Nudibranchs, often called sea slugs, are brightly colored marine gastropod molluscs found in seas worldwide. Their striking appearance raises questions about how they survive without a hard shell. Unlike their snail relatives, nudibranchs shed their shells after the larval stage, leaving their soft bodies exposed to predators.
This vulnerability necessitates a powerful alternative defense strategy, often involving chemical warfare. Their striking coloration serves as a clear warning sign (aposematism) to potential predators that they are unpalatable or toxic. This defense allows them to thrive in open marine environments despite lacking physical armor.
Poisonous Versus Venomous
Understanding the nudibranch’s defense requires distinguishing between poisonous and venomous toxins. A creature is venomous if it actively injects a toxin, typically through a bite or sting. An organism is poisonous if its body contains a toxin that is harmful only when ingested or absorbed through surface contact.
Nudibranchs are classified as poisonous; the toxins are contained within their tissues and are not actively delivered. They lack the apparatus, like fangs or stingers, to inject a substance into a predator. For a marine animal or human to be affected, they must ingest the animal or break its skin. This makes their defense a passive deterrent rather than an offensive weapon.
Acquiring and Storing Chemical Defenses
Nudibranchs do not usually produce toxins internally. Instead, they acquire them from their diet through a process called dietary sequestration. These specialized feeders consume inherently toxic organisms, such as sponges, hydroids, or sea anemones. They ingest these toxic prey items without harming themselves, effectively turning their food’s defense into their own.
Sponges are a primary source of defense, producing complex secondary metabolites to deter predators. Dorid nudibranch species that feed on sponges selectively absorb and store these chemical compounds in specialized skin glands or mantle dermal formations. These compounds, which can include potent cytotoxins like Latrunculin A, are concentrated in the outer tissues to be effective against a predator’s initial bite.
Aeolid nudibranchs feed on cnidarians and employ a unique form of sequestration known as kleptocnidae, or “stolen stinging cells.” They ingest the undischarged stinging capsules, called nematocysts, from their prey. These capsules travel through the digestive tract, avoiding discharge, and are stored in specialized sacs called cnidosacs at the tips of the dorsal appendages (cerata).
When threatened, the aeolid nudibranch deploys these stolen nematocysts to sting a predator, providing a powerful mechanical-chemical defense. While most defenses are sequestered, some species can produce a limited number of unique defensive chemicals de novo (from scratch). However, the vast majority of their chemical arsenal results directly from their specialized feeding habits.
Risk Assessment for Human Contact
Despite their potent chemical defenses, the risk of harm to humans from nudibranchs is generally low. The toxins are primarily designed to be a deterrent to other marine life, causing a foul taste or an irritating effect rather than a fatal injury. A person would need to ingest the animal for the chemical toxins to be absorbed into the body in a harmful concentration.
Even for the aeolid species that steal and reuse stinging cells, simple skin contact is unlikely to cause a severe reaction in humans. While the nematocysts can be discharged upon contact, the effect is typically minor compared to the sting of their original host, like a jellyfish or Portuguese man-of-war. The most common advice for divers and marine enthusiasts is simply to observe these animals without touching them.
Handling a nudibranch should be avoided to prevent irritation and to protect the animal from stress. Since their defense mechanisms are primarily a taste or an irritant, the most practical approach is to respect their vibrant warning colors and appreciate them from a distance. The beautiful coloration is a signal of their chemical nature, but it is a warning meant for predators, not a threat to casual human observers.