Phylum Echinodermata “spiny” “skin” www.onacd.ca

Major Classes of Echinoderms ECHINOIDEA: Includes the sea urchins and sand dollars ASTEROIDEA: includes the starfish (sea stars)

OPHIUROIDEA: includes the brittle stars (below: a fossilized brittle star) HOLOTHUROIDEA: includes the sea cucumbers

Characteristics of Echinoderms Coelomate and radially symmetrical Body is usually 5-rayed (pentaradial) Have a calcareous skeleton Possess a water vascular system used for gas exchange, feeding, and locomotion Complete digestive system Open/reduced circulatory system (no heart) with a central ring and vessels Simple radial nervous system (no brain) Reproductive system consists of gonads which take up a significant amount of space in the body cavity Sexual reproduction with fertilization outside body (most) Have ciliated, free-swimming larvae Abundant on most sea coasts, especially on rocky shores

Taking a closer look at Sea Stars Possess 5 or more arms which radiate from a central disk Mouth is located on oral surface (underneath) Sea stars are found all over the world in a variety of shapes and colors Most possess a simple light sensitive eye spot at the end of each ray (arm)

Digestion Sea stars have 2 stomachs that perform different functions The cardiac stomach can be pushed outside of the mouth to engulf and digest food, then digested food is brought inside to the pyloric stomach The pyloric stomach further digests food and passes it on to the intestine and the anus which is located on the aboral surface (top) This allows a sea star to consume food that would normally not fit into its mouth!!!

The Water Vascular System The water vascular system consists of a central ring and a series of canals that eventually end in tube feet which you can see protruding from the underside of this sea star. These tube feet are used for movement, feeding, and bringing in water for gas exchange.

Here are the tube feet of the sea urchin which protrude out from the spines. Sea stars and urchins alternately contract muscles that force water into the tube feet which extends and pushes them into the ground. The muscles are then relaxed in order for the tube feet to retract.

A Closer Look at Sea Urchins Most sea urchins possess spines of 1-3 cm but can be found up to 20 cm long Globoid shape with no rays or arms

Sea Urchin mouth Sea Urchin Test (shell) Have ball and socket joints on the spines which allow them to move. If you try to touch the spines of an urchin, the surrounding spines will converge there. Possess a mouth with 5 calcium carbonate teeth (see above). Also have a fleshy tongue within. This structure is termed Aristotle’s Lantern.

Defense systems of Echinoderms Spines are observed on many echinoderms and can be soft or hard and sharp. Their presence deters predators Pedicellariae are pincer-like structures that protrude from the body surface between the spines Pedicellariae keep the body surface clear of encrusting organisms Echinoderms also camouflage with their natural color into their environment

Ecological Roles of Echinoderms Starfish and brittle stars prevent the growth of algal mats on coral reefs Ossified skeletons of echinoderms are major contributors to many limestone formations Part of the food chain, larvae produce food for other creatures The death of echinoderms often is followed by an increase in seaweed growth and reef destruction

Sea urchin grazing reduces the rate of colonization of bare rock Burrowing of sand dollars and sea cucumbers depletes the sea floor of nutrients and encourages deeper penetration which increases levels of oxygen and ecological tiering Sea urchins bore into rocks, releasing nutrients Sea cucumbers provide habitats for parasites Echinoderms feed on dead and decaying organisms

Super Cool Echinoderm Facts Sea stars are capable of regeneration of body parts as long as part of the central disk remains The gonads of Sea urchins and cucumbers are considered a delicacy in SE Asia Sea cucumbers will eviscerate (shoot out parts of their internal organs) when they sense extreme danger, then re-grow the organs Sea urchins can replace spines that are damaged or lost