Turbellarian taxonomic database

Abstract / Notes

"Symbiosis is a dominant trait in the Platyhelminthes. The Neodermata (Aspidogastrea, Monogenea, Digenea,
Udonellidea, Cestoda) are wholly parasitic and even the predominantly free-living Turbellaria have almost 200
species from 35 families living in permanent associations with other animals. In the simplest turbellarian
symbioses, ectosymbiotes such as the Temnocephalida, some other Rhabdocoela and a few Tricladida live on the
body surfaces or in the branchial chambers of their mainly arthropodan or chelonian hosts. They feed on the
same types of prey as their free-living relatives but supplement their diet by opportunistic commensalism.
Their digestive physiology and food reserves are the same as in free-living species. The entosymbiotic
Umagillidae, Graffillidae, Pterastericolidae, Fecamplidae and Acholadidae live in internal body cavities or
body wall derivatives of echinoderms, molluscs or arthropods and show increasing metabolic dependence on their
hosts. Patterns of digestive physiology and food storage generally differ markedly from those of ectosymbiotic
and free-living species. Some umagillids, in echinoids, feed as entozoic predators on co-symbiotic protozoa,
supplemented by opportunistic ingestion of the hosts' ingesta, gut cells or coelomocytes. Others, in
holothurians, feed mainly on gut cells, which also provide some digestive enzymes, and to a lesser extent on
host ingesta and co-symbiotes. Graffillids, in molluscs, lack endogenous digestive enzymes and rely entirely
on those taken in with host ingesta and gut tissues. Pterastericolids, in asteroids, similarly utilise gut
tissues both as food and enzyme sources. The climax to metabolic dependence occurs in the Fecamplidae and
Acholadidae. The former, in crustacean haemocoels and myzostomid tissues, lack conventional alimentary systems
and absorb soluble nutrients through the epidermis. In the latter the only known species lives in the tube
feet of its asteroid host, lacks a normal endodermal gut, but has a modified epidermis performing both
digestive and absorptive functions. Most of these entosymbiotes show a shift from the lipid storage
characteristic of free-living and ectosymbiotic species to the glycogen storage predominating in the
Neodermata. In both groups this emphasis on carbohydrate metabolism is often independent of the PO2 of their
environment. Both groups also show high fecundity and it is suggested that there is a direct relationship
between this and glycogen storage. High fecundity, while clearly of adaptive value in entosymbiotes, is
arguably primarily related to the assured food supply conferred by the entosymbiotic habit and thus can be
viewed as a consequence of the latter rather than a prerequisite for it. Some entosymbiotic Turbellaria have
evolved physiologically active haemoglobins, allowing them to abstract oxygen preferentially from host
tissues; some have also evolved facultative glycolytic mechanisms comparable to those of the Cestoda. All
these adaptations to ecto- and entosymbiotic life in the Turbellaria exemplify possible pathways to wholly
parasitic lifestyles, with total metabolic dependence on the hosts, which may have been followed during the
evolution of the Neodermata."

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