A Different Kind of Underwater Life
Some divers are surprised to discover a sea slug inside bubble algae — a behavior that looks almost impossible at first glance.
Some sea slugs don’t just live on algae. They live inside it.
How It Works: Life Inside a Bubble
These tiny slugs — often from the genus Ercolania — interact with algae in a very unusual way.
They puncture large algal bubble cells (such as Valonia ventricosa), then crawl inside head-first. Once inside, they begin feeding — slowly consuming the chloroplast-rich inner layers of the cell.
The process is gradual:
- Over a few hours, the inner contents are partially eaten
- Over a couple of days, the cell becomes almost empty
- Eventually, the same space turns into a nursery filled with eggs
And the slug? It remains inside the entire time.
What’s even more surprising — the bubble itself doesn’t collapse.
When everything is finished, the slug simply exits and moves on to the next cell.
Why It’s So Hard to See
These slugs are extremely small — often just a few millimeters long — and perfectly adapted to their environment.
Finding them requires:
- Slow, careful observation
- Attention to small details on algae
- Often, a good macro setup
Photographing them is even harder. But when you do find one inside a bubble, it’s one of those moments that reminds you why macro diving never gets boring.
Identifying What You’ve Found
Finding these slugs is one thing. Understanding what you’re looking at is another.
Many species from the genus Ercolania look very similar at first glance, and their behavior is often the key to correct identification. Details like host algae, feeding patterns, and egg placement can be just as important as morphology.
If you want to go deeper into identification, we’ve compiled a dedicated photo guide covering nudibranchs and related sea slugs, including many lesser-known and hard-to-spot species.
→ Link to the most up-to-date nudibranch guide Nudibranchs of the Coral Triangle
A Strange Parallel
For some reason, this behavior always brings to mind The Little Prince.
Each slug lives inside its own tiny “planet.”
But unlike the Little Prince, who carefully tends his world, these creatures consume theirs from the inside.
A Small Reminder
We don’t get that option.
We only have one planet — and unlike these sea slugs, we can’t just move on to the next one.
