Echinoderms ( literally "spiny skins" ) are among the strangest animals on the planet. They start out as bilaterally symmetric larvae, but grow into a 5-way body symmetry as adults. Some species show six, seven, or even higher levels of symmetry. Worm-like Sea Cucumbers have re-evolved a bilateral body plan over the underlying 5-way plan. Because of certain developmental traits, it is felt that despite their strangeness, echinoderms are actually more closely related to chordates than any other group.
All echinoderms have a water-hydraulic system which controls hundreds of tube-feet with which the animal moves and feeds. They are also capable of stiffening their skins at will by creating new calcium bonds, which are later dissolved when no longer needed. This is how sea urchins lock their spines out when disturbed.
Starfish or Sea Stars are found on any solid structure, from inter-tidal rocks to deep wrecks. They are so common that one forgets just how bizarre they really are. These animals have 5-way body symmetry *, hydraulic tube feet, and ejects its stomach through its mouth to feed. ( Oddly, the planktonic larvae have bilateral symmetry, like higher animals. ) the "eye" is called a madreporite, and is actually the exhaust vent for the hydraulic system. They are capable of regenerating an entire animal from just a fragment. Starfish come in a rainbow of colors from yellow to lavender.
Forbes' Asterias ( above right ) are common in all environments from rivers to deep seas. The skin is covered with tiny spines, giving a rough texture. They grow to an arm length of 5 inches. Northern or Boreal Sea Stars Asterias vulgaris are generally similar to Forbeses, but rather larger ( arms to 8" ) and flabbier. They are more commonly found offshore. Blood Stars Henricia sanguinolenta ( right ) have a much smoother skin. They are found only in deeper waters offshore.
Brittle Stars Ophioderma spp ( below right ) have thin delicate arms that are very flexible and easily broken. They are capable of much
faster movements than other starfish, crawling, and in some cases even swimming. Brittle stars grow as large as 6 inches, but most are much smaller. They are usually found on muddy bottoms, but may turn up anywhere.
Starfish often move so slowly that they don't seem to move at all. Large specimens from deep waters are surprisingly soft and soggy when you bring them up - collect small ones if you want souvenirs.
Sea Star anatomy
Close-up of skin and madreporite of a Forbes Asterias. The madreporite of the Forbes Asterias is usually orange as shown, while that of the Northern Star is paler.
Northern Sea Star on the left, with Forbes on the right.
Northern Sea Stars
Feeding exclusively on sponges, the Blood Star Henricia sanguinolenta reaches
an arm length of about two inches, and is found as far south as Cape Hatteras.
A small Forbes attacking a mussel in an aquarium
Contrary to the common conception of a drawn-out tug-of-war between the starfish and the mussel, the starfish does not pry open its victim by brute strength. Instead, the starfish inserts its stomach through whatever tiny gap or opening there may be between the shells of the bivalve, which it may widen only slightly by pulling. The digestive process then weakens the bivalve, eventually attacking the adductor muscles that hold the shell closed, and allowing the starfish to open it fully.
Rolling Starfish
Peterson's states that starfish can "float free and drift on strong currents," but the behavior you see here is not widely documented. These starfish are balled up and rolling in the surge. This appears to be a deliberate mode of transportation, quite a bit faster than they would otherwise travel. Video courtesy of Dan Crowell / SDP-video.com
( The annoying tapping sound in the video is not the starfish - it is the boat's depth finder. )
Arbacia punctulata
Size: to 3"
Sea Urchins may be found on any solid structure, from tidal rocks to deep wrecks, although in our region they are more common in shallow estuarine waters than offshore.
This animal is related to the starfish, and shares many of its traits, with one exception. Starfish are predators, feeding upon clams and mussels, while these sea urchins are strictly vegetarians. I once ordered Sea Urchin at a sushi bar, but I never even tasted it, it smelled so bad. Nonetheless, it is a delicacy in much of the far east. Unlike tropical black urchins, our northern ones are not poisonous, and the spines are not even particularly sharp.
Sea urchin anatomy
The underside, showing mouth, tube feet among the spines, and 5-way symmetry
Echinarachnius parma
Size: to 3"
Think of the Sand Dollar as a flattened Sea Urchin. They live on and slightly buried in sandy bottoms, at any depth.
Soak them in a bleach mixture and then rub off the spines with a rag to get a nice white keepsake. The tiny spines are like cactus needles and can be very irritating. Live Sand Dollars are pink; they turn green in death, and the indelible green pigment will rub off on anything it touches.
The spines are used like a conveyer belt to move grains of sand along the upper and lower surface of the animal, allowing it to crawl around the bottom and bury itself. They feed on organic material found in the sand, which is passed to the mouth by the tiny tube feet all over the body.
Sclerodactyla briareus
Size: to 6", larger offshore
The Sea Cucumber is not a worm, but an echinoderm, related to the starfish. If you look closely, the 5-way symmetry is still evident. Sea Cucumbers live in buried in sandy or muddy bottoms less than 60 ft deep. Find them by the volcano shaped mounds they make.
North Atlantic Sea Cucumber
I make no claim as to the accuracy, validity, or appropriateness of any information found in this website. I will not be responsible for the consequences of any action that is based upon information found here. Scuba diving is an adventure sport, and as always, you alone are responsible for your own safety and well being.
Copyright © 1996-2008 Rich Galiano
unless otherwise noted
