Visitors to our lodge are often surprised by the large number of brightly colored sea stars inhabiting the low-tide zone on Kodiak Island. Sea stars are prolific throughout the Pacific Northwest and are critical to the health of intertidal and subtidal communities. Scientists have identified more than 120 species of sea stars in Alaska, including the sunflower sea star, one of the largest sea stars in the world.
Sea stars are often called starfish, but since they aren’t fish, biologists prefer the name sea star. Sea stars belong to the phylum Echinodermata. Other echinoderms include sea cucumbers, sea urchins, sand dollars, and brittle stars. Echinoderms usually have pentamerous radial symmetry, meaning the body can be divided into five parts around a central axis. This five-parted symmetry is easy to see in a sea star with five arms, but it is also apparent if you look at the bottom of a sand dollar or the pen of a sea urchin. Some sea stars have more than five arms. A sunflower sea star has twenty arms, but the animal is still divided into five equal parts around the central disk.
Sea Stars are flattened in appearance and may range in size from 1 inch (2.54 cm) to over a yard (1 meter) in width. A sea star has an internal skeleton which is somewhat flexible. The skeleton consists of small calcareous plates bound together with connective tissue. Sea stars may look rigid and sedentary, but the connective tissue between the plates allows them to bend to attack prey, flee predators, and right themselves when they are turned upside down.
A sea star’s anus is in the center of the top side, or the aboral surface of the animal. A circular madreporite is located just off center on the aboral surface, and this madreporite is a critical part of the circulation system of the sea star. Instead of a circulatory system, a sea star has a water vascular system, and the madreporite acts as a trap door through which water can move in and out in a controlled manner. The mouth of a sea star is located in the center of its underneath or oral surface. Open furrows containing tube feet extend from the mouth along the length of each leg.
Sea stars do not have eyes, but they have eyespots that can detect light at the tip of each arm. Interestingly, scientific studies have shown some species of sea stars move toward light while others move away from the light. Neurosensory cells which are sensitive to both touch and chemical tastes cover the surface of a sea star and are particularly dense in the suckers of the tube feet. Many species of sea stars are covered by clusters of tiny, calcareous pincers. These tiny pincers deter predators and keep the surface of the sea star free of parasites and debris. Also on the surface, thin-walled gills protrude between the calcareous plates and serve to exchange respiratory gases and excrete liquid wastes.
The internal anatomy of a sea star includes the water vascular system, digestive tract, reproductive organs, and nervous system. The water vascular system uses muscles and hydraulics to power a sea star’s tube feet. The tube feet not only allow a sea star to move but are used to grasp prey, and the combined force of numerous tube feet is strong enough to pry apart a clam shell. Most seas stars move very slowly, and their pace is measured in inches per hour, but giant sunflower sea stars can travel at a speed of two feet per minute.
The mouth of a sea star opens into two stomachs connected to paired, lobed organs called pyloric caeca. The pyloric caeca extend into each arm and aid in the digestion of food. Sea stars are either male or female, and their reproductive organs, or gonads, lie between the pyloric caeca in each arm. In the spring, sea stars broadcast either eggs or sperm through pores in their arms into the water where chance fertilization occurs. Sea stars have no brain or central nervous system, but they have a nerve ring in the central disk connected to radial nerves running the length of each arm. The radial nerves are connected to a diffuse network of nerve cells scattered throughout the skin. Sea stars have the ability to regenerate lost arms.
Sea stars utilize a range of habitats and may be found from the shoreline to depths greater than 13,450 ft. (4,100 m). Sea stars consume a wide variety of prey, including sponges, snails, clams, mussels, sea cucumbers, barnacles, anemones, scallops, fishes, and even other sea stars. Some species of sea stars feed on plankton, while other species prefer dead organisms. Sea stars have few predators and are believed to have a lifespan of only a few years.
Next week, I will post about sea star wasting syndrome, a devastating disease that has killed millions of sea stars in the last few years from California to Alaska.
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