The eye of the takahe, Porphyrio hochstetteri

Abstract

We describe the ocular features and normal physiological parameters of the South Island takahe (Porphyrio hochstetteri), using three birds. Both eyes face slightly forward such that there is some anatomic possibility of binocular vision but with restriction particularly of the superior and posterior visual fields. The pupil is round under both dim room light and very high illumination and the centre of the pupil is displaced <= 0.5 mm nasally with regard to the corneal centre. The iris is light brown and has several areas of more prominent vascularity. Their refractive error in the distance by retinoscopy is between 0 and +1 dioptre, although they sometimes accommodate down to -1 dioptre during examination. The average intraocular pressure is 6.75 +/- 0.88 mmHg using an iCare tonometer and the average corneal thickness is 340 +/- 32 mm. The average corneal curvature is 73.63 +/- 0.82 dioptres. On B scan the eye appears flattened in its antero-posterior axis; the axial length is 13.67 +/- 0.61 mm, the width 16.36 +/- 0.30 mm. The distance from the posterior pole of the lens to the retina is 7.37 +/- 0.26 mm. The pecten is visible inferiorly but no os opticus can be identified.