New research reveals how aeolid nudibranchs neutralise and repurpose nematocysts from scyphozoan polyps.
The study examined the predator–prey interactions between two nudibranch species, Caloria militaris and Flabellina affinis (shown), and various cnidarians, focusing on scyphozoan polyps from common eastern Mediterranean medusae.
Some of the ocean’s most striking sea slugs—aeolid nudibranchs—possess an extraordinary ability: they can feed on stinging cnidarians without being harmed, and then capture and reuse their prey’s stinging cells, known as nematocysts, for their own defence.
New scientific findings have now shed light on how they do it, offering a clearer picture of this remarkable biological strategy.
Feeding without being stung
A recent study has examined how aeolid nudibranchs prey on scyphozoan polyps, confirming that they can consume tissue loaded with active nematocysts without triggering discharge. The research shows that the slug’s digestive system has multiple protective adaptations that prevent nematocysts from firing during ingestion.
Mucus secretions, specialised epithelial linings and controlled pharyngeal movements appear to ensure that nematocysts pass through the gut intact. This finding is significant because scyphozoan polyps contain dense batteries of stinging cells normally capable of immobilising small predators.
Borrowed defence
Once inside the nudibranch, the unfired nematocysts move through the digestive glands and into the cerata, the finger-like extensions on the animal’s back. At the tip of each ceras lies a specialised chamber called the cnidosac, where nematocysts are sorted and stored.
The study confirms that the nudibranch distinguishes between discharged and undischarged nematocysts, retaining only those capable of firing. These are incorporated into the cnidosac’s defensive tissues, forming a functional arsenal known as kleptocnides.
When threatened, the nudibranch can release these captured nematocysts to deter predators—a form of borrowed defence that remains one of the most unusual examples of biological recycling in the animal kingdom.
