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A brachiolaria is the second stage of larval development in many starfishes. It follows the bipinnaria. Brachiolaria have bilateral symmetry, unlike the adult starfish, which have a pentaradial symmetry. Starfish of the order Paxillosida (Astropecten and Asterina) have no brachiolaria stage, with the bipinnaria developing directly into an adult.
Starfish sometimes have negative effects on ecosystems. Outbreaks of crown-of-thorns starfish have caused damage to coral reefs in Northeast Australia and French Polynesia. [59] [70] A study in Polynesia found that coral cover declined drastically with the arrival of migratory starfish in 2006, dropping from 50% to under 5% in three years.
The Asterozoa are a subphylum in the phylum Echinodermata, within the Eleutherozoa.Characteristics include a star-shaped body and radially divergent axes of symmetry. The subphylum includes the classes Asteroidea (the starfish or sea stars), Ophiuroidea (the brittle stars and basket stars), Somasteroidea (early asterozoans from which the other classes most likely evolved), and Stenuroidea ...
In being multiple-armed, it has lost the five-fold symmetry (pentamerism) typical of starfish, although it begins its lifecycle with this symmetry. The animal has true image-forming vision. [3] Adult crown-of-thorns starfish normally range in size from 25 to 35 cm (10 to 14 in). [4] They have up to 21 arms.
Internal features can also show symmetry, for example the tubes in the human body (responsible for transporting gases, nutrients, and waste products) which are cylindrical and have several planes of symmetry. Biological symmetry can be thought of as a balanced distribution of duplicate body parts or shapes within the body of an organism.
Of all echinoderms, the Ophiuroidea may have the strongest tendency toward five-segment radial (pentaradial) symmetry. The body outline is similar to that of starfish, in that ophiuroids have five arms joined to a central body disk. However, in ophiuroids, the central body disk is sharply marked off from the arms.
These starfish have between 6 and 16 long, attenuated arms which they use for suspension feeding. [3] Other characteristics include a single series of marginals, a fused ring of disc plates, the lack of actinal plates, a spool-like ambulacral column, reduced abactinal plates, and crossed pedicellariae . [ 4 ]
The genus contains a total of eight species in all. All species have five arms and are native to shallow oceanic areas (the littoral zone) of cold to temperate parts of the Holarctic. These starfish have planktonic larvae. Asterias amurensis is an invasive species in Australia and can in some years become a pest in the Japanese mariculture ...