How do sea anemones control their stings?

He Lily S, Qi Yujia, Allard Corey AH, Valencia-Montoya Wendy A, Krueger Stephanie P, Weir Keiko, Seminara Agnese, Bellono Nicholas W (2023) Molecular tuning of sea anemone stinging eLife 12:RP88900

Cnidarians are a group of predatory aquatic animals, including corals, sea anemones, and jellyfish, that use stinging cells to capture prey and defend from predators. These cells, called cnidocytes or nematocytes, contain tiny spring-laden harpoons tipped with powerful neurotoxins. Nearly all cnidarians have these stinging cells—but luckily only a few are dangerous to humans!

If you’ve got a weapon, you’d better know how to use it

Partially-fired nematocyst. The red dye stains venom. Image source: David Brand, Wikimedia Commons

To use their stinging cells, cnidarians first need to sense their prey. Touch probably seems like the most obvious method. If you touch a jellyfish, it stings you, right? However, from the jellyfish’s perspective, it doesn’t want to fire too easily. Let’s say it brushes up against a piece of seaweed and fires all of its cnidocytes… now the jellyfish has just wasted a lot of expensive venom, because cnidocytes are one-use only! Therefore, cnidarians need a mechanism to detect prey and predators without wasting those cnidocytes.

Also, different species live in different habitats and interact with different prey and predators. Therefore, they may have different needs and requirements for their stinging. In a recent study, scientists looked at two species of anemones that differ in their lifestyle and stinging sensitivity, and figured out that these behavioral differences could be tracked back to a single gene.

One anemone, Nematostella vectensis, spends its life burrowed in the mud, and mainly uses its stinging cells to capture prey. The other anemone, Exaiptasia diaphana, does not burrow and is more exposed to predators. This species gains extra nutrition from symbiotic algae (symbiotic meaning “living together”). Just like how corals have symbionts that perform photosynthesis, this anemone uses these algae to supplement its diet, and doesn’t rely as much on hunting prey. For these reasons, E. diaphana mainly uses its cnidocytes for defence.

Shoot first, ask questions later?

The different ecological needs of the two species are accompanied by differences in their stinging behavior. N. vectensis is much less sensitive, meaning touch alone is not enough to get it to sting. Instead, this species requires both touch and chemical signals , such as smells from prey. This allows it to optimize its hunting strategy while conserving energy. On the other hand, E. diaphana stings in response to touch alone, without the chemical cue. This allows E. diaphana to better protect itself from predators.

Exaiptasia diaphana. Image source: Chris Alice Kratzer, Wikimedia Commons

Amazingly, a single gene seems to be responsible for these different stinging strategies! N. vectensis uses a type of protein called an “ion channel” to control cnidocytes. When the animal touches something, it activates a mechanical sensor that causes changes in calcium concentration, which activates the ion channel and causes the cnidocyte to fire. In N. vectensis, the channel is inactive by default and only becomes active when the animal detects certain chemicals; therefore, it needs both chemical signals and touch to work.

E. diapahana uses the same ion channel for its cnidocytes, but it’s much more sensitive. That’s why this species responds to touch even without a chemical stimulus. The scientists examined the genetic sequences of the ion channel in both species, and found that a specific part of the gene differs between the two species and is responsible for the increased sensitivity in E. diaphana.

From molecules to ecology

This study is a cool example of how small molecular differences can cause major changes in behavior and ecology. Sometimes even complex behaviors are regulated by relatively simple mechanisms. When you think about how organisms evolve, you can see how a single mutation could have a big impact!



Cover image: closeiup of Nematostella vectensis mouth and tentacles. Image source: Cory Berger


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