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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 common blood worm, Glycera dibranchiata), are extensively harvested along the Northeastern coast of the United States for use as bait in fishing. Another common species is the tufted gilled bloodworm, G. americana.
Bloodworms have a creamy pink color, as their pale skin allows their red body fluids that contain hemoglobin to show through. This is the origin of the name "bloodworm". At the 'head', bloodworms have four small antennae and small fleshy projections called parapodia running down their bodies. Bloodworms can grow up to 35 centimetres (14 in) 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 salt levels in the water, and also poor oxygen levels. Bloodworms and all water worms have adapted to life in the sand and silt for the protection it offers.
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 a non-feeding stage called the epitoke. With enlarged parapodia, they swim to the surface of the water where both sexes release gametes, and then die.
The first stage in many forms of bloodworm is a zooplanktonic stage followed by the benthic instar where the familiar segmented red larvae develop protected by silk tubes made in the bottom silt. These larvae progress from tiny pale opaque worms to the larger red larvae of 3 to 10 centimeters in length or longer over a period of as short a period as 2 – 3 weeks in optimum conditions. 
The animals are unique in that they contain a lot of copper without being poisoned. Their jaws are unusually strong since they too contain the metal in the form of a copper-based chloride biomineral, known as atacamite. And unlike the clamworm (Nereis limbata), whose jaws contain the metal zinc, the copper in the mineral in the jaws of Glycera is actually present in its crystalline form. It is theorized that this copper is used as a catalyst for its poisonous bite.
Use by humans
Glycera worms are sold commercially by nurseries and aquatic stores in both live, frozen and freeze dried form as a food for aquarium fish and frogs. They are also commonly sold in tackle shops as bait for saltwater fishing.
Popular culture references
- Dirty Jobs host Mike Rowe went digging for bloodworms in Maine in episode 5.2 from December 2006.
- The 1976 film Squirm featured bloodworms attacking the people of a small town.
- Chien PK; Rice MA (1985). "Autoradiographic localization of exogenously supplied amino acids after uptake by the polychaete, Glycera dibranchiata Ehlers". Wasmann Journal of Biology 43: 60–71. ISSN 0043-0927. OCLC 6322423.
- Qafaiti M; Stephens GC (1988). "Distribution of Amino Acids to Internal Tissues After Epidermal Uptake in the Annelid Glycera dibranchiata" (PDF). Journal of Experimental Biology 136 (1): 177–191.
- Bloodworm: Uses and applications as a fishing bait
- Lichtenegger HC, Schöberl T, Bartl MH, Waite H, Stucky GD (October 2002). "High abrasion resistance with sparse mineralization: copper biomineral in worm jaws". Science 298 (5592): 389–92. doi:10.1126/science.1075433. PMID 12376695.
- Lichtenegger HC, Schöberl T, Ruokolainen JT et al. (August 2003). "Zinc and mechanical prowess in the jaws of Nereis, a marine worm". Proc. Natl. Acad. Sci. U.S.A. 100 (16): 9144–9. doi:10.1073/pnas.1632658100. PMC 170886. PMID 12886017.
- Fauchald, K.; Bellan, G. (2009). Glycera Savigny, 1818. In: Fauchald, K. (Ed) (2009). World Polychaeta database. Accessed through the World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=129296 on 2009-03-12.