Leptothecata, or thecate hydroids, are an order of hydrozoans in the phylum

Cnidaria 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, ...

. Their closest living relatives are the athecate hydroids, which are similar enough to have always been considered closely related, and the very

apomorph In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ha ...

Siphonophorae Siphonophorae (from Ancient Greek σίφων (siphōn), meaning "tube" and -φόρος (-phóros), meaning "bearing") is an order within Hydrozoa, a class of marine organisms within the phylum Cnidaria. According to the World Register of Marin ...

, which were placed outside the "Hydroida". Given that there are no firm rules for synonymy for high-ranked

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

, alternative names like Leptomedusa, Thecaphora or Thecata, with or without the ending emended to "-ae", are also often used for Leptothecata. In the sessile stage, Leptothecata are surrounded by a chitinous outer layer as their exoskeleton, including the gonophores, their reproductive organ. Leptothecata exhibit radial symmetry, and their gonads can be found in the radial canals of the medusa stage. Their habits range from benthic to planktonic. The polyps and colonial forms are benthic, whilst the medusae are planktonic. Leptothecata exhibit extensive, complex variation. Thecata colonies also have extensive specialization due to their polyps' function and variation. Most Leptothecata possess statocysts, which are used for defence and protection. The classes that have lost their statocysts have been changed ancestrally over time rather than a direct loss. Halecium-muricatum

The approximately 1,900

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of Leptothecata are characterized by a number of features: Their polyps are always living in colonies with the hydranths set in hydrotheca which are usually permanent and often long enough so the animal can fully retract into it; some have very reduced hydrothecae resembling Anthoathecata. There is a single

whorl A whorl ( or ) is an individual circle, oval, volution or equivalent in a whorled pattern, which consists of a spiral or multiple concentric objects (including circles, ovals and arcs). In nature File:Photograph and axial plane floral diagra ...

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 main ...

s. The gonophores are borne on much reduced hydranths and usually protected in a peridermal gonotheca.

Medusae Jellyfish, also known as sea jellies or simply jellies, are the medusa-phase of certain gelatinous members of the subphylum Medusozoa, which is a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals, althoug ...

forming on fully developed hydranths are extremely rare; usually the gonophores develop into medusae or into sessile sporosacs. The medusae have a shallow bell, bear the

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

s on their radial canals, and usually have

statocyst The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, ctenophorans, echinoderms, cephalopods, crustaceans, and gastropods, A similar structure is also found in '' Xenoturbella''. T ...

s which are formed only from epidermal tissue and more than four tentacles and. The cnidome never has stenoteles.

Characteristics

Colony architecture among Leptothecata comprises extensive diversity found in the hydrozoans. Their life cycles have been found to be connected with changes in colony shapes. Zooid polymorphism within the colonies are usually specialized. Polyps that make up the colonies tend to have three specialities and functions. First being the gastrozooid, which has their speciality of nutrition and digesting the food. Second, the gonozooid which is the reproductive polyp. Third, being the dactylozooid which function works in defense for the colony. The dactylozooid recently had become more highly variable with not being present in some thecata colony forms, and only possessing the gastrozooid and gonozooid polyps. Thecata colonies have detectable shapes and arrangements allowing for distinguishing classification between one another. One major shape is when the colonies are erected off a branched colony. Another major shape of thecata is where the colonies can be erected off an unbranched stem. Stolonal colonies are a final major type where their polyps are connected to the creeping part of the colony. Where most cases of the erected branched shape have been found to be derived over time. Medusae tend to be pelagic. But there are specific medusae species while at the medusa stage can still remain benthic. Polyps can also be free floating, which are called pelagic polyps. Similarly, their gastroular and nervous system have great complexity, as well as their shape. While medusae due tend to lack any presence of visible sense organs. Leptothecata have significant synapomorphies that are present in most of all their species. In regards to their gastrozooids, Lephtothecata have the theca layer on their polyps. Which has allowed the synapomorphy for Leptothecata to form hydrothecae that is also made of theca, that surrounds the gonozooid.

