De Blainville, 1830
The adult body of Asteroidea (sea stars) is stellate, typically pentamerous, but many asteroids have more than five arms. The arms are not set off from the central disc by distinct articulations. as in the brittle stars [Class Ophiuroidea]. Adult sea stars are benthic.
The earliest larval stage of Asteroidea is called an auricularia, followed by bipinnaria and brachiolaria stages.
Larval development is illustrated with Asterias rubens, a common sea star in the North Sea. After fertilisation, the asteroid embryo becomes free swimming at some point between the blastula and gastrula stages. At first, the entire larval surface is beset with cilia, but during development the ciliation becomes confined to definitive ciliary bands, the serve for locomotion. The first stage with such peripherical ciliary bands is the typically slab-sided auricularia larva (then the larva is some six days old). In the next stage, projections called arms develop from the body surface; the ciliary locomotor bands extend along the arms. At this stage, the larva is called a bipinnaria [Asteroidea bipinnaria-1 ]; this stage may be attained after several weeks, the length of the larva is then about 1.5-1.6 mm. [Asteroidea bipinnaria-2 ]. After 30 days the bipinnaria larvae transforms into the brachiolaria larval form, characterised by the brachiolaria complex of three elaborate arms tipped with adhesive papillae and an adhesive disc between the bases of the three arms [Asteroidea brachiolaria]. The length of the larva is then about 2.5 mm. Larvae of Asterias rubens are planktotrophic prior to metamorphosis.
When the rudimentary juvenile (juv.) is completely formed at the posterior end of the body, and the brachiolar arms are fully developed (at the anterior side of the body), the larvae is ready to adhere to a suitable substratum with help of the brachiolar arms and adhesive disc, and to undergo metamorphosis. The final external stage in metamorphosis is the resorption of the brachiolaria complex after which the young starfish (the rudiment) has all the radially symmetrical adult structures; the juvenile breaks from the larval body that is lost, and starts a free-living benthic life. The typical body size of the juvenile after metamorphosis is 0.75-0.85 mm, considerably smaller than the last brachiolar stage that can be some 4.5 mm in length.
Spawning of Asterias rubens can begin as early as in March and the bipinnaria larvae are most abundant in early April, followed by a peak of the subsequent brachiolaria larvae in mid-June (Barker and Nichols, 1983).
The following asteroid species are included: