Bright blues, vibrant yellows, blazing oranges, striking reds, rich purples, electric greens, hot pinks, and some milky pastels, all in pops, spots, stripes, and bold color fields. These colors belong to around 3,000 species of nudibranchs, ‘designer’ sea slugs that can be found all over the globe.
Though some nudibranchs camouflage, these soft-bodied creatures stand out with high contrast markings and colors that warn predators of their defense mechanisms.
Some ward off predators thanks to “toxic or bad-tasting chemicals from their prey” but others use unfired nematocysts, microscopic harpoon-like stinging cells collected from the tentacles of their ingested hydroid prey. From KQED Science:
This Jorunna sea slug looks like a tiny, fluffy “sea bunny”, Glaucilla marginata: Beautiful blue sea slugs or nudibranches, and brightly colored flatworm filmed in the waters of Sulawesi, Indonesia.
The nudibranch’s gut has fingerlike branches that extend up into the long cerata on its back. The unfired stingers travel up into the cerata and concentrate in little sacs at the tips, where they continue to develop.
If a fish or crab tries to bite the nudibranch, it squeezes those sacs and shoots out the stingers, which immediately pop in the predator’s mouth. It doesn’t take long for predators to avoid the brightly colored nudibranchs.