This chapter aims to show that morphological and ecological adaptations of some animal species are highly intricate, formed by evolutionary processes over many millions of years. Disturbances upsetting the value of such adaptations may lead to the extinction of species that cannot easily be replaced, at least in some habitats. As an example we use two species of Aspidogastrea studied over many years by light and electron microscopy, DNA analyses and autecological approaches including studies of life cycles, effects on hosts, niche selection and population dynamics.

The Aspidogastrea (= Aspidobothrea = Aspidobothria) is a small group of trematodes (flukes) with about 80 species. They represent the sister group of the digeneans. Cladistic analyses using a total evidence approach, i.e., 18S rDNA sequences, morphology, ultrastructure, life-cycle data and hosts, suggest that they separated from the digeneans over 400 million years ago (Littlewood et al., 1999). They parasitize mollusks and vertebrates. Mollusks either serve as intermediate hosts containing the larvae and juveniles, or the entire life cycle can be completed in them. In all known life cycles, vertebrates become infected by eating infected mollusks (snails and/or bivalves). There are four families, and three of them, each with a single genus and one or two species, infect chondrichthyan fishes (sharks, rays and chimaeras); species of the fourth family infect teleost fishes and turtles. The greater number of host families infected suggests that chondrychthyans are the original hosts of aspidogastreans. Whereas digeneans multiply in the mollusk intermediate hosts, aspidogastreans do not: one egg never gives rise to more than one larva.