| Platyctenida | |
|---|---|
 | |
| Coeloplana astericola on the surface of a seastar | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Ctenophora |
| Class: | Tentaculata |
| Order: | Platyctenida |
| Families | |
| |
Platyctenida is an order of comb jellies in the class Tentaculata.
Platyctenida is the only benthic group of organisms in the phylum Ctenophora. Platyctenida are considered to be a phylogenetically young group along with the orders Lobata and Beroida and are believed to have stemmed from an ancestral version of the order Cydippida, after some kind of bottleneck effect in the phylum. This has been supported by strong morphological and developmental data, specifically the sharing of what has been termed a "Cydippida-like" larva form in all 4 orders. Platyctenida is thought to be a polyphyletic group. [1]
Ranging in size 15 cm and below, they have dorso-ventrally flattened, oval and secondarily bilaterally symmetrical bodies. Platyctenids look very much like nudibranchs or flatworms and are often confused for them. All but one species of platyctenids lack the iconic ctene rows (the ciliated comb-rows) that distinguishes the ctenophores but they still possess the pair of tentilla-bearing tentacles and adhesive collocytes that also characterize the phylum in pores along the dorsal surface. [2] They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot".
They are usually cryptically colored, live on the seabed and rocks, and as epibionts on other benthic organisms like algae, soft coral, and other invertebrates; primarily certain species of cnidarians and echinoderms (primarily the genus Coeloplana sp.). [3] They are often revealed by their long tentacles with many side branches, seen streaming off the back of the animal into the current. They tend to be ectosymbiotic with the organisms they live on. [4]
Whereas most ctenophores are hermaphroditic, certain platyctenids have been found to be asexual, and furthermore, where other ctenophores have been found to reproduce using external fertilization, certain species of platyctenids have been found to use brood pouches. [5]
Cnidaria is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in fresh water and marine environments, including jellyfish, hydroids, sea anemones, corals and some of the smallest marine parasites. Their distinguishing features are a decentralized nervous system distributed throughout a gelatinous body and the presence of cnidocytes or cnidoblasts, specialized cells with ejectable flagella used mainly for envenomation and capturing prey. Their bodies consist of mesoglea, a non-living, jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. Cnidarians are also some of the only animals that can reproduce both sexually and asexually.
Placozoa is a phylum of marine and free-living (non-parasitic) animals. They are simple blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, placozoans are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among the most basal animals, thus, constituting a primitive metazoan phylum.
Ctenophora comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming, and they are the largest animals to swim with the help of cilia.
A nerve net consists of interconnected neurons lacking a brain or any form of cephalization. While organisms with bilateral body symmetry are normally associated with a condensation of neurons or, in more advanced forms, a central nervous system, organisms with radial symmetry are associated with nerve nets, and are found in members of the Ctenophora, Cnidaria, and Echinodermata phyla, all of which are found in marine environments. In the Xenacoelomorpha, a phylum of bilaterally symmetrical animals, members of the subphylum Xenoturbellida also possess a nerve net. Nerve nets can provide animals with the ability to sense objects through the use of the sensory neurons within the nerve net.
Lobata is an order of transparent marine invertebrates belonging to the phylum of Ctenophora in the class Tentaculata, and are commonly referred to as comb jellies or sea gooseberries. There are currently 19 extant known species in the order of Lobata. Members of Lobata exhibit a compressed body in the vertical plane and a pair of oral lobes. They are known to inhabit marine pelagic surfaces and the marine shores.
Siphonophorae is an order within Hydrozoa, which is a class of marine organisms within the phylum Cnidaria. According to the World Register of Marine Species, the order contains 175 species described thus far.
Coelenterata is a term encompassing the animal phyla Cnidaria and Ctenophora. The name comes from Ancient Greek κοῖλος (koîlos) 'hollow', and ἔντερον (énteron) 'intestine', referring to the hollow body cavity common to these two phyla. They have very simple tissue organization, with only two layers of cells, along with a middle undifferentiated layer called mesoglea, and radial symmetry. Some examples are corals, which are typically colonial; hydrae, jellyfish, sea anemones, and Aurelia, which are solitary; Pennatula; Portuguese man o' war; Gorgonia; and Physalia. Coelenterata lack a specialized circulatory system, relying instead on diffusion across the tissue layers.
Mnemiopsis leidyi, the warty comb jelly or sea walnut, is a species of tentaculate ctenophore. It is native to western Atlantic coastal waters, but has become established as an invasive species in European and western Asian regions. Three species have been named in the genus Mnemiopsis, but they are now believed to be different ecological forms of a single species M. leidyi by most zoologists.
Marine invertebrates are the invertebrates that live in marine habitats. Invertebrate is a blanket term that includes all animals apart from the vertebrate members of the chordate phylum. Invertebrates lack a vertebral column, and some have evolved a shell or a hard exoskeleton. As on land and in the air, marine invertebrates have a large variety of body plans, and have been categorised into over 30 phyla. They make up most of the macroscopic life in the oceans.
Hydroidolina is a subclass of Hydrozoa and makes up 90% of the class. Controversy surrounds who the sister groups of Hydroidolina are, but research has shown that three orders remain consistent as direct relatives: Siphonophorae, Anthoathecata, and Leptothecata.
