Phylum Ctenophora, commonly known as comb jellies, includes 7 orders, with over 200 currently known species of biradially symmetrical, acoelomate organisms that resemble cnidarians. Ctenophore species are largely planktonic, exclusively marine animals, found throughout the world’s oceans, and comprise a significant portion of the planktonic biomass in their range. Comb jellies have a wide variety of body shapes, from small, roughly spherical species of less than a centimeter in diameter, to flattened, ribbon-shaped forms that reach lengths of up to two meters. (Appeltans, et al., 2012; Brusca and Brusca, 2003; Zhang, 2011)
Although the majority of ctenophores, up to 75%, live in warm waters, they are found globally in marine environments, including in Arctic seas, where certain species are found in abundance. Species of this phylum are native globally; however, they may be locally invasive. In one famous case, the ctenophore Mnemiopsis leidyi was accidentally introduced to the Black Sea in the early 1980s; within 10 years, it had destroyed the fishing industry in the entire region, outcompeting native planktonic fishes and disrupting the food chain. (Brusca and Brusca, 2003; Mills, 2010; Shiganova, 1998; Wrobel, 2012)
Ctenophores are exclusively marine animals. They may be found from coastal areas to open waters and are most typically planktonic, from surface waters down to 2,000 to 3,000 meters. A few species are epibenthic or benthic, and may be found in warm waters or on deep, cold, ocean floors; these are often found in association with echinoderms, cnidarians, or sponges. (Brusca and Brusca, 2003; Mills, 2010; Waggoner, et al., 2006; Wrobel, 2012)
Evolutionary relationships within Ctenophora are largely unresolved. Based on morphological and molecular analyses there is at least one polyphyletic order within phylum Ctenophora, Cydippida. The relationships of ctenophores to other metazoan phyla remain an open question, with different analyses alternately finding sister lineage to cnidarians, bilaterians, a clade containing cnidarians, placozoans, and bilaterians, or to all other animal phyla. (Dunn, et al., 2008; Harbison, 1985; Podar, et al., 2001; Ryan, et al., 2010)
Comb jellies have a wide variety of body shapes, from small, roughly spherical species of less than a centimeter in diameter, to flattened, ribbon-shaped forms that reach lengths of up to two meters. They are of biradially symmetrical and acoelomate. Most ctenophores are colorless, but some deep sea species are highly pigmented (often colored similarly to the invertebrates with which they are associated). It was once assumed that all ctenophores are bioluminescent; however, recent research shows that some species (members of genus Pleurobrachia for example) are not capable of producing light. Bioluminescent ctenophores produce calcium-activated phytoproteins. (Brusca and Brusca, 2003; Haddock and Case, 1995; Mills, 2010)
Body shape and robustness is directly related to the environment in which a species lives. Coastal species need to be tougher to withstand the force associated with wave motions, while pelagic species are often very fragile. Species in order Cydippida are typically round or oblong, with small (less than 3 cm in diameter), solid bodies. They have two tentacles used for capturing prey, which are usually branched. Species in order Lobata are generally larger than cydippids, and have expandable, sticky oral lobes used for capturing prey. The lobes have small tentacles within them. Members of order Beroida have cylindrical or flat, sac-like bodies, which open to engulf prey. Ctenophores of order Platyctenida are benthic organisms and most resemble sea slugs, but have branched feeding tentacles, similar to those of cydippids. (Brusca and Brusca, 2003; Mills and Haddock, 2007; Mills, 2010; Wrobel, 2012)
Ctenophore bodies are composed of two layers of epithelial tissue: an outer epidermis and inner gastrodermis. These sandwich the mesoglea, a jelly-like layer of mesenchymal tissue. True muscular cells, arranged in longitudinal and radial fibers, are found within the mesenchyme and provide the majority of support to the body and assist in movements associated with feeding. At some point in the life history of all comb jelly species (usually at all stages), the outer body bears eight plates of long, fused cilia, called “ctenes". These ciliary bands are the primary means of locomotion, and their beating is coordinated by an apical sense organ containing a calcareous statolith. In species with tentacles, these structures are armed with colloblasts, cells that discharge adhesive substances to aid in subduing prey. (Brusca and Brusca, 2003)
Development is indirect, but non-metamorphic, with fertilized eggs of most species (with the exception of beroids) rapidly growing into ciliated cydippid larvae, which gradually attain adult sizes and morphologies. Lobates and cestids lose the characteristic paired feeding tentacles as they grow, while cydippid species retain them. Beroids lack tentacles at any developmental stage. (Brusca and Brusca, 2003; Wrobel, 2012)
Reproductive tissues develop within the meridional gastrovascular canals and gametes are expelled from the mouth, fertilization usually occurs in the water. In two benthic genera, Coeloplana and Tjalfiella, gametes are taken in through the mouth and fertilization is internal. Ctenophores are able to self-fertilize, although cross-fertilization with other individuals is also common. (Brusca and Brusca, 2003)
Most ctenophores are simultaneous hermaphrodites, although some dioecious species are known, such as members of genus Ocyropsis. Members of order Platyctenida are also known to reproduce asexually, with small fragments that break off as the animal moves, developing into fully-developed adults. Most ctenophores are capable of reproduction before they reach adulthood (paedogenesis). As adults, ctenophores release gametes daily for periods of weeks. Gamete production may slow or cease if food becomes scarce. (Brusca and Brusca, 2003; Jaspers, et al., 2012; Mills, 2010)
As hermaphroditic broadcast spawners, ctenophores exhibit no parental investment beyond the production of gametes. (Brusca and Brusca, 2003)
While little information is available regarding average ctenophore lifespans, individual species may have lifespans of anywhere from less than a month to three years. (Kasuya, et al., 2002; Mills, 1984; Siferd and Conover, 1992)
