What is the contribution of thermal selection to the maintenance of a shell color cline in Littorina saxatilis?

The study of clines, or geographical variations of a given trait, can help understand how the interactions of genetics and local environments determine phenotypic diversity. The marine snail Littorina saxatilis (Olivi, 1792) exhibits a gradual change in the relative frequencies of shell color morphs across populations in the estuary system of the Rias Baixas (Galicia, NW Iberian Peninsula). A consistent pattern of distribution occurs across these four Rias, with the interior, sheltered regions dominated by a light fawn-like morph (fulva), and the exterior, wave-exposed locations by a darker lineated morph (lineata). Measurements of rock surface temperature along one of the Rias confirmed a general environmental temperature gradient during summer. The potential role of thermal adaptation driving this distribution pattern was tested by comparing shell thermal tolerance and performance between color morphs. The two color morphs (fulva and lineata) were collected from both sympatric and allopatric populations within the cline to account for the potential influence of either population or region-related traits. Laboratory experiments revealed no significant differences in the heating rate of shell temperature between color morphs in sympatric populations, although fulva snails showed higher recovery rates after exposure when combining sites from two Rias. As allopatric color morphs did not differ in thermal tolerance or performance, and sympatric differences were not consistent across Rias, we conclude that thermal effects represent a minor contribution to the maintenance of this color cline.