Aggregating anemone

The aggregating anemone, also known as clonal anemone, (Anthopleura elegantissima) is a species of sea anemone, of the family Actiniidae. It can be found on rocky, tide swept shores.

This species of anemone reproduces both sexually and asexually through binary fission. Over time, a single individual can form a colony of genetically identical polyps. Colonies are essentially immortal. When one colony encounters another, the two colonies will wage territorial battles.

The aggregating anemone has specialized tentacles called acrorhagi that inflate and have stinging cells but are not used in food gathering. Instead, they are used to attack enemy anemones. It is difficult to study these anemones in the wild as they live in the tide zone. When the tide is out, they are closed up. When the tide is up, the action of the waves blocks viewing.

A study of two colonies on a boulder removed from the shore and brought into a laboratory revealed that hostilities between neighboring colonies follow the tides. As water rushed into the tank, some of the polyps inflated their acrorhagi, tripled their body length and began reaching into an empty swath of rock between the colonies. Occasionally, a polyp from one of the colonies would move into the no-mans zone between the two colonies, acting as a scout, and would be attacked by the warrior polyps. If the scout polyp received enough stings, it would be attacked upon returning to its own colony. This is likely because it has picked up so many stinging cells from the enemy colony that its own colony mates no longer recognize it.

Anthopleura elegantissima