''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 "unisp ...

genus of

hydrozoans Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is 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 spe ...

in the family

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

. 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 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, which are not related to cnidarians at all. Some groups of Nemerteans also produce larva ...

) 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 toge ...

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 gastr ...

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 as well as ctenophores that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material f ...

. 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) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...

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 Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and different ...

to become its adult form. During

, 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 (aquatic environment), the earthy material that exi ...

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 main ...

Nervous system

The planulae demonstrates

cephalization Cephalization is an evolutionary trend in animals that, over a sufficient number of generations, concentrates the special sense organ (biology), organs and nerve ganglia towards the front of the body where the mouth is located, often producing a ...

with concentration of

neurons A neuron (American English), neurone (British English), or nerve cell, is an membrane potential#Cell excitability, excitable cell (biology), cell that fires electric signals called action potentials across a neural network (biology), neural net ...

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

, 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, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotransmitters are rele ...

cnidarian Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...

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 hypostome (also called the maxilla, radula, or Labium (insect), labium) is a Os ...

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 (simplified nomenclature for all-''trans''-retinoic acid) is a metabolite of vitamin A1 (all-''trans''-retinol) that is required for embryonic development, male fertility, regulation of bone growth and immune function. All-''trans ...

(RA) and

Citral Citral is an acyclic monoterpene aldehyde. 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 or polliwog (also spelled pollywog) is the Larva, larval stage in the biological life cycle of an amphibian. Most tadpoles are fully Aquatic animal, aquatic, though some species of amphibians have tadpoles that are terrestrial animal, ...

-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 (physiology), stimulus of light. This is advantageous for phototrophic organisms as they can orient themselves most e ...

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 a bilaterian animal belonging to the phylum Chordata ( ). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics ( synapomorphies) that distinguish them from ot ...

) on this

phototropic In biology, 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 ligh ...

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 tables ...

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

stolon In biology, a stolon ( from Latin ''wikt:stolo, stolō'', genitive ''stolōnis'' – "branch"), also known as a runner, is a horizontal connection between parts of an organism. It may be part of the organism, or of its skeleton. Typically, animal ...

and begins to emerge once the environment is safe again. Regeneration of their colonies follows this time period. Because of this ability to become dormant, they have been found to live in the

Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...

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 ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, s ...

species including ''

Ascophyllum nodosum ''Ascophyllum nodosum'' is a large, common cold water seaweed or brown alga ( Phaeophyceae) in the family Fucaceae. Its common names include knotted wrack, egg wrack, feamainn bhuí, rockweed, knotted kelp and Norwegian kelp. It grows only in the ...

'' and ''

Fucus vesiculosus ''Fucus vesiculosus'', known by the common names bladderwrack, 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 ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

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 Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...

. This species also houses other

invertebrates Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum ...

. One known species is the larvae of ''

Pycnogonum litorale ''Pycnogonum'' is a genus of sea spiders in the family Pycnogonidae. It is the type genus of the family. Etymology The generic name literally means “dense knees”. ''Pycnogonum'' combines the prefix ' (from ‘dense’ in Greek) with the G ...

'', a sea spider who lives as an

ectoparasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...

on ''C. multicornis'' either in its

or hydrocaulus.

Phylogeny

''Clava multicornis'' was originally specified under the family Clavidae 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, a stolon ( from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as a runner, is a horizontal connection between parts of an organism. It may be part of the organism, or of its skeleton. Typically, animal stolons ar ...

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 Filifera is a Order (biology)#Hierarchy of ranks, suborder of hydrozoans in the Order (biology), order Anthoathecata. They are found in marine, brackish and freshwater habitats. Characteristics Members of this suborder are characterised by the ...

. Ecomare - knotspoliep (pcd02028-knotspoliep-sd).jpg Clava multicornis 3299.jpg

References

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