FlabellinaWhere and when to see them

Approaching a delicate aeolid nudibranch requires impeccable buoyancy control, as even a minor disturbance from a fin kick can dislodge the tiny creature from its hydroid perch or kick up silt that ruins visibility. Divers should position themselves parallel to the substrate, using a single finger placed carefully on bare, uncolonized rock if necessary to steady their position without crushing fragile marine life. For underwater photographers, a dedicated macro lens (such as a 60mm or 105mm) paired with dual strobes or continuous snoot lighting is essential to illuminate the rich purples and pinks without blowing out the translucent cerata. Use a narrow aperture between f/11 and f/22 to achieve a sharp depth of field across the rhinophores and cerata tips. Maintain a respectful distance with camera ports, ensuring no physical contact is made with the animal or its delicate hydroid habitat.

The prime season for spotting Flabellina nudibranchs in the Mediterranean Sea, particularly around the Medes Islands and the Costa Brava in Spain, runs from April to June and again from September to October when hydroid colonies thrive in cooler, nutrient-rich waters. In the Ligurian Sea and around Portofino, May and June provide ideal conditions with blooming hydrozoan populations. For divers exploring the temperate northeastern Atlantic Ocean, including the coasts of Cornwall, the Hebrides in Scotland, and the fjords of Norway, the optimal window spans June to September, when seawater temperatures rise slightly, algal cover thins, and visibility peaks. In the subtropical Canary Islands, these colourful sea slugs can be spotted virtually year-round, with winter and spring months (December to April) yielding particularly high densities of reproducing adults on wave-sheltered rocky reefs and overhangs.

Flabellina nudibranchs are found across a depth range spanning from shallow intertidal surge channels down to deep circalittoral reefs exceeding 50 metres, though they are most frequently encountered between 5 and 30 metres. They show a pronounced affinity for current-swept rocky reefs, vertical drop-offs, sea caves, boulder fields, and artificial substrates such as historic shipwrecks and pier pilings where hydroid colonies flourish. The waters this animal calls home encompass the entirety of the Mediterranean Sea, the eastern Atlantic Ocean from Norway and the British Isles down to the Canary Islands, as well as the cold-temperate waters of the northwestern Atlantic and temperate zones of the Pacific Ocean. They require stable salinities and clean, oxygenated coastal waters with moderate water movement.

Flabellina nudibranchs are specialist carnivores that feed almost exclusively on colonial hydroids, particularly species within the genera *Eudendrium*, *Bougainvillia*, and *Pennaria*. Using a specialized, tooth-lined feeding structure called a radula, the nudibranch rasps away the individual hydroid polyps and tissues. A remarkable biological feat of this genus is cleptocnidy: when ingesting hydroids, the slug avoids triggering the hydrozoan's stinging cells (nematocysts). Instead, it transports the immature, unexploded capsules through its digestive tract into specialized sacs called cnidosacs at the tips of its cerata. Here, the stolen stinging cells are stored and weaponized against potential predators such as wrasses and crustaceans. Grazing occurs continuously around the clock, with peak feeding and movement dictated by water flow that brings fresh polyps into reach.