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Introduction

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).

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