The Functional Anatomy of the Cornea and Anterior Chamber in Lampreys: Insights From the Pouched Lamprey, Geotria australis (Geotriidae, Agnatha).

H Barry Collin,Julian Ratcliffe,Shaun P Collin

doi:10.3389/fnana.2021.786729

H Barry Collin, Julian Ratcliffe + Show 1 more

Open Access

https://doi.org/10.3389/fnana.2021.786729

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Abstract

Extant lampreys (Petromyzontiformes) are one of two lineages of surviving jawless fishes or agnathans, and are therefore of critical importance to our understanding of vertebrate evolution. Anadromous lampreys undergo a protracted lifecycle, which includes metamorphosis from a larval ammocoete stage to an adult that moves between freshwater and saltwater with exposure to a range of lighting conditions. Previous studies have revealed that photoreception differs radically across the three extant families with the Pouched lamprey Geotria australis possessing a complex retina with the potential for pentachromacy. This study investigates the functional morphology of the cornea and anterior chamber of G. australis, which is specialised compared to its northern hemisphere counterparts. Using light microscopy, scanning and transmission electron microscopy and microcomputed tomography, the cornea is found to be split into a primary spectacle (dermal cornea) and a scleral cornea (continuous with the scleral eyecup), separated by a mucoid layer bounded on each side by a basement membrane. A number of other specialisations are described including mucin-secreting epithelial cells and microholes, four types of stromal sutures for the inhibition of stromal swelling, abundant anastomosing and branching of collagen lamellae, and a scleral endothelium bounded by basement membranes. The structure and function of the cornea including an annular and possibly a pectinate ligament and iris are discussed in the context of the evolution of the eye in vertebrates.

Highlights

Lampreys (Petromyzontiformes) and hagfishes (Myxiniformes) are the extant representatives of the agnathan stage in vertebrate evolution
The cornea of the Pouched lamprey (Geotria australis) possesses a primary spectacle and a scleral cornea separated by a narrow mucoid layer, which widens toward the periphery (Figure 1)
In the caudal periphery of the dermal cornea, there is a dorso-ventral seam of connective tissue, which indicates the insertion of the tendon of the caudal cornealis muscle

Summary

Introduction

Lampreys (Petromyzontiformes) and hagfishes (Myxiniformes) are the extant representatives of the agnathan (jawless) stage in vertebrate evolution. Lampreys have a protracted larval phase, where the larvae are microphagous and photophobic (Hardisty and Potter, 1971a,b), burrowing in freshwater rivers for many years, before they undergo a radical metamorphosis to become adults. Anadromous species such as the Pouched lamprey, Geotria australis migrate downstream to enter their marine phase, where they are found in high numbers throughout the austral summer in the cold and clear waters surrounding South Georgia (Potter et al, 1979; Prince, 1980). G. australis return to their natal freshwater river and migrate upstream, where they spawn and die (Potter et al, 1983)

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