Scientific name : Echinoidea
Sea urchins belong to the phylum Echinodermata, class Echinoidea. All Echinoderms are original animals, adopting unique survival strategies. Even if far from the main evolutionary line, their success is clear, looking at the number of species and at the abundance in some environments.
Like most Echinodermata, sea urchins have a radial simmetry (that is they have no head and tail, but a rounded body, mouth in the center of the lower side, anus at the top) Their simmetry is pentaradial, that is the majority of the organs is repeated along 5 sectors.
The body wall is a hard skeleton, compact, with articulated, very mobile spines.
5 radial rows of pedicellaria exist, highly mobile appendages of different kinds: there are pedicellaria with a small sucker at the tip, used for movement and to hold item for hiding; pince shaped pedicellaria, to grasp; and venomous pedicellaria, with a small spine at the tip, that can inject a dose of poison in the aggressor.
The pedicellaria movement uses a unique system in the animal kingdom: an aquiferous system, with vessels similar to the blood vessels, but containing circulating sea water from the external environment. Pumping water the urchin can inflate the pedicellaria and keep their mobility.
The mouth has 5 large pyramidal teeth, with the tip downward. Many urchins can attack and browse hard benthic organisms with this powerful browsing apparatus.
Having no special apparatus, breathing is through the pedicellaria skin. In few species the rectal end is estroflected in an anal papilla, with breathing functions.
Sea urchins are obviously benthic animals, living in all the marine environments, even if most species live on shallow bottoms, in lagoon, bay, back reef.
Many species are primarily herbivorous, feeding on algae, seagrasses, detritus, but actually ingesting also benthic invertebrates.
Even if living in shallow, well lit environments, most species seem to avoid light and cover themselves with various items (rubble, detritus) to hide. In tropical seas most species are nocturnal, and spend the day hidden in holes or crevices. Most are solitary, few species live in aggregations.
Fecundation is external, eggs and sperm emission is synchronised and the planktonic larvae, after many developmental stages, will settle on the bottom.
The unique sea urchin shape allows no confusion.
The so called irregular sea urchins have a slightly different body shape. They are flattened, elongate, with a polarity (anterior and posterior parts). Some species have no teeth, but live inside sandy bottoms swallowing detritus.
Contact with sea urchins can be an unpleasant experience. Spines get easily broken in the wound, that can get infected. Some speciesw are venomous, and can inject the poison both through spines and through modified pedicellaria.
Echinoderm poison gives initially a very acute local pain. With some species other symptoms can occurr later: swelling, local paralysis, in extreme case also death.
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