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The common starfish, common sea star or sugar starfish ( Asterias rubens) is the most common and familiar starfish in the north-east Atlantic. Belonging to the family Asteriidae, it has five arms and usually grows to between 10–30 cm across, although larger specimens (up to 52 cm across) are known. The common starfish is usually orange or brownish in color, and sometimes violet; specimens found in deeper waters are pale. The common starfish is found on rocky and gravelly substrates where it feeds on mollusks and other Benthos invertebrates.
The common starfish is dioecious, which means that each individual is either male or female. In the spring, the females release their eggs into the sea. A moderate sized starfish is estimated to be able to produce 2.5 million eggs. The males shed their sperm and fertilisation takes place in the water column. The larvae are planktonic and drift for about 87 days before settling on the seabed and undergoing metamorphosis into juveniles. Common starfish are believed to live for about seven to eight years. When well fed, the juveniles can increase their radius at the rate of slightly more than per month during the summer and autumn and slightly less than per month in the winter. An adult common starfish can survive starvation for several months although it loses weight in the process. One specimen shrank from a radius of to a radius of after starvation for five months.
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 , 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 and the winter host may be Leptasterias spp.. The ciliate has been found in the testes of all these species. When inside the gonad, it Phagocytosis the sperm thus rendering the starfish infertile. Researchers have found a change in the sex ratios of affected populations with fewer males than females being present with the males being consistently smaller than the females.
The common starfish produces a saponin-like substance designed to repel predators, which causes a reaction in the common whelk ( Buccinum undatum), a common prey species. At dilute concentrations it caused the whelk to take evasive action and at higher concentrations it produced a series of convulsions.
During Storm Frank in the early hours of 31 December 2015 hundreds of starfish were stranded along the coast of Portsmouth, Hampshire.
This is not a unique phenomenon and other mass strandings have occurred in Britain and elsewhere at such places as near Sandwich in Kent in 2008, and near Brighton ten days later. A similar occurrence occurred on the shore of the White Sea where vast numbers of starfish came ashore on a nine-mile stretch of beach in 2004. It was said that people could not "walk around them because the whole shore was full of starfish". Russian experts expressed mystification as to the cause of the stranding.
Mass strandings
In January 2013, large numbers of common starfish were washed up near Cleethorpes Pier on the east coast of England along with many . There were estimated to be four thousand starfish along the stretch of shore. The cause of this mass stranding was unknown but bad weather and storms out at sea coupled with higher than usual tides may have been to blame.
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