How an Ecological Race Is Forming: Morphological and Genetic Disparity among Thermal and Non-Thermal Populations of Aquatic Lymnaeid Snails (Gastropoda: Lymnaeidae)

Abstract

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.

Coordinated response of endemic gastropods to Late Glacial and Holocene climate-driven paleohydrological changes in a small thermal pond of Central Europe

“…Shell size decreases while the aperture circumference increases. This type of dwarfism is characteristic in thermal water habitats 28 .…”

“…Understanding the underlying reasons for phenotypic plasticity and the resulting morphological disparity is thus one of the key topics of evolutionary biological and paleobiological research on mollusks 1 – 3 , 5 – 11 The phenotypic plasticity of extant and fossil melanopsids (the family Melanopsidae H. & A. Adams, 1855) has been widely documented in various lacustrine and fluvial habitats 12 – 27 . However, studies of small thermal water habitats harboring highly endangered endemics and with geological records spanning several millennia is rare 28 .…”

“…Thermal ponds represent unique habitats with a special temperature regime and chemical composition 28 . Temperatures are higher than in other habitats and can fluctuate frequently.…”