''Asterina gibbosa'', commonly known as the starlet cushion star, is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...

in the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

Asterinidae The Asterinidae are a large family of sea stars in the order Valvatida. Description and characteristics These are generally small sea stars, flattened dorsally and bearing very short arms, often giving a pentagonal shape in the body ;example: ...

. It is native to the northeastern

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

and the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ea ...

Description

''Asterina gibbosa'' is a pentagonal starfish with short blunt arms and an inflated appearance. The aboral (upper) surface is clothed in groups of short, blunt spines. This starfish grows to a diameter of about and may be brown, green or orange. It is sometimes blotched with colour and individuals from deeper sea locations tend to be paler in colour. It can be distinguished from the closely related ''

Asterina phylactica ''Asterina phylactica'' is a species of sea star. It can be found in geographically widespread sites around the British Isles and in the Mediterranean Sea. It has five arms, is about 1.5 cm across and is of a green colour with central brown ...

'' by the fact that it has two small spines on each of the plates surrounding its mouth, ''A. phylactica'' having no spines on these plates and having a plain olive-green aboral surface with a brown central star. ''A. gibbosa'', at is considerably bigger than ''A. phylactica'' which seldom exceeds . At one time the two were believed to be the same species but it was realised in 1979 that besides the differences in appearance, the two occupied different ecological niches and had different reproductive strategies.

Biology

''Asterina gibbosa'' is mainly nocturnal, spending the day underneath boulders, overhangs or in caves, or hidden away under algae in rock pools. It is an opportunistic scavenger but the bulk of its diet comes from the film of bacteria and

diatom A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

s that exist on the surface of rocks. It feeds by everting its stomach (turning it inside out) against the surface of the rock and secreting enzymes which digest the film. Other foods found in its stomach included decaying toothed wrack (''

Fucus serratus ''Fucus serratus'' is a seaweed of the north Atlantic Ocean, known as toothed wrack or serrated wrack. Description and reproduction ''Fucus serratus'' is a robust alga, olive-brown in colour and similar to ''Fucus vesiculosus'' and ''Fucus spir ...

''), periwinkle faeces and bits of dead molluscs such as mussels (''

Mytilus edulis The blue mussel (''Mytilus edulis''), also known as the common mussel, is a medium-sized edible marine (ocean), marine bivalve mollusc in the family (biology), family Mytilidae, the mussels. Blue mussels are subject to commercial use and intensiv ...

''), oysters (''

Ostrea edulis ''Ostrea edulis'', commonly known as the European flat oyster, is a species of oyster native to Europe. In Britain and Ireland, regional names include Colchester native oyster, mud oyster, or edible oyster. In France, ''Ostrea edulis'' are known ...

'') and periwinkles (''

Littorina littorea The common periwinkle or winkle (''Littorina littorea'') is a species of small edible whelk or sea snail, a marine gastropod mollusc that has gills and an operculum, and is classified within the family Littorinidae, the periwinkles.Reid, David ...

''), but 95% of the stomachs contained no large particles indicating the importance in its diet of microscopic organisms. ''Asterina gibbosa'' is a

protandric hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have sepa ...

. This means that it is born a male and later changes sex and becomes a female. Researchers found that at

Plymouth Plymouth () is a port city and unitary authority in South West England. It is located on the south coast of Devon, approximately south-west of Exeter and south-west of London. It is bordered by Cornwall to the west and south-west. Plymouth ...

, United Kingdom, the sex change happens when it has an arm length somewhere between . Other researchers at

Naples Naples (; it, Napoli ; nap, Napule ), from grc, Νεάπολις, Neápolis, lit=new city. is the regional capital of Campania and the third-largest city of Italy, after Rome and Milan, with a population of 909,048 within the city's adminis ...

, Italy found that Mediterranean populations did not have such a clearcut change of sex. In the first year cohort, with arms averaging , about 80% were male. In the second year batch, with arms averaging , about 30% were male. By the third year, with arms, males were very scarce, but in even larger individuals of unknown age, about 15% were male. ''Asterina gibbosa'' may live for six years or more. The eggs of ''Asterina gibbosa'' are laid in a mass and glued to the substrate by their jelly-like coating. Each developing embryo feeds on its egg yolk and hatches directly into a brachiolaria larva, without the intervening mobile

plankton Plankton are the diverse collection of organisms found in Hydrosphere, water (or atmosphere, air) that are unable to propel themselves against a Ocean current, current (or wind). The individual organisms constituting plankton are called plankt ...

ic phase of most starfish larvae. The larva has a pair of asymmetric arms which gradually lengthen, and an adhesive disc with which it cements itself to the seabed. It then undergoes

metamorphosis Metamorphosis is a biological process by which an animal physically develops including birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Some inse ...

, its arms are reabsorbed and

tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on britt ...

develop and take on the role of anchoring it to the seabed. Later the mouth of the juvenile starfish develops and it is able to start feeding.

Distribution and habitat

''Asterina gibbosa'' is found in the northeastern Atlantic Ocean and the Mediterranean Sea. Its range extends from

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

, Scotland, Ireland and the southern

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

southwards to the Mediterranean Sea, Morocco, Tunisia, the Azores, Cape Verde Islands, the Canary Islands and Madeira. It is found from the lower shore down to a depth of about in pools, on rocks, under boulders and overhangs.

References

{{Taxonbar, from=Q2513886 gibbosa Fauna of the Atlantic Ocean Fauna of the Mediterranean Sea Starfish described in 1777 Taxa named by Thomas Pennant