Ocular ultrasonography of sea turtles

Abstract

Background: Environmental changes contribute to the development of ophthalmic diseases in sea turtles, but information on their eye biometrics is scarce. The aim of this study was to describe ophthalmic ultrasonographic features of four different sea turtle species; Caretta caretta (Loggerhead turtle; n = 10), Chelonia mydas (Green turtle; n = 8), Eretmochelys imbricata (Hawksbill turtle; n = 8) and Lepidochelys olivacea (Olive ridley; n = 6) under human care. Corneal thickness, scleral ossicle width and thickness, anterior chamber depth, axial length of the lens, vitreous chamber depth and axial globe length were measured by B-mode sonography with a linear transducer. Carapace size and animal weight were recorded. A sonographic description of the eye structures was established.

Results: The four species presented an ovate eyeball, a relatively thin cornea, and a small-sized lens positioned rostrally in the eye bulb, near the cornea, resulting in a shallow anterior chamber. The scleral ossicles did not prevent the evaluation of intraocular structures, even with a rotated eye or closed eyelids; image formation beyond the ossicles and measurements of all proposed structures were possible. B-mode sonography was easily performed in all animals studied. The sonographic characteristics of the eye were similar among the four species. Since there was a correlation between the size of the eye structures and the size of the individual, especially its carapace size, the differences found between E. imbricata and Caretta caretta are believed to be due to their overall difference in size.

Conclusions: Sonography is a valuable tool in ophthalmic evaluation of these species. Only minor differences were found between the species in this study, reinforcing their phylogenetic proximity and their similar functions and habitats.

Keywords: Caretta caretta; Chelonia mydas; Eretmochelys imbricate; Eye; Lepidochelys olivacea; Ultrasound.

Fig. 1

Ultrasound images of sea turtle eyes. A Examination of a Caretta caretta eye. B Chelonia mydas eye—coloured lines show the axial globe length (a), corneal thickness (b), anterior chamber depth (c), lens axial length (d), vitreous chamber depth (e). C Caretta caretta eye—slightly oblique image from which scleral ossicle width and thickness were measured (between arrows). D Eretmochelys imbricata eye showing oval shape with central optic axis (line) smaller than the equatorial diameter (dashed line), indicating the posterior shading of the orbit bone (asterisk). E Lepidochelys olivacea eye—note that despite the posterior ossicle artefacts, visualization of the posterior portions is not impeded. F Caretta caretta eye in power Doppler mode showing segments of blood vessels (in yellow–orange) distributed in the scleral cartilage. G Part of the salt gland (between arrows) adjacent to the bulb (*) of Lepidochelys olivacea. H Caretta caretta eye in power Doppler mode showing a large blood vessel (*) adjacent to the salt gland (between arrows), where few and small vessels were identified (arrow head)

Fig. 2

Ultrasonographic images of the eyes of four different species of sea turtle with different body weights: Caretta caretta (A), Chelonia mydas (B), Eretmochelys imbricata (C), and Lepidochelys olivacea (D), taken at the same transducer frequency and the same scale size (note the ruler to the right of each image). Similar structures at the level of the central optic axis are indicated by coloured lines as follows: corneal thickness (a), anterior segment depth (b), lens axial length (c), vitreous chamber depth (d), and axial globe length (e). Note the evident difference in size, especially the depth of the vitreous chamber and the axial length of the globe