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Ctenophore
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The Ctenophora , commonly known as comb jellies, is a phylum of animals that live in all types of marine waters world-wide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size. Like cnidarians', their bodies consist of a mass of jelly with one layer of cells on the outside and another lining the internal cavity.
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Encyclopedia
The Ctenophora , commonly known as comb jellies, is a phylum of animals that live in all types of marine waters world-wide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size. Like cnidarians', their bodies consist of a mass of jelly with one layer of cells on the outside and another lining the internal cavity. However in ctenophores these layers are two cells deep while those of cnidarians are only one cell deep. Ctenophores also resemble cnidarians in having a decentralized nerve net rather than a brain. Some authors combined ctenophores and cnidarians in one phylum, Coelenterata, as both groups rely on water flow though the body cavity for both digestion and respiration. However increasing awareness of the differences persuaded more recent authors to classify them in separate phlya.
All are predators, taking prey ranging from microscopic larvae and rotifers to the adults of small crustaceans – except that juveniles of two species live as parasites on the salps on which adults of their species feed. In favorable circumstances ctenophores can eat ten times their own weight in a day. There are only 100–150 valid species and possibly another 25 that have not been fully described and named. The textbook examples are cydippids with egg-shaped bodies and a pair of retractable tentacles fringed with tentilla ("litte tentacles") that are covered with colloblasts, sticky cells that capture prey. However the phylum has a wide range of body forms, including the flattened deep-sea platyctenids, in which the adults of most species which lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as "teeth". These variations enable different species to build huge populations in the same area, because they specialize in different types of prey, which they capture by as wide a range of methods as spiders.
Most species are hermaphrodites, in other words they can produce eggs and sperm at the same time. Fertilization is generally external fertilization, although platyctenids' eggs are fertilized inside their parent bodies and kept there until they hatch. The young are generally planktonic and in most species look like miniature cydippids, gradually changing into the adult shape as they grow. The exceptions are the beroids, whose young are miniature beroids with large mouths and no tentacles, and the platyctenids, whose young live as cydippid-like plankton until they reach near-adult size, but then sink to the bottom and rapidly metamorphose into the adult form. Juveniles are capable of reproduction before reaching the adult size and shape. The combination of hermaphroditism and early reproduction enables small populations to grow at an explosive rate.
Predation by ctenophores controls the populations of small planktonic populations such as copepods, which migh otherwise wipe out the phytoplankton (plankotonic plants), which are a vital part of marine food chains. However one ctenophore, Menemiopsis has accidentally been introduced into the Black Sea, where it is blamed for causing fish stocks to collapse by eating both fish larvae and organisms that would otherwise have fed the fish. The situation was aggravated by other factors, such as over-fishing and environmental changes that promoted the growth of the Mnemiopsis population. The later accidental introduction of Beroe helped to mitigate the problem, as Beroe preys on other ctenophores.
Despite their soft, gelatinous bodies fossil ctenophores, apparently with no tentacles but many more comb-rows than modern forms, have been found in lagerstätten as far back as the early Cambrian, about . The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilaterians are more closely related to each other than either is to ctenophores. A recent molecular phylogenetics analysis concluded that the common ancestor of all modern ctenophores was cydippid-like, and that all the modern groups appeared relatively recently, probably after the Cretaceous-Tertiary extinction . Evidence accumulating since the 1980s indicates that the "cydippids" are not monophyletic, in other words do not all and only the descendants of a single common ancestor, because all the other traditional ctenophore groups are descendants of various cydippids.
Distinguishing features
Ctenophores form an animal phylum that is more complex than sponges, about as complex as cnidarians (jellyfish, sea anemones, etc.), and less complex than bilaterians, which include almost all other animals. Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like basement membranes; muscles; nervous systems; and some have sensory organs. Ctenophores are distinguished from all other animals by having colloblasts that capture prey by squirting glue on them, although a few ctenophore species lack them.
Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. However both ctenophores and cnidarians have a type of muscle which, in more complex animals, arises from the middle cell layer. As a result some recent text books classify ctenophores as triploblastic, while others still regard them as diploblastic.
Ranging from about to in size, ctenophores are the largest non-colonial animals that use cilia ("hairs") as their main method of locomotion. Most species have up to eight strips that run the length of their bodies and bear comb-like bands of cilia running at intervals across the strips. The name "ctenophore" means "comb-bearing", from the Greek ?te?? (stem-form ?te?-) meaning "comb" and the Greek suffix -f???? meaning "carrying".
| | Sponges | Cnidarians | Ctenophores | Bilateria |
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| Cnidocytes | No | Yes | No |
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| Colloblasts | No | In most species | No |
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| Digestive and circulatory organs | No | Yes |
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| Number of main cell layers | Two, with jelly-like layer between them | Debate about whether two or three | three |
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| Cells in each layer bound together | No, except that Homoscleromorpha have basement membranes. | Yes: inter-cell connections; basement membranes |
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| Sensory organs | No | Yes |
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| Number of cells in middle "jelly" layer | Many | Few | (not applicable) |
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| Cells in outer layers can move inwards and change functions | Yes | No | (not applicable) |
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| Nervous system | No | Yes, simple | Simple to complex |
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| Muscles | None | Mostly epitheliomuscular | Mostly myoepithelial | Mostly myocytes |
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Description
For a phylum with relatively few species, ctenophores have a wide range of body plans. Coastal species need to be tough enough to withstand waves and swirling sediment particles, while oceanic species are so fragile that it is very difficult to capture them intact for study. In addition oceanic species do not preserve well, and are known mainly from photographs and from observers' notes. Hence most attention has until recently concentrated on three coastal genera – Pleurobrachia, Beroe and Mnemiopsis. At least two textbooks base their descriptions of ctenophores on the cydippid Pleurobrachia.
Common features
Body layers
Like those of cnidarians, (jellyfish, sea anemones, etc.), ctenophores' bodies consist of a relatively thick, jelly-like mesoglea sandwiched between two epithelia, layers of cells bound by inter-cell connections and by a fibrous basement membrane which they secrete. However the epithelia of ctenophores have two layers of cells rather than one, and the cells in the upper layer generally have several cilia per cell.
The outer layer of the epidermis (outer skin) consists of: sensory cells; cells that secrete mucus, which protects the body; and interstitial cells, which can transform into other types of cell. In specialized parts of the body the outer layer also contains colloblasts, used in capturing prey, or cells bearing multiple large cilia, for locomotion. The inner layer of the epidermis contains a nerve net, and myoepithelial cells which act as muscles.
The internal cavity forms: a mouth that can usually be closed by muscles; a pharynx ("throat"); a wider area in the center that acts as a stomach; and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx; the roots of the tentacles, if present; all along the underside of each comb row; and four branches round the sensory complex at the far end from the mouth – two of these four branches terminate in anal pores. The inner surface of the cavity is lined with an epithelium, the gastrodermis. The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in vacuoles (internal compartments), germ cells that produce eggs or sperm, and photocytes that produce bioluminescence. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea.
Feeding, excretion and respiration
When prey is swallowed, it is liquidized in the pharynx by enzymes and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia, and digested by the nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The anal pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.
Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the gastrodermis may help to remove wastes from the mesoglea, and may also help to adjust the animal's buoyancy by pumping water into or out of the mesoglea.
Locomotion
The outer surface bears several rows of "combs", which are used for swimming. The rows run from near the mouth to the opposite end, and are usually spaced evenly round the body. The "combs" run across across each row, and each consists of thousands of unusually long cilia, up to . These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is pointing, unlike jellyfish. When trying to escape predators, they can accelerate to six times times their normal speed.
It is uncertain how ctenophores control their buoyancy, but possibly they rely on osmotic pressure to adapt to water of different densities. Their body fluids are normally as concentrated as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea to increase its bulk and decrease its density, to avoid sinking. Conversely if they move from brackish to to full-strength seawater, the rosettes may pump water out of the mesoglea to reduce its volume and increase its density.
