Hot (geothermal) pools and streams, a specific type of aquatic biotopes having almost
worldwide distribution, maintain rich faunas of animals belonging to various taxa. Snails (Gastropoda) represent one of such groups, which form populations in geothermal waterbodies of all continents. Some freshwater snail species produce morphologically distinct hot-water populations,
whose rank is often debated (full species or thermal ‘ecotype’, or ‘race’). In this study, we used six
species of pond snails (family Lymnaeidae) to investigate the morphological and genetic consequences of infiltration of freshwater snails into geothermal habitats. In particular, we aimed at studying the changes in shell shape and proportions as well as the formation of unique hot-water haplotypes and the occurrence of the latter beyond geothermal waterbodies. All six species studied
demonstrate diminutive body size in hot sites accompanied, in four species, by apparent alterations
in shell proportions. A phenomenon of phenotypic ‘juvenilization’, when adult and able to reproduce individuals demonstrate shell proportions characteristic for full-grown individuals living under ‘normal’ conditions, is described. Unique ‘thermal’ haplotypes, not found beyond the geothermal sites, were normally restricted to a single locality, and no signs of frequent ‘travels’ of snails
from one thermal habitat to another are seen. In the vast majority of cases, these exclusive haplotypes are separated from their ancestors by only a few (1–3) mutational steps, which may indicate
their relatively recent origin. We are inclined to relate both size reduction and ‘juvenilization’ to the
life-cycle re-adjustment following the penetration of lymnaeids to thermal habitats. The ecological
(‘thermal’) intraspecific races of different species, forming in geothermal habitats, exhibit, to a great
extent, evolutionary predictability (= convergent evolution; = parallelisms). The dilemma ‘ecological
race vs. young species’ in application to the taxonomy of these hot-water populations is briefly discussed.