''Clava'' is a

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

genus of

hydrozoans Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specializ ...

in the family

Hydractiniidae Hydractiniidae is a cnidarian family of athecate hydroids. Genera The World Register of Marine Species The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive list of name ...

. It contains only one accepted species, ''Clava multicornis''. Other names synonymous with ''Clava multicornis'' include ''Clava cornea'', ''Clava diffusa'', ''Clava leptostyla'', ''Clava nodosa'', ''Clava parasitica'', ''Clava squamata'', ''Coryne squamata'', ''Hydra multicornis'', and ''Hydra squamata''. The larvae form of the species has a well developed nervous system compared to its small size. The adult form is also advanced due to its ability to stay

dormant Dormant, "sleeping", may refer to: Science *Dormancy Dormancy is a period in an organism's life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolic activity and therefore helps ...

during unfavorable periods.

Anatomy

The larval form (

planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores. Some groups of Nemerteans also produce larvae that are very similar to the planula, whi ...

) and adult form show two different body plans. The planula is a small, free-living larva, and is

diploblastic Diploblasty is a condition of the blastula in which there are two primary germ layers: the ectoderm and endoderm. Diploblastic organisms are organisms which develop from such a blastula, and include cnidaria and ctenophora, formerly grouped togeth ...

with two layers: the

endoderm Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gast ...

and

ectoderm The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...

with an additional

mesoglea Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges. Desc ...

. The

thickness decreases from the anterior to posterior poles. Further, the

has mucous gland cells for secretory purposes, support, and sense along with

cnidocytes A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida () or nematocyst) that can deliver a sting to other organisms. The presence of this ce ...

with nematocysts in the posterior end. The planula remain free-living for a short time, after which they settle onto hard substrate and then complete metamorphosis to become its adult form. During metamorphosis, the species destroys most of its

and

tissues as it undergoes a massive reorganization of its body. In this form it stays attached to a

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

as a polyp. Features on the polyp include a mouth and

tentacles In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work mai ...

Nervous system

The planulae demonstrates

cephalization Cephalization is an evolutionary trend in which, over many generations, the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region. This is associated with movement and bilateral symmetr ...

with concentration of

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...

and sensory cells in a dome shape at the anterior end. These

vary in types and function. These sensory cells may include photoreceptors. Additionally, it is believed that these planulae crawl by secretion of mucous gland cells which are secreted using

. During metamorphosis, these

move within the organism. This is demonstrated in screening of GLWamide-immunoreactive

and RFamide-immunoreactive

. GLWamide-immunoreactive

display as

neurotransmitters A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neuro ...

cnidarian Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in Fresh water, freshwater and Marine habitats, marine environments, predominantly the latter. Their distinguishing feature is cnidocyt ...

organisms. These exist specifically in the polyp form of ''C. multicornis'' in the

hypostome In zoology, the hypostome can refer to structures in distinct animal groups: *Hypostome (trilobite), the ventral mouthpart plate in trilobites *Hypostome (tick), the barbed attachment structure associated with the mouthparts of parasitic arachnids ...

near the mouth. These will move along the body either upwards above the

or downwards into the column of the polyp. RFamide-immunoreactive

demonstrate

used for sensory purposes. ''C. multicornis'' have less of these neurons than the GLWamide-immnoreactive neurons. These exist in the adult polyp form in the

. These move along the

and into the column of the polyp. GLWamide-immunoreactive

and RFamide-immunoreactive neurons can be affected by treatment of

retinoic acid Retinoic acid (used simplified here for all-''trans''-retinoic acid) is a metabolite of vitamin A1 (all-''trans''- retinol) that mediates the functions of vitamin A1 required for growth and development. All-''trans''-retinoic acid is required in ...

(RA) and

Citral Citral is an acyclic monoterpene aldehyde, and being a monoterpene, it is made of two isoprene units. Citral is a collective term which covers two geometric isomers that have their own separate names; the ''E''-isomer is named geranial (''trans' ...

. RA prevents sensory cells to differentiate between GLWamide-immunoreactive and RFamide-immunoreactive

. This leads to the inability to react to light.

causes the embryos of ''C. multicornis'' to be shorter and unable to move. This is from a reduction in GLWamide-immunoreactive and RFamide-immunoreactive cells.

Life cycle

''Clava multicornis'' reproduces sexually as the larvae are fertilized in the gonophores on an adult female. The planulae hatch 48–72 hours after. They then develop on the polyp where they are hatched into a 600–800-μm-long

tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found ...

