|Posted on 31 July, 2015 at 13:00|
For this week's Friday Fish fact, I have a whole treasure trove of facts for you about starfish.
Starfish (more correctly known as Sea Stars) are not fish. They are Echinoderms. They are related to sea urchins, sea cucumbers and even sand dollars. All Echinoderms have 5 point radial symmetry, which means that their body has 5 sections (or multiples thereof) arranged around a central disk. Next time you're on a beach or diving, see if you can find a sea star, sand dollar and sea urchin and find the 5 sections in each.
Not all sea stars have 5 arms. While the five-armed varieties of sea star are the most well-known, not all sea stars have 5 arms. Some have many more. Take our local waters Sun Star for instance, which has up to 40 arms! (Picture below)
Amazingly, sea stars can regenerate lost arms. This is useful if the sea star is threatened by a predator - it can drop an arm, get away and grow a new arm. Sea stars house most of their vital organs in their arms, so some can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc. It won't happen too quickly, though. It takes about a year for an arm to grow back.
Sea stars do not have blood. Instead of blood, sea stars have a water vascular system, in which the sea star pumps sea water through its sieve plate, or madreporite, into its tube feet to extend them. Muscles within the tube feet retract them.
Sea stars have eyes. While they can't see as well as we do, sea stars have an eye spot at the end of each arm. This is a very simple eye that looks like a red spot. The eye doesn't see much detail, but can sense light and dark.
Our sea stars are disappearing. I recall diving as little as 5 years ago and encountering dozens of large sea stars on every dive I went on.
Lately, I am very hard pressed to recall a dive I've done where I've even seen one. It appears that there is a virus that is circulating between the sea star population that is causing them to die rapidly. You can learn more about what is happening at this link. Virus killing Sea Stars!