[2] Thecacera pennigera has 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.

[1] It has been reported to have 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.

The fertilised eggs are deposited in strings draped across the substrate, usually bryozoans of the genus Bugula, on which the sea slug feeds.