''Pennaria disticha'', also known as the Christmas tree hydroid, is a species of athecate hydroid in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Pennariidae ''Pennaria'' is a genus of hydrozoans. It is the only genus within the 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 includ ...

Colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...

are common in the Mediterranean Sea growing on rocks close to the surface. This species has been used in research into prey capture.

Description

Colonies of ''Pennaria disticha'' consist of numerous much-branched stems up to high. The branching system is alternate. The polyps are tiny and are supported by a hydrocaulus, a fairly stiff hollow tube with a

perisarc 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 ...

(sheath) made of chitin and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

. This is dark brown or blackish, but is often colonised by algae and diatoms giving it a muddy appearance. The whitish polyps are tinged with red. Each polyp has ten to eighteen slender, filiform

tentacle 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 ma ...

s at its base and up to twelve tentacles with knobbed tips surrounding its terminal mouth.

Distribution

''Pennaria disticha'' probably originated in the western Atlantic Ocean but has spread and now is widely distributed in warm waters throughout the world. It was first detected in Hawaii in 1928. It is found in shallow water attached to hard surfaces, on both rocks and man-made structures, in locations with some water movement. On reefs, it tends to be in less-exposed positions and in crevices. This hydrozoan forms part of the community of organisms known as the

fouling community Fouling communities are communities of organisms found on artificial surfaces like the sides of docks, marinas, harbors, and boats. Settlement panels made from a variety of substances have been used to monitor settlement patterns and to examine s ...

, and has spread around the world as a result of man's maritime activities.

Ecology

The polyps of ''Pennaria disticha'' spread out their tentacles to catch any small

zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...

that float by. The prey is often captured and immobilised by

nematocysts 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 c ...

on the threadlike tentacles at the base of the polyp. The crown bends over to receive the item, which is then killed by the more powerful nematocysts at the tip of the crown tentacles and thrust into the mouth. The colony grows by

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

, during which process new feeding polyps are formed. Reproduction is by the budding of reproductive polyps known as

gonophore A gonophore is a reproductive organ in Hydrozoa that produces gametes. It is a sporosac, a medusa or any intermediate stage. The name is derived from the Greek words (, that which produces seed) and (, -bearing). Gonophores are borne on branch ...

s, the

medusa In Greek mythology, Medusa (; Ancient Greek: Μέδουσα "guardian, protectress"), also called Gorgo, was one of the three monstrous Gorgons, generally described as winged human females with living venomous snakes in place of hair. Those ...

stage in the

life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...

. Gonophores may remain attached to the parent colony, but usually become detached. Each colony produces gonophores of one sex. Male gonophores soon eject white sperm into the water and females emit up to six eggs. After fertilisation, each embryo develops within a day or so into a free-swimming

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, which ...

larva. After drifting with the

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

for some time, this finds a suitable hard surface on which to settle and start a new colony.

Toxicity

It has been reported that an acute, painful

dermatitis Dermatitis is inflammation of the skin, typically characterized by itchiness, redness and a rash. In cases of short duration, there may be small blisters, while in long-term cases the skin may become thickened. The area of skin involved can ...

occurred in several people who came into contact with a rope on which ''Pennaria disticha'' was growing as a fouling organism.

References

{{Taxonbar, from=Q2591429 Pennariidae Animals described in 1820