''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 Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

larval stage inhabiting

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

hosts and a free-living adult stage that swims in open water.

Taxonomy

Nectonematoidea is one of two orders within the phylum

Nematomorpha Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in siz ...

, the other being Gordioidea. The latter is likewise in a single class

Gordioida Gordioidea is an order (sometimes placed at superfamily level) of parasitic horsehair worms. Its taxonomy remains uncertain, but appears to be contained in the monotypic class Gordioida and contains about 320 known species. Biology Gordioidean ...

, 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

protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's mem ...

s 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'' 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 Nectonematoi ...

'' * ''

Nectonema svensksundi ''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 Nectonematoi ...

'' * ''

Nectonema zealandica ''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 Nectonematoi ...

'' Three species are known from the

North Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the "Old World" of Africa, Europe and ...

, including ''N. agile'' from the North American and European coasts, as well as in the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

and

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, ...

. ''N. munidae'' has been recorded in fjords near

Bergen Bergen (), historically Bjørgvin, is a city and municipality in Vestland county on the west coast of Norway. , its population is roughly 285,900. Bergen is the second-largest city in Norway. The municipality covers and is on the peninsula o ...

Norway Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...

in the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

, and ''N. svensksundi'' is known from Svalbard. Additional unconfirmed reports of possible ''Nectonema'' specimens have come from Western

Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is ...

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

and

Sierra Leone Sierra Leone,)]. officially the Republic of Sierra Leone, is a country on the southwest coast of West Africa. It is bordered by Liberia to the southeast and Guinea surrounds the northern half of the nation. Covering a total area of , Sierra ...

. In the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

, ''N. melanocephalum'' was recorded off the Balabalagan Islands in the

Makassar Strait Makassar Strait is a strait between the islands of Borneo and Sulawesi in Indonesia. To the north it joins the Celebes Sea, while to the south it meets the Java Sea. To the northeast, it forms the Sangkulirang Bay south of the Mangkalihat Pe ...

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

, while ''N. zealandica'' has been recorded off the coast of

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 List of islands of New Zealand, smaller islands. It is the ...

. 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 The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, in ...

, nectonematids possess an additional dorsal nerve cord. Nectonematids also possess a blindly-ending

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans a ...

and double rows of dorsal and ventral cuticular natatory bristles. In males, sperm sacs attached to the dorsal

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the ...

are the

gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces ...

s, 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 organs or

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in th ...

. The digestive system does not primarily fulfil the role of nutrient uptake, which instead likely occurs through the cuticle, but rather the storage of substances that are taken up through the cuticle. Layers of cuticle have been observed, with an adult cuticle forming underneath the larval cuticle, in addition to a cellular epidermis. The

nervous system In Biology, biology, the nervous system is the Complex system, highly complex part of an animal that coordinates its Behavior, actions and Sense, sensory information by transmitting action potential, signals to and from different parts of its ...

consists of a

circumesophageal nerve ring A circumesophageal or circumpharyngeal nerve ring is an arrangement of nerve ganglia around the esophagus/ pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trach ...

that functions as a simple brain along with two longitudinal nerve cords located dorsally and ventrally, though the dorsal part of the nerve ring reduces as individuals mature. The

sensory system The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved ...

is either largely absent or poorly understood. Bristles and probable

cilium The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...

have been observed on the cuticle, which appear to be connected to the nervous system in a sensory role. 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). The cells appear to be connected to the nerve chord via

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action ...

s, supporting this interpretation. However, their potential sensory role remains unclear. Species exhibit

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

in their size, with males growing to lengths from to depending on species, while females of all species are longer than males, growing to between and in length. 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.

Ecology and life cycle

Nectonematids spend the larval stage of their life cycle as parasites of

decapod crustaceans The Decapoda or decapods (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crabs, lobsters, crayfish, shrimp and prawns. Most decapods are scavengers. The order is estima ...

. At least 28 host species have been identified, including

hermit crabs Hermit crabs are anomuran decapod crustaceans of the superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit crab, most of which possess an a ...

, crabs,

caridean shrimp The Caridea, commonly known as caridean shrimp or true shrimp, are an infraorder of shrimp within the order Decapoda. This infraorder contains all species of true shrimp. They are found widely around the world in both fresh and salt water. Many ot ...

and '' Eusergestes'' prawns; a single ''N. agile'' individual has also been found within an

American lobster The American lobster (''Homarus americanus'') is a species of lobster found on the Atlantic coast of North America, chiefly from Labrador to New Jersey. It is also known as Atlantic lobster, Canadian lobster, true lobster, northern lobster, Can ...

(''Homarus americanus'') specimen. Larvae inhabit hosts' body cavity, especially in the region of the thorax; typically, a decapod will be host to a single nectonematid, however as many as nine have been observed inhabiting a single crab. Evidence is conflicting on possible correlations between the size and sex of hosts and nectonematid infection rates and growth sizes. 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 ''Pagurus bernhardus'' is the common marine hermit crab of Europe's Atlantic coasts. It is sometimes referred to as the common hermit crab or soldier crab. Its carapace reaches long, and is found in both rocky and sandy areas, from the Arctic ...

'', ''Anapagurus hyndmanni'' and '' Cancer irroratus'', while Brinkmann (1930), Nouvel & Nouvel (1934) and Nielsen (1969) did not observe any tissue alteration. 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.

References

{{Taxonbar, from1=Q3820188, from2=Q10595387, from3=Q10595388, from4=Q18547778, from5=Q10335955 Nematomorpha Parasitic protostomes Animals described in 1879 Taxa named by Addison Emery Verrill