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Photoreceptors of cnidarians

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Abstract:



Cnidarians are the most primitive present-day invertebrates to have multicellular light-detecting organs, called ocelli (eyes). These photodetectors include simple eyespots, pigment cups, complex pigment cups with lenses, and camera-type eyes with a cornea, lens, and retina. Ocelli are composed of sensory photoreceptor cells interspersed among nonsensory pigment cells. The photoreceptor cells are bipolar, the apical end forming a light-receptor process and the basal end forming an axon. These axons synapse with second-order neurons that may form ocular nerves. A cilium with a 9 + 2 arrangement of microtubules projects from the receptor-cell process. Depending on the species, the membrane covering the cilium shows several variations, including evaginating microvilli. In the cubomedusae stacks of membranes fill the apical regions of the photoreceptor cells. Pigment cells are rich in pigment granules, and in some animals the distal regions of these cells form tubular processes that project into the cavity of the ocellus. Microvilli may extend laterally from these tubular processes and interdigitate with the microvilli from the ciliary membranes of photoreceptor cells. Photoreceptor cells respond to changes in light intensity with graded potentials that are directly proportional to the range of the changes in light intensity. In the Hydrozoa these cells may be electrically coupled to each other through gap junctions. Light affects the behavioral activities of cnidarians, including diel vertical migration, responses to rapid changes in light intensity, and reproduction. Medusae with the most highly modified photoreceptors demonstrate the most complex photic behaviors. The sophisticated visual system of the cubomedusan jellyfish Carybdea marsupialis is described. Extraocular photosensitivity is widespread throughout the cnidarians, with neurons, epithelial cells, and muscle cells mediating light detection. Rhodopsin-like and opsin-like proteins are present in the photoreceptor cells of the complex eyes of some cubomedusae and in some neurons of hydras. Neurons expressing glutamate, serotonin, -aminobutyric acid, and RFamide (Arg-Phe-amide) are found in close proximity to the complex eyes of cubomedusae; these neurotransmitters may function in the photic system of the jellyfish. Pax genes are expressed in cnidarians; these genes may control many developmental pathways, including eye development. The photobiology of cnidarians is similar in many ways to that of higher multicellular animals.

Les cnidaires sont les invertébrés les plus primitifs à posséder des organes pluricellulaires à fonction photoréceptrice appelés ocelles (yeux). Ces photorécepteurs sont de simples taches oculaires ou des coupes de pigments, ou encore des coupes complexes de pigments avec un cristallin, ou alors ce sont des yeux semblables à des appareils-photo avec cornée, cristallin et rétine. Les ocelles sont composés de cellules sensorielles photoréceptrices dispersées parmi des cellules pigmentaires non sensorielles. Les cellules photoréceptrices sont bipolaires; leur extrémité apicale émet un processus photorécepteur et leur extrémité basale forme un axone. Les axones font des synapses avec des neurones de second ordre, formant parfois des nerfs oculaires. Un cil avec des microtubules disposés selon un arrangement 9 + 2 prend son origine sur le processus des cellules réceptrices. La membrane qui recouvre le cil est très variable selon l'espèce et porte parfois des microvillosités évaginées. Chez les cuboméduses, des empilements de membranes remplissent les régions apicales des cellules photoréceptrices. Les cellules pigmentaires sont riches en granules de pigments et, chez certaines espèces, la région distale de ces cellules émet des processus tubulaires dans la cavité de l'ocelle. Il arrive que des microvillosités issues de ces processus tubulaires s'étendent latéralement et s'entrecroisent avec les microvillosités issues des membranes ciliaires des cellules photoréceptrices. Les cellules photoréceptrices réagissent aux changements d'intensité lumineuse par des potentiels gradués directement proportionnels à l'étendue de ces changements. Chez les hydrozoaires, ces cellules peuvent s'unir pour former des couples électriques au niveau des jonctions intercellulaires de type adherens (gap junctions). La lumière a des effets sur les activités comportementales des cnidaires, notamment les migrations verticales, les réactions aux changements brusques de l'intensité lumineuse et la reproduction. Les méduses qui on

Document Type: Research Article

Publication date: October 1, 2002

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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