Carotenoid esters are common in plants and animals in aquatic environments. In seaweeds and microalgae, acetate esters are abundant (e.g. in the characteristic carotenoids fucoxanthin and peridinin). Esterification of C(19) hydroxy groups with fatty acids of 4–12 carbons is also a common feature. Acyl esters with longer-chain fatty acids (16–20 carbons) are not often found in algae, except in secondary carotenoids that accumulate in oil droplets outside the chloroplast of some green algae under stress conditions. Such long-chain acyl esters (e.g. of astaxanthin) are more common in fish and invertebrate animals, but carotenoid acyl esters consumed in the diet are generally de-esterified and the free xanthophyll re-esterified before deposition in tissues. Several examples of novel carotenoids isolated from invertebrate animals are esters, including sulphates found in several sponges. In fish, carotenoids in the integument (skin tissues) are typically present as esters, but the free form predominates in other tissues, though findings with salmonid fishes should not be extrapolated to other species. The carotenoids present in carotenoprotein complexes in crustaceans and some other invertebrates are not esterified, though carotenoid may be deposited as esters in storage and other tissues. The carotenoids enter the food chains from the primary producers, such as phytoplankton and other algae, but may be modified structurally by metabolism within the animal.