Cuthona caerulea

(Montagu, 1804)

The body of this strikingly beautiful nudibranch may reach 26 mm in length. The oral tentacles and rhinophores are white, suffused with lemon-yellow. Opaque white epidermal pigment often forms an indistinct streak running from the head to the posterior tip of the foot (dividing to pass on either side of the pericardium). The most vivid and diagnostic features are associated with the cerata. These are set in up to twelve rows (usually 8-10), with up to eight (usually six) cerata per row. Variation in the colour of the hepatic lobe within each ceras and in the pigmentation of the ceratal epidermis bring about a wide morphological range. The hepatic lobe may be almost black, various shades of blue or bright green. The epidermal pigment consists maximally of three bands of colour: (a) a subterminal red, orange or yellow band fading towards the colourless tip is invariably present; (b) usually, a blue, turquoise or green band encircles the ceras below this (sometimes completely absent); (c) a lower band, similar to (a) but less vivid. The cerata are usually stout, but this is a variable feature.

This species feeds upon the hydroids Sertularella polyzonias , S. gayi , Halecium halecinum and Hydrallmania falcata . Mediterranean specimens have been recorded feeding on Eudendrium .

It is known from the Mediterranean Sea (to Naples), Portugal and France to southern Norway, to 270 m (Distr. C. caerulea). It also occurs on the American coast.