Symbiotic unicellular algae (zooxanthellae) in reef-building corals provide food and cause coral to deposit its calcium carbonate skeleton by photosynthesis. Further, the (UV) ultraviolet-absorbing agents called the mycosporins¹, produced by the symbionts, protect shallow-water corals from the damaging effects of UV radiation.

MAAs, when present in marine invertebrates having symbiotic algae (e.g. Pacific staghorn coral Acropora formosa) are freely diffusable (i.e. soluble but not associated with any protein species) whereas, in invertebrates without symbiotic algae where MAAs are obtained mainly through diet, (e.g. crown-of-thorns starfish - Acanthaster planci) the MAAs are protein 'associated'. In these invertebrates protein-associated MAA's are distributed in most tissues but are concentrated on the outer epidermal tissues, and in the case of telostean fish, in their eye lenses as well.

Mesopelagic fish have been shown to possess a variety of largely unidentified short wave absorbing pigments within their lenses. Among the identified pigments are carotenoids, kyourenine (6) and 3-hydroxy kynurenine(7). The tryptophan derived pigments (6), (7), also found in terrestrial vertebrates, have been isolated from a deep sea species and several closely related tropical fresh water species, respectively².

The lenses of many shallow water fish also absorb short wave radiation and the pigments in these lenses have been identified as MAAs³. These pigments appear more widespread and have been isolated from lenses of fish from a variety of habitats³. Pigments extracted from 52 species have been identified as palythine(2) asterina-330(3) palythinol(4) and palythene(5). In the majority of species asterina-330 was the most abundant pigment.

Gadusol(9), which is structurally related to the MAAs, is present in the fish ovaries that contain ripe eggs just before spawning, and is a rninor component in the fish lenses.

The molecular weights of the two proteins are characteristic of b and g crystallins, respectively, which are two of the three major soluble crystallin protein present in fish lenses.

Thus, it appears that while the highly unstable complex is an association of asterina-330 and b crystallin, the second complex probably is a Schiff's base formed by the 'reaction' of gadusol and g crystallin. It is interesting to note that this latter 'association' is similar in nature to the visual pigment Rhodopsin, (lmax498 nm) a Schiff's base which contains 11-cis retinal (lmax382 nm) as a prosthetic group and a lipoprotein, opsin (lmax280 nm). It is noteworthy that while gadusol was present in the ripe fish eggs, asterina-330 was absent. Asterina-330 may be assimilated through diet and/or may have been synthesised by the animal from the mycospoorins present in the diet.