Reproduction and development

Leptothecata have distinguishing factors in the presence of morphological dimorphism. The dimorphism in the species classes has led to great complexity within their taxonomic identification. Majority of the thecate hydroids use asexual reproduction in response to budding. They have also been known for their plasticity, allowing them to adapt and grow in their given environment. Thecata's branch has a mutual similarity based on mature gamete localization, where they possess their mature gametes located through their radial canals.

Distribution

Among Leptothecata's diversified species, they have great variability within their organization and life cycles. Leptothecata can be found worldwide in all marine environments. The location of where Leptothecata are found ranges from shallow waters to the deep sea, most being marine species. In their polyp and medusa form, due to natural factors they can travel outside their native location. This is usually done by currents or if attached to other vertebrates. Due to the hydroids' broad range of locations, they also have been known to play in many ecosystem factors. They provide shelter and protection and are a known food source for other marine species. Leptothecata has been the main attraction in many great expeditions and studies due to the broad spectrum of thecate hydroids within their order. These developments have led to further discoveries of finding locations where thecate hydroids can be relatively rich in the marine environment. Allowing researchers to study a range of differences based on the colonies, the hydrotheca and even the pairs of thecae themselves.

Notable species of Leptothecata

* Air fern (''Sertularia argentea''), sold dried as novelty "plants" and aquarium ornaments * Crystal jelly (''Aequorea victoria''), a bioluminescent hydrozoan * Sea fur (''Obelia'' ''spp.''), a common coastal polyp and

medusa In Greek mythology, Medusa (; ), also called Gorgo () or the Gorgon, was one of the three Gorgons. Medusa is generally described as a woman with living snakes in place of hair; her appearance was so hideous that anyone who looked upon her wa ...

Taxonomy and systematics

The thecate hydroids were formerly placed in the

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

" Hydroida" as the

suborder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...

Leptomedusa. Currently, the following families are classified within the order Leptothecata: * Basal and ''incertae sedis'' ** Leptothecata * Family ** Family Aequoreidae ** Family Barcinidae ** Family Blackfordiidae ** Family Bonneviellidae ** Family Campanulariidae ** Family Campanulinidae ** Family Cirrholoveniidae ** Family Clathrozoidae ** Family Dipleurosomatidae ** Family Eirenidae ** Family Haleciidae ** Family Hebellidae ** Family Lafoeidae ** Family Laodiceidae ** Family Lineolariidae ** Family Lovenellidae ** Family Malagazziidae ** Family Melicertidae ** Family Mitrocomidae ** Family Octocannoididae ** Family Orchistomatidae ** Family † Palaequoreidae ** Family Phialellidae ** Family Phialuciidae ** Family Staurothecidae ** Family Sugiuridae ** Family Symplectoscyphidae ** Family Syntheciidae ** Family Teclaiidae ** Family Tiarannidae ** Family Tiaropsidae ** Family Zygophylacidae * Superfamily Plumularioidea ** Family

Aglaopheniidae Aglaopheniidae is a family (biology), family of hydrozoans. Genera According to the World Register of Marine Species, these genus, genera belong to this family: * ''Aglaophenia'' Lamouroux, 1812 * ''Aglaophenopsis'' Fewkes, 1881 * ''Carpocladus' ...

** Family Halopterididae ** Family Kirchenpaueriidae ** Family Phylactothecidae ** Family Plumaleciidae ** Family Plumulariidae ** Family Schizotrichidae * Superfamily Sertularioidea ** Family Sertularellidae ** Family Sertulariidae ** Family Thyroscyphidae

References

* (2008)
Leptomedusae
Retrieved 2008-JUL-08. * (2005a)

Retrieved 2008-JUL-08. * (2005b)

Retrieved 2008-JUL-08. * * * * * {{Taxonbar, from1=Q5122184, from2=Q138147 Cnidarian orders