These are the largest known Troons. They live in colonies of up to 1000, maintaining a territory of about 10 square kilometres. Each one is capable of biting off your hand.
Much like their smaller cousins, they are a dark blacky/browny colour, with small black eyes and two large teeth. Their gills are soft but serrated and their stalks have a diameter of 3-3.5cm. They are about 10cm high, and quite heavily built, weighing around 1kg. One in Pen Island Zoo once grew to 50cm tall, weighing 4 kg!