Starfish walk on glue and hide a hydraulic city inside
About 1,900 asteroid species carpet seabeds as star-shaped echinoderms—usually five arms, sometimes dozens in Antarctic Labidiaster. Tube feet run on a water-vascular system that enters through the madreporite, stretches via ampullae, and grips with adhesive chemistry rather than suction.
A starfish walks on water pressure. Seawater enters through the madreporite, a sieve-like plate on the upper surface, runs down the stone canal to a ring around the mouth, and branches into a radial canal along each arm. When muscles squeeze the bulb-like ampullae, water swells the tube feet so they stretch to the ground. They look like suckers but hold on with adhesive chemicals, and other secretions release them, so the animal ripples forward as some feet grip while others let go. The same canals serve as extra gills, moving oxygen and nutrients in both directions.
Feeding is just as unusual. Primitive species such as Astropecten and Luidia swallow prey whole and spit out the shells, while advanced species push part of the stomach out through the mouth to digest food outside the body, then haul it back with ligaments anchored in the arms, a movement triggered by a neuropeptide called NGFFYamide. Branches of the stomach reach into every arm.
The skin hides a skeleton of calcite ossicles, honeycomb lattices of tiny crystals forming plates, granules and spines. Some groups carry pedicellariae, pincer-like spines that clear debris, and a few species use them to catch prey, while thin papulae poking through the body wall help with breathing. Most starfish have five arms, but some have ten to fifteen and the Antarctic Labidiaster annulatus can exceed fifty. Descended from bilateral ancestors, they often lead with particular arms when crawling, and a flipped starfish rights itself by planting three arms and levering the other two over.
Around 1,900 species live from the shore to 6,000 metres down; the ochre sea star shapes whole communities, while the crown-of-thorns devours Indo-Pacific coral.
Source: Starfish
