Echinoidea | EchinoID

Echinoidea or sea urchins

Sea urchins are very visible and frequently see in shallow waters. They probably correspond the best to the etymological definition of echinoderms: spiny skin. Their skeleton is composed of 10 series of plates arranged radially around a mouth facing down and an anus facing up. A typically large number of spines articulate on these plate to give the spiny appearance of the urchins. Some urchins, have dramatically modified their morphology to adapt to soft sediment. They are are irregular in shape, some flattened (the sand dollars) some similar to a mammal heart (the heart sea urchins). In this irregular urchins, the spines are much finer and resemble a fur made of stiff bristles.
Sea urchins either graze on the algae growing at the surface of hard substrates (regular sea urchins) or feed on the fine organic particles that settles on and within the sediments (irregular sea urchins). A few urchins are carnivorous.
The feeding apparatus of the regular sea urchins is complex and composed of a series of 5 identical skeletal structures arranged radially around a central mouth and called the Aristotle's lantern. The 5 teeth of the lantern are used to scrape the surface of the substate from its algae and micro algae.
Like all echinoderms, they gave a well developed water vascular system composed externally of numerous tube-feet that are used for locomotion, respiration and attachment to the substrate.
There are divided into a series of super-orders corresponding to major differences in body plan.
The Cidaroidea or pencil urchins are typically predatory and characterized by large, thick spines. So far, only one species was observed in Oman.
The Regular sea urchins are divided into two main groups: the Diadematacea (long spines urchins) and Echinacea the taxonomy of which is based on the structure of the teeth, the spines and the different types of pedicellaria.
The Irregular urchins are either spantagoids (heart sea urchins) or clypeasteroids (sand dollars). Their fauna is very sketchy so far, because they typically live in the sediment and remain invisible during the day. Skeletons are sometimes found on beaches after storms and indicate the presences of at least some species nearby.

There are about 20 known species of Echinoids in the shallow waters of Oman, but more irregular urchins are likely to be found in both shallow and deeper sediments.
The structure of the identification key is not based on the taxonomy of these organisms but on their appearance live in the ocean.