| Aspidogastrea | |
|---|---|
 | |
| Multicalyx elegans | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Class: | Trematoda |
| Subclass: | Aspidogastrea Faust & Tang, 1936 |
| Orders | |
TheAspidogastrea (Ancient Greek:ἀσπίςaspis “shield”,γαστήρgaster “stomach/pouch”) is a small group offlukes comprising about 80species. It is asubclass of thetrematoda, and sister group to theDigenea. Species range in length from approximately one millimeter to several centimeters. They areparasites offreshwater andmarinemolluscs andvertebrates (cartilaginous andbonyfishes andturtles). Maturation may occur in themollusc orvertebrate host. None of the species has anyeconomic importance, but the group is of very great interest tobiologists because it has several characters which appear to be archaic.
Shared characteristics of the group are a largeventral disc with a large number of smallalveoli ("suckerlets") or a row of suckers and ategument with short protrusions, so-called "microtubercles". Aspidogastreans are an understudied class of parasitic flatworms that possess unique anterior attachment structures and are found exclusively in freshwater and marine environments, infecting a variety of hosts including fish, amphibians, and reptiles.
Larvae of some species haveciliated patches. Those ofMulticotyle purvisi have four patches on theanterior side of theposterior sucker and six at the posterior side, those ofCotylogaster occidentalis have an anterior ring of eight and a posterior ring of six, while larvae ofAspidogaster conchicola,Lobatostoma manteri,Rugogaster hydrolagi lack cilia altogether. Larvae of some species hatch from eggs, others do not.
Like mostplatyhelminthes, aspidogastreans useflame cells as anexcretory mechanism. The two excretorybladders are locateddorsally, on the anterior side of the posterior sucker, connected to ducts, and three flame cell "bulbs" on each side of the body; the ducts containcilia to aid the flow of excreta.
Aspidogastreans have anervous system of extraordinary complexity, greater than that of related free-living forms, and a great number ofsensory receptors of many different types. The nervous system is of great complexity, consisting of a great number of longitudinalnerves (connectives) connected by circular commissures. Thebrain (cerebral commissure) is located dorsally, in the anterior part of the body, theeyes dorsally 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-microscopic studies revealed 13 types of receptors.[1][2][3]
Theirlife cycle is much simpler than that ofdigeneantrematodes, including amollusc and a facultative or compulsoryvertebratehost. There are no multiplicativelarval stages 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 offreshwater bivalves belonging to several families, as well asfreshwater snails, many species offreshwater fishes of several families, and freshwatertortoises.[4]
Life cycles have been elucidated for a number of species.Lobatostoma manteri is an example of a species which has obligate vertebrate hosts. Adultworms live in thesmall intestine of the snubnosed dart,Trachinotus blochi (Teleostei,Carangidae), on theGreat Barrier Reef. They produce large numbers ofeggs which are shed in thefaeces. If eaten by various prosobranchsnails, larvae hatch in thestomach, and—depending on the species of snail—stay there ormigrate to thedigestive gland where they grow up to the preadult stage which has all the characteristics of the adult including a testis and ovary.[5]
Digenean trematodes have beencultured in 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 physiologicalsaline solution). For example, adultA. conchicola survived in water for a fortnight, and in a mixture of water and saline solution for up to five weeks.L. manteri extracted fromfish could be kept alive for up to 13 days in dilute sea water in which they laid eggs containing larvae infective to snails.[4] This has led to the suggestions that aspidogastreans are archaic trematodes, not yet welladapted to 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.
Synapomorphies of the trematodes are presence of aLaurer'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.[6]
This view is supported by the evolutionary relationships of the hosts which these two subclasses utilise. The hosts of aspidogastreans includechondrichthyan fishes (sharks,rays andchimaeras), a group that is 450 million years old, whereas the digeneans, are known fromteleost fishes (210 million years old) as well as from various "higher" vertebrates; very few species have invaded chondrichthyans secondarily.
Rohde (2001) distinguish four families of Aspidogastrea:
Gibson further recognized two orders, theAspidogastrida with the single family Aspidogastridae, and theStichocotylida including the Stichocotylidae, Multicalycidae and Rugogastridae.[7] However, similarities between species of these two orders are so great that distinction at the level of orders does not seem justified.