Photoreception et Vision Chez Les Annelides

Abstract

In Annelids, the photoreceptors are located not only on the head but are also present in other parts of the body. They are uni- or pericellular organs. The sensory cells, in the majority of cases, possess numerous microvilli which are arranged either on the apical process (all prostomial Polychaete eyes) or either around an internal vacuole called phaosome or “Binnenkörper” (Oligochaete and Leech photoreceptors, some Polychaete photoreceptive structures). However a narrow connection with the extracellular space has been demonstrated for Hirudo by lanthanum impregnation. In addition to microvilli, a varying number of cilia (9 + 0) is present in Oligochaetes. Sometimes, a rudimentary cilium is also found at or close to the tip of apical processes of the prostomial Polychaete eyes. Ciliary photoreceptors have been described in the brain of Nereidae and Phyllodocidae and in the eyes of branchial filaments of Sabellidae. In the latter case, the photosensory structures are stacked lamellar “sacs” which project into an extracellular space. Every “sac” is attached to a kinetosome (9 + 0).

Absent in Oligochaete photoreceptors, the pigmented cells in the leeches form a pigment cup around the photoreceptoral cells whereas in the cerebral Polychaete eyes which have many cells, they are placed side by side of sensory cells and often called supportive cells. When the cerebral eye has only one pigmented cell, it forms a cup around the light- sensitive organelles of the receptoral cell. The shape of the supportive cells and particularly that of their apical processes changes with the family and sometimes within the same family with the species. In some cases, the apical processes constitute the lens.

Present in Polychaete cerebral eyes, the lens can be very diverse; it can be secreted by special cells (secretory cell of Vanadis, lenticular cell of Eulalia) or derived from pigmented cells which either secrete a hyalinematerial (Autolytus) or develop slender interdigitated apical processes (Nereis, Syllis). The branchial eyes of Sabellidae also possess a lens. In Dasychone and Potamilla the lens are secreted whereas in Branchiomma it is constituted of a single cell which is sitting at the top of the receptoral cell. It contains basally a dish-shaped nucleus and more centrally an ovoid mass of vesicles.

The electroretinograms have been recorded in Nereidae and in Alciopidae. They are cornea negative, but in T. Candida (Alciopidae which has two retinas) a reversal of polarity of ERG appears with wavelength and at 560 nm with position of the active electrode on the cornea. The cornea negative response is from the main retina while the cornea positive response is from an accessory retina.

In Torrea, the primary retina peaks in sensitivity at 400 nm, the secondary retina at 560 nm. The Vanadis eye (another Alciopidae) peaks in unit at 460 to 480 nm. Nereis mediator is also maximally sensitive at 480 nm.

In the medicinal leech (Hirudo medicinalis) the absorption of light increases the conductance of the microvillar membrane. Therefore a flow of current appears; it flows inward through the microvillar membrane and outward through the external membrane.

In general, Annelids are photonegative in their behaviour but they become photopositive under some conditions. It is the case for the blood-sucking leeches, several Polychaete families which are pelagic during the breeding season and L. terrestris which crawls toward dim lights and away from strong ones.