Viviparous eelpouts grow to a maximum length of 52 centimetres (20 in) and a maximum weight of 510 grams (18 oz). They typically live at water depths up to 40 metres (130 ft). The fish live to a maximum age of 10 years.
These eelpouts live in a temperate climate in the marine waters of the Northeast Atlantic including the seas such as the Baltic, Barents, Irish,North, and White Seas. They also live in some brackish inlets, such where the River Somme meets the English Channel. The fish stay towards the rocky shorelines and tidepools among the stones and algae. The fish are capable of living out of water under rocks and seaweed due to their ability to breathe air directly. The fish eat eggs and fry of fish and macroinvertebrates such as gastropods and crustaceans.
Adults mate during the months of August and September using internal fertilization. The fish are notably viviparous, giving birth to 30–400 live developed young. Unusually, it does so during winter when water temperatures are extremely cold. Among fish it has one of the longest known pregnancies, lasting approximately six months. It has been discovered that the eelpout suckles its young embryos while still within their mother's body, making it the only fish species to suckle its offspring. The embryos actually suckle from ovarian follicles, ingesting nutrients and gases from these internal structures. After depleting the egg's yolk reserves, the eelpouts attach their mouths to an ovarian follicle, which has a canal in its tip via which fluid and nutrients can flow. This follicle fluid is rich in proteins, fatty acids and glucose. It is also saturated in oxygen, which helps ventilate the gills of the developing fish. Each embryo latches onto a single follicle. This ensures an equal distribution of nutrients .