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 miniature adults Bipinnaria larva The bipinnaria is free-living, swimming as part of the zooplankton. When it initially forms, the entire body is covered by cilia, but as it grows, these become confined to a narrow band forming a number of loops over the body surface.
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Older brachiolaria larva of Asterias sp. from below, anterior end at top From Ernst Haeckel's Kunstformen der Natur (1904) 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.
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In the north of its range, breeding takes place from April to June. Large numbers of small eggs are produced and fertilisation is external. The bipinnaria larvae that develop from these form part of the zooplankton and disperse with the currents.
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Bat star bipinnaria at fourth day of development Photographed using confocal microscopy. Histones are labeled in blue, actin filaments in orange. The bat stars reproduce through spawning. The male casts sperm and the female drops eggs; each has pores at the base of the rays for this purpose.
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Little is known of the reproduction of Labidiaster annulatus, but the larvae pass through at least one bipinnaria and one brachiolaria stage as has been demonstrated by DNA analysis. The larvae are planktonic and spend many months drifting with the currents before settling on the seabed and undergoing metamorphosis into juveniles.
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The starfish in any area may synchronise their emission of gametes which will increase the chance of fertilization taking place. The eggs hatch into bipinnaria larvae which form part of the zooplankton and develop over a period of about three weeks before settling on the seabed and undergoing metamorphosis into juvenile starfish.
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A tornaria larva A tornaria is the planktonic larva of some species of Hemichordata such as the acorn worms. It is very similar in appearance to the bipinnaria larvae of starfishes, with convoluted bands of cilia running around the body. It is an oval shaped, transparent larva. The diameter of the body is about 3 mm.
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Among them fall the Bipinnaria and the Brachiolaria of the sea-stars, the Auricularia of the sea-rollers and the Plutei of the sea-hedgehogs, and the sand stars. Also the Doliolaria of the Pelmatozoa (sea-lilies and feather stars) can be attributed to the same basic pattern, even if they do not have a mouth opening.
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Both females and males liberate gametes into the sea where fertilisation takes place. The bipinnaria larvae that hatch from the eggs are planktonic. They are bilaterally symmetrical and have a pair of body processes, a gut and two bands of cilia which are used for swimming. They feed and grow but do not pass through a brachiolaria stage as do most starfish larvae.
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It is thought that groups of stars use environmental signals to coordinate spawning to increase the chances of fertilization. Once fertilization has occurred, the zygote develops into a planktonic larva which feeds on small algae. The larva proceeds through several developmental phases. It becomes a gastrula in 2 to 3 days, a bipinnaria in 5 days, and finally a brachiolaria.
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S. australis, like the majority of other starfish, exhibits two life stages, planktonic and benthic. Once the eggs of the invertebrate are fertilized, bilaterally symmetrical planktonic larvae are formed. At this stage, these larvae are known as bipinnaria. The planktotrophic larval stage for S. australis will last about 6 months, with larvae typically settling any time from May through July.
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The sea stars liberate their gametes into the sea where fertilisation takes place. The eggs hatch into bipinnaria larva which drift with the plankton. In about 25 days they have grown considerably and settle on the sea bed before undergoing metamorphosis into juvenile sea stars. In polluted waters, the nine-armed sea star has been found to concentrate heavy metals into its tissues.
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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.
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The affinities of Balanoglossus with echinoderms were first suggested by Metschinkoff (1865). There are no similarities between adults of echinoderms and Balanoglossus except in the presence of intraepidermal nervous system. However, the striking resemblance between the larval forms of the two groups misled Muller to regard tornaria larva as a larva of starfish. The similarities between tornaria larva and bipinnaria and other larval forms of echinoderms are as follows: 1) They are pelagic and transparent.
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The larvae of echinoderms pass through a number of stages and these have specific names derived from the taxonomic names of the adults or from their appearance. For example, a sea urchin has an 'echinopluteus' larva while a brittle star has an 'ophiopluteus' larva. A starfish has a 'bipinnaria' larva but this later develops into a multi-armed 'brachiolaria' larva. A sea cucumber larva is an 'auricularia' while a crinoid one is a 'vitellaria'.
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Echinaster are able to start reproduction by broadcast spawning gametes into the water, where externally fertilized eggs develop into planktonic larvae. Most species only produce brachiolaria larvae which are lecithotropic, non-feeding larvae. However, some Echinaster produce larvae which first go through a smaller, feeding bipinnaria stage, and then through the second brachiolaria larval stage. Their eggs vary in size depending on parental investment, the amount of nutrients found in eggs, and the surrounding habitat.
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A pair of short, stubby arms soon develop on the body, with the ciliated bands extending into them. In addition to propelling the larva through the water, the cilia also catch suspended food particles, and deliver them to the mouth (more correctly called a stomodeum). Eventually, three additional arms develop at the front end of the larva; at this point it becomes a brachiolaria. In some species, including the common starfish Asterias, the bipinnaria develops directly into an adult.
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The initiation of oogenesis occurs from August to February and the eggs take about 18 months to mature. Spawning takes place between May and September and may be linked to seasonal changes in light levels, sunrise taking place in McMurdo Sound in August. The larval development is also slow with the first, bipinnaria, stage lasting 2 months. The larvae remain near the seabed during this time but become pelagic for up to 6 months as brachiolaria larvae which allows them to disperse widely.
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In all other species, the egg develops into a free-swimming larva, typically after around three days of development. The first stage of larval development is known as an auricularia, and is only around in length. This larva swims by means of a long band of cilia wrapped around its body, and somewhat resembles the bipinnaria larva of starfish. As the larva grows it transforms into the doliolaria, with a barrel- shaped body and three to five separate rings of cilia.
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The chunky body shape makes them easy to handle and wounds heal well; individuals recover easily from surgery and can be used repeatedly for further experiments. The oocytes are large and transparent and remain viable when removed from the gonads. They have been used for studies of oocyte maturation, fertilization, and larval development. It has been shown that if the embryo of this starfish is dissociated into its constituent cells, a collection of these cells is capable of re-aggregation into a viable bipinnaria larva.
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In some species, these develop directly into adults, but in others, there is a free-swimming intermediate stage referred to as a tornaria larva. These are very similar in appearance to the bipinnaria larvae of starfishes, with convoluted bands of cilia running around the body. Since the embryonic development of the blastula within the egg is also very similar to that of echinoderms, this suggests a close phylogenetic link between the two groups. After a number of days or weeks, a groove begins to form around the larval midsection, with the anterior portion eventually destined to become the proboscis, while the remainder forms the collar and trunk.
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