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Echinoderm
http://e-echinoderm.blogspot.com/
Avoiding predation
Despite their low nutrition value and the abundance of indigestable calcite, many organisms, such as Crabs, sharks, sea birds and larger starfish, make a living by feeding on echinoderms. Defensive strategies employed include the presence of spines, toxins, which can be inherent or delivered through the tube feet, and the discharge of sticky entangling threads by sea cucumbers. Being stabbed by a sea urchin may result in painful injury.
Publicat de health victorro la 04:34 0 comentarii
Etichete: Avoiding, predation
Ecology
Echinoderms provide a key ecological role in ecosystems. For example, the grazing of sea urchins reduces the rate of colonization of bare rock; the burrowing of sand dollars and sea cucumbers depleted the sea floor of nutrients and encouraged deeper penetration of the sea floor, increasing the depth to which oxygenation occurs and allowing a more complex ecological tiering to develop. Starfish and brittle stars prevent the growth of algal mats on coral reefs, which would obstruct the filter-feeding constituent organisms. Some sea urchins can bore into solid rock; this bioerosion can destabilise rock faces and release nutrients into the ocean.
The echinoderms are also the staple diet of many organisms, most notably the otter; conversely, many sea cucumbers provide a habitat for parasites, including crabs, worms and snails. The extinction of large quantities of echinoderms appears to have caused a subsequent overrunning of ecosystems by seaweed, or the destruction of an entire reef.
Publicat de health victorro la 04:33 0 comentarii
Etichete: crabs, Ecology, nutriens
Evolution
The first universally accepted echinoderms appear in the Lower Cambrian period (Paul and Smith 1984). Echinoderms left behind an extensive fossil record. Despite this, there are numerous conflicting hypotheses on their phylogeny. Based on their bilateral larvae, many zoologists argue that echinoderm ancestors were bilateral and that their coelom had three pairs of spaces (trimeric).
Some have proposed that radial symmetry arose in a free-moving echinoderm ancestor and that sessile groups were derived several times independently from free-moving ancestors. Unfortunately, this view does not address the significance of radial symmetry as an adaptation for a sessile existence.
The more traditional view is that the first echinoderms were sessile, became radial as an adaptation to that existence, and then gave rise to free-moving groups. This view perceives the evolution of endoskeletal plates with stereom structure and of external ciliary grooves for feeding as early echinoderm developments.
The extinct members of Class Homalozoa, commonly referred to as carpoids, had stereom ossicles but were not radially symmetrical, and the status of their water-vascular system is not known. Further, extinct members of the Class Helicoplacoidea possessed three, true ambulacral grooves, and their mouth was on the side of their body.
Attachment to a substratum would have selected for radial symmetry and may have marked the origin of the Class Crinoidea. Members of Crinoidea, along with the extinct members of Class Cystoidea, were primitively attached to a substratum by an aboral stalk. An ancestor that became free-moving might have given rise to Asteroidea, Ophiuroidia, Holothuroidea, and Echinoidea.
Publicat de health victorro la 04:32 0 comentarii
Etichete: endoskeletal, Evolution, Helicoplacoidea
Economic importance
Most humans know the Echinoderms rather from the unpleasant side: if one finds oneself near the coast, on a rocky shore or reef, one must beware the prick of a sea urchin. The fine structure of the spines of certain species of sea urchins means that if the spine pierces the flesh, it may break off when an attempt is made to remove it. It may require patience — or the physician — to fully remove the remaining piece of spine. However in the kitchens of some countries, echinoderms are regarded as a delicacy; and for children sea-urchin skeletons are as popular a collecting object as brightly coloured starfish are fascinating.
The economic impact of Echinoderms is primarily local. Around 50,000 tons of sea urchins are captured each year, the gonads of which are consumed particularly in Japan, Peru and in France. The taste is described as soft and melting, like a mix of seafood and fruit. The quality depends on the color, which can range from light yellow to bright orange.
Sea cucumbers are also considered a delicacy in some countries of south east Asia; particularly popular are the pineapple roller Thelenota ananas (susuhan) and the red Halodeima edulis. They are well known as bêche de mer or Trepang in China and Indonesia. The sea cucumbers are dried, and the potentially poisonous entrails removed. The strong poisons of the sea cucumbers are often psychoactive, but their effects are not well studied. It does appear that some sea cucumber toxins restrain the growth rate of tumour cells, which has sparked interest from cancer researchers.
The calcareous tests or shells of echinoderms are used as a source of lime by farmers in areas where limestone is unavailable; indeed 4,000 tons of the animals are used annually for this purpose. This trade is often carried out in conjunction with shellfish farmers, for whom the starfish pose a major irritation by eating their stocks.
Publicat de health victorro la 04:32 0 comentarii
Etichete: Economic, importance
Classification
Echinoderms, like chordates, are deuterostomes and are therefore thought to be the most closely related of the major phyla to the chordates, being a sister group to chordates plus hemichordates. (Some believe that acorn worms are more closely related to echinoderms than chordates.) Because of a controversial interpretation of Homalozoa, a minority of classifiers place the echinoderms into the Chordata). Williamson (2003) disputes the links to hemichordates and chordates. They are based on larvae, which (Williamson claims) were later additions to life-histories. And pteropod hemichordates have larvae resembling trochophores, which would link them with annelids and molluscs. The phylogeny below is based on Smith (2005). It should be noted that this topology is not unilaterally agreed upon. Asteroids and Ophiuroids are frequently supported as sister groups using fossil evidence and molecular data.
Recent Posts
Avoiding predation
Despite their low nutrition value and the abundance of indigestable calcite, many organisms, such as Crabs, sharks, sea birds and larger starfish, make a living by feeding on echinoderms. Defensive strategies employed include the presence of spines, ...
Ecology
Echinoderms provide a key ecological role in ecosystems. For example, the grazing of sea urchins reduces the rate of colonization of bare rock; the burrowing of sand dollars and sea cucumbers depleted the sea floor of nutrients and encouraged deeper ...
Evolution
The first universally accepted echinoderms appear in the Lower Cambrian period (Paul and Smith 1984). Echinoderms left behind an extensive fossil record. Despite this, there are numerous conflicting hypotheses on their phylogeny. Based on their bilat...
Economic importance
Most humans know the Echinoderms rather from the unpleasant side: if one finds oneself near the coast, on a rocky shore or reef, one must beware the prick of a sea urchin. The fine structure of the spines of certain species of sea urchins means that ...
Classification
Echinoderms, like chordates, are deuterostomes and are therefore thought to be the most closely related of the major phyla to the chordates, being a sister group to chordates plus hemichordates. (Some believe that acorn worms are more closely related...
