larvae can determine their depth by color. The color unlike brightness stays almost constant independent of time of day or the weather, for instance if it is cloudy. Panopsins are also expressed in the brains of some insects. The panopsins of mosquito and pufferfish absorb maximally at 500 nm and 460 nm, respectively. Both activate in vitro Gi and Go proteins. The panopsins are sister to the TMT-opsins. Teleost Multiple Tissue (TMT) Opsin The first TMT-opsin was found in many tissues in Teleost fish and therefore they are called Teleost Multiple Tissue (TMT) opsins. TMT-opsins form three groups which are most closely related to a fourth group the panopsins, which thus are paralogous to the TMT-opsins. TMT-opsins and panopsins also share the same introns, which confirms that they belong together. Opsins in cnidarians Cnidaria, which include jellyfish, corals, and sea anemones, are the most basal animals to possess complex eyes. Jellyfish opsins in the rhopalia couple to Gs-proteins raising the intracellular cAMP level. Coral opsins can couple to Gq-proteins and Gc-proteins. Gc-proteins are a subtype of G-proteins specific to cnidarians. The cnidarian opsins belong to two groups the xenopsins and the nessopsins. The xenopsins contain also bilaterian opsins, while the nessopsins are restricted to the cnidarians. However, earlier studies have found that some cnidarian opsins belong to the cilopsins, rhabopsins, and the tetraopsins of the bilaterians. Rhabdomeric opsins Rhabdomeric opsins (rhabopsins, r-opsins) are also known as Gq-opsins, because they couple to a Gq-protein. Rhabopsins are used by molluscs and arthropods. Arthropods appear to attain colour vision in a similar fashion to the vertebrates, by using three (or more) distinct groups of opsins, distinct both in terms of phylogeny and spectral sensitivity. The rhabopsin melanopsin is also expressed in vertebrates, where it regulates circadian rhythms and mediates the pupillary reflex. Unlike cilopsins, rhabopsins are associated with canonical transient receptor potential ion channels; these lead to the electric potential difference across a cell membrane being eradicated (i.e. depolarization). The identification of the crystal structure of squid rhodopsin is likely to further our understanding of its function in this group. Arthropods use different opsins in their different eye types, but at least in Limulus the opsins expressed in the lateral and the compound eyes are 99% identical and presumably diverged recently. Melanopsin Melanopsin (OPN4) is involved in circadian rhythms, the pupillary reflex, and color correction in high-brightness situations. Phylogenetically, it is a member