The seastar belongs to a group of some six thousand species called echinoderms. These include sea urchins, brittle stars, sea feathers and sea cucumbers. We struggle with these names, which hint of form but not of function or lifestyle. We barely comprehend their existence, extended over 550 million years. We often forget how abundant they are today. We don’t quite appreciate exactly what they are as our relatives.

My friend and I make a special expedition to the beach in Lennox to do some wildlife photography. Our subject is a small marine animal, often overlooked. We quickly locate a couple of specimens and gently set them inside the ‘wet studio’: a clear plastic dish of seawater on a blue cloth over the sand. They tense and twist, reaching around. What to make of these two seastars?

The largest seastar is less than a 100mm in length. Perhaps it’s better to say it’s about 50mm in any direction from its centre. Unlike most other animals, seastars are not bilateral. Nor do they have a ‘head’. They do, however, have two sides. The oral, with a mouth is ‘below’. The aboral, the opposite, is the ‘top’.

The aboral surface is colouful and rough. The thin skin is stretched over a skeleton of small jointed plates made of calcium carbonate. The plates are held together with collagen, a stringy protein that works like a muscle, stretching and retracting.

The skin is also covered with many very small two blade pincers called pedicellariae. These snip away, preventing any algae or debris from settling on the seastar.

Another unique part of the aboral surface is its link inside to the internals of the seastar. A small white round disk near the centre is a miniature sieve called a madreporite. The seastar uses it to regulate the flow of seawater in and out of itself.

I flip over one seastar and position it near the other. I want to photograph seastar hydraulics. Running down the centre length of each ‘arm’ is an open groove in the hard skeleton/skin combo. Along this length, waving about, are little tentacles. These are tube feet, complete with suction cup tips.

The tube feet are the visible part of the single soft internal tubular hosing system. All tube feet are linked to a length of flexible pipe which joins in a ring circling the centre of the animal. Along the top of these waterworks is a complex string of nerves.

Without any central coordination such as a brain, the nerves manage the plumbing. The tube feet are used not only for moving about but also for smelling and tasting.

Living marine water is full of molecular messages too subtle for us humans. But using its hydraulics/nerves combo, the seastar senses what is nearby. It can follow the scent of its preferred food. Depending on the species of seastar, this includes shellfish, snails, sponges and algae.

The hydraulics/nerves combo secures the meal by stabilising the seastar in position on top of food. Holding on is critical because of the seastar’s eating habits. At the centre of the oral side is a round mouth. The first section of its stomach is pushed through the mouth. Digestive juices break down the food so that it can be absorbed into the second stomach.

If the meal is a shellfish, the tube feet steadily pull on the two shells. The pull is rather weak but the seastar simply outlasts the effort of shellfish. The shellfish muscles may be stronger but are more quickly exhausted.

The seastar belongs to a group of some six thousand species called echinoderms. These include sea urchins, brittle stars, sea feathers and sea cucumbers.

We struggle with these names, which hint of form but not of function or lifestyle. We barely comprehend their existence, extended over 550 million years. We often forget how abundant they are today. We don’t quite appreciate exactly what they are as our relatives.

Echinoderm eggs develop in a pattern like ours, all of us animals with spinal cords. Every other group of animals’ eggs develops differently. This difference begins at a very early moment; when the fertilised eggs, having divided into four cells, are about to become eight.

For echinoderms and us, the cells stack one layer atop of the other. More divisions result in a hollow ball. The first opening in that ball develops into an anus. The second opening created becomes a mouth. This pattern is called deuterostome, Greek for ‘second mouth’. 

Every other animal has eggs which divide into in a spiral pattern. The hollow ball develops its first opening into a mouth. These protostomes, Greek for ‘first mouth’, grow into insects, snails, crabs, prawns and worms.

But specialists find that worms differ.  One group, the hemichordates, has internal supporting rods called notochords. A kind of spine. Unlike other worms, they are also deuterostomes.

Chances are good that the origins of all vertebrates are with these worms. The echinoderms, way back when, went in another direction.

So here we all are today, joined and separated by patterns of development going back to the third cell division of our eggs. I search through my photographs for that telling image. I want one that can bridge deep time with life as it begins not only for each animal but for the planet too.

There isn’t any single image. There isn’t a single story, either. For all that I untangle in one article, there is still more to tell. That’s why my team and I created an online resource, a website called Tangle of Life  www.tangleoflife.org.

This week, a new section opens about animals. You can read more about eggs.

Look at eggs dividing. A seastar developing.

Tell your friends. The wealth of the world really is in the kingdoms of life.