Brachiolaria
Encyclopedia
A brachiolaria is the second stage of larva
l development in many sea star
s; it follows the bipinnaria
. Brachiolaria have bilateral symmetry, unlike the adult sea stars, 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.
The brachiolaria develops from the bipinnaria larva when the latter grows three short arms at the underside of its anterior end. These arms each bear sticky cells at the tip, and they surround an adhesive sucker. The larva soon sinks to the bottom, attaching itself to the substrate, firstly with the tips of the arms, and then with the sucker. Once attached, it begins to metamorphose
into the adult form.
The adult starfish develops only from the hind-part of the larva, away from the sucker. It is from this part that the arms of the adult grow, with the larval arms eventually degenerating and disappearing. The digestive system of the larva also degenerates, and is almost entirely rebuilt. A new mouth forming on the left side of the body, which eventually becomes the lower, or oral, surface of the adult. Similarly, a new anus forms on the right side, which becomes the upper, or aboral, surface.
The coelom
, or body cavity is divided into three chambers in the larva, two of which form the water vascular system
, while the other remains as the adult body cavity. Once the tube feet
develop from the water vascular system, the larva frees itself from the bottom. At around the same time, the skeleton begins to develop, initially in a ring around the anus; at this point the larva has developed into an adult, although it will continue to grow for some years before reaching sexual maturity.
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
l development in many sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...
s; it follows the bipinnaria
Bipinnaria
A bipinnaria is the first stage in the larval development of most starfish, and is usually followed by a brachiolaria stage. Movement and feeding is accomplished by the bands of cilia. Starfish that brood their young generally lack a bipinnaria stage, with the eggs developing directly into...
. Brachiolaria have bilateral symmetry, unlike the adult sea stars, which have a pentaradial symmetry. Starfish of the order paxillosida
Paxillosida
Paxillosida are a large order of sea stars.-Characteristics:Paxillosida adults lack an anus and have no suckers on their tube feet. They do not develop the brachiolaria stage in their early development. They possess marginal plates, and have sessile pedicellariae...
(Astropecten
Astropecten
Astropecten is a genus of sea star of the family Astropectinidae.-Species:* Astropecten alligator Perrier, 1881* Astropecten americanus * Astropecten antillensis Lütken, 1860 * Astropecten armatus Gray, 1840...
and Asterina) have no brachiolaria stage, with the bipinnaria developing directly into an adult.
The brachiolaria develops from the bipinnaria larva when the latter grows three short arms at the underside of its anterior end. These arms each bear sticky cells at the tip, and they surround an adhesive sucker. The larva soon sinks to the bottom, attaching itself to the substrate, firstly with the tips of the arms, and then with the sucker. Once attached, it begins to metamorphose
Metamorphosis
Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation...
into the adult form.
The adult starfish develops only from the hind-part of the larva, away from the sucker. It is from this part that the arms of the adult grow, with the larval arms eventually degenerating and disappearing. The digestive system of the larva also degenerates, and is almost entirely rebuilt. A new mouth forming on the left side of the body, which eventually becomes the lower, or oral, surface of the adult. Similarly, a new anus forms on the right side, which becomes the upper, or aboral, surface.
The coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
, or body cavity is divided into three chambers in the larva, two of which form the water vascular system
Water vascular system
The water vascular system is a hydraulic system used by echinoderms, such as sea stars and sea urchins, for locomotion, food and waste transportation, and respiration. The system is composed of canals connecting numerous tube feet...
, while the other remains as the adult body cavity. Once the tube feet
Tube feet
Tube feet are the many small tubular projections found most famously on the oral face of a sea star's arms, but are characteristic of the water vascular system of the echinoderm phylum which also includes sea urchins, sand dollars and sea cucumbers and many other sea creatures.Tube feet function in...
develop from the water vascular system, the larva frees itself from the bottom. At around the same time, the skeleton begins to develop, initially in a ring around the anus; at this point the larva has developed into an adult, although it will continue to grow for some years before reaching sexual maturity.