Sea urchins feed mainly on algae, but can also feed on sea cucumbers, and a wide range of invertebrates such as mussels, polychaetes, sponges, brittle stars and crinoids. Sea urchins are one of the favorite foods of sea otters and wolf eels. Without predators to eat them, urchins breed in large numbers and eat almost all the nearby kelp, creating an "urchin barren", devoid of macroalgae and associated fauna. Sea otters have re-entered British Columbia, dramatically improving coastal ecosystem health by eating sea urchins.
The first echinoid fossils are from the Lower Ordovician period. The earliest forms had flexible tests, with plates that could slide over each other. Echinoids were a relatively unimportant part of the biota in the Palaeozoic. Only one group survived the P/Tr extinction event to form the basis of all later echinoids. The group which survived into the Triassic, the Cidaroids, radiated into all other modern groups, which are known as the Euechinoids.
The break with perfect symmetry, which happened in the Jurassic, gave them definite front and back ends. This opened up new habitats, in particular, the burrowing habitat. Sand dollars and heart urchins have been highly successful. The living echinoids are now important members of the biota, especially in shallow and in-shore waters.
Some humans eat the reproductive organs of sea urchins (which they call roe). This is popular in Korea. In Japan, this kind of sushi is called "uni".
Brittle star
Brittle stars are Echinoderms of the Class Ophiuroidea. The adults are superficially like starfish, but they are a different group, with different larvae. Like other echinoderms, they do have a skeleton of calcareous plates, and radial symmetry. There are about 1500 different species.
Usually they have 5 "arms", which can grow long, up to 60 cm, in the largest species. The central disk of brittle stars can be three to eight inches in diameter. Unlike starfish, they live deeper in the ocean. Ophiuroids are known even from abyssal (>6000 m) depths. However, brittle stars are also common members of reef communities, where they hide under rocks and even within other living organisms. Generally, brittle stars live below 500m of depth.
There are two related groups of Ophiuroids: the "Ophiurida" (brittle stars) and the basket stars ("Eurialida"). Basket stars usually live in the deeper parts of the range. Brittle stars are mostly scavengers and detrivores, able to catch some small prey on occasion.
Many brittle stars are more fragile than starfish, which is how they got their name. When they are picked up, their arms may fall off. This might make the animal seem brittle, but they often do this as a defense to escape from predators.
Brittle stars do not use their tube feet for movement, but instead use muscles in their long, thin arms to go around rocks. Ophiurids move quickly when disturbed, and are faster movers than starfish. In living ophiuroids the plates are linked by longitudinal muscles. Ophiurid arms only move horizontally. The way their plates interlock prevents the arms from moving in any other plane. In contrast, basket star arms can move vertically. These movement patterns are distinct, and separate the groups.
Ophiuroids in general are mostly scavengers or detritivores. Small organic particles are moved into the mouth by the tube feet. Ophiuroids may also prey on small crustaceans or worms. Brittle stars will eat small suspended organisms if available. In large, crowded areas, brittle stars eat suspended matter from prevailing seafloor currents.
Basket stars (Euryalids) are large brittle stars with many rays. Their arms branch, and can be moved in any direction. This distinguishes them from the Ophiurids. Basket stars may be capable of suspension feeding, using the mucus coating on their arms to trap plankton and bacteria. They extend one arm out and use the other four as anchors. The arms form so many branches that they are almost feathery on the ends. They get their name because when they stretch their rays out to catch plankton, they look like the bottom of a basket.
Basket stars can be large and heavy, the biggest Ophiuroids. The largest, "Gorgonocephalus stimpsoni", weigh up to 5 kg (11 lbs) and has arms 70cms long, with a disk diameter of 14 cm.
Bivalve
The bivalves are a large class of molluscs, also known as pelecypods.
They have a hard calcareous shell made of two parts or 'valves'. The soft parts are inside the shell. The shell is usually bilaterally symmetrical.
There are over 30,000 species of bivalves, including the fossil species. There are about 9,200 living species in 1,260 genera and 106 families. All of them live in the water, most of them in the sea or in brackish water. Some live in fresh water. All are filter feeders: they lost their radula in the course of evolution. A few are carnivorous, eating much larger prey than the tiny microalgae eaten by other bivalves.
The best known examples of bivalves are clams, mussels, scallops and oysters.
Bivalves have two shells or valves connected by a hinge with hinge teeth. They are made of a calcareous mineral, calcite or aragonite. The valves are covered by a periostracum, which is an organic horny substance. This forms the familiar coloured layer on the shell.
The shells are usually held shut by strong adductor muscles. Scallops can use their muscles to flap the valves and swim.
A bivalve takes in water that has plankton and other things floating in it.
Some (but not all) molluscs have a part of their mantle known as the siphon (a tube). Siphons, if they exist, come in pairs, one to suck in, one to expel.
Anything that is small enough to fit inside the hole of its incurrent siphon enters the bivalve. When the floating material comes in, it gets stuck in slimy mucus that is on the surface of the bivalve's gills. The food is moved down to the mouth, which is on the other side of the siphon. Food is digested in the bivalve's stomach and intestine, and everything that is not digested goes out through the other siphon, with water.
The siphons are an adaptation of burrowing molluscs. Those that live above the substrate (sediment), such as scallops and oysters, do not need them.
There are four feeding types, defined by their gill structure:
As a group, the bivalves are adapted to penetrate into, and to move along horizontally along, soft ground such as mud and sand. Common examples of this are razor shells, which can dig themselves into the sand with great speed to escape enemies, and cockles.
Scallops and file clams can swim to escape a predator, clapping their valves together to create a jet of water. Cockles can use their foot to leap from danger. However these methods quickly exhaust the animal. In the razor shells the siphons can break off only to grow back later.
The file shells can produce a noxious secretion when threatened, and the fan shells of the same family have a unique, acid-producing organ.
Bivalves are superficially similar to brachiopods, but the construction of the shell is completely different in the two groups. In brachiopods, the two valves are on the dorsal and ventral surfaces of the body, while in bivalves, they are on the left and right sides.
Bivalves appeared late in the Cambrian explosion and came to increase in the Palaeozoic, and dominate over brachiopods during the Mesozoic. So, it was thought that bivalves are better adapted to aquatic life than the brachiopods, and this caused brachiopods to be out-competed and relegated to minor niches.
However, after the Permian-Triassic extinction event bivalves had a huge adaptive radiation while brachiopods were devastated, losing 95% of their diversity.
The upshot was that bivalves took over the desirable inshore habitats. Brachiopods now live in deeper waters where food is scarcer.
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