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Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (/ ˌ æ s t ə ˈ r ɔɪ d i ə /). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish are also known as asteroids due to being in the class
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 ciliate has been found in the testes of all these species. When inside the gonad, it phagocytoses the sperm thus rendering the starfish infertile. Researchers have found skewed sex ratios of affected populations with fewer males than females being found, and males being consistently smaller than females.
The starfish body plan consists of five to six arms radiating from a central disk. Regenerative ability differs greatly among starfish species, but can generally be classified within three categories: unidirectional regeneration, disk-dependent bidirectional regeneration, and disk-independent bidirectional regeneration.
As the adductor muscle of the bivalve relaxes, more stomach is inserted and when digestion is complete, the stomach is returned to its usual position in the starfish with its now liquefied bivalve meal inside it. Other starfish evert the stomach to feed on sponges, sea anemones, corals, detritus and algal films. [95]
Culcita novaeguineae (common name, cushion star) is a species of starfish. It has short arms and an inflated appearance and resembles a pentagonal pincushion . It is variable in colour and can be found in tropical warm waters in the Indo-Pacific .
Pisaster ochraceus, generally known as the purple sea star, ochre sea star, or ochre starfish, is a common seastar found among the waters of the Pacific Ocean. Identified as a keystone species , P. ochraceus is considered an important indicator for the health of the intertidal zone .
Sea urchins move by walking, using their many flexible tube feet in a way similar to that of starfish; regular sea urchins do not have any favourite walking direction. [13] The tube feet protrude through pairs of pores in the test, and are operated by a water vascular system ; this works through hydraulic pressure , allowing the sea urchin to ...