Impact of habitat and life trait on character evolution of pallial eyes in Pectinidae (Mollusca: bivalvia)

Abstract

Pectinidae, a large group of marine bivalves comprising more than 300 species worldwide, inhabit a diverse array of habitats, enabling an enormous radiation, and yielding many different life forms and adaptations. This apparent diversity led to the distinction of ecotypes based on shell morphology and lifestyle. Eyes in Pectinidae (Bivalvia, Pteriomorphia) have long sparked scientific interest and have been described for various species over the past two centuries. These eyes are morphologically and functionally highly complex. Despite this complexity, studies have focused mostly on functional aspects with only few examining the relationships associated with different environmental or evolutionary traits. Here, the pallial eye structure within the Pectinidae was examined using Masson Goldner Trichrom staining, and ancestral character estimation with BayesTraits was performed to reconstruct macro-evolutionary patterns. To evaluate the connection of substrate type and lifestyle to the evolution of eyes, we compared eyes within the major subgroups of Pectinidae while considering the different lifestyles and substrate types as well as different depth ranges. The results indicate a tendency towards a taxon-/clade-specific evolution in respect to characters such as the cornea and lens while depth specific adaptations occur mainly in the light sensitive compartments of the retina. Successive reduction of eyes seems to occur from shallow to deep water species and ends in a total reduction of all structures in deep sea species.