By Bill Barker
If a science fiction writer was seeking inspiration, the sea provides almost endless examples of strange creatures that could provide models for alien beings. The density of the watery medium in which sea creatures live means that many of them have been able to dispense with a skeleton, as their body mass is supported by the water. They then can grow into all manner of strange shapes. Another factor making for greater variety is that the sea is, in many places, a soupy mix of nutrients that enable animals to live simply by attaching themselves permanently to a hard surface and sieving out the food as it goes past.
An interesting group, found only in the sea, is the spiny-skinned ‘tribe of five’. These ‘echinoderms’, to give them their scientific name, include the familiar sea urchins and sea stars, along with less familiar relatives such as brittle stars, feather stars and sea cucumbers.
On land we expect animals to have a front and a back and a left and a right. The sea permits quite different structures, including the five-sided structure adopted by the echinoderms. The five sided layout is easiest to see in the sea stars, most of which have five arms, though some have six, seven, eight or many more.
They have no eyes, but are voracious predators, hunting by picking up chemical clues as to the whereabouts of their prey. Their mouths are located in the centre of the underside of the animal and they have the bizarre technique of feeding by extruding their stomach through their mouth and digesting their meal outside their body. Next time you are fossicking around a rock pool, pick up one of the small pentagonal sea stars that are often found there. Chances are you will see a clear jelly-like tissue hanging out of the centre of the underside of the animal and enveloping its meal.
At first glance, it might be difficult to see much resemblance between a sea star and the spiky ball that is a sea urchin. But look closer at the ‘test’ or shell of a sea urchin, which is the external skeleton that is left when the spines and tissues have come away. These tests are often to be found on the beach and around rock pools. If you pick one up, you will see that it is made up of five plates, from which in the live animal the spines protrude. Sometimes also you can find the jaws of a sea urchin, a curious assemblage of five teeth, which in live specimens are found at the end of a short muscular tube that makes up the mouth.
The other classes of animal in the spiny skinned group all share in some way the five sided structure. These are the feather stars and basket stars, which wave their arms in the current to catch passing plankton; the brittle stars, which slither around under rocks and can shed their arms if needed for defence; and the sea cucumbers, which are like underwater vacuum cleaners sucking in material from the sea bottom and filtering out any food material.
A unique feature most of the spiny-skins share is a hydraulic system that pumps seawater through tubes leading to suction-tipped ‘tube feet’ which are used to enable them to move around, to pass food to their mouths and even to force open the shells of their prey. If you look on the centre of the upper surface of a sea star you will see a little ‘grate’, just as you might have on the bathroom floor, that permits the water to enter this system. On the underside, the tube feet are clearly visible.
The spiny skins are very successful as predators and competitors. In Batemans Marine Park waters black sea urchins in particular are very common, often to the extent of crowding out other marine life.
Whenever you see members of this group, they are worth a closer look, not only for their own interest, but also because they often host other creatures, fish, worms, molluscs or crustaceans, with which they live commensally. Just remember, always leave things as you find them, particularly if you are lifting rocks on the shore.