Meet your ocean cousins: The fascinating link between sea anemones, urchins, and humans

The anemone consists of a single column-like polyp attached by a sticky foot to the sea bed or sand; a split at the top, that opens and closes, serving as both the mouth and the anus. It is surrounded by the tentacles which immobilize prey and draw them into the mouth.

Sea anemones maintain cordial bilateral relationships with several other creatures, including two types of green algae which live among their tentacles. The algae provide the anemone with oxygen – via photosynthesis – and food in the form of glucose, glycol and alanine, and in return are assured of an adequate amount of sunlight and protection from predators. Other creatures that they have symbiotic relationships with include clownfish – which again, like some of the other creatures are provided protection and are covered with a mucous which immunizes them from the venom, and on whose leftovers and detritus the anemone feeds. The anemone also gets a cleaning service from some of these like the hermit crab and in fact rides on top of their shells and sea snails. The boxing crab holds them in its claws like a sort of first line of defense.

A few weeks ago, I met my second lot of anemones – and they could not have been more different. There were three of them, sitting at the bottom of a shallow tide pool and looked like little crowns of black thorns waiting to be placed on a martyr’s head! At first, I wondered what they were – perhaps sea urchins – which I had not as yet met, but my tide-pooling friend, Sejal Mehta confirmed that they were indeed sea anemones. Though I was quite close to them I stared at them through the binoculars and felt the same calm hypnotic trance come over me, it was hard to take your eyes off them.

Sea anemones maintain cordial bilateral relationships with several other creatures, including two types of green algae which live among their tentacles.

Later, I checked up on sea urchins, aka ‘sea hedgehogs’. They are spiny globular creatures, found in oceans all over the world from shallow tide pools to the deepest ocean depths, and are important herbivores keeping sea algae in check. They may live (eg. the Canadian red sea urchin) for up to 200 years, and in many cuisines are considered a delicacy. They are also relished by sea otters, wolf eels and trigger fish. Largely nocturnal vegetarian algae grazers, they may also indulge in slow moving animals like sponges. If allowed to overgraze, they may denude the sea bed of seaweed – areas which are called ‘urchin barrens’.

They have a rigid hollow internal skeleton (called a ‘test’) to which the spines are attached, and live in a sort of bottoms up way – the mouth being at the bottom, and intriguingly called “Aristotle’s lantern”, the anus at the top! Actually, the term Aristotle’s lantern referred to the entire sea urchin which apparently was shaped like the lanterns used in Aristotle’s time.

The spines, which may be venomous, may be long (primary) or short (secondary) the latter being at the poles, the former at the equator and are used for locomotion and defense. Other creatures such as crabs and shrimp may take cover between the spines even wearing the same colour as the urchin.

They have a conventional sex life – sperm being released and fertilizing eggs which are also released giving rise to larvae which sink to the bottom of the sea and develop further.

I saw my first sea urchins in an aquarium. They were jet black pincushions, not the sort of creatures you would like to put your foot on. Do so and you will be in pain and may develop rashes.

But now for the kicker: both the (the starlet) sea anemone and the (purple) sea urchin are more closely related to us than perhaps we would like. The starlet sea anemone contains nearly half of a full set of human genes – of its nearly 18,000 protein coding genes as many as 7,766 are shared with us. Also, they contain versions of 283 human disease-oriented genes, including one that is related to breast cancer. They are closer to us in kinship than they are to flies and worms – so respect it due – we both descended from a common ancestor.

As for purple sea urchins, they have 23,500 genes, which is more than what we have, and again are more closely related to us than are beetles, flies, crabs and clams. Other creatures too have closer connections to us than we would find flattering – the genes used to build our brains are the same that are used to build the brains of species such as flies, worms and jellyfish.

No matter how different we may be in appearance and lifestyle, we are closely related to these so-called primitive forms of life — many of which (like the sea urchin; fossils of which date back 450 million years) evolved much before us. We are all members of the same family tree. Which is why perhaps, I was so entranced watching those anemones sway hypnotically from side to side: ‘hello cousin!’