The genus Glycera is a group of polychaetes (bristle worms) commonly known as blood worms. They are typically found on the bottom of shallow marine waters, and some species (e.g. the proboscis worm, Glycera dibranchiata), are extensively harvested for use as bait in fishing. Another common species is the tufted gilled bloodworm, G. americana.

Bloodworms are a creamy pink colour, because their pale skin allows their red body fluids to show through: this is the origin of the name "bloodworm". They have small fleshy projections called parapodia on their bodies, and at the "head" end, four small antennae. They breathe through gills located on the parapodia. They grow up to 35cm in length.

Bloodworms are poor swimmers but good burrowers, living on the sandy or silty bottoms of the intertidal or subtidal regions. Though usually marine, they can tolerate low levels of salt in the water, and also poor oxygen levels.

Bloodworms are carnivorous. They feed by extending a large proboscis that bears four hollow jaws. The jaws are connected to glands that supply poison which they use to kill their prey, and their bite is painful even to a human. They are preyed on by other worms, by bottom-feeding fish and crustaceans, and by gulls.

Reproduction occurs in midsummer, when the warmer water temperature and lunar cycle among other factors triggers sexually mature worms to transform into non-feeding stage called Epitoky with enlarged parapodia and swim to the surface of the water where both sexes release gametes, and then die.

They are sold commercially by nurseries and acquatic stores in both live and freeze dried form as a food for aquarium fish.

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Annelids