Retinal

Retinal, also called retinaldehyde or vitamin A aldehyde, is one of the many forms of vitamin A (the number of which varies from species to species). Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision. Bound to proteins called type 1 rhodopsins, retinal allows certain microorganisms to convert light into metabolic energy.

Vertebrate animals ingest retinal directly from meat, or produce retinal from one of four carotenoids (beta-carotene, alpha-carotene, gamma-carotene, and beta-cryptoxanthin), which they must obtain from plants or other photosynthetic organisms (no other carotenoids can be converted by animals to retinal, and some carnivores cannot convert any carotenoids at all). The other main forms of vitamin A, retinol, and a partially active form retinoic acid, may both be produced from retinal.

Invertebrates such as insects and squid use hydroxylated forms of retinal in their visual systems, which derive from conversion from other xanthophylls.