Habitat: Habitats could range from tropical coral reefs, kelp forests to deep-sea floor, although none of them live within the water column all species of sea stars found are living as benthos. Asterias is a common genus found in European waters and on the eastern coast of the United States Pisaster, along with Dermasterias ("leather star"), are usually found on the western coast. The greatest variety of sea stars is found in the northern Pacific Ocean. Name: As these creatures are echinoderms and not actually fish, most marine biologists prefer to replace the term starfish with the less misleading term sea star.ĭistribution: There are about 1,800 living species of sea star, and they occur in all of the Earth's oceans. The water vascular system has many projections called tube feet, located on the ventral face of the sea star's arms, which function in locomotion and aid with feeding. Skeleton: Sea stars do not have movable skeletons, but instead possess a hydraulic water vascular system. In fact, their evolutionary ancestors are believed to have had bilateral symmetry, and sea stars do exhibit some superficial remnant of this body structure. Starfish typically have five or more "arms" which radiate from an indistinct disk (pentaradial symmetry). They exhibit a superficially radial symmetry. The names sea star and starfish are also used for the closely related brittle stars, which make up the class Ophiuroidea. Gametes (sperms or eggs) are discharged through the gonophores which open on the aboral surface (Boolootian 1966 Ruppert, Fox & Barnes 2004).Sea stars or starfish are marine invertebrates belonging to Kingdom Animalia and phylum Echinodermata, class Asteroidea. Two gonads are situated on either side of the ambulacra within each arm and each gonad is connected to its own gonophore (Boolootian 1966 Lawrence 1987). In other words, they have separate male and female individuals (Yamaguchi 1977). The blue sea stars, which belong to the genus Linckia, have remarkable regenerative ability though have not yet shown to undergo autotomy (Williams 2000). Species of the genus Linckia is known to be capable of casting off part of their body when under threat/stress (i.e. Linckia laevigata is able to reproduce both asexually and sexually. A simplified version of the digestive system, image adapted from 'Invertebrate zoology : a functional evolutionary approach' by Ruppert, Fox & Barnes 2004 In addition, the madreporite that is located on the aboral surface of a sea star is also joined with the water vascular system via the stone canal (Ruppert, Fox & Barnes 2004).įigure 3). The presence of the valves is for the buildup of the hydrostatic pressure that is required for the movement of the tube feet (Nichols 1967). Each tube foot has a bulging structure called the ampulla and a valve is present between the tube foot and the radial canal (Nichols 1967 Ruppert, Fox & Barnes 2004). The tube feet are joined with the radial canal through the lateral canal. The ring canal is then connected to the five radial canals that extend into each arm. As seen in the figure below, a circular ring canal is located inside the central disc of the sea star. In particular, the water vascular system of a sea star consists of the tube feet and a series of internal coelomic fluid-filled vessels that runs through the central disc and the arms (Ruppert, Fox & Barnes 2004). Water vascular system (WVS) is a highly derived coelomic cavity (i.e.
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