Going to war

Sea anemones vary in widely size from half an inch wide to six feet in diameter (1.3 cm to 1.8 m), although most are less than two inches (5 cm). NOAA.

Close relatives of coral take up residence on rocky reefs. These are sea anemones and hydroids, the latter of which may or may not eventually transform into jellies. Both are armed with stinging cells in their tentacles. Anemones look like large corals. And, like corals, they’re found in many different environments and come in many shapes and sizes. They’re sometimes found in groups, since they increase their numbers by budding and dividing in half, although others, like the common great green anemones, are solitary.

Like many corals, anemones also house symbiotic bacteria that make food for them. Some will move towards light to help their algae photosynthesize and one, the snakelocks anemone, will track the sun across the sky as sunflowers do.

While anemones are often mistaken for plants, they’re actually animals. They look like they’re rooted to rocks and are immobile, but they can slowly crawl away like snails, seeking a better spot to sit for a while. They also crawl when dividing themselves in half. One side moves one way, while the other heads in the opposite direction and they slowly stretch until they tear in half. That can take around ten minutes. Then each one regenerates and heals up, resulting in two clones. And if they’re involuntarily moved from their spot, any little bits of tissue left behind will grow into new clones.

If they want to get somewhere and walking is too slow, they’ll release themselves altogether and roll off or float away.

These coral polyps are open for feeding. Their body structure is similar to that of anemones, but they're much smaller. G.P. Schmahl, NOAA.

For corals that live in coral gardens with other types of corals, they prefer to keep to themselves. Even though the polyps can’t move, they are territorial and will use the stinging cells in their tentacles to kill polyps of other species that try to settle close to them. They also release poisonous chemicals to ward off other unrelated polyps and those corals respond in the same manner. This establishes a no-man’s land boundary between them. This prompted physician and author Lewis Thomas to refer to them as “biologically self-conscious.”

Some types of their larger relatives, sea anemones, are the same. This is something like how our immune system protects us from external cells that might harm us by distinguishing self from non-self. In the anemones’ case, they seem to be protecting their living space.

Lots of hungry anemone mouths.J.R. Ferrer Paris, CC BY-SA 2.0.

These anemone species reproduce by budding where a new anemone sprouts from its side, dividing by tearing itself in half either vertically or horizontally; or by fragmentation where bits are left behind when walking or are torn off and grow into new anemones. They all recover within three weeks. These methods create a group of clones living closely together, and they can tell by touch when an anemone is a clone or genetically different and they immediately attack those that are different. Marine biologists near Bodega Bay in California have closely studied this in anemones that live at the low tide line, so those are the ones I’ll describe here.

In a colony of clones, the anemones around the edge are mostly ready for battle, while those near the center are more focused on reproduction. The warriors make up the outer three or four rows of individuals. Also along the edge are well-armed scouts and poorly armed free-edge anemones, both of which are small. Further back are some small reserves that might be able to transform into warriors or reproductives as needed. This division of duties is similar to that in social insects, such as ants and bees. When two groups live close together, battle lines are drawn and a no-man’s-land two-anemones wide forms.

These particular anemones are above water when it’s low tide. During this time they release most of the water they contain and shrivel up, turning in on themselves, and they look like green lumps with sand and shell fragments stuck to them, which helps them retain moisture.

Once the tide rises, they expand and unfurl their tentacles. The first thing they do is check their border. Those about four rows back or so extend their trunks so their tentacles can reach no-man’s-land and they, along with those in the first rows, start searching for enemy scouts that are venturing into no-man’s-land, searching for empty space they can take occupy. If they find one, they touch it with specialized weaponized tentacles, leaving behind stinging cells that then fire their harpoons and toxins into the scout causing damage that looks like a cigarette burn. And the cells continue firing for some time, as the area around the wound puckers inward.

These fighting tentacles are easily discernible during a fight, since they’re thick and are in a collar below their thin feeding tentacles, but when not in fighting mode these tentacles are deflated and may just appear as a row of bumps.

The scout might fight back or retreat. Usually it retreats, but if it receives too many patches of tissue from attackers, its clonemates might mistake it for the enemy and attack it as well. Scouts can die from the injuries caused by one side or both. They can also die if they retreat, but are too badly damaged, so may give up the ghost from their wounds while they’re back inside their group. Or they might become too weak to hold on and then the waves will wash them away.

Free-edge anemones usually inhabit community edges that don’t face an opposing force, but occasionally they inadvertently venture into no-man’s-land where they’re attacked, but since they’re poorly armed, they don’t usually fight back. If attacked repeatedly over time, they can transform into warriors, since all of the anemones can change roles.

The attacks aren’t always just on scouts. Sometimes a number of warriors become involved and it can become a battle. Generally the anemones with the most weapons win. All of this occurs in the first hour after the tide rises and the fighting usually lasts twenty to thirty minutes, then things quiet down until the next rising tide sets them off again. The borders are dynamic, varying by the success of one group over the other. They may change rapidly, but can settle into long periods of entrenchment—sometimes lasting years.

Each group behaves differently, varying in organization and reaction to signals. The warriors also seem to remember their opponent’s style of attack, so are able to react quicker in their next bout.

Most anemones live by themselves, but they don’t like being touched by others either. If the loser of a fight vacates its spot, the winner might bend itself over with its mouth just above the surface and place a clone from inside its body to occupy that space.

Anemones have disorganized nerves and muscles, but nothing like a brain or any senses other than the ability to sense chemicals, yet they’re clearly able to make decisions. All of this is fairly complex behavior for animals that Rick Grosberg, a marine biologist who works with them, describes them as “gelatinous bags of snot”.

 

If you like this, please subscribe below to receive an email the next time I post something wondrous. It's free.