Friday, 11 November 2016

12) Anemonefish

 Of course, the other famous fish in the latest animated movie this year is the anemonefish or clownfish. There are at least 30 species of these in the world’s oceans, all but one of them in the genus Amphiprion. The species currently on show are a pair of Percula Clownfish, A.percula.

These fish are famous for their mutualistic associations with various tropical sea anemones, especially in the genera Entacmea, Heteractis and Stichodactyla, which gives them their alternative common name. An individual anemonefish species is usually restricted to a single species or genus of sea anemone, but this is not reversed – a single sea anemone species may host different anemonefish in different parts of its range. In captivity anemonefish species live perfectly well, and even breed, without anemones, but in the wild they do not appear able to survive without their host.

The benefit to the anemonefish from association with an animal with powerful defensive stings is pretty obvious. Less obvious is what the anemone gets out of the deal. These anemones are quite capable of killing and consuming small fish which lack the protective mucous coat that anemonefish use to disarm the anemone’s stinging cells, but whatever trick the anemonefish use is not confined to them. Several other unrelated species, including some gobies and cardinalfish, also use anemones as shelters. More common are fish and crustaceans which use more mobile coelenterates such as jellyfish as shelters, and presumably have similar means of preventing the host stinging them. It looks as though the “off switch” for coelenterate nematocysts is comparatively easy for other species to duplicate. It appears that the sea anemone does benefit in some ways from having residents – the faeces of the fish provide scarce nutrients to the symbiotic algae in the anemone, and possibly attract smaller fish without defences as prey.
P.biaculeatus - Andaman Island form
Anemonefish are absent from the Atlantic, but are found from the Red Sea eastwards to the Pacific. The taxonomy is still being clarified, and it appears that the large Spine-cheeked Anemonefish Premnas biaculeatus is part of a clade with A.percula and A.latizonatus, which makes Amphiprion paraphyletic.
The ecology of anemonefish species is fairly uniform. They mainly feed on plankton with some algae (more algae in a few species such as A. perideraion). Usually they live in groups, with a single breeding pair and some juveniles sharing a host anemone. The large species such as P. biaculeatus, which can reach 17cm, tend to be more territorial. The social structure is conditioned by their reproductive biology. They are protandrous hermaphrodites, with juveniles becoming sexually mature as males and changing to females as they age. The transformation is controlled by their status in the social hierarchy. When the breeding female dies, the breeding male, which is invariably much smaller, grows rapidly and changes gender to replace her, with one of the older juveniles maturing as a new male.
Anemonefish breed by laying a batch of eggs on a smooth surface close to their host anemone, which is carefully guarded and kept clean by the male until they hatch 6 to 10 days later. Hatching takes place after sunset and the larvae immediately disperse into the plankton. They generally do not have a long larval period, as they are quite large on hatching compared to the related damselfish for example. As a result, many species have comparatively small ranges compared to other marine fish whose larvae travel long distances before settling on a reef.
Various anemonefish species are among the commonest fish in the marine aquarium trade, but several have very limited range or are only found in protected areas and are seldom seen in captivity. Trade in anemonefish is now complicated by the successful commercial production of several species, some of which are now produced in domesticated varieties in the same way that many freshwater aquarium fish are. If at all possible, any hobbyist should only buy captive raised specimens as these invariably do better in a home aquarium than a wild caught individual. If it is wished to show the mutualism of the anemonefish and their host, it is important to remember that many sea anemones do very badly in captivity and are harder to care for than the fish. Entacmea or Bubble-Tip anemones seem to do best and make suitable hosts for most commonly traded species.
A.percula - normal and black variant
Images from wikipedia

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