Nervous system and senses
Ctenophores have no brain or central nervous system, but instead have a nerve net (rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth.
The largest single sensory feature is the aboral organ (at the opposite end from the mouth). Its main component is a statocyst, a balance sensor consisting of a statolith, a solid particle supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead its responses is determined by the animals' "mood", in other words the overall state of the nervous system. For example if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey.
Symmetry and axes
Since the body of many species is almost radially symmetrical, the main axis is oral to aboral (from the mouth to the opposite end). However since only two of the canals near the statocyst terminate in anal pores, ctenophores have no mirror-symmetery, although many have rotational symmetry, in other words if the animal rotates in a half-circle it looks the same as when it started.
Cydippids
Cydippids, such as the "sea gooseberry" Pleurobranchia, have egg-shaped bodies with the mouth at the narrow end. From opposite sides of the body extends a long, slender tentacle, each housed in a sheath into which it can be withdrawn. Cydippids' bodies may be slightly flattened so that they are wider in the plane of the tentacles.
Each tentacle is fringed with tentilla ("little tentacles") that bear many colloblasts that capture prey by sticking to it. Colloblasts are specialized mushroom-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with vesicles (chambers) that contain adhesive; a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea; and a spiral thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart. In addition members of the genus Haeckelia , which feed mainly on jellyfish, incorporate their victims' stinging nematocytes into their own tentacles. Euplokamis tentilla are coiled in their relaxed stated, and have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 milliseconds); they can wriggle, which may lure prey by behaving like small planktonic worms; and they coil round prey. The flicking movement is powered by striated muscles, a type that is not known in other ctenophores. The wriggling motion is produced by smooth muscles, but of a highly specialized type. Coiling round prey is largely a return to the normal state of the tentilla, but the coils may be tightened by smooth muscle.
There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. The "combs" beat in a sequence rather like that of a Mexican wave. From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs all the way along the comb rows. This forms a mechanical system for transmitting the beat rhythm from the balancers to the combs, via water disturbances created by the cilia.
Lobates
The Lobata have a pair of lobes, muscular extensions of the body that project ahead of the mouth. Their small tentacles run in convoluted grooves over the inner surface of the lobes. Between the lobes and surrounding the mouth are four auricles, projections fringed with cilia that produce water currents that wash microscopic prey into the mouth. This combination of structures enables lobates to feed continuously on suspended prey such as plankton.
There are eight comb-rows, two on each lobe and two on each side beteeen the lobes. Members of the genera Leucothea and Ocyropsis also use their large lobes for swimming. Unlike cydippids, the movements of lobates' combs are co-ordinated by nerves rather than by via water disturbances created by the cilia, and combs on the same row beat together rather than in Mexican wave style. This may have enabled lobates to grow larger than cydippids and to have shapes that are less egg-like. In addition members of the lobate genera Bathocyroe and Ocyropsis can escape from danger by clapping their lobes, so that the jet of expelled water drives them backwards very quickly.
An unusual species first described in 2000 has been classified as a lobate, although the lobes are "primitive" and the body is medusa-like when floating and disk-like when resting on the sea-bed.
Beroids
The Beroida, also known as Nuda, have no feeding appendages, but their large pharynxes, just inside their large mouths, bear "macrocilia", fused bundles of several thousand large cilia that bite off pieces of prey that is is too large to swallow whole - almost always other ctenophores. Just behind the ring of macrocilia is an internal ridge that runs all round the mouth and "zips" it shut when the animal is not feeding, by forming intracellular connections with the opposite part of the ridge. This tight closure streamlines the front of the animal when it is pursuing prey.
Other body forms
The Ganeshida have a pair of small oral lobes and a pair of tentacles. The body is circular rather than oval in cross-section, and the pharynx extends over the inner surfaces of the lobes.
The Thalassocalycida, only discovered in 1978 and known from only one species, are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth. They capture prey by movements of the bell and possibly by using two short tentacles.
