The primary events in phototransduction in invertebrate photoreceptor cells are initiated at rhodopsins which share structural features common to the subgroup of G-protein coupled receptors specialized for the absorption of light. Evaluation of cephalopod rhodopsin indicates that the seven transmembrane helices of invertebrate rhodopsins are spaced around the retinal chromophore in a 3D structure very similar to that of vertebrate rhodopsins. Nevertheless, from a multitude of information obtained from studies of phototransduction pathways in model systems from the phyla of molluscs (squid, octopus, scallop) and arthropods (Drosophila, Limulus, crayfish) it is obvious that evolution has led to some divergence (i) within invertebrates and (ii) between vertebrates and invertebrates. These modifications are manifested at the level of rhodopsin function as well as in the individual steps of the signalling cascade initiated through photon absorption by rhodopsin. This divergence specifically concerns the formation of stable metarhodopsin states, the coupling to different types of G-proteins, the mechanisms of amplification and termination of signalling, the supramolecular organization of the signalling cascade and non-visual, intracellular signalling pathways for morphogenesis, membrane renewal and apoptosis.
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