Osteological and histopathological details of unilateral microphthalmia and anophthalmia in juvenile common carp (Cyprinus carpio)

Abstract We describe and compare normal eye, unilateral microphthalmia (small eye), and unilateral anophthalmia (eye incompletely developed) in young (5–7 month-old) common carp, Cyprinus carpio , (Cypriniformes: Cyprinidae). Five of 1058 (0.47%) common carp had grossly-observed eye deformities that we characterized using osteology and histopathology: 2 (81 and 98 mm fork length) had unilateral microphthalmia and 3 (103, 123, and 137 mm in fork length) had unilateral anophthalmia. Grossly, the eye of the microphthalmic common carp was opaque and slightly protruding from a reduced orbit; whereas, only a slight integumental concavity was observed on the deformed side of the anophthalmic common carp. Osteologically, microphthalmic and anophthalmic common carp had an irregular orbit with orbital bones that were laterally-expanded and that formed a discontinuous, uneven orbital perimeter compared to the narrow orbital bones forming a nearly circular orbit in normal common carp. Histologically, microphthalmia manifested as an eye having a thickened cornea with a retina and attached optic nerve but no lens (aphakia); whereas, anophthalmia manifested as lacking a globe, cornea, lens, or retina but having a nerve fascicle and capillaries representing probable optic nerve and its associated choroid. No histological sign of infection was observed in the ocular region of any common carp studied. The etiological agent associated with microphthalmia and anophthalmia is indeterminate, and the hypothesis that electroporation caused the ocular deformity was not tested herein. We suspect that the microphthalmic eye could detect light but not form a focused image. A list of putatively similar abnormalities in other fishes as well as anisophthalmia (incomplete eye development) and symmetrical cyclopia (medially fused eye) in larval and juvenile fishes is provided. This is the first published report providing osteological or histopathological details of microphthalmia or anophthalmia in a non-embryonic or non-larval fish.