Is It a Flower or an Animal? A Stomach With Tentacles
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An anemone is a predatory animal built as a column with a mouth on top surrounded by stinging tentacles, and almost nothing about its body plan resembles that of animals people are familiar with. Several live in partnerships that are genuinely remarkable.
How the animal is built
The body is a hollow column attached at the base, with a single opening at the top surrounded by a ring of tentacles. That opening serves as both mouth and exit, since there is no gut running through the animal and no second opening. Inside, the cavity is divided by radiating partitions that increase the surface available for digestion and that carry the reproductive tissue. There is no brain, no heart, no blood and no skeleton, with the shape maintained by water pressure inside the cavity against a flexible wall. Nerves form a net rather than a centralised system, so the animal responds to touch anywhere without anything coordinating a response centrally.
How the stinging works
The tentacles carry cells found only in this group of animals and their relatives:
- •Each cell contains a coiled thread under enormous internal pressure
- •A trigger on the cell surface responds to touch and to chemical cues together
- •Firing everts the thread, turning it inside out at extraordinary acceleration
- •The thread penetrates the prey and delivers toxin
- •Each cell fires once and is then discarded and replaced
- •The whole event takes a few microseconds and is among the fastest in biology
The partnerships
Several species live in close association with other animals in arrangements that benefit both. Clownfish live among the tentacles of particular large anemones, protected from predators by stings they are immune to, while defending the anemone from fish that eat it and improving water circulation around it, and the immunity is achieved by a mucus coating that the fish acquires gradually by careful contact. Hermit crabs carry anemones on their shells, gaining protection from the stings while the anemone gains mobility and scraps of food, and crabs transfer their anemones deliberately when they change shells. Many anemones also host photosynthetic algae within their own tissue, which supply a substantial share of their food.
The animals they are related to
Placing them properly clarifies what an anemone actually is. They belong with jellyfish, corals and hydroids in a group whose defining feature is the stinging cell, and every member is built on the same radial plan with a single opening. Corals are essentially anemones that build a hard skeleton and live colonially, which is why a reef is an animal structure. Jellyfish are the same body plan inverted and swimming, with the mouth downwards and the tentacles trailing. Several species in the group alternate between an attached form and a swimming one during their life. The group appeared over half a billion years ago and is among the earliest animal lineages with a recognisable body plan.
What they do that animals are not supposed to
Several capabilities are unusual enough to have attracted serious research attention. Regeneration is extensive, with a fragment of an animal capable of regrowing a complete individual, and the genes involved overlap with those governing development in animals generally. Several species show no measurable signs of ageing, with mortality not rising with time and individuals in captivity recorded for many decades. Reproduction happens both sexually and by splitting, so a colony may consist of genetically identical individuals covering a rock. Some species move, slowly, by creeping on the base or by detaching and drifting. And a few swim actively to escape predatory starfish, which is a startling thing to watch.
The takeaway
The body is a hollow column with one opening serving as mouth and exit, divided inside by radiating partitions, with no brain, heart, blood or skeleton and a nerve net rather than a central system. Tentacles carry cells containing coiled threads under pressure that fire in microseconds. Clownfish and hermit crabs live in partnerships with them, and several species regenerate completely and show no measurable ageing.