Morphological and biochemical variation in populations of Dugesia (G.) tigrina (Turbellaria, Tricladida, Paludicola) from the western Mediterranean: biogeographical and taxonomical implications

Dugesia (Girardia) tigrina is a Nearctic freshwater triclad native to North America, now amply distributed in most European countries as the result of a reportedly unique introduction into north-west Europe early this century and its dispersal southwards and eastwards in the last decades. All European populations described so far are externally spotted, with a pigmented pharynx, and, apart from two sexual populations recently described in England, they are fissiparous. This uniformity contrasts with the variability reported in its native area, thus supporting the hypothesis of a single introduction. However, thorough morphological, karyological and biochemical comparative studies of European populations have never been properly done.
We have undertaken such a study comparing 24 populations present in a restricted area of the western Mediterranean. The results show an unexpectedly ample variation in external and pharyngeal pigmentation patterns, the presence of sexually reproducing populations, and ample variation in their biochemical patterns as revealed by isoelectric-focusing. This is an indication that European Dugesia (G.) tigrina is far more diverse than previously thought, suggesting that several, independent introductions from its native area have recently occurred, and the implications of these findings in biogeographical and taxonomic terms are discussed.     

Myxosoma cerebralis (Myxozoa: Myxosporea) Etiologic Agent of Salmonid Whirling Disease Requires Tubificid Worm (Annelida: Oligochaeta) in its Life Cycle

Studies of the life cycle of Myxosoma cerebralis showed that development of infectivity did not occur endogenously but that the spore “aging” process required participation of an aquatic tubificid oligochaete. Data suggestive of such involvement were derived from trials in which spores were “aged” in an array of inert, sterilized, pasteurized, or natural aquatic substrates and from examination of aquatic soils from trout hatcheries in which whirling disease was epizootic. The role of the aquatic oligochaete was confirmed two ways. First, signs of whirling disease developed, and M. cerebralis spores were produced in young rainbow trout (Salmo gairdneri) that had been fed oligochaetes harvested from pond soil taken from two hatcheries where whirling disease was epizootic. Second, when containers of pasteurized soil were populated with four genera of oligochaetes–Aeolosoma, Dero, Stylaria, or Tubifex– from a biological supply house, or with tubificid worms from trout hatcheries free of whirling disease, and then seeded with M. cerebralis spores and “aged” for 4 months, whirling disease occurred only in trout held with Tubifex and with hatchery tubificids.