While they may have locally high population densities, ctenophores are solitary animals. Movement is achieved through the use of cilial motion, coordinated by an apical sense organ containing a calcareous statolith. Depending on the comb jellies' orientation, this statolith presses on one of four balancers (tufts of cilia supporting the statolith), causing the downside ctenes to beat more vigorously and right the animal. Benthic ctenophores may "creep" along a surface, using their flattened bodies as a sort of foot. Some ctenophores exhibit diurnal migration, following the movement of their planktonic prey into deeper water during the day and shallower water at night. The beating motion of their cilia causes light diffraction, which causes waves of color change down the comb rows, not to be confused with bioluminescence. Many, but not all species of ctenophores are also bioluminescent, however, producing blue-green light. (Brusca and Brusca, 2003; Haddock and Case, 1995; Marinova, et al., 2004; Mills and Haddock, 2007; Mills, 1984; Mills, 2010; Nicol, 1960)
The ctenophore nervous system is a non-centralized net, much like that of cnidarians, although it differs in many important specific aspects and is generally more specialized. Ctenophores have a diffuse subepidermal net of non-polar neurons; beneath the comb rows, these neurons form an elongate mesh resembling nerve fibers. A large concentration of nerve tissue is also found around the mouth. Outside of the apical sense organ, no other sensory organs have been confirmed, although some species possess oval tracts of cilia called polar fields on the aboral surface, which may play some sensory role. Areas near the mouths of some ctenophore species have chemoreceptive cells, aiding in prey detection and capture. ("Ctenophores", 2012; Brusca and Brusca, 2003; Falkenhaug and Stabell, 1996; Kass-Simon and Hufnagel, 1992)
All known ctenophore species are carnivorous, feeding on rotifers, small crustaceans (including copepods, amphipods, and euphausiids), and the planktonic larvae of many other species (including clams and snails). Beroids are known to feed on other ctenophores. Depending on the body structure of the specific species, prey may be captured with long tentacles or with a mucosal layer on the body surface, which carries the prey to the mouth by ciliary currents. Colloblasts, located on the animals' tentacles or lobes, aid in prey capture; species of genus Haeckelia do not have colloblasts, and instead use sequestered nematocysts from their cnidarian prey. Species of Euplokamis have prehensile side branches on their tentacles, which wrap around and snare prey. (Brusca and Brusca, 2003; Haddock, 2007; Mills, 2010)
Known predators of ctenophores include other ctenophores, cnidarians, medusae, other invertebrates, fishes, whales, sea turtles, and ocean sunfish. (Mills, 2010; Purcell and Cowan Jr, 1995; Soulanille, 2012; Wrobel, 2012)
Ctenophores may host a variety of parasites, including endoparasitic trematodes, cestodes, nematodes, ectoparasitic isopods, dinoflagellates, and amphipods. Some species may also host a parasitic sea anemone. They may serve as intermediate hosts to digenean flukes, due to their placement on the food chain. A few species of ctenophores may themselves be parasitic on salps. (Boero and Bouillon, 2005; Martorelli, 2001; Mills and McLean, 1991; Reitzel, et al., 2007; Selander, et al., 2010; Yip, 1984)
Beyond the potential for scientific research and display in public aquaria, there are no known positive economic effects of ctenophores on humans. (Brusca and Brusca, 2003; Mills, 2010)
The introduction of the North American species Mnemiopsis leidyi into the Black Sea in the early 1980s, most likely in ballast water from ships originating in the northwestern Atlantic, completely disrupted this ecosystem's natural food chain. As a rapidly reproducing, generalized feeder, it spread throughout the area, outcompeting native planktonic fishes and completely destroying the region's fishing industry within 10 years of its introduction. Since then, another ctenophore, Beroe ovata, has been introduced as well (likely by the same means). A voracious predator, B. ovata has reduced populations of M. leidyi and native fauna populations have rebounded since its introduction, however, the long term effects of this second invasion are unknown. Mnemiopsis leidyi and Beroe ovata have moved into the Caspian Sea from the Black Sea; the ecological ramifications of this introduction remain to be seen. As of 2009, M. leidyi had spread to most European coastlines as well. (Mills, 2010; Shiganova, 1998)
There is currently no concern that ctenophores will become threatened or endangered, on either a local or global scale. (Mills, 2010)
Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.
lives on Antarctica, the southernmost continent which sits astride the southern pole.
the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.
the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
on or near the ocean floor in the deep ocean. Abyssal regions are characterized by complete lack of light, extremely high water pressure, low nutrient availability, and continuous cold (3 degrees C).
reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents
Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
uses smells or other chemicals to communicate
the nearshore aquatic habitats near a coast, or shoreline.
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
active at dawn and dusk
fertilization takes place outside the female's body
union of egg and spermatozoan
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.
Found in northern North America and northern Europe or Asia.
fertilization takes place within the female's body
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
eats mollusks, members of Phylum Mollusca
having the capacity to move from one place to another.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
active during the night
Areas of the deep sea floor where continental plates are being pushed apart. Oceanic vents are places where hot sulfur-rich water is released from the ocean floor. An aquatic biome.
found in the oriental region of the world. In other words, India and southeast Asia.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).
generates and uses light to communicate
an animal that mainly eats plankton
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.
a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).
structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.
mainly lives in oceans, seas, or other bodies of salt water.
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
breeding takes place throughout the year
young are relatively well-developed when born
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