanthocephala

Acanthocephalans have complex life cycles, involving a number of hosts, for both developmental and resting stages. Embryos are released with the faeces of the definitive host. Development starts when the embryo is ingested by an invertebrate, almost always an arthropod. Several species have broad host and microhabitat choice and exhibit alternative transmission strategies . The alternate transmission pathway includes an additional intermediate host called paratenic. Habitat choice refers to the ability to infect different host species and/or to occupy different part of the gut. Indeed, some individuals are restricted to the pyloric caeca of the same fish while other individuals are restricted to the intestine or capable of inhabiting both locations. Although potential examples of environmentally induced variation were identified, it has been proposed that phenotype plasticity is often phylogenetically based and can be attributed to cryptic speciation [2].