title: Eye retraction in the giant guitarfish, Rhynchobatus djiddensis (Elasmobranchii: Batoidea): a novel mechanism for eye protection in batoid fishes.
abstract: Eye retraction behavior has evolved independently in some vertebrate linages such as mudskippers (fish), frogs and salamanders (amphibians), and cetaceans (mammals). In this paper, we report the eye retraction behavior of the giant guitarfish (Rhynchobatus djiddensis) for the first time, and discuss its mechanism and function. The eye retraction distance was nearly the same as the diameter of the eyeball itself, indicating that eye retraction in the giant guitarfish is probably one of the largest among vertebrates. Eye retraction is achieved by unique arrangement of the eye muscle: one of the anterior eye muscles (the obliquus inferior) is directed ventrally from the eyeball and attaches to the ventral surface of the neurocranium. Due to such muscle arrangement, the obliquus inferior can pull the eyeball ventrally. This mechanism was also confirmed by electrical stimulation of the obliquus inferior. The eye retraction ability of the giant guitarfish likely represents a novel eye protection behavior of elasmobranch fishes.
authors.affiliation: The Hokkaido University Museum, 3-1-1, Minato-cho, Hakodate, Hokkaido 041-9611, Japan. Electronic address: t-tomita@okichura.jp.
authors.forename: Taketeru
authors.initials: T
authors.lastname: Tomita
authors.affiliation: Okinawa Churaumi Aquarium, 424, Motobu-cho, Okinawa 905-0206, Japan.
authors.forename: Kiyomi
authors.initials: K
authors.lastname: Murakumo
authors.affiliation: Okinawa Churashima Research Center, Okinawa Churashima Foundation, 888, Motobu-cho, Okinawa 905-0206, Japan.
authors.forename: Kei
authors.initials: K
authors.lastname: Miyamoto
authors.affiliation: Okinawa Churashima Research Center, Okinawa Churashima Foundation, 888, Motobu-cho, Okinawa 905-0206, Japan.
authors.forename: Keiichi
authors.initials: K
authors.lastname: Sato
authors.affiliation: Okinawa Churashima Research Center, Okinawa Churashima Foundation, 888, Motobu-cho, Okinawa 905-0206, Japan.
authors.forename: Shin-ichiro
authors.initials: S
authors.lastname: Oka
authors.affiliation: Okinawa Churaumi Aquarium, 424, Motobu-cho, Okinawa 905-0206, Japan.
authors.forename: Haruka
authors.initials: H
authors.lastname: Kamisako
authors.affiliation: Okinawa Churashima Research Center, Okinawa Churashima Foundation, 888, Motobu-cho, Okinawa 905-0206, Japan.
authors.forename: Minoru
authors.initials: M
authors.lastname: Toda
mesh_terms: D000818:Animals; D005123:Eye / Q000033:anatomy & histology; D009799:Ocular Physiological Phenomena; D009801:Oculomotor Muscles / Q000033:anatomy & histology* / Q000502:physiology*; D012860:Skates, Fish / Q000033:anatomy & histology* / Q000502:physiology*
keywords: Batoids; Eye retraction; Giant guitarfish