Aspidogastrea

{{Taxobox | color = pink | name = Aspidogastrea | image = Lobalostoma_manteri.jpg | image_width = 250px | image_caption = Lobalostoma manteri, an Aspidogastrean parasite of Australian fishes. | regnum = Animalia | phylum = Platyhelminthes | classis = Trematoda | subclassis = Aspidogastrea | subdivision_ranks = Orders | subdivision = incertae sedis, see text }} The Aspidogastrea (gr. aspid - shield, gaster - stomach/pouch) is a small group of flukescomprising about 80 species. It is a subclassof the trematoda, and sister group to the Digenea. Species range in length from approximately one mm to several cm. They are parasitesof freshwaterand marinemolluscsand vertebrates(cartilaginousand bonyfishesand turtles). Maturation may occur in the molluscor vertebratehost. None of the species has any economicimportance, but the group is of very great interest to biologistsbecause it has several characters which appear to be archaic.

Morphology

Shared characteristics

Shared characteristics of the group are a large ventraldisc with a large number of small alveoli("suckerlets") or a row of suckers and a tegumentwith short protrusions, so-called "microtubercles".

Larval physiology

Larvaeof some species have ciliatedpatches. Those of Multicotyle purvisi have four patches on the anteriorside of the posteriorsucker and six at the posterior side, those of Cotylogaster occidentalis have an anterior ring of eight and a posterior ring of six, while larvae of Aspidogaster conchicola, Lobatostoma manteri, Rugogaster hydrolagi lack cilia altogether. Larvae of some species hatch from eggs, others do not.

Excretory system

Like most platyhelminthes, aspidogastreans use flame cellsas an excretory mechanism. The two excretory bladdersare located dorsally, on the anterior side of the posterior sucker, connected to ducts, and three flame cell "bulbs" on each side of the body; the ducts contain ciliato aid the flow of excreta.

Nervous system

Aspidogastreans have a nervous systemof extraordinary complexity, greater than that of related free-living forms, and a great number of sensoryreceptorsof many different types. The nervous system is of great complexity, consisting of a great number of longitudinal nerves(connectives) connected by circular commissures. The brain(cerebral commissure) is located dorsally, in the anterior part of the body, the eyesdorsally attached to it. A nerve from the main connective enters the pharynx and also supplies the intestine. Posteriorly, the main connective enters the sucker.

Sensory receptors are scattered over the ventral and dorsal surface, the largest numbers occurring on the ventral surface, at the anterior end and on the posterior sucker. Electron-microscopicstudies revealed 13 types of receptors (Rohde and Watson, 1990a, b, c).

Life cycles

Their life cycleis much simpler than that of digeneantrematodes, including a molluscand a facultative or compulsory vertebratehost. There are no multiplicative larvalstages in the mollusc host, as known from all digeneans.

Host specificity of most aspidogastreans is very low, i.e., a single species of aspidogastrean can infect a wide range of host species, whereas a typical digenean trematode is restricted to few species (at least of molluscs). For example, Aspidogaster conchicola infects many species of freshwaterbivalvesbelonging to several families, as well as snails, many species of freshwater fishesof several families, and freshwater tortoises(Rohde, 1972).

Life cycles have been elucidated for a number of species. Lobatostoma manteri is an example of a species which has obligate vertebrate hosts. Adult wormslive in the small intestineof the snubnosed dart, Trachinotus blochi (Teleostei, Carangidae), on the Great Barrier Reef. They produce large numbers of eggswhich are shed in the faeces. If eaten by various prosobranch snails, larvae hatch in the stomach, and - depending on the species of snail - stay there or migrateto the digestivegland where they grow up to the preadult stage which has all the characteristics of the adult including a testis and ovary (Rohde, 1973).

