The stars of echinoderms
In my last article I wrote about sea cucumbers, now let’s explore another famous echinoderm class: the sea stars or starfishes. Sea stars make up the echinoderm class asteroidae, and there are around 2’000 species of them. As most echinoderms, sea stars can be found in pretty every marine habitat, from shallow waters down to the deep sea in over 7’000 meter, from polar marine regions to tropical seas. Their class name asteroidae (meaning “star like”) derives from their typically star shape, mostly resembling a five-pointed star with five arms, but a few can have up to 50 arms. Similar to their relatives the sea cucumbers, they come in a variety of shapes and colors and like sea cucumbers, sea stars are masters of regeneration. While sea cucumbers like to spit out their guts and internal organs, sea stars can self-amputate their arms and regrow the missing limb afterwards. Many sea stars are able to regrow a whole new clone of a broken off limb. This is possible, as most species possess a duplicated set of organs in each of their limbs. Cloning not only happens when they come under attack, they can also split themselves into two halves on purpose to reproduce asexually. They are true echinoderms (“spiny skins”) in the sense that their skin is covered with tiny pincers to keep unwelcomed guests away and prevent algae and parasites to grow on them. Like sea cucumbers and the rest of the echinoderms, starfishes possess a water vascular system whereas their “blood” is slightly modified sea water and like sea cucumbers they can move through their tube feet, hold onto objects and climb up walls. Typically to echinoderms, sea stars are supported by a calcareous endoskeleton made up of calcium carbonate plates called ossicles. And last but not least, like all their relatives of the phylum echinodermata, star fish are brainless but possess a so called “distributed brain”, an internal ring of neurons and nerves at the center of their body. This decentralized brain and nervous system provides sea star with a powerful sense of touch, let them detect undersea smells and let them see through primitive eye-spots located at the end of each of their arms.
Sea stars however may be the most offensive echinoderms of all. First of all: if you dive above them what they show you right away is their butthole, the other body orifice the mouth points usually towards the ground. Second, in sharp contrast to the peaceful filter feeders the sea cucumbers and their other relatives the algae eating sea urchins, sea stars are real predators. Starfishes hunting for clams, crabs and even attack other sea cucumbers. They are especially skilled at eating shellfish and are able to wrap their arms around a clam to to pry apart the shell with their tube feet. After that, the ugly part of the drama starts. As soon as a small gap between the shells of clam opens, the sea star emits its stomachs out of its body and into its prey’s shell. There the sea star’s stomach secretes digestive enzymes that dissolve the clams tissues into a liquid which then can be slurped up. This liquefaction process can take hours. After the shell organism is sucked up into the sea stars emitted stomach, the sea star simply pulls its stomach back into its body, “bon appetite”! Sea stars literally are dining out.
Here an interesting youtube video describing the clam eating process of sea stars!