Aggregating Anemones (Anthopleura elegantissima)

Contributed by Biologist Bonnie Becker

Where found:  On exposed surfaces of rocks, especially where sand is being deposited.

Interesting facts:  These animals form beds that cause the rocks to feel so “squishy” and slippery when you step on them.

Adaptations:  Anemones protract their tentacles when exposed and cover themselves with bits of rock and debris. Although this makes a great camouflage, the anemone does this to avoid desiccation. Their distribution is limited by desiccation stress. Although they appear to be quite delicate, these animals are very hardy and can tolerate pollution and slightly hypoxic (low oxygen) conditions.

Food:  Anemones are big eaters and will eat almost anything, even small crabs. Their tentacles are covered with stinging cells called nematocysts, which paralyze small animals. Their digestive enzymes are spectacular, and researchers have seen an anemone devour a chiton in 15 minutes. Anemones are often mistaken for plants, but they are animals. Interestingly, many species contain microscopic plants (dinoflagellates called zooxanthellae and/or unicellular algae called zoochlorellae) living within their tissues in a symbiotic relationship. By definition, the plants are photosynthetic (they produce food using sunlight), some of which is released to the anemone, which contributes nutrients to the plants through excretion of wastes.

Life history:  Anthopleura reproduces both sexually and asexually. During the summer and fall, males and females release gametes to be fertilized in the water. The animals can also “clone” themselves through longitudinal fission. Both have evolutionary benefits–sexual reproduction allows for more genetic diversity, through the mixing of genes and dispersal of larvae. This progeny drifts away from the parents and, if it survives the difficult trip, settles in a brand new area. Once it settles, it can clone itself many times to quickly create a colony that spreads over the new rock. These animals are good competitors for space. Therefore, it is quite common to see large colonies of Anthopleura that are all genetically identical clones.

Between two aggregations there will often be a line of bare rock, aptly named the “war zone”. The anemones on the edge of the colony will aggressively sting non-clone mates to keep them away from its twin “brothers” or “sisters”. The anemones on the edge of the colony tend to be smaller and non-sexual, since they take such a beating from neighboring clones. Colonies can persist for decades.

Classification:
Phylum:           Cnidaria (Jellies, Corals, etc.)
Class:              Anthozoa (Sea anemones, Corals, Sea Pens, etc.)
Subclass:         Zoantharia
Order:             Actinaria
Family:            Actiniidae

Last revised 11-Jun-13