IYOR Intern's Take on Singapore's Marine Life - How sea anemones evade predators?

Now that you got to know a bit more about the sea anemones, so how sea anemones evade predators? Let's find out in this post!

The Game of Survival 

Stealthy, quiet, and well-camouflaged, the sea anemones are likened to ninjas of the sea. Photo credits: Grae Hunter. 

A typical sea anemone (aka sea ninja) has to evade its predators, battle other anemones, and find its next meal in an ocean of scarcity and competition. Toxins are a sea anemone’s most vital weapon because they play a part in capturing prey, repelling predators, digestion, and interspecies competition.

The typical strategy for capturing prey involves the sea ninja patiently waiting for its victim(s). Once the anemone wraps its tentacles around the prey, nematocysts discharge and inject a lethal toxin concoction into the prey. The prey experiences intense pain and paralysis due to the mixture of neurotoxins, pore-forming toxins, and toxin enzymes.

Once ensnarled by the tentacles of a sea anemone such as the Haeckel's Anemone, the prey will become immobilised by the painful stings released by the anemone. After which, they will become the meals of the day.

But wait! The victim has to deal with another horrifying surprise—spirocysts - a type of cnidae cells. The spirocysts ensure that the prey stays stuck until the anemone pulls the tentacle into its mouth for digestion. However, it gets worse for our victim. The anemone continues to inject the living, breathing prey with even more venom while it is being digested in the gastrovascular cavity.

Even though anemones have a scrumptious scheme for capturing prey, they still have to compete with other anemones for food and prey. The sea ninja not only engages in combat with other species but anemones with species as itself as well. Some anemones use acrorhagi (specialised tentacles used solely to deter other anemones/colonies from encroaching into its space) as toxic bombs, while others use catch-tentacles as their sword.

Acrorhagi - highly specialised tentacles with a sole purpose to 'fight' against other anemones or organisms. Photo credit: Emery Hodge.

Certain species of anemones in the family Actiniidae have acrorhagi in a ring around the base of the tentacles for intraspecific competition. The acrorhagi have tiny sacs with large nematocysts packed with toxins. Once the sea ninja comes in contact with the opponent, the acrorhagi attaches to the enemy’s skin and nematocyst discharge. At best, the victim suffers from necrosis; at worst the victim dies within a couple of days. Researchers have found that the type of toxins produced in acrorhagi is different from the ones produced in the tentacles. The acrorhagi toxins are also more lethal compared to toxins found in other parts of the anemone’s body.

Check out this video of Anemones Fighting with their Acrorhagi: https://www.shapeoflife.org/video/cnidarians-anemones-fight

The Lined bead anemone (Diadumene lineata) has got some long catch tentacles!

Some anemones aren't happy with just one set of tentacles. No, they need another set of longer and wider tentacles known as catch tentacles! Normal tentacles tend to be more translucent but catch tentacles are opaque. Catch tentacles will extend and start ‘searching’ for something. Once the catch tentacle finds its target, the tip of tentacle attaches to the body of the opponent and the tentacle breaks off slightly. The punishment for competing with a sea ninja for food or territory is being tortured by the attached catch tentacle tip, as its large nematocysts discharge and inject venom into the foe... until it dies a slow and painful death.

Capturing prey - The sea anemone uses their tentacles around their mouth to catch prey, which becomes triggered when it comes in contact with prey. This trigger is stimulated on a stinging cell called cnidocyte, which shoots a harpoon-like nematocyst into the prey injecting toxins. Photo credit: Erica Patterson.

Of course, there is always someone bigger or stronger in the ocean, whether it’s Lysmata shrimp or scrawled cowfish. Anemones use both offensive and evasive strategies to deal with predators. Anemones can rapidly withdraw via burrowing into the sand or mud flats if there is any kind of sudden mechanical stimulation. The anemone’s body column acts as a hydrostatic skeleton, as its powerful retractor muscles contract and force all of the water to the base or pedal. Then the anemone goes through a cycle of contracting, elongating and contracting until digs deep enough to find a rock or shell fragment.

Acontium tissues found in the gastrovascular (stomach) cavity of Exaiptasia anemone. Taken from: Lam et al. (2017). 

If swimming away or burrowing isn’t an option, then some sea anemones can use their acontia to deter predators. Acontia are thread-like tissue packed with nematocysts, found in the stomach cavity. Anemones can eject and retract acontia by manipulating the water pressure in its gastrovascular cavity. Once the acontia comes in contact with the predator, the nematocysts discharge and inject toxins due to physical contact or specific molecules. Sometimes, an acontial attack is enough to stop a predator from feeding on the anemone.

Ultimately, the sea ninja will do whatever it takes - strike, sting and strangle - to win the game of survival.

About the writer:
Shabdita Vatsa recently graduated from the National University of Singapore with a Bachelors in Life Sciences (Molecular and Cell Biology). She spent most of her undergraduate days obsessing over sea anemones and their toxins. Her fascination became research projects, working with scientists from the Reef Ecology Lab and Protein Chemistry Lab to study anemone toxins.

A passionate writer, Shabdita loves creative writing, popular science writing, and scientific writing. You can find more of her writings on her personal page: https://lifesciencepotluck.tumblr.com/

References used:

• Magnum (1970) Burrowing behaviour of sea anemone Phyllactis. Biological Bulletin 138: 316-325.
• Edmunds et al. (1976) Defensive behaviour of sea anemones in response to predation by the opisthobranch mollusc Aeolidia papillosa (L.). Journal of Marine Biology Association of United Kingdom 56: 65-83.
• Purcell (1977) Aggressive function and induced development of catch tentacles in the sea anemone Metridium senile (Coelenterata, Actiniara). Biological Bulletin 153: 355-368.
• Watson & Mariscal (1983) The development of a sea anemone tentacle specialized for aggression: Morphogenesis and regression of the catch tentacle of Haliplanella luciae (Cnidaria, Anthozoa). Biological Bulletin 164: 506-517.
• Shick (1991) Functional biology of sea anemones. Springer.
• Anthony (1997) Prey capture by the sea anemone Metridium senile (L.): Effects of body size, flow regime, and upstream neighbours. Biological Bulletin 192: 73-86.
• Honma et al. (2005) Novel peptide toxins from acrorhagi, aggressive organs of the sea anemone Actinia equina. Toxicon 46: 768-774.
• Frazão et al. (2012) Sea anemone (Cnidaria, Anthozoa, Actiniara) toxins: An overview. Marine Drugs 10: 1812-1851.
• Lam et al. (2017) A detailed observation of the ejection and retraction of defense tissue acontia in sea anemone (Exaiptasia pallida). PeerJ 5: e2996.