The ocular morphology of the southern hemisphere lamprey Geotria australisgray with special reference to optical specialisations and the characterisation and phylogeny of photoreceptor types

This paper describes the ocular morphology of young adults of the southern hemisphere lamprey Geotria australis, the sole representative of the Geotri idae, and makes comparisons with those of holarctic lampreys (Petromyzontid ae). As previously reported for the holarctic lamprey Ichthyomyzon unicuspi s [Collin and Fritzsch, 1993], the lens of G. australis is non-spherical an d possesses a cone-shaped posterior that may be capable of mediating variab le focus. The avascular retina of G. australis is well differentiated, cont aining three retinal ganglion cell populations, three layers of horizontal cells and three photoreceptor types, in contrast to petromyzontids that con tain only two photoreceptor types (short and long), G. australis possesses one rod-like (R1) and two cone-like (C1 and C2) photoreceptors. Although th e rodlike receptor in G. australis may be homologous with the short recepto rs of holarctic lampreys, the two cone-like receptors have morphological ch aracteristics that differ markedly from those of the long receptors of thei r holarctic counterparts. The features which distinguish the two cone-like receptors from those of the long receptor type in holarctic lampreys are th e characteristics of the mitochondria and the presence of large amounts of two different types of stored secretory material in the endoplasmic reticul um of the myoid (refractile bodies). The endoplasmic reticulum of each rece ptor type has a different shape and staining profile and is polymorphic, ea ch showing a continuum of distension. It is proposed that the presence of t wo cone-like photoreceptors with different characteristics would increase t he spectral range of G. australis and thus be of value during the parasitic phase, when this lamprey lives in the surface marine waters. The irideal f lap, present in G. australis but not petromyzontids, would assist in reduci ng intraocular flare during life in surface waters. The results of this stu dy, which are discussed in the context of the proposed evolution of lamprey s, emphasise that it is important to take into account the characteristics of the eyes of southern hemisphere lampreys when making generalizations abo ut the eyes of lampreys as a whole.