When it comes to anemones, there’s more than meets the eye.

By Ben Farmer, Waterfront Assistant, The School for Field Studies, South Caicos

Anemones, perhaps the most frequently mispronounced animals in the sea, are fascinating creatures. The vibrant colors and swaying tentacles characteristic of anemones are staples in the backdrop of tropical coral reefs and tide pools around the world. While the tropics are where most anemones are found, they can also be spotted in just about any marine habitat on Earth, from cold-water rocky shorelines to deep ocean habitats. Anemones are able to populate so many habitats because of their incredible variation, spanning over 1,000 known species.

The tentacles of a Giant Caribbean Sea Anemone sway with the current.

What is an anemone?

You may have seen them in the Turks & Caicos Islands and wondered what makes these animals tick. In a lot of ways, anemones are similar to corals, and even jellyfish—they are all grouped into the phylum Cnidaria. Cnidarians are considered some of the simplest organisms in the animal kingdom because they lack what we would consider organs. While you and I have a heart to distribute blood throughout our bodies and lungs to transfer oxygen into our blood, cnidarians lack these organs. Ultimately, cnidarians are made up of just two sets of membranes, or body parts—the epidermis (similar to our skin) and the gastrodermis (similar to our stomach). So, if they don’t have gills or lungs, how do cnidarians get their oxygen? Cnidarians’ thin membranes are the key—these membranes are in constant contact with water and allow diffusion of oxygen into the body.

Simple yet deadly

What really sets cnidarians apart are their potent stinging cells. If you have ever accidentally stepped on a jellyfish on the beach or brushed against one underwater, you may have had first-hand experience with this! These cells, called nematocytes, are used by cnidarians to capture their food as well as protect themselves. Nematocytes stay spring-loaded at all times, ready to fire out with such incredible acceleration that this event is considered one of the fastest found in nature. When nematocytes on the tentacles of a cnidarian are triggered by contact with another object, such as a small tasty crustacean, they launch hundreds of hooked barbs into the object and begin releasing venom. Prey that are unfortunate enough to be on the receiving end of these barbs are incapacitated and then eaten by the cnidarian.

In the case of anemones, they have a mouth in the center of an oral disc, into which their tentacles guide the food. One species here in the Turks & Caicos Islands is in fact a voracious eater. The Giant Caribbean Anemone, Condylactis gigantea, is capable of eating not only crustaceans and other invertebrates, but even small fish!

Anemone menagerie

The mouth of this Giant Caribbean Sea Anemone is located in the center of the oral disc.

Anemones come in countless colors, shapes and sizes, but perhaps the Giant Caribbean Anemone is the type that comes to mind for most people. Even within this species, however, exists a variety of color morphs. In Jamaica, the Giant Caribbean Anemone has been spotted with pink-tipped tentacles in deeper waters, and green-tipped tentacles in shallower waters. These different morphs might be linked to genetic differences in how well the anemone absorbs sunlight. Many anemones require the sun to sustain them, however they also need protection from the irradiation of sunlight. Here at The School for Field Studies Center for Marine Resource Studies on South Caicos, we have documented several color morphs of the Giant Caribbean Anemone. Whether or not these morphs are genetically distinct awaits to be seen. In terms of other anemone species typically seen around South Caicos and the other islands, a common one is a type of corkscrew anemone. These are often seen with symbiotic cleaner shrimp, which use the anemone as both a shelter and a cleaning station.

Symbiosis galore

This cleaner shrimp is using the anemone for protection.

Part of what makes anemones so fascinating are the several forms of symbiosis in which they take part. When most people think of symbiosis, they think of a relationship between two organisms which benefits both organisms. However, symbiosis literally means “living together,” and that does not always bode well for both parties. Types of symbiosis include mutualism (a relationship where both parties benefit from each other), commensalism (one party benefits, the other is neutrally affected), and parasitism (one party benefits, the other is negatively affected).

Thankfully for the anemone, it enjoys many mutualistic forms of symbiosis. Perhaps the most well-known is the anemone’s relationship with the clownfish. As made famous by “Finding Nemo,” the clownfish uses the anemone for shelter, benefiting because the nematocytes employed by the anemone keep predators away. Additionally, the clownfish benefits because it supplements its regular diet with the anemone’s leftovers, as well as parasites on the anemone’s tentacles.

But how does the clownfish avoid injury from the venomous nematocytes of the anemone? Strangely enough, the answer lies in the fish’s mucus—clownfish have a protective coating of mucus surrounding their body which protects them from the anemone’s harmful stings. Meanwhile, the anemone benefits because clownfish are very territorial and fend off fish which eat anemones, and also clean the anemone of sand and waste.

While clownfish are not native to the Caribbean, anemones here have a few other incredible forms of symbiosis. One is the relationship between the anemone and the cleaner shrimp. A species of corkscrew anemone, Bartholomea annulata, is commonly seen in the Turks & Caicos Islands inhabited by Pederson’s cleaner shrimp, Ancylomenes pedersoni. While it is not clear that the anemone derives any benefit from the relationship, the shrimp certainly benefits—a form of commensalism! Fish know to come to anemone “cleaning stations,” where shrimp get a nice meal by cleaning the fish.

Another symbiosis in which most tropical anemones take part is one of the most fascinating mutualistic relationships in nature: that of the zooxanthellae and the cnidarian. Zooxanthellae, a type of tiny algae, live inside the anemone’s membranes and perform photosynthesis. The energy produced by photosynthesis is taken up by the anemone, supplementing its carnivorous diet. The zooxanthellae in turn benefit by living in a safe environment, as well as receiving nutrients that they need for photosynthesis.

Next time you see an anemone, remember there is a lot more than meets the eye!

For more information, contact SFS Center Director Heidi Hertler, PhD at hhertler@fieldstudies.org.