Nudibranchs

Nudibranchs are sea slugs that have given up their shell and advertise instead, using colour as a warning that they taste appalling. Four species make up most of what Maltese divers photograph. The dotted sea slug, Peltodoris atromaculata, is a firm white oval 5 to 7 cm long with irregular dark brown blotches, almost always sitting on the pale brown stony sponge Petrosia ficiformis, which is the only thing it eats. The regal doris, Felimare picta, is the showpiece: up to 13 cm, deep blue-black with bright yellow lines and elongated rings, feeding on Dysidea sponges.

The other two are aeolids, slimmer slugs whose backs are covered in finger-like cerata rather than a flat mantle. Flabellina affinis is translucent violet-pink with purple-tipped cerata and grows to about 5 cm. Cratena peregrina is smaller, three or four centimetres, white and blue-tinged with two orange patches on the head and orange-tipped rhinophores. Both graze the polyps of Eudendrium hydroids and are frequently found on the same colony, often in the top 5 to 20 m on dim, shaded walls.

Around the Maltese islands the reliable places are shaded rock and cave mouths: Reqqa Point, the Blue Hole and Anchor Reef in Gozo, the overhangs at Ċirkewwa, and anywhere a good Petrosia sponge or a healthy clump of Eudendrium is growing.