title: Repeated eye reduction events reveal multiple pathways to degeneration in a family of marine snails.
abstract: Eye reduction occurs in many troglobitic, fossorial, and deep-sea animals but there is no clear consensus on its evolutionary mechanism. Given the highly conserved and pleiotropic nature of many genes instrumental to eye development, degeneration might be expected to follow consistent evolutionary trajectories in closely related animals. We tested this in a comparative study of ocular anatomy in solariellid snails from deep and shallow marine habitats using morphological, histological, and tomographic techniques, contextualized phylogenetically. Of 67 species studied, 15 lack retinal pigmentation and at least seven have eyes enveloped by surrounding epithelium. Independent instances of reduction follow numerous different morphological trajectories. We estimate eye loss has evolved at least seven times within Solariellidae, in at least three different ways: characters such as pigmentation loss, obstruction of eye aperture, and "lens" degeneration can occur in any order. In one instance, two morphologically distinct reduction pathways appear within a single genus, Bathymophila. Even amongst closely related animals living at similar depths and presumably with similar selective pressures, the processes leading to eye loss have more evolutionary plasticity than previously realized. Although there is selective pressure driving eye reduction, it is clearly not morphologically or developmentally constrained as has been suggested by previous studies.
authors.affiliation: School of Biological Sciences, Queen's University Belfast, Belfast, BT9 7BL, Northern Ireland. lsumnerrooney01@qub.ac.uk.|Queen's University Marine Laboratory, Queen's University Belfast, Portaferry, Co. Down, BT22 1PF, Northern Ireland. lsumnerrooney01@qub.ac.uk.
authors.forename: Lauren
authors.initials: L
authors.lastname: Sumner-Rooney
authors.affiliation: School of Biological Sciences, Queen's University Belfast, Belfast, BT9 7BL, Northern Ireland.|Queen's University Marine Laboratory, Queen's University Belfast, Portaferry, Co. Down, BT22 1PF, Northern Ireland.
authors.forename: Julia D
authors.initials: JD
authors.lastname: Sigwart
authors.affiliation: Queen's University Marine Laboratory, Queen's University Belfast, Portaferry, Co. Down, BT22 1PF, Northern Ireland.
authors.forename: Jenny
authors.initials: J
authors.lastname: McAfee
authors.affiliation: Core Research Facility, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom.
authors.forename: Lisa
authors.initials: L
authors.lastname: Smith
authors.affiliation: Department of Life Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom.
authors.forename: Suzanne T
authors.initials: ST
authors.lastname: Williams
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005123:Eye / Q000033:anatomy & histology*; D010802:Phylogeny; D010858:Pigmentation / Q000235:genetics; D012908:Snails / Q000033:anatomy & histology / Q000145:classification / Q000235:genetics*
keywords: Deep sea; Solariellidae; Vetigastropoda; eye degeneration; eye evolution; eye loss; vision