The Aspidogastrea is a small group of flukes comprising about 80 species. It belongs to the Trematoda, which comprises the two subclasses Aspidogastrea and Digenea. Species range in length from approximately one mm to several cm. They are parasites of freshwater and marine molluscs and vertebrates (cartilaginous and bony fishes and turtles). Maturation may occur in the mollusc or vertebrate host. None of the species has any economic importance, but the group is of very great interest to biologists because it has several characters which appear to be archaic. For example, the hostsof aspidogastreans include chondrichthyan fishes (sharks, rays and chimaeras), a group that is 450 million years old, whereas the sister group of the aspidogastreans, the digeneans, are known from teleost fishes (210 million years old) as well as from various "higher" vertebrates; very few species have invaded chondrichthyans secondarily.
Aspidogastreans have a nervous system of extraordinary complexity, greater than that of related free-living forms, and - likewise - they have a very great number of sensory receptors of many different types. Their life cycle is much simpler than that of digenean trematodes, including a mollusc and a facultative or compulsory vertebrate host. There are no multiplicative larval stages in the mollusc host, as known from all digenean trematodes. Furthermore, host specificity of most aspidogastreans is verylow, i.e., they infect a wide range of hosts, whereas a typical digenean trematode is restricted to few species (at least of molluscs). Aspidogastreans may survive for many days or even weeks outside a host in simple media (water, saline solution). All this hasled to the suggestions that aspidogastreans are archaic trematodes, not yet well adapted to specific hosts, which have given rise to the more "advanced" digenean trematodes, and that the complex life cycles of digenean trematodes have evolved fromthe simple ones of aspidogastreans (Rohde, 1972, further references therein).
The Families of the Aspidogastrea
Gibson (1987) and Rohde (2001) distinguish four families of Aspidogastrea:
- The Rugogastridae include a single genus, Rugogaster, with two species from the rectal glands of holocephalan fishes. It is characterised by a single row of rugae (transverse thickenings of the tegument) developing from the posterior wall of the anteriorly located ventral sucker, 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 separated by transverse septa.
- The Aspidogastridae includes species infecting molluscs, teleosts and turtles. The ventral adhesive disc bears either three or four rows of alveoli. Rohde distinguishes three subfamilies of Aspidogastridae, the Rohdellinae, Cotylaspidinae and Aspidogastrinae. The first subfamily includes the single species Rohdella siamensis from freshwater teleosts; the terminal male and female genital ducts are united to form a hermaphroditic duct, which is absent in the other two subfamilies. The Cotylaspidinae have an adhesive disc bearing three, and the Aspidogastrinae have an adhesive disc bearing four longitudinal rows of alveoli. The genera Cotylogaster, Cotylaspis and Lissemysia of the Cotylaspidinae differ in the absence or presence of a cirrus pouch and the number of testes (one or two), the genera Multicotyle, Lobatostoma, Aspidogaster, Lophotaspis, Sychnocotyle, and Neosychnocotyle of the Aspidogastrinae also differ in the number of testes (one or two), the absence or presence of a cirrus pouch, and - in addition - in the absence or presence of head lobes and/or papillae on the adhesive disc.
Shared characteristics of the group are a large ventral disc with a large number of small alveoli (suckerlets) (Fig. 1, see also Structure of the juvenile and adult)or a row of suckers and a tegument with short protrusions, so-called microtubercles.
Figure 1. Whole mount of Multicotyle purvisi (redrawn from Rohde, 1972).
Larvae have a posterior sucker and in some species a short posterior appendage; they either possess a number of ciliated patches, or they are non-ciliated (Fig.2), see Structure of the larvae page.
Figure 2. Larva of Lobatostoma manteri (redrawn from Rohde, 1973a).
Most studies deal with taxonomy and light as well as electron-microscopic structure. Embryology and development has been studied in few species. Host specificity is usually low, and several species were shown to be able to survive for long periods outside the host (see Host specificity and survival outside the host). Little is known on the effects on the host. The life cycles of several species are known.
Evolution and Systematics
Discussion of Phylogenetic Relationships
The tree presented here is a preliminary one and must be confirmed by DNAstudies. It shows four families, the Aspidogastridae infecting teleosts andturtles, and the Stichocotylidae, Multicalycidae and Rugogastridaeinfecting chondrichthyans (holocephalans and elasmobranchs). A synapomorphyfor all Aspidogastrea is the presence of an adhesive disc, a synapomorphyof the Aspidogastridae is an adhesive disc which is subdivided into threeor four rows of alveoli, a synapomorphy of the other three families is anadhesive disc consisting of a single longitudinal row of suckers, rugae ordeep alveoli. An adhesive disc consisting of a row of suckers is apomorphicfor the Stichocotylidae, an adhesive disc consisting of a single row ofdeep alveoli is apomorphic for the Multicalycidae, and an adhesive discconsisting of rugae is apomorphic for the Rugogastridae. The plesiomorphic state within the Aspidogastrea is likely to be a posterior, undivided sucker as found in larval aspidogastreans anddigeneans.
========= Aspidogastridae (hosts:Teleostei, Chelonia) | | ====== Multicalycidae (hosts:Elasmobranchii, Holocephali) =====| | ===|===== Rugogastridae (hosts:Holocephali) | ====== Stichocotylidae (hosts:Elasmobranchii)
The four families of aspidogastreans are also recognized by Gibson (1987) and Rohde (2001). Gibson (1987, also Gibson and Chinabut 1984) further recognized twoorders, the Aspidogastrida with the single family Aspidogastridae, and theStichocotylida including the Stichocotylidae, Multicalycidae andRugogastridae. However, similarities between species of these two ordersare so great that distinction at the level of orders does not seemjustified.
The sister group of the Aspidogastrea is the Digenea, both comprising theTrematoda. Synapomorphies of the trematodes are presence of a Laurer'scanal, a posterior sucker (transformed to an adhesive disc in theAspidogastrea), and life cycles involving molluscs and vertebrates. DNAstudies have consistently supported this sister group relationship. Thequestion of whether vertebrates or molluscs are the original hosts of thetrematodes, has not been resolved (see discussion in Rohde 2001).
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