The bilaterally symmetrical larva, called an echinopluteus, is planktonic and may not feed, relying on its yolk for nourishment before descending to the seabed and undergoing metamorphosis.
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Most deepwater echinoderms feed on algal debris and detritus. Examination of the gut contents of this species also showed small shells and the skeletons of hydroids. The eggs are small, which implies this species has echinopluteus larvae which are planktonic.
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The first larval stage is called a prism. After this stage the embryo will develop two arms transforming itself into an echinopluteus larva. This is followed by the development of arms, until it reaches 8 arms all together. After this the larva develops an echinus or juvenile rudiment, which will become the juvenile.
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London: MacMillan & Co. It includes a blastula, gastrula and a four armed echinopluteus stage that forms an important part of the zooplankton.MacBride, E.W., 1903. The development of Echinus esculentus together with some points on the development of E. miliaris and E. acutus. Philosophical Transactions of the Royal Society of London, Series B, 195, 285-327.
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Echinometra mathaei uses its spines and teeth (part of the mouthparts known as Aristotle's lantern) to dig itself into the basaltic and calcareous rock where it lives. It emerges from these hollows at night to graze on algae. As with other sea urchins, breeding involves releasing gametes into the water column. Fertilisation is external and the echinopluteus larvae are planktonic.
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The spawning period varies along the Chilean coast. Spawning period occurs later in the year as latitude increases; at 23° S it occurs in June, at 45° S (Chiloé) it occurs in November–December. The populations inhabiting the Magellan Region ( 53° S) are an exception since their spawning period lasts from July to September. The echinopluteus larvae form part of the zooplankton for about thirty days, feeding on phytoplankton.
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Pluteus larva has bilateral symmetry. In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The unfertilized egg meets with the free-floating sperm released by males, and develops into a free-swimming blastula embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva.
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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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It does not seem to be correlated with the phases of the moon as in some other sea urchin species. Fertilisation is external and the echinopluteus larvae are planktonic. When these settle, they undergo a rapid metamorphosis into juvenile sea urchins. Researchers have studied the likely effect on the reef urchin of a rise in the carbon dioxide levels which are likely to increase ocean acidification by the end of the 21st century.
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An echinopluteus larva with larval arms The development of an echinoderm begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. Gastrulation marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host the skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are mostly planktonic but in some species the eggs are retained inside the female and in some, the larvae are also brooded by the female.
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The eggs clump together and may adhere in a viscous mass to the adult's spines and be brooded there. The sperm also form a mucous mass, and it may be that a form of pseudo-copulation takes place with the pressing together of the gamete masses of adjacent individuals. In the laboratory, embryos developed over a period of 5 months into echinopluteus larvae measuring over 3000μm which were fed on unicellular algae. During their early stages the larvae developed 2 and then 4 larval arms before the mouth developed at about 11 days.
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As is the case with most other sea urchins, the sexes are separate in this species, and adults liberate eggs and sperm into the water column. After fertilisation, the echinopluteus larvae spend several months swimming and drifting with the plankton. Metamorphosis is stimulated by the detection by the larvae of suitable locations to settle on the seabed, cues being provided by the presence on the substrate of films of micro-algae or of microbes. In the absence of a suitable habitat, the larvae can continue swimming, but metamorphosis will eventually happen even in the absence of these cues.
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The species is herbivorous, grazing on marine algae. It mostly feeds on the red leafy alga Acrosorium polyneurum between December and March at the times of year when it is abundant, and migrating to areas of crustose coralline algae at other times of year when the red alga is scarce. As is the case with other sea urchins, H. pulcherrus liberates eggs and sperm into the water column and the echinopluteus larvae spend several months drifting with the plankton. They are stimulated to settle on the seabed by the attachment of diatoms and by the presence in the water of particles of an alga such as Hizikia fusiformis which grows around the coasts of Japan.
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