The Ocular Morphology of the Southern Hemisphere Lamprey Geotria australis Gray, with Special Reference to Optical Specialisations and the Characterisation and Phylogeny of Photoreceptor Types

The lens of the southern hemisphere lamprey Geotria australis possesses a cone-shaped posterior that may mediate variable focus.

• The avascular retina of G. australis contains three populations of retinal ganglion cells, three layers of horizontal cells, and three types of photoreceptors.
• In addition to a rod-like receptor, G. australis has two distinct cone-like photoreceptors, differing from the two types found in holarctic lampreys.
• The two cone-like receptors in G. australis have unique mitochondrial characteristics and large amounts of different stored materials in their endoplasmic reticulum.
• The shape and staining profile of the endoplasmic reticulum vary among receptor types, indicating polymorphism.
• These adaptations may enhance the spectral range of G. australis during its parasitic phase in surface marine waters.

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