The Cestoida ("belt animals"), are ribbon-shaped planktonic animals, with the mouth and aboral organ in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla on the oral edge. Cestoids can swim by undulating their bodies as well as by means of their comb-rows. The two known species, Cestum veneris ("Venus' girdle") and Velamen parallelum, are the largest ctenophores – up to and long respectively.
The Platyctenida have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows.
Reproduction and development
Adults of most species can regenerate tissues that are damaged or removed. However only platyctenids reproduce by cloning, splitting off from the edges of their flat bodies fragments that develop into new individuals.
Almost all species are hermaphrodites, in other words function as both males and females at the same time - except that in two species of the genus Ocryopsis individuals remain of the same single sex all their lives. The gonads are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is external in most species, but platyctenids use internal fertilzation and keep the eggs in internal brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, and it is thought that most of the hermaphroditic species are self-fertile.
Development of the fertized eggs is direct, and juveniles of all groups generally resemble miniature cydippid adults. However in the genus Beroe the juveniles, like the adults, lack tentacles and tentacle sheaths. In most species the juveniles gradually develop the body forms of their parents. However in some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae, as they: live among the plankton and thus occupy a different ecological niche from their parents; and attain the adult form by a more radical metamorphosis, after dropping to the sea-floor.
Juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for long as they have sufficient food. If they run short of food, first they stop producing eggs and sperm, and then they shrink in size. When the food suppply improves, they grow back to normal size and then resume reproduction. The features make ctenophores capable of increasing thier populations very quickly.
Colors and bioluminescence
Most ctenophores that live near the surface are colorless and almost transparent. However some deeper-living species are strongly colored, for example the species known as "Tortugas red", which has been named Agmayeria tortugensis but has not been described in detail. Platyctenids generally live attached to other sea-bottom organisms, and have similar colors to these organisms'. The gut of the deap-sea genus Bathocyroe is red, which hides the bioluminescence of copepods it has swallowed.
Many planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescence but by the scattering of light as the combs move. Most species are also bioluminescent, but the light is usually blue or green and can only been seen in darkness. However some significant groups, including all known platyctenids and the cydippid genus Pleurobrachia, appear incapable of bioluminescence.
When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions that luminesce at much the same wavelengths as their bodies. Juveniles can luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any correlation between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters.
Ecology
Distribution
Ctenophores are found in most marine environments: from polar waters to the tropics; near coasts and in mid-ocean; from the surface waters to the ocean depths. However the best-understood are the genera Pleurobrachia, Beroe and Mnemiopsis, as these planktonic coastal forms are easiest to study.
Prey and predators
Almost all ctenophores are predators – there are no vegetarians and only one genus that is partly parasitic. If food is plentiful, they can eat 10 times their own weight per day. While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplankton (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepods, amphipods, and even krill. Members of the genus Haeckelia prey on jellyfish and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblasts. Ctenophores have been compared to spiders in their wide range of techniques from capturing prey - some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid jumping spiders, and some dangle a sticky droplet at the end of a fine thread, as Bola spiders do. This variety explains the wide range of body forms in a phylum with rather few species. The two-tentacled "cydippid" Lampea feeds exclusively on salps, close relatives of sea-squirts that form large chain-like foating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. Members of the cydippid genus Pleurobrachia and the lobate Bolinopsis often reach high population densities at the same place and time because they specialize in different types of prey: Pleurobrachias long tentacles mainly capture relatively strong swimmers such as adult copepods, while Bolinopsis generally feeds on smaller, weaker swimmers such as rotifers and mollusc and crustacean larvae.
Ctenophores used to be regarded as "dead ends" in marine food chains because it was thought their low ratio of organic matter to salt and water made them a poor diet for other animals. In addition it is often difficult to identify the remains of ctenophores in the guts of possible predators, although the combs sometimes remain intact long enough to provide a clue. However detailed investigation of chum salmon, Oncorhynchus keta, showed that these fish digest ctenophores 20 times