Few marine creatures capture the imagination quite like nudibranchs. These vibrant, soft-bodied sea slugs are favourites among divers and underwater photographers, thanks to their stunning colours and bizarre forms. But in addition to being the stars of our marine social media feeds, nudibranchs also serve a crucial role as indicator species for climate change. They provide key insights into ocean health, making them essential subjects of marine research. Cape RADD contributes to this effort by including nudibranchs in its marine invertebrate surveys around False Bay, South Africa. So what makes these tiny creatures so important? And how can studying them help us to better understand changes in our marine neighbourhood?
What Are Nudibranchs?
Nudibranchs (pronounced “new-dee-branks”) are a type of sea slug belonging to the mollusc class Gastropoda, making them distant relatives of land snails. Lacking the protective armour of their shelled cousins, they rely on alternative defence mechanisms, such as toxic secretions or incorporating the stinging cells of their prey. Their most distinctive feature is their external gills, which often appear as feathery plumes or tentacle-like appendages called cerata. These structures give nudibranchs their name, which means “naked gills” in Latin. However, what truly sets them apart is their dazzling array of colours and patterns, which serve as a warning to predators and delight keen-eyed divers.
Over 3,000 known nudibranch species exist, with new discoveries still occurring — the most recent in December 2024. Their incredible diversity is thought to stem from their ability to adapt to specific ecological niches and their notoriously picky diets (more on that later). As a result, many species are endemic to relatively small regions. In the Cape Town area alone there are more than 75 species, most of which are endemic to this section of the coast, including the flashy gasflame nudibranch and the elegant cape dorid (below).
False Bay’s varied habitats, from rocky outcrops to sea bamboo kelp forests, host a unique assortment of nudibranchs. Different locations often reveal entirely distinct species, making each dive an exciting opportunity for discovery. As it turns out, this specialisation also explains why nudibranchs are excellent indicators of changes in their local environment.
Why Are Nudibranchs Indicator Species for Climate Change?
An indicator species is an organism that reflects the health or quality of its ecosystem, often by reacting quickly to changes in its environment. This makes them invaluable for tracking trends in pollution, habitat shifts, and climate-related impacts. By spotting these trends early on, we can more quickly signal a call to action and have a greater chance of reducing any long-term habitat degradation.
Nudibranchs are considered to be an indicator species for climate change for three key reasons:
1. They Are Highly Sensitive to Ocean Water Conditions
Rather than relying on sight, Nudibranchs perceive their surroundings using sensory organs called rhinophores. Much like a finely tuned sense of smell, these horn-like structures detect faint levels of dissolved chemicals in the water. They can even sense water pressure changes and vibrations! Nudibranchs use these organs to navigate, hunt for food, avoid predators, and find mates. Because of this acute sensitivity, nudibranchs respond quickly to ocean changes caused by pollution, ocean acidification, and shifting ocean currents.
Nudibranchs are also cold-blooded, meaning their internal temperature fluctuates with the surrounding water. Many species have very specific thermal tolerances, so even minor temperature shifts affect their populations. As ocean temperatures rise, scientists have observed nudibranchs expanding their ranges into previously cooler waters. These movements provide valuable real-time evidence of ocean warming and its impact on marine biodiversity.
2. They Have Short Life Cycles
Long-lived marine species may take years to show climate-related shifts, but nudibranch populations can change within a single season. This is because most nudibranchs live only a few months, allowing researchers to track environmental changes on short timescales. In one study from 2014, unusually warm waters in the Northeast Pacific Ocean caused a rapid northward shift in nudibranch populations. Within a year, all nudibranch species in the study were observed in higher numbers at the northern, cooler, end of their range, with 30% of species found at record northernmost locations. Such a rapid and dramatic biological response can be an early signal of ecological stress and help researchers to predict downstream impacts.
3. They Rely on Very Specific Food Sources
Nudibranchs are famously picky eaters — many species’ diet consists of only a single type of sponge, hydroid, or coral. This close relationship to their prey is thought to be one of the drivers for nudibranch speciation. Some species have even adapted to spend their entire life cycle, from larva to adulthood, directly on the coral or sponge that is their food source. Because of this dependence, nudibranch populations directly reflect the health of their prey. If sponges and corals decline due to ocean acidification or habitat destruction, nudibranchs disappear as well. By monitoring these shifts, scientists gain insights into broader ecosystem changes.
How You Can Get Involved
Nudibranchs do much more than dazzle underwater photographers — they provide essential data on our changing oceans. Studying their movements, population shifts, and ecological interactions helps scientists better understand the impact of climate change on marine life. For this reason, nudibranchs are a popular subject of study in marine biology and conservation science.
The best part? You don’t need to be a scientist to contribute. Nudibranch research thrives on citizen science. Divers worldwide help track nudibranch populations simply by sharing their sightings.
If you love the ocean, here’s how you can get involved:
- Log Your Sightings: If you spot a nudibranch while diving, take a photo and upload it to platforms like iNaturalist (look for projects specific to your area!) or local marine research databases.
- Support Conservation Efforts: Volunteer, donate, or advocate for organisations protecting marine habitats. Spreading awareness about nudibranchs and their role in climate monitoring strengthens conservation efforts.
- Join a Cape RADD Activity: Participate in a Snorkel for Science program or dive deeper into marine conservation by enrolling in the Marine Science Field Course!
Nudibranchs may be small, but they tell a big story about ocean health. Whether you’re a scientist, a diver, or simply an ocean enthusiast, you can play a role in protecting these fascinating creatures and the ecosystems they call home. So next time you’re in the water, keep an eye out—you might be witnessing an important clue in our planet’s changing climate.
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