Cydippida is an order of comb jellies. They are distinguished from other comb jellies by their spherical or oval bodies, and the fact their tentacles are branched, and can be retracted into pouches on either side of the pharynx. The order is not monophyletic, that is, more than one common ancestor is believed to exist.
Pleurobrachia bachei is a member of the phylum Ctenophora and is commonly referred to as the Pacific sea gooseberry. These comb jellies are often mistaken for medusoid Cnidaria, but lack stinging cells.
ParaHoxozoa is a clade of animals that consists of Bilateria, Placozoa, and Cnidaria. The relationship of this clade relative to the two other animal lineages Ctenophora and Porifera is debated. Some phylogenomic studies have presented evidence supporting Ctenophora as the sister to Parahoxozoa and Porifera as the sister group to the rest of animals. Other studies have presented evidence supporting Porifera as the sister to Parahoxozoa and Ctenophora as the sister group to the rest of animals, finding that nervous systems either evolved independently in ctenophores and parahoxozoans, or were secondarily lost in poriferans. If ctenophores are taken to have diverged first, Eumetazoa is sometimes used as a synonym for ParaHoxozoa.
Collocyte is a term variously applied in botany and zoology to cells that produce gluey substances, or that bind or capture prey or assorted objects by securing them with gluey materials and structures, or that simply look smooth and gelatinous. Literally the word means "glue cell", and it has a number of poorly distinguished synonyms, such as colloblast.
Thalassocalyce is a genus of ctenophore, or comb jellies, known from the California Coast, Gulf of Mexico, and west north Atlantic. It is represented by a single species, Thalassocalyce inconstans, which is the only species in the family Thalassocalycidae and the order Thalassocalycida. T. inconstans is a pelagic ctenophore typically occurring in upper-mesopelagic depths, but has been observed at depths up to 3,500 m in Monterey Canyon.
Bathyphysa conifera, sometimes called the flying spaghetti monster, is a bathypelagic species of siphonophore in the family Rhizophysidae. It is found in the northern Atlantic Ocean and off the coast of Southwestern Africa and California.
Coeloplana astericola, the creeping comb jelly, is a species of benthic comb jelly from the tropical western Indo-Pacific region that lives as an episymbiont on starfish such as Echinaster luzonicus.
Calycophorae is a suborder of Siphonophores alongside two other suborders Physonectae and Cystonectae. This suborder includes the giant siphonophore, ; one of the longest lengthwise extant creatures (40–50m). While the Physonectae have a pneumatophore, nectophore, and a siphosome, Cystonectae lack a nectophore, and Calycophorae lack a pneumatophore. From the bell-shaped nectophores, Physonectae and Calycophorae are called Codonophores or Greek for bell-bearers. The distribution, morphology, and behaviors of Calycophorae species are vast and greatly depend on the species. Calycophoraes typically consist of two nectophores with a siphosome that have many tentacles that grow out of the siphosome. The Calycophoraes move by propelling water out of the nectophore much like how jellyfishes move. The tentacles act as fishing nets where the nematocysts on the tentacles paralyze their prey which are then later fed on. Calycophorae have three life stages, which are the larval development stage, the polygastric stage, and the eudoxid maturation stage. Each Calycophorae colony forms from one fertilized egg.
Beroe abyssicola is a species of beroid ctenophore, or comb jelly. It is largely found in deep waters in the North Pacific Ocean, and is common in Japan and the Arctic Ocean. A predator, Beroe feeds mostly on other ctenophores by swallowing them whole. Like other ctenophores, B. abyssicola has a simple nervous system in the form of a nerve net, which it uses to direct its movement, feeding, and hunting behaviors.
Euplokamis is a genus of ctenophores, or comb jellies, belonging to the monotypic family Euplokamididae. It shares the common name sea gooseberry with species of the genus Pleurobrachia. Despite living for hundreds of millions of years in marine environments, there is minimal research regarding Euplokamis, because they are uncommon. Research on the evolution of the basic body structures of diploblastic metazoans revealed that there are four major phyla, including the Ctenophores. Although the morphology of Euplokamis often resembles the medusa stage of Cnidarians, their eight rows of combs are one distinguishing feature that led to the official classification of Ctenophores. After being originally described by Chun (1879), the family Euplokamididae was expanded by Mills (1987) due to the discovery of a new species, Euplokamis dunlapae. Further research indicated that Euplokamis should be identified from Mertensiidae due to the rows of combs and some compression. They may also be distinguished from the genus Pleurobrachia due to their more elongated shape. Additionally, various adaptations of Euplokamis have been observed such as the use of tentacles for movement/feeding, a complex nervous system, and bioluminescent capabilities. Other characteristics including a defined mesoderm, lack of stinging cells, developmental differences, and symmetry supported the reclassification of these organisms.
Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. p. 176. ISBN 978-0-03-056747-6.
1)Rudman, W. B. 1999. Benthic ctenophores. Sea Slug Forum, Australian Museum, Sydney
3)Harbison, G. R. 2001. Ctenophora. Encyclopaedia of Life Sciences
| Authority control databases: National |
|---|
