Phenotypic divergence in viviparid snails in a recently converted freshwater lagoon

Abstract

Freshwater mollusks often show high intraspecific variability in shell morphology, which may cause incongruences between molecular phylogeny and morphology-based species taxonomy. However, it remains unclear how morphological variations in freshwater snails are associated with different habitats such as rivers, ponds, and lakes. In the present study, we investigated the morphological variation in the shells of the viviparid freshwater snail Sinotaia quadrata in the Kahoku Lagoon in Japan, which was converted from a brackish to a freshwater lagoon 60 years ago. This newly established freshwater habitat is particularly well-suited for studies on how morphology is associated with environments. We found clear differences in shell morphologies between the S. quadrata populations sampled inside and outside of the lagoon. The S. quadrata sampled in the lagoon had thicker and more elongated shells with wider apertures than those sampled outside of the lagoon. No relationship was found between shell morphology and the distance between sampling sites. This suggests that the divergence in shell morphology occurred over a fairly short time period by rapid adaptation or phenotypic plasticity. Although it is unclear whether these differences are caused by phenotypic plasticity or adaptation, the present findings suggest that the shell morphologies of freshwater snails easily change under the influence of the environment. The high flexibility in shell morphology should be carefully considered when managing or studying freshwater mollusks.

New insights from museum specimens: a case of Viviparidae (Caenogastropoda: Mollusca) in Iwakawa’s collection preserved in the National Museum of Nature and Science, Tokyo

“…material 1 ). In particular, the adult shell width tends to be relatively smaller than that of other Japanese Viviparidae ( Masuda and Uchiyama 2004 ; for example, under 24 mm except for one exception in Kagawa et al (2019) ).…”

“…The distinctive (see diagnosis and taxon discussion) shell morphology of H. longispira is considered to be a consequence of adaptation to its ancient lake habitat and this evolutionary change in morphology may have originated multiple times, based on examination of fossil specimens ( Hirano et al 2019b ). Furthermore, plastic and/or adaptive morphological changes to the environment seem to occur easily both within and amongst species in viviparid gastropods ( Hirano et al 2019b , Kagawa et al 2019 , Stelbrink et al 2020 ). Such morphological changes might have occurred in those H. japonica that have the elongated shell shape, as two of the three localities from where these specimens were collected are large lakes.…”

Role of ancient lakes in genetic and phenotypic diversification of freshwater snails

“…In addition, our results showed multiple diversifications of shell morphology in Lake Biwa. Previous studies have suggested that shell morphology might be associated with habitat (Kagawa, Saito, Uchida, & Chiba, ; Miura et al, ), and the habitat of Heterogen sp. is clearly different from the habitat of H. japonica and H. longispira .…”

A new species of the genus <i>Heterogen</i> (Mollusca: Caenogastropoda: Viviparidae) from the Pleistocene Katata Formation of the Kobiwako Group, Shiga Prefecture, central Japan

“…is found on the 7 m down of the rocky shore of Chikubushima Island in Lake Biwa (Hirano et al 2019). In addition, shell morphologies of viviparid snails easily change under the influence of the environment even within species [individuals of Sinotaia quadrata histrica (Gould, 1859) sampled in the lagoon had thicker and more elongated shells with narrower apertures than those sampled outside of the lagoon (Kagawa et al 2019)]. 1.2 Ma, Lake Biwa was shallower than it is in the present day (Miura et al 2019).…”