Stars on the sea floor

Last month I wrote about the underwater starry night sky of bioluminescence this time we are going for the upclose encounters of Sea stars. A favourite for many divers and snorkelers due to their iconic shape and the fact that they can be seen in many different marine environments but although it may be a common sighting for many they are far from common creatures.
Sea stars, not starfish as many people call them (they are not in the fish family) are in a group called the Echinoderms which means spiny skin.  This group consists of other creatures such as brittle stars, sea urchins, sand dollars and sea cucumbers; although they share many similar features, today I'm going to focus on the sea stars.

When you first look at a sea star it can be difficult to figure out what parts you are looking at - the arms are obvious but where is the head, the eyes, the nose, the backside?  Well seastars don't have a head!  They also have a pretty simple nervous, digestive, excretory and respiratory system.  They do not have blood - seastars use sea water to help move nutrients around their body. The seawater gets taken in through a sieve like plate, called a madrepore, on their back - this is located just off the centre of their central disc. The madrepore takes in water, this is then moved around their body using a water vascular system - there is a network of fluid filled canals and muscles pump the water around the body.  The constant flow of water also contin oxygen thus allowing the animal to breath through gas exchange in the cell walls.  They can also breathe through their skin - most seastars have small papulae on their backs which look like little fleshy extrusions which also allow gas exchange.

Tube feet - we can't think about seastars without thinking about how they move.  On their underside they have hundreds if not thousands of tube feet - it is often thought that these work like a suction cup but the gripping function is actually a chemical reaction with a glue being released and broken down each time.  The tube feet help with movement but they also help it to breathe because they act as accessory gills.

On the underside of a seastars body lies an opening that can act as both a mouth and a backside.  Through this opening seastars can extrude their stomach this allows them to use digestive enzymes to break down prey and then when it is consumed bring their stomach back inside their body.  This allows seastars to eat prey much bigger than themselves.  When I lived in Tofino I would often see ochre seastars covering huge mussels; they would entire into a war of attrition with the mussels trying to stay shut and the seastars trying to pry them open but eventually the seastar could open a small gap between the mussels shell - it would then secrete an enzyme to turn the insides into soup which it can then take into its stomach.  Most seastars are opportunistic feeders and although they seem to prefer things like clams and oysters some species have been known to eat small fish, algae and even other seastars!

The seastar may not have true eyes but it does have eyespots at the end of each arm which are light sensitive photoreceptors.  The tube feet, spines and papillae are also sensitive to touch and the tube feet can detect chemicals in the water which can help them to find food sources.

Some species of starfish have the ability to regenerate lost arms and can regrow an entire new limb given time.  A few can regrow a complete new disc from a single arm, while others need at least part of the central disc to be attached to the detached part.
  Regrowth can take several months or years, and starfish are vulnerable to infections during the early stages after the loss of an arm. A separated limb lives off stored nutrients until it regrows a disc and mouth and is able to feed again.  The loss of a body part may happen unexpectedly due to part being detached by a predator, or part may be actively shed by the starfish in an escape response.  The loss of parts of the body is achieved by the rapid softening of a special type of connective tissue in response to nervous signals. This type of tissue is called catch connective tissue and is found in most echinoderms.  It is made of a type of collagen that unlike that found in our body has the ability to unwind and thus loose its strength.

Here around La Paz there is a wide variety of seastars to find, whether you are diving, snorkelling or just beachcombing.  We have the beautiful panemic cushion seastar, the wonderfully named chocolate chip seastar, the aptly named spiny seastar, the dangerous crown-of-thorns and the multiarmed gulf sun star to name a few.  Some of these you will find in the shallow intertidal zone and others much deeper.  As always in my opinion once we know a little more about a creature it allows us to enjoy seeing it that bit more.

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