Contrasting body sizes in Nerita yoldii living on shores with different salinity regimes in South China

Morphological plasticity is common in intertidal molluscs. Populations of the same species can diverge in morphological traits such as body size, shape or colour under biological (e.g. predation: Trussell, 1996; Dalziel & Boulding, 2005), physical (e.g. wave exposure: Etter, 1988; Forrester et al., 2016; temperature: Miura et al., 2007; Irie & Morimoto, 2016; Zhao et al., 2019) or combinations of these factors along environmental gradients (Kitching et al., 1966; Vermeij, 1972; Crothers, 1983; Trussell, 2000; Hollander et al., 2006). In some cases, these morphological variations are subject to selection by local environmental conditions, resulting in different morphs across habitats and/or geographic locations (Janson, 1982; Boulding & Hay, 1993; Johannesson & Johannesson, 1996; Parsons, 1997). One key physical factor shaping plasticity and local adaptation in intertidal molluscs is salinity, which has been shown to alter morphological, physiological and behavioural traits across populations (Cheung, 1997; Ramajo et al., 2016; Piwoni-Piórewicz et al., 2021). Physiological tolerance to hyposaline stress determines species survival and distribution, suggesting salinity is a strong environmental filter (Gainey & Greenberg, 1977; Kangas & Skoog, 1978; Covernton & Harley, 2020; Krug et al., 2021). In terms of shell morphology, molluscs in marine environments tend to have thicker and more robust shells compared to those found in freshwater (Vermeij & Covich, 1978), likely due to the higher potential for biomineralization in marine water (Marshall & Tsikouras, 2024), or the lower pH and thus stronger shell dissolution in freshwater (Marshall et al., 2008). Metabolic responses to salinity variation may induce further energy demand for maintenance (Koehn & Bayne, 1989), leading to reduced growth (Forbes, 1991; Bashevkin & Pechenik, 2015; Ramajo et al., 2016). Populations of the same species living in habitats with different salinity may, therefore, demonstrate plasticity in body size or shape in association with variations in physiological and metabolic responses.