Evolutionary relationships

Digenean trematodes have been culturedin various, complex, media. However, their parasitic stages die soon in water. Aspidogastreans may survive for many days or even weeks outside a host in simple physiological salinesolution). For example, adult A. conchicola survived in water for a fortnight, and in a mixture of water and saline solution for up to five weeks. L. manteri extracted from fishcould be kept alive for up to 13 days in dilute sea water in which they laid eggs containing larvae infective to snails (Rohde, 1972). This has led to the suggestions that aspidogastreans are archaictrematodes, not yet well adaptedto specific hosts, which have given rise to the more "advanced" digenean trematodes, and that the complex life cycles of digenean trematodes have evolved from the simple ones of aspidogastreans.

Synapomorphiesof the trematodes are presence of a Laurer's Canal, a posterior sucker (transformed to an adhesive disc in the Aspidogastrea), and life cycles involving molluscs and vertebrates. DNA studies have consistently supported this sister group relationship. The question of whether vertebrates or molluscs are the original hosts of the trematodes, has not been resolved (Rohde, 2001).

This view is supported by the evolutionary relationships of the hosts which these two subclasses utilise. The hosts of aspidogastreans include chondrichthyan fishes(sharks, raysand chimaeras), a group that is 450 million years old, whereas the digeneans, are known from teleostfishes (210 million years old) as well as from various "higher" vertebrates; very few species have invaded chondrichthyans secondarily.

Families within the Aspidogastrea

Rohde (2001) distinguish four families of Aspidogastrea:

• The Rugogastridae include a single genus, Rugogaster, with two species from the rectal glands of holocephalanfishes. It is characterised by a single row of rugae (transverse thickenings of the body surface), numerous testes, and two caeca. Species of all other families have a single caecum and either one or two testes.

• The Stichocotylidae include the single species Stichocotyle nephropis from the intestine of elasmobranchs. It has a single ventral row of well separated suckers.

• The Multicalycidae include the single genus Multicalyx from the intestine of holocephalans and elasmobranchs. It is characterised by a single ventral row of alveoli.

• The Aspidogastridae includes species infecting molluscs, teleostsand turtles. The ventral adhesive disc bears either three or four rows of alveoli. Rohde distinguishes three subfamilies of Aspidogastridae, the Rohdellinae, Cotylaspidinae and Aspidogastrinae.

Gibson (1987) further recognized two orders, the Aspidogastrida with the single family Aspidogastridae, and the Stichocotylida including the Stichocotylidae, Multicalycidae and Rugogastridae. However, similarities between species of these two orders are so great that distinction at the level of orders does not seem justified.

References

• Gibson, D. I. (1987). Questions in digenean systematics and evolution. Parasitology 95, 429-460.

• Rohde, K. (1972). The Aspidogastrea, especially Multicotyle purvisi Dawes, 1941. Advances in Parasitology 10, 77 - 151.

• Rohde, K. (1973). Structure and development of Lobatostoma manteri sp. nov. (Trematoda: Aspidogastrea) from the Great Barrier Reef, Australia. Parasitology 66, 63-83.

• Rohde, K. (2001). The Aspidogastrea, an archaic group of Platyhelminthes.In: Interrelationships of the Platyhelminthes, pp. 159-167 (eds. Littlewood, D.T.J. and Bray, R.A.). Taylor and Francis, London and New York.

• Rohde, K. and Watson, N. A. (1990a). Non-ciliate sensory receptors of larval Multicotyle purvisi (Trematoda, Aspidogastrea). Parasitology Research 76, 585-590.

• Rohde, K. and Watson, N. A. (1990b). Uniciliate sensory receptors of larval Multicotyle purvisi(Trematoda, Aspidogastrea). Parasitology Research 76, 591-596.

• Rohde, K. and Watson, N. (1990c). Paired multiciliate receptor complexes in larval Multicotyle purvisi (Trematoda, Aspidogastrea). Parasitology Research 76, 597-601.

External links

• Aspidogastrea from Tree of Life
• Checklist of European aspidogastreans
• Fishdisease.net

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Aspidogastrea Wikipedia article Aspidogastrea.