[3] The following classification shows the place of Nectonematoida within the protostomes according to Minelli (2008)[4] and Tedersoo (2017):[5] Platyzoa Spiralia Scalidophora Nematoda Gordioidea Nectonematoidea Panarthropoda Chaetognatha Within Nectonematoida only a single genus, Nectonema, is known, with five species so far described:[6] 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.

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.

[3][6] Additionally, giant cells with a diameter up to 400 μm have been observed in the anterior cavity of three species (N. agile, N. munidae and N. zealandica) which have been posited to play a role in sensory perception by Ward (1892) and Bresciani (1991).

Larval nectonematids have only been described once, with the smallest being 350 μm in length and possessing rings of spines as well as cuticular structures denoted "jaws" on the anterior.

Conflicting observations also exist on whether the parasites cause internal damage to their hosts, with Mouchet (1931) and Leslie et al. (1981) reporting damage to male reproductive organs in host species Pagurus bernhardus, Anapagurus hyndmanni and Cancer irroratus, while Brinkmann (1930), Nouvel & Nouvel (1934) and Nielsen (1969) did not observe any tissue alteration.

[6][10] After emerging from their hosts, adult nematomorphs use their dorsal and ventral double rows of bristles to swim through open water.

In preparation for reproduction, mature females' body cavities become filled with eggs, while males form sperm sacs.

Unlike gordiids, nectonematids copulate, with males inserting their posterior end into the genital opening of the female.