Evolution of large eyes in stromboidea (Gastropoda): Impact of photic environment and life history traits

Eyes within the marine gastropod superfamily Stromboidea range widely in
size, from 0.2 to 2.3 mm - the largest eyes known in any gastropod.
Despite this interesting variation, the underlying evolutionary pressures
remain unknown. Here, we use the wealth of material available in museum
collections to explore the evolution of stromboid eye size and structure.
Our results suggest that depth is a key light-limiting factor in stromboid
eye evolution; here, increasing water depth is correlated with increasing
aperture width relative to lens diameter, and therefore an increasing
investment in sensitivity in dim light environments. In the major clade
containing all large-eyed stromboid families, species observed active
during the day and the night had wider eye apertures relative to lens
sizes than species observed active during the day only, thereby
prioritising sensitivity over resolution. Species with no consistent diel
activity pattern also had smaller body sizes than exclusively day-active
species, which may suggest that smaller animals are more vulnerable to
shell-crushing predators, and avoid the higher predation pressure
experienced by animals active during the day. Within the same major clade,
ancestral state reconstruction suggests that absolute eye size increased
above 1 mm twice. The unresolved position of Varicospira, however, weakens
this hypothesis and further work with additional markers is needed to
confirm this result.