By Kelly Stewart
Starfish are iconic animals of the sea, immediately recognizable by everyone. Although many insist that the proper term for these calcareous creatures is “sea star,” starfish is more commonly used. Starfish are Echinoderms (along with sea cucumbers and sea urchins) and many belong to the Class Asteroidea. There are hundreds of different species around the world in all kinds of habitats, but there are 3 or 4 commonly seen starfish in our area.
Starfish have incredible features and complexity. Their bodies are made up of calcium carbonate materials, which give the exoskeleton a hard structure. Internally, they are all hydraulics. By controlling the amount of water in their system, and moving it around their bodies, they can move and feed very effectively.
The madreporite, which is a calcareous porous plate on the top of the starfish just off-center of the central disk, is used to regulate the amount of water in the vascular system. Sea stars move along the seafloor, gliding on tube feet. Tube feet extend from the internal water system out through spaces in the hard exoskeleton, and water pressure is used to move the feet, distributing pressure to different areas where it’s needed. Water can be pumped out into the feet, or contracted back into the body.
Most starfish have five arms (pentaradial symmetry) that radiate from a central disk. Many people are familiar with the amazing regeneration capabilities of the starfish. As long as part of the central disk is present, a starfish can regenerate its entire body (although it takes some time). Arms lost to predators are able to grow again.
Oftentimes when you pick up a starfish in a tidepool, you’ll see a jelly-like protrusion on the underside, coming out of the central part of the body. Often there will be a mussel or other hard-shelled creature caught in the jelly. This is the stomach. Many species are able to push one of their stomachs (cardiac stomach) to the outside of their body to feed on hard-shelled items like mussels and urchins. The digested food is then passed to their pyloric stomach, which stays inside the animal.
Starfish, although technically eyeless and earless, have many senses. They can sense light and dark using ocelli (light detecting pigmented skin cells) on the ends of their arms. Tube feet in this area are equipped with chemical sensors, which help the starfish detect odors of food. Sea stars play a critical role in the ecosystem as predators, and are known as keystone species, because they help regulate fast-growing populations of invertebrates and maintain biodiversity.