Summer is the perfect season of the year to go and enjoy the sea and you probably are one of these people who practise snorkel or who dive. In this case, I want to explain which are the main features of starfishes, with which animals can be confused and some examples of the Mediterranean sea.
Starfishes (Asteroidea) are included in the phylum of Echinoderms, together with sea urchins (Echinoidea), sea cucumbers (Holothuroidea), crinoids (Crinoidea) and brittle stars or ophiuroids (Ophiuroidea).
Echinoderms are all marine animals, which present the following main features:
- Body with pentamerous radial symmetry in the adult phase, but with bilateral symmetry in the larval phase (there is just one symmetry axis).
- They are all mobile, except some sessile species (affixed on the sea-floor) of crinoids.
- Endoskeleton consisting of ossicles.
- Water vascular system: system projected from the body wall with some expansions similar to tentacles called podium, which can be spread out due to de liquid pressure. Normally, they present an opening called madreporite.
It is important to remember that they are marine animals and, if you want to observe them, you must not extract them from the water because they begin to die in just 10 seconds.
There is about 1,500 species of starfishes, which are all included in the Asteroidea class. Starfishes live in sandy, muddy, rocky and coral reefs seafloors, depending on the species. They can measure from some centimetres to one metre.
Externally, starfishes have a central disc from which the arms are originated. From the mouth, which is placed in the lower part (or oral part), and throughout the arms there is the ambulacral ridge, from which the podiums are originated. The upper part (or aboral part) is usually coarse and with spines. In the base of this spines there is structures called pedicellarie, which function is to remove the particles that remove debris from the body surface and in some cases are used to capture small fishes. Gas exchange takes place through papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Anus and madreporite are placed in the aboral surface.
An important feature of echinoderms is the water vascular system. In the case of starfishes, it plays an important role in locomotion, in food capture, in excretion and in breathing.
Many starfishes are carnivorous and feed on molluscs, crustaceans, worms, echinoderms and other invertebrates, sometime on small fishes too. Some starfishes can feed on small particles of plankton or other organic particles.
WATCH OUT! Starfishes can be confused with ophiuroids, but ophiuroids have thinner and more mobile arms than starfishes, in addition to the absence of anus and the fact that they do not use podium to get around, so they move the arms.
SOME EXAMPLES OF THE MEDITERRANEAN SEA
This group includes 6 Mediterranean species, which live in sandy and muddy seafloors and with 5 arms covered by scales and spikes. The most common is the red comb-star (Astropecten aranciacus), which present two lines of big and sharp spines, and with a red – orange colouration.
BLUE SPINY STARFISH (Coscinasterias tenuispina)
This starfish measures between 7 and 10 cm and usually present between 7 and 9 arms of different sizes, which are covered by small spines. It is bluish white and brown. The most common reproduction system of this species is fission, and for this reason a large section of the coast can be occupied by the same genetic individual. The blue spiny starfish usually lives under the stones.
SPINY-STARFISH (Marthasterias glacialis)
The spiny-starfish, which can measure 80 cm of diameter, always presents 5 arms, which are covered by hard spikes. Its colouration is greenish or brownish, with light spots when they live deeper. It can live in rocky or sandy seafloors, until 180 metres deep.
RED STARFISH (Echinaster sepositus)
Red starfish, which can measure 30 cm, has a red to orange body, with 5 long and cylindrical arms, covered by small spines. It can be found utill 1000 metres deep, always in rocky bottoms.
HACELIA (Hacelia attenuata)
This starfish, with also 5 cylindrical arms, has a red to orange body. It can be confused with the red starfish, but this has the papulae in longitudinal lines, while in Echinaster their distribution is irregular.
STARLET (Asterina gibbosa)
The arms of this little star (from 2 to 4 cm) are not many differentiated from the central disc. It can present different colours, from greenish grey to red. It can be observed on rocks, sand or behind sea-grass.
- Club d’Immersió de Biologia: Estrelles de mar
- Corbera & Muñoz-Ramos (1991). Els invertebrats litorals dels Països Catalans. Ed. Pòrtic
- Hickman, Roberts, Larson, l’Anson & Eisenhhour (2006). Principios integrales de Zoología. Ed. McGraw Hill (13 ed)
- Martin (1999). Claves para la clasificación de la fauna marina. Ed. Omega
- Riedl (1986). Fauna y Flora del Mar Mediterráneo. Ed. Omega
- Foto de portada: Coscinasterias tenuispina (Foto: F Cardigos ImagDOP)