Hydrozoa
Hydrozoa Temporal range:
| |
---|---|
Siphonophorae | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Cnidaria |
Subphylum: | Medusozoa |
Class: | Hydrozoa Owen, 1843 |
Subclasses and orders[1] | |
Hydrozoa (hydrozoans; from
Some examples of hydrozoans are the freshwater jelly (
Anatomy
Most hydrozoan species include both a
Polyps
The hydroid form is usually colonial, with multiple polyps connected by tubelike hydrocauli. The hollow cavity in the middle of the polyp extends into the associated hydrocaulus, so that all the individuals of the colony are intimately connected. Where the hydrocaulus runs along the substrate, it forms a horizontal root-like stolon that anchors the colony to the bottom.
The colonies are generally small, no more than a few centimeters across, but some in Siphonophorae can reach sizes of several meters. They may have a tree-like or fan-like appearance, depending on species. The polyps themselves are usually tiny, although some noncolonial species are much larger, reaching 6 to 9 cm (2.4 to 3.5 in), or, in the case of the deep-sea Branchiocerianthus, a remarkable 2 m (6.6 ft).[2]
The hydrocaulus is usually surrounded by a sheath of chitin and proteins called the perisarc. In some species, this extends upwards to also enclose part of the polyps, in some cases including a closeable lid through which the polyp may extend its tentacles.[2]
In any given colony, the majority of polyps are specialized for feeding. These have a more or less cylindrical body with a terminal mouth on a raised protuberance called the
All colonial hydrozoans also include some polyps specialized for reproduction. These lack tentacles and contain numerous buds from which the medusoid stage of the life cycle is produced. The arrangement and type of these reproductive polyps varies considerably between different groups.
In addition to these two basic types of polyps, a few colonial species have other specialized forms. In some, defensive polyps are found, armed with large numbers of stinging cells. In others, one polyp may develop as a large float, from which the other polyps hang down, allowing the colony to drift in open water instead of being anchored to a solid surface.[2]
Medusae
The medusae of hydrozoans are smaller than those of typical jellyfish, ranging from 0.5 to 6 cm (0.20 to 2.36 in) in diameter. Although most hydrozoans have a medusoid stage, this is not always free-living and in many species exists solely as a sexually reproducing bud on the surface of the hydroid colony. Sometimes, these medusoid buds may be so degenerated as to entirely lack tentacles or mouths, essentially consisting of an isolated gonad.[2]
The body consists of a dome-like umbrella ringed by tentacles. A tube-like structure hangs down from the centre of the umbrella and includes the mouth at its tip. Most hydrozoan medusae have just four tentacles, although a number of exceptions exist. Stinging cells are found on the tentacles and around the mouth.
The mouth leads into a central stomach cavity. Four radial canals connect the stomach to an additional, circular canal running around the base of the bell, just above the tentacles. Striated muscle fibres also line the rim of the bell, allowing the animal to move along by alternately contracting and relaxing its body. An additional shelf of tissue lies just inside the rim, narrowing the aperture at the base of the umbrella, and thereby increasing the force of the expelled jet of water.[2]
The nervous system is unusually advanced for cnidarians. Two nerve rings lie close to the margin of the bell, and send fibres into the muscles and tentacles. The genus
Life cycle
In hydrozoan species with both polyp and medusa generations, the medusa stage is the sexually reproductive phase. Medusae of these species of Hydrozoa are known as "hydromedusae". Most hydromedusae have shorter lifespans than the larger scyphozoan jellyfish. Some species of hydromedusae release gametes shortly after they are themselves released from the hydroids (as in the case of fire corals), living only a few hours, while other species of hydromedusae grow and feed in the plankton for months, spawning daily for many days before their supply of food or other water conditions deteriorate and cause their demise.
Additionally, some hydrozoan species (particularly in Turritopsis genus) share an unusual life cycle among the animals - they can transform themselves from sexually mature medusae stage back to their juvenile hydroid stage.[4]
Systematics and evolution
The earliest hydrozoans may be from the
Hydrozoan systematics are highly complex.[6] Several approaches for expressing their interrelationships were proposed and heavily contested since the late 19th century, but in more recent times a consensus seems to be emerging.
Historically, the hydrozoans were divided into a number of
As far as can be told from the
In any case, according to this classification, the hydrozoans can be subdivided as follows, with taxon names emended to end in "-ae":[1]
Class Hydrozoa
- Subclass Hydroidolina
- Order Anthoathecata (= Anthoathecata(e), Athecata(e), Anthomedusae, Stylasterina(e)) — includes Laingoimedusae but monophyly requires verification
- Order Leptothecata (= Leptothecata(e), Thecaphora(e), Thecata(e), Leptomedusae)
- Order Siphonophorae
- Subclass Trachylinae
- Order Actinulidae
- Order Limnomedusae — monophyly requires verification; tentatively placed here
- Order Narcomedusae
- Order Trachymedusae — monophyly requires verification
- Order
In addition, there exists a cnidarian
Other classifications
Some of the more widespread classification systems for the Hydrozoa are listed below. Though they are often found in seemingly authoritative Internet sources and databases, they do not agree with the available data.[
The obsolete classification mentioned above was:
- Order Actinulidae
- Order Anthoathecatae
- Order Hydroida
- Suborder Anthomedusae
- Suborder Leptomedusae
- Suborder Limnomedusae
- Suborder
- Order Laingiomedusae
- Order Polypodiozoa
- Order Siphonophorae
- Order Trachylina
- Suborder Narcomedusae
- Suborder Trachymedusae
A very old classification that is sometimes still seen is:
- Order Hydroida
- Order Milleporina
- Order Siphonophorae
- Order Stylasterina(= Anthomedusae)
- Order Trachylinida
Catalogue of Life uses:
- Order Actinulida
- Order Anthoathecata (= Anthomedusae)
- Order Hydroida
- Order Laingiomedusae
- Order Leptothecata (= Leptomedusae)
- Order Limnomedusae
- Order Narcomedusae
- Order Siphonophorae
- Order Trachymedusae
Animal Diversity Web uses:
- Order Actinulida
- Order Anthoathecata
- Order Laingiomedusae
- Order Leptothecata
- Order Limnomedusae
- Order Narcomedusae
- Order Siphonophorae
- Order Trachymedusae
References
- ^ a b c Schuchert, Peter. "World Hydrozoa Database". Retrieved 2016-02-05.
- ^ ISBN 978-0-03-056747-6.
- ISBN 2-85653-580-1. 591pp. + 1 cd-rom
- ISSN 0362-4331. Retrieved 2022-08-29.
- ^ Waggoner, Ben M.; Smith, David. "Hydrozoa: Fossil Record". UCMP Berkeley. Retrieved 19 January 2019.
- S2CID 82373673.
- ^ Zrzavý & Hypša 2003
- Schuchert, Peter (2011). "Hydrozoan Phylogeny and Classification". The Hydrozoa Directory. Natural History Museum of Geneva. Archived from the original on 2013-06-04.
- Zrzavý, Jan & Hypša, Václav (2003): Myxozoa, Polypodium, and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of Buddenbrockia. Cladistics 19(2): 164–169. (HTML abstract)
External links
- J. Bouillon; M.D. Medel; F. Pagès; J.M. Gili; F. Boero; C. Gravili (2004). "Fauna of the Mediterranean Hydrozoa" (PDF). .
- Les Hydraires à la Réunion et dans l'océan Indien at the Wayback Machine (archived March 15, 2011)
- Bo Johannesson; Martin Larsvik; Lars-Ove Loo; Helena Samuelsson (2000). "The hydroid colony and its life cycle". Aquascope. Tjärnö Marine Biological Laboratory.
- Schuchert, P. (2016). "World Hydrozoa Database".
- Claudia E. Mills (2012-09-07). "Hydromedusae". University of Washington.
- Ben M. Waggoner (1995-07-21). "Introduction to the Hydrozoa". University of California Museum of Paleontology.