Nectonema is a genus of marine horsehair worms first described by Addison E. Verrill in 1879. It is the only genus in the family Nectonematidae described by Henry B. Ward in 1892, in the order Nectonematoidea, and in the class Nectonematoida. The genus contains five species; all species have a parasitic larval stage inhabiting crustacean hosts and a free-living adult stage that swims in open water. Taxonomy Nectonematoidea is one of two orders within the phylum Nematomorpha, the other being Gordioidea. The latter is likewise in a single class Gordioida, which is a significantly larger taxon, with over 300 known species. Nematomorpha are known as horsehair worms or Gordian worms, and form a sister-group to the nematodes. The following classification shows the place of Nectonematoida within the protostomes according to Minelli (2008) and Tedersoo (2017): Within Nectonematoida only a single genus, Nectonema, is known, with five species so far described: Nectonema agile Nectonema melanocephalum Nectonema munidae Nectonema svensksundi Nectonema zealandica Three species are known from the North Atlantic, including N. agile from the North American and European coasts, as well as in the Mediterranean Sea and Black Sea. N. munidae has been recorded in fjords near Bergen, Norway in the North Sea, and N. svensksundi is known from Svalbard. Additional unconfirmed reports of possible Nectonema specimens have come from Western Greenland, Brazil and Sierra Leone. In the Pacific Ocean, N. melanocephalum was recorded off the Balabalagan Islands in the Makassar Strait of Indonesia, while N. zealandica has been recorded off the coast of New Zealand. There are also reports of Nectonema from Japan. Description Nectonema has not been extensively studied, and most of what is known about the genus is based on the two best-studied species, N. agile and N. munidae. There are several unique features that distinguish marine Nematomorphs (nectonematids) from freshwater Nematomorph species (gordiids). Studies indicate differences in muscle cell structure as well as an anterior body cavity. While gordiids possess a single longitudinal ventral nerve cord, nectonematids possess an additional dorsal nerve cord. Nectonematids also possess a blindly-ending intestine and double rows of dorsal and ventral cuticular natatory bristles. In males, sperm sacs attached to the dorsal epidermis are the gonads, while females possess a vesicle-rich tissue called a gono-parenchyne during early developmental stages. Additionally, spines are formed on nectonematid eggs after they make contact with seawater. Like all horsehair worms, there is a lack of excretory