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The stars in the sea!

When you hear the word stars,what comes to your mind immediately could be a starry sky. There are falling stars or shooting stars, twinkling stars and various other star groups. It's such a fascinating sight, isn't it? Do you know how to differentiate stars from planets? Blinking lights in the sky are stars and the others are planets.

And do you know what the biggest star in the universe is? It's the Sun. Now, that's enough about the stars up in the sky.Let's look at some other stars that are directly below the sky... in the sea. I'm sure most of you will be amazed to hear about stars in the sea, but there are stars in the sea. However, they are not like the stars up in the sky. These stars are alive. They are known as starfish.

Think of beach souvenirs, and what are the things that first spring to your mind? Conch shells, hermit crabs, and starfish? You can find starfish skeletons on numerous coastlines, but unfortunately not much can be found in Sri Lanka. Though we call them starfish, they are actually not fish. Technically they are classed as invertebrates. They lack both vertebrae[spine]and fins and yet many marine biologists are pushing to have them reclassified as Sea Stars.

Starfish belong to a group of animals called echinoderms, (this means "spiny skin" in Greek) which includes sea cucumbers, brittle stars, feather stars and sea urchins. Many echinoderms have spiny skin. There are over 6000 species.

Echinoderms usually have five arms, but some have as many as 44 arms! While many echinoderms are only found in the sea, and not in fresh water, the 1800 or so species of starfish flourish throughout the world's waters, with the northern Pacific Ocean and the Red Sea boasting a great variety. Most of these measure only 20-30 centimetres, but they can span from a minute one centimetre to as much as 65 centimetre across.

There are two sub-types of sea stars or starfish, the Asteroideas which are true sea stars and sun stars, and ophiuroideas which are brittle stars and basket stars.

The differences between both groups lie in how the arms connect to the central disk. Ophiuroids have arms that do not connect with each other.

There is a distinct (noticeably different) boundary between the arm and central disk. Asteroids have arms that are connected to each other. Also, it is hard to tell with asteroids where the central disk ends and the arms begin. Their common name implies they are incredibly unusual.

These creatures are called starfish because they have star shaped bodies, comprising arms that project out of the central disk. Its mouth is located at the underside of the fish. Sounds interesting, isn't it? They are really colourful. If you feel like calling them flower fish instead of starfish, that's not surprising at all.

The most fascinating thing about the starfish is, that their interiors are filled with fluid, and although they have no front or back, they are able to move in any direction without turning. Rather than using muscles to move their hundreds of tiny tube-like legs (which are situated under each tentacle and resemble a hair brush), starfish use a complex hydraulic system to shift around or cling to rocks. The intake valve of this system is generally located on the top of the starfish, as can be seen clearly on the leather star variety.

If you ever try to remove a starfish from a rock someday, you will really appreciate the effectiveness of its hydraulic system.

Most species of starfish shed their eggs and sperm freely into the water, so fertilisation takes place externally. The very small chance of fertilisation is compensated by the enormous amounts of eggs they discharge into the water, with a mean (average) diameter of 0.16-0.19 mm.

Starfish have hundreds of tiny feet on the bottom of each tentacle.

Starfish have an enduring fascination for many people, but only a few of us understand their complex nature. We mentioned that they have their mouths at the bottom, but did you ever wonder where the eyes of these creatures are?

Their eyes are not in the traditional form or place. Instead, they have eyespots at the tip of each arm or tentacle, which act as light sensors. But starfish actually lack the capability to see images and also the sense of hearing.

Their primary sense is chemoreception. Starfish use chemical concentration information (which may be actual concentration or some other correlated parameter, such as slope or height of odour[a smell] peaks) to compare concentration between their rays and move in the direction of the highest concentration. Sometimes this moves them upstream, while at other times, it moves them across-stream towards the plume axis.

And know what? They don't have a brain, which means they lack a coordinating nervous centre. They have a radial nerve running down the length of each ray, connected to the other radial nerves by a circumoral nerve ring in the body. Though the nerve ring provides connections, it is not a processing centre. The structure of the nerve ring is much simpler than the radial nerves.

More about starfish: varieties, eating habits and enemies next week.

- Janani Amarasekara.

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