The photoreceptors of the Atlantic deep sea scallop, Placopecten magellanicus (Gmelin 1791) (Bivalvia - Pteroconchida)

Powell, Catherine Vonnie Loretta (1990) The photoreceptors of the Atlantic deep sea scallop, Placopecten magellanicus (Gmelin 1791) (Bivalvia - Pteroconchida). Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The morphology and development of the pallial eyes and larval photoreceptors of the Atlantic Deep Sea Scallop, Placopecten magellanicus (Gmelin 1791) wore investigated, for the first time, by the correlative techniques of light and electron microscopy. -- The structure of the pallial eye in Placopecten magellanicus is similar to that of other scallops. Each eye is borne on an optic tentacle whose outer columnar epithelium is modified into a pigmented iris and transparent cornea. Beneath the cornea are located a cellular lens, a two-layered retina, a reflecting argentea and a pigmented tapetum. P. magellanicus eyes differ from those of other scallops in that the distal retina has fewer receptor cells than the proximal retina and a common optic nerve is not present. The absence of synapses within the retinae is confirmed and the three-dimensional structure of the photoreceptive cilia on the distal retinal receptor cells is elucidated. The presence of cilia and associated structures in the proximal retinal receptor cells is established and the implications of this are discussed. -- The development of the pallial eye of Placopecten magellanicus is described. The presence of a papilla-like prospective optic tentacle containing the retinal anlage and components of the tapetum and argentea is conventional. However, the sequence of appearance of the optical components differs from that reported for other scallops. Morphogenesis of the pallial eye of P. magellanicus proceeds from the collective differentiation of the tapetum, argentea and proximal retina to the formation of the distal retina, proximal optic nerve, iris, lens, distal optic nerve and cornea. Aspects of the formation of certain optical structures (i.e. rhabdomeric microvilli, photoreceptive cilia, nerves) within the pallial eye are described and their relationship and significance to specific differentiation events is discussed. The mode of synthesis of the reflecting crystals in the developing argentea cells is described. Cells are added to the differentiating retinae from the peripheral anlage and mitosis does not occur in the retinae or in the differentiating lens. The possible role subserved by the short cilia and their extensive root systems in the differentiating proximal receptor cells is discussed in relation to existing theories. The mesodermal origin of prospective lens cells is confirmed and the presence of rudimentary cilia in prospective lens cells is established. -- The fine structure of the photoreceptors in the pediveliger larvae of Placopecten magellanicus is described. Each ocellus consists of a single pigmented cell with long microvilli at the distal end forming a rhabdomere and a presumptive axon at the base, and non-pigmented cells which bear at least 3 cilia, each with a 9x2+2 axoneme pattern. The pigmented cell forms a cup that sheaths the photosensory organelles. A lens is absent. A ciliary component in the form of a basal body is located in the distal cytoplasm of the pigmented cell and its significance is discussed. Arguments that the pigmented cell is the photosensory cell are presented. Morphological comparisons are made between the photoreceptors in the scallop pediveliger and ocelli found in other invertebrates and protochordates. The possible function of the larval scallop’s photoreceptors is discussed.

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