The starfish or sea stars are animals from the phylum Echinodermata (phylum to which also belong animals like the sea urchin and the sea cucumber) and, within this, from the class Asteroidea. They are invertebrate animals that live exclusively in the marine environment and have radial symmetry. They can be found all over the world, mainly in coastal areas and occupying different substrates. They may occur in a variety of sizes and with different colours.
Its name is likely to have arisen because of its shape (which resembles a star) – a central disc from which arms radiate. Most species have five arms, but some species have a higher number (usually a multiple of 5), up to 50 arms (as in starfish-sun: Pycnopodia helianthoides)
Locomotion and feeding
In the lower part of the arms, arranged in rows, we can find the tube feet – short tubular structures, usually with a suction cup at the end. The starfish move by using these tube feet. They use the suction cups to attach to the substrate (such as rocks) and to capture food – grabbing, with the suction cups, for example a bivalve (such as mussels), the starfish can apply enough force to open its shell. The starfish releases his stomach to the outside (eversion of stomach) and releases digestive enzymes on the prey.
Reproduction
In the starfish, reproduction can be sexual or asexual.
In the case of sexual reproduction, gametes are released into the water and the fertilization is external (it happens outside the body). An egg is formed and then usually gives way to a larval form which then undergoes a metamorphosis that originates a body similar to an adult starfish.
Regarding the asexual reproduction, two processes may occur in starfish: fission or fragmentation. If a starfish’s arm (together with part of the central disc) detaches from the rest of the body, it has a capacity for regeneration which allows the formation of a new starfish. The “mother starfish” can also regenerate the missing arm. This fragmentation can happen voluntarily or accidentally.
Currently, some species of starfish are threatened with extinction due to over harvesting of these animals, which are traded for decorative use.
References
– CORGOSINHO, P.; BORGES, M.; NEVES, F. (2010). Zoologia de Invertebrados. Montes Claros:Editora Unimontes
– Jesus, D. As Estrelas-do-mar. Available: http://naturlink.sapo.pt/NaturSAPO/Biodiversidade/Artigos/content/As-Estrelas-do-mar?bl=1&viewall=true.
