Morphological study of the eye bulb of the hammerhead shark, Sphyrna lewini (Elasmobranch: Carcharhinidae) / Estudio morfológico del bulbo ocular del tiburón martillo, Sphyrna lewini (Elasmobranch: Carcharhinidae)

ABSTRACT

The hammerhead sharks shows a head laterally expanded with eyes and nostrils on the edges, which gives the species a hammer appearance. Another strand of studies indicates that the hypothesis of head shape alterations for better adaptions in the sensorial system with the development of structures associated to binocular sight and others sensorial organs. Given the variety of characteristics, the study aim was to describe the anatomical constitution of the hammerhead shark eye bulb. The bulb and its annexes exenteration was performed, and further dissection; the morphological description of the muscle insertions and eye bulb components were based on direct observation and were further photographed and catalogued. The eye bulb fragments and its annexes were histological technical. Between the sclera and the choroid it was possible to observe, by the electronical scanning microscopy, the thin layer called supra-choroid, in this region, spaces often filled by lymphatic vessels allied to a matrix formed by loose conjunctive tissue are found. In the choroid, a layer which is rich in blood vessels, loose conjunctive tissue and collagen fibers, was observed, besides pigmentary cells full of melanin in its interior, which result in its layer's dark color. Ciliary body is a choroid's dilatation; it has the aspect of a thick ring in finger-like shape, pigmented, covering the sclera surface and containing pigmentary cells. The crystalline capsule, which shows an acellular covering that, is hyaline and homogeneous. In the electronical scanning microscopy, it was observed that the capsule is extremely thick especially in the anterior face. The capsule is very elastic, constituted mainly by thin lamellae of collagenous fibers, as illustrated by the electronically scanning microscopy. Anatomic variations related mainly to the position of the eye bulb in the skull, fibrous tunica and lens call the attention and must be related to its habitat.

RESUMEN

El tiburón martillo tiene la cabeza lateralmente expandida con los ojos y la nariz en sus márgenes. Estudios indican que alteraciones de la forma de la cabeza de estos animales son adaptaciones que mejoran el desarrollo del sistema sensorial y estructuras asociadas a la visión binocular y otros órganos sensoriales. Dada la variedad de características, el objetivo del estudio fue describir la constitución anatómica del bulbo ocular. Se disecó el bulbo ocular junto a sus anexos. La descripción morfológica de las inserciones musculares y componentes del bulbo ocular se basaron en la observación directa, siendo éstas fotografiadas y catalogadas. Entre la esclera y la coroides se observó através de microscopía electrónica de barrido, una delgada capa supra-coroides, y vasos linfáticos junto a la matriz formada por tejidos conectivos. La coroides, era abundante en vasos sanguíneos y fibras de tejido conectivo. Se observaron escasas células pigmentarias llenas de melanina, lo que se asemeja a un anillo en forma de dedo, de manera que cubren la superficie de la esclera y células que contienen pigmentos. En la cápsula del lente, fue posible observar un recubrimiento celular, hialino y homogéneo. En la microscopía electrónica de barrido, se observó una cápsula muy gruesa, principalmente en la región frontal. La cápsula elástica,estaba constituida por láminas delgadas, principalmente, por fibras de colágeno. Se concluyó que el bulbo ocular de estos animales posee variaciones anatómicas relacionadas, principalmente, con la posición del bulbo ocular en el cráneo, túnica fibrosa y lente, lo que puede estar directamente relacionado con su hábitat.