MUSEUM OF NATURE

Press, Volume CVIII, Issue 31736, 20 July 1968, Page 5

MUSEUM OF NATURE

Tube-Footed Marine Animals

(Contributed by the Canterbury Museum) Biologists have estimated that a sea star is able to walk about 6 inches a minute; a brittle star, 6 feet a minute; and a sea urchin, travelling at top speed, 5 feet a minute.

These racing times depend on one of the most curious walking systems nature has devised, and one that is unique to this group of animals. the Echinodermata. It is a kind of hydrostatic mechanism known scientifically as the “water-vascular” system, meaning water-filled tubes. Sea water is taken into this system through a sieve plate on the upper surface of the sea star, in the central disc between the arms. From the sieve plate, the water passes to a central ring canal, and from here it is distributed along each of the sea star’s arms in yet another system of canals. Along the arm canals, numerous side branches give rise to pairs of tube feet—hollow, thinwalled cylinders. The tube feet emerge to the outside of the sea star from a groove along the underside of each arm.

The inside part of the tube foot ends in an oval, muscular. bulb and this operates like a rubber syringe. When the bulb contracts, it squeezes water into the tube foot which elongates and a little suction disc on the end of the foot adheres firmly to the rock beneath. When muscles in the foot Itself contract, water is pushed back into the bulb and the foot is shortened and lifted from the rock. Foot by foot, feet by feet, arm by arm, the sea star creeps inches over the ground. Hard To Move One tube foot by itself is a weak structure, but hundreds together provide such strength that it is virtually impossible to shift a sea star if he decides to stay put. Making such a decision may seem impossible for a creature that we discover has not a single brain like a human. In the place of a brain, his nerves and sense organs are spread along each of his arms, very close to his tube feet This means that he is really very firmly in touch with whatever he is doing. Further, as we have already described, the sea star actually takes the outside seawater into the water vascular system and so he is truly immersed in the sights, smells and sounds of his environment. Meat-Eaters If a sea star senses a delicious morsel of food near “at arm”, the tube feet of that arm dominate the movement of the animal until the food is grabbed by the feet at the tip of the arm. It is then passed by a chain gang of feet to the mouth which lies right in centre of the arms on the underside of the sea star. Most sea stars are voraci-

ous, determined and efficient meat-eaters and sometimes cannibals. Few things prove too great an adversary for the sea star and he can even tackle large shellfish by crawling on to the top of the two valves of the shell and pulling them apart with his tube feet. Once this is achieved, he pushes his stomach outside his own mouth and into the soft body of the shellfish. There he digests the unfortunate shellfish, and upon finishing his meal sucks the shellfish juices and his own stomach back inside his mouth Large sea stars may eat small shellfish whole, and other sea

stars too. The undigested hard parts are later ejected in a little pellet In Europe, geologists have found fossil pellets believed to have been ejected by sea stars. One such pellet, which must have been ejected by a particularly cannibalistic sea star, contained nine different kinds of small sea star, two kinds of brittle star and one sea urchin. Teeth Replaced Sea urchins are practically omnivorous. They possess most powerful toothed jaws which scrape off seaweeds, sponges, tubeworms, and

even barnacles, growing on rocks, and crunch them up with uncanny determination. The teeth wear away with such drastic browsing but are replaced by new growth.

Like sea stars, the sea urchins also have hundreds of tube feet, which are longer and slimmer and extend through small pores in the urchin’s hard shell. The sea urchin can climb quite vertical surfaces by applying his long suction feet to the rock and pulling himself upwards. In addition, the spines of the sea urchin can be raised and lowered with strong muscles, and he may use them like

stilts to walk across a sandy bottom. Brittle stars, the third of our fascinating charges, feed mainly on small particles of plant and animal food that they pick up from the mud and sand in which they live. They too have tube feet, but they are small and usually without the strong suction tips. Most often, brittle stars move by lashing their snakelike arms, but sometimes two brittle stars are seen to abandon single sculls and join together in pairs, whence they row themselves rapidly over the ground.—M.M.D. and J.T.D.