Einige Beispiele für Sacculinisierung, d.h. für die regressive Evolution als eine Folge des Parasitismus, werden gegeben. Jedoch ist das Phänomen nicht bei allen Parasiten zu finden. Dieser Knol diskutiert die sehr grosse Zahl und Vielfalt der Rezeptoren, und die grosse Komplexität des Nervensystems bei zwei Arten der Aspidogastrea (Trematoda), die bedeutend grösser als bei verwandten freilebenden Platyhelminthes (Plattwürmern) sind.
All species use restricted niches, and the niches of all species are segregated, although often with much overlap, by the use of different habitats, different geographic areas and seasons, different food resources, etc. Hypotheses explaining the causes of niche restriction and segregation are discussed.
Chaotic fluctuations in population sizes are reduced in metapopulations consisting of several largely independent subpopulations with different reproductive rates.This may suggest that chaotic fluctuations are much stronger in single large economies, or a single completely globalized economy, than in the world economy consisting of national economies that are to a degree separated.
We discuss the functional morphology of two species of Aspidogastrea, and the distribution of one of them, and conclude that interspecific competition and equilibrium assumptions leading to short-term optimisation cannot explain the morhological and biological features of the species. An autecological approach is best suited to analyse the patterns found.
The Aspidogastrea, a “primitive” group of flukes, has been studied extensively using experimental techniques as well as molecular biology, transmission and scanning electron microscopy. They belong to the best known groups of helminths (parasitic worms) and can therefore serve as a model group for parasitological studies. This knol discusses the morphology, taxonomy, phylogeny and life cycles of aspidogastreans. Two further knols will deal with the light aspidogastreans cast on the supposed “retrograde” evolution of parasites, i.e. their “sacculinisation”, and their functional morphology and ecology, respectively.
Some examples of sacculinisation, of retrograde evolution due to parasitism, are given. However, this phenomenon is not universal among parasites. This knol discusses the very great variety of sensory receptors and their great numbers, as well as the complexity of the nervous system, in two species of Aspidogastrea. The number of receptors of the Aspidogastrea is vastly greater than in any free-living flatworm that has been examined, and the variety of receptor types is at least as great or greater. Complexity of nerves also is greater, i.e., there is no evidence for sacculinisation in these parasites.
The present state of our knowledge about biodiversity and problems encountered with giving estimates of species numbers are discussed. Most species of multicellular, unicellular and micro-organisms have not been described. Even approximate estimates of global diversity are therefore premature. Urgently needed are extensive and intensive surveys of all groups from a multitude of habitats. In particular, tropical habitats, the meiofauna, deep-sea fauna, parasites and microorganisms need attention.
Many marine parasite species are found in man, although none needs humans as obligatory hosts. Important infections are anisakiasis acquired by eating raw fish and invertebrates, trichinosis aquired by eating insufficiently cooked meat from walrus and polar bears, angiostrongylosis acquired by eating infected invertebrates such as shrimp, schistosome dermatitis acquired when swimming in infected waters, symptoms caused by various tapeworms and small flukes acquired by eating various marine animals, and by unicelluar parasites (protistans) acquired by eating marine invertebrates and ingesting polluted water. The various infections including modes of infection and symptoms are discussed.
The gyrocotylid cestodes, a small group of about 10 known species, are discussed. They are without economic importance, but have found much interest because of some unique biological and morphological features. Life cycles are not known.
Monogeneans (ectoparasitic Platyhelminthes) are of great economic importance as agents of fish diseases, particularly in aquaculture. Their morphology and phylogeny, life cycles and mortalities caused by them, are discussed. Special ecological aspects considered are: within or between host speciation, niche restriction and segregation, reaction to magnetic stimuli, competition and reinforcement of reproductive barriers.