Polynoidae

Polynoidae is a family of scaled Polychaete worms known as 'scale worms'. More than 900 species are currently recognised belonging to 18 subfamilies and 167 genera . They are active hunters, but generally dwell in protected environments such as under stones. The group is widely distributed from shallow intertidal waters to hadal trenches . They are the most diverse group of polychaetes in terms of genus number and second most diverse in terms of species number . Most species are short and flattened, but can reach as much as 20 cm in length and 10 cm width in Eulagisca gigantea.Individuals are covered almost entirely by scales known as elytra, which can be shed and regenerated as a means of defence in many species. The scales of some species are faintly bioluminescent, and leave glowing traces around the mouthparts of their predators, making those predators more likely to be attacked in turn. The first deep-sea species was collected at 1230m during the Challenger Expedition and at least 13 of the 18 known subfamilies appear to be fully restricted to the deep sea below 500m . Species have colonised submarine caves and hydrothermal vents. Deep sea species are characterised by a partial or complete loss of antennae, a reduction in jaws and delicate elytra . The phylogeny of the Polynoidea is still contentious and has been understudied, with most studies only considering the group as part of the larger phylogeny of the Annelida. One of the main deep sea subfamilies, the Marcellicephalinae has been consistently recovered as paraphyletic and Bonifácio & Menot found that ten Polynoid subfamilies could be synonymized with it to create a homogeneous clade characterised by a lack of lateral antennae .