Thecacera pennigera, also known as the winged thecacera, is a species of sea slug in the family Polyceridae. It has a cosmopolitan distribution, being found in temperate waters on either side of the North Atlantic Ocean, in the Mediterranean Sea, around South and West Africa, Brazil, Japan, Korea, Pakistan and more recently in Australia and New Zealand. There is a significant difference in colouring between Atlantic populations and Pacific specimens, however. Thecacera pennigera has a typical adult length between 15 millimetres (0.6 in) and 30 millimetres (1.2 in), featuring a short, wide head with two lateral flaps and two sheathed olfactory organs called rhinophores. The body is wedge shaped, being wide at the front and ending in a slender foot with a lateral keel on either side. The general colour of the body is translucent white and the upper side is covered with orange splotches and small black spots. Like other sea slugs, T. pennigera is a hermaphrodite with internal fertilisation and a mating mechanism whereby pairs of animals exchange packets of sperm. This T. pennigera was photographed in the Mar Piccolo of Taranto, Italy.