-like larvae. Its body shape is similar to a

in that it tapers from the anterior to the posterior pole. However, its swimming stays close to the

and moves by ciliary gliding toward light. It also crawls by using its mucous gland cells. The larvae attaches on the anterior end once it has found an adequate environment to settle. The posterior end then forms the mouth and

. This

never exhibits a medusa life stage. Sexual reproduction can only be done in the prime temperature range of the species. ''Clava multicornis'' also reproduces asexually through budding from its polyp form. The asexual reproduction cycle reaches maximum reproduction rates at about 39 days in temperatures on the higher scale of their prime range. However, the species can reproduce asexually in a wider range of temperatures than during sexual reproduction.

Behavior

Locomotion

As planulae crawl along the

with ciliary gliding, they move with their anterior end forward. This movement demonstrates a left and right bending in order to glide forward. Once in the adult polyp form, they stay attached to the

throughout the rest of their life. Additionally, these planulae exhibit positive

phototactic Phototaxis is a kind of taxis, or locomotory movement, that occurs when a whole organism moves towards or away from a stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most efficiently to receive l ...

behavior (moving towards the light source). They have become a model for determining effects of

(used to regulate physiological processes in

chordates A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...

) on this

phototropic Phototropism is the growth of an organism in response to a light stimulus. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. The cells on the plant that are farthest from the light contain a hor ...

behavior.

Environment

During settlement of the planulae, ''C. multicornis'' tends to settle in colonies. Additionally, they settle during periods of higher

tides Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tabl ...

rather than lower

. This is because they prefer to settle when algae are in a sloped or vertical position. This position is likely when sea level rises and

fronds A frond is a large, divided leaf. In both common usage and botanical nomenclature, the leaves of ferns are referred to as fronds and some botanists restrict the term to this group. Other botanists allow the term frond to also apply to the lar ...

float vertically. In unfavorable time-periods of their environment, adult forms of ''C. multicornis'' can become

in order to survive. Their hydranth, a feeding zooid, rests in the

stolon In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external ...

and begins to emerge once the environment is safe again. Regeneration of their colonies follows this time period. Because of this ability to become

, they have been found to live in the

Arctic The Arctic ( or ) is a polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada ( Yukon, Northwest Territories, Nunavut), Danish Realm ( Greenland), Finland, Iceland ...

Habitat

''Clava multicornis'' are sensitive to temperature. In their prime temperature range of 12 °C to 17 °C, the species can withstand many

salinities Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal to ...

ranging from 16 ppt to 40 ppt. However, the species can flourish in 32 ppt environments. One specific environment that they have been found in is the

. They attach to

algal Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from un ...

species including ''

Ascophyllum nodosum ''Ascophyllum nodosum'' is a large, common cold water seaweed or brown alga ( Phaeophyceae) in the family Fucaceae, being the only species in the genus ''Ascophyllum''. It is a seaweed that only grows in the northern Atlantic Ocean, also known ...

'' and ''

Fucus vesiculosus ''Fucus vesiculosus'', known by the common names bladder wrack, black tang, rockweed, sea grapes, bladder fucus, sea oak, cut weed, dyers fucus, red fucus and rock wrack, is a seaweed found on the coasts of the North Sea, the western Baltic Sea ...

''. The species tends to live in sheltered areas on

algae Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...

away from shores because the planulae cannot settle when there is too much wave action. They can only feed efficiently in lesser currents because their tentacles become deformed from intense waves, causing their size to diminish. Further, when the

larvae develop, they settle close to the bottom and middle of the

since they are fragile against waves. They also may live away from shores because they are fragile to

solar radiation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

. This species also houses other

invertebrates Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

. One known species is the larvae of ''

Pycnogonum litorale ''Pycnogonum litorale'' is a marine arthropod in the family Pycnogonidae, the sea spiders. It is found in the northern Atlantic Ocean, the North Sea, the English Channel and the western Mediterranean Sea. Description ''Pycnogonum litorale'' has ...

'', a sea spider who lives as an

ectoparasite 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 ...

on ''C. multicornis'' either in its

or hydrocaulus.

Phylogeny

''Clava multicornis'' was originally specified under the family

Clavidae Hydractiniidae is a cnidarian family of athecate hydroids. Genera The World Register of Marine Species includes the following genera in the family: *'' Bouillonactinia'' Miglietta, McNally & Cunningham, 2010 *'' Clava'' Gmelin, 1788 *'' Clavact ...

but it was moved to family

due to its similarities to other members of this family. The similarities that placed them in the family include having

stolons In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external s ...

growing off of its skeleton and a variation in the formation of polyps. The most recent study from 2015 confirmed this, placing the species also in Filifera III. Ecomare - knotspoliep (pcd02028-knotspoliep-sd).jpg Clava multicornis 3299.jpg

References

{{Taxonbar, from1=Q3326513, from2=Q4000828 Hydractiniidae Taxa named by Johann Friedrich Gmelin