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Asterias Rubens Video Stock Footage
The common starfish normally has five arms, broad at their base and gradually tapering to a point at their tips, which are often turned up slightly. There is a line of short white spines running along the centre of the aboral (upper) surface of the arms with low, soft mounds called papulae on either side. The oral (lower) surfaces of the arms have rows of small tube feet, used in locomotion and feeding. The starfish is usually orange or brick red on the aboral surface and paler on the oral surface but can also be purple or pale brown. Individuals from deep water are usually paler. It grows to a maximum diameter of about 52 centimetres (20 in) but a more normal size is 10 to 30 cm (4 to 12 in). The ciliate protozoan Orchitophrya stellarum is sometimes a parasite of the common starfish. It normally lives on the outer surface of the starfish feeding on sloughed-off epidermal tissue. It appears to become parasitic when the host starfish has ripe gonads and is a male. It enters the starfish through the gonopores, the orifices where gametes are released. There may be a pheromone that alerts it to the fact that the testes are ripe and causes it to change its behaviour. As different species of starfish breed at different times of year, Orchitophrya stellarum may move from one species to another in accordance with their reproductive cycles. In the Atlantic Ocean, it may alternate between parasitising Asterias forbesi and Asterias rubens during the spring and summer...
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