Vitamin a in the Invertebrates

1 Vitamin A has been measured in invertebrates by biological, physico‐chemical and physical methods. 2 Some Mollusca and Crustacea, two species of Nematoda and one of Chaeto‐gnatha are known to contain the vitamin, and it may possibly be present in certain starfish, an annelid, and an insect larva. Its presence has never been established in any Protozoa, Porifera, Coelenterata, or in most Annelida, Echinodermata, Insecta and Tunicata. 3 In the Mollusca vitamin A occurs in small concentrations in a chiton, severallamellibranchs and gastropods, and in all the species of cephalopods examined. Certain pteropods are rich in the vitamin. 4 Vitamin A has not been found in any branchiopod, ostracod or cirripede crustacean, but does occur in three out of a number of copepod species analysed. In the Malacostraca it is present in some and absent from other species of Isopoda, Amphipoda, Mysidacea and Decapoda. In the last three groups the pelagic species tend to be richer in the vitamin than the benthic and littoral forms. All species of Euphausiacea so far examined contain vitamin A, in concentrations usually higher than in other Crustacea. The presence of vitamin A in the eye of the honey bee has been demonstrated. 5 In the lamellibranchs and gastropods vitamin A, when present, occurs mainly in the hepatopancreas or in the visceral mass. In Patella vulgata it is also found in the gonads, the testes being invariably richer than the ovaries. I n the cephalopods the distribution of the vitamin is similar to that in vertebrates, with most of it in the liver, smaller amounts in the eyes and other parts of the body, and high concentrations in the ripe eggs. 6 The Crustacea with vitamin A have most of it in their eyes, except the Mysidacea and certain Caridea, especially the species of Acanthephyra. Further investigation of the rich reserves in the eyes of euphausiids indicates that most of the vitamin occurs in the region of the rhabdomes, with a smaller amount in the photophore. 7 The source of vitamin A for the herbivores is probably mostly β‐carotene in the phytoplankton or other plants they eat. Provitamins A for the carnivorous invertebrates include j? ‐carotene in their herbivorous prey, and echinenone, especially in sponges, echinoderms and molluscs. It is also likely that astaxanthin may be a vitamin A precursor at least for some crustaceans. Many carnivorous and necrophagous invertebrates, particularly crustaceans and cephalopods, get the vitamin A preformed in their food. 8 Vitamin A accumulates throughout life in some Crustacea, although there are seasonal variations in concentration associated with increased feeding during plankton outbursts. There is evidence of a build‐up of vitamin A reserves in maturing female Crangon which are passed on to the eggs when these are laid. A similar decrease in the liver stores of vitamin A at egg‐laying occurs in the cephalopods. 9 In Mollusca the form of vitamin A in species other than cephalopods varies. In many it is all in the ester form, but in others it may be present mainly as the alcohol, especially in the hepatopancreas. In the cephalopods the picture is similar to that in vertebrates, with the ester normally in excess of the alcohol in the liver, the aldehyde predominant in the eyes, and the eggs rich in the alcohol. 10 In Crustacea most of the vitamin A is esterified, especially in the eyes, where only 10% or less may be present as the alcohol. The aldehyde has also been detected in the eyes of some crustaceans and of the honey‐bee. 11 Most of the vitamin A in crustacean eyes is as the 11‐cis geometric isomer. Nothing is known of the isomeric form in other invertebrates, except in the eyes and liver of cephalopods where it is all‐trans vitamin A, apart from the I14 s retinene combined in rhodopsin. 12 No experimental evidence has yet been produced of a dietary requirement for vitamin A by any invertebrate, although it seems likely that the higher crustaceans, the insects and the cephalopods need vitamin A at least for visual purposes. 13 In the molluscs, except the cephalopods, nothing is known of the functions, if any, of vitamin A, although it seems unlikely that it is involved in vision. In cephalopods the vitamin undoubtedly participates as retinene in the rhodopsin of the eye, but the large reserves of vitamin A in the liver and eggs may serve other functions just as in vertebrates. 14 In the Arthropoda there is no evidence of any other function for vitamin A than in vision. In the Crustacea the vitamin is concentrated mainly in the rhabdomes, which are the photoreceptors of the eye. It is mostly in the 114 s form, of which the aldehyde is the chromophore of vertebrate rhodopsin, and all the animal's reserves of the vitamin are stored in the eye. 15 Many invertebrates are a source of vitamin A for their predators. Most important are the cephalopods and the euphausiids. Both groups contain high concentrations and are the food of economically valuable whales and fishes. Euphausiids in turn are important components in the diet of cephalopods and so may be regarded as the base of a vitamin A pyramid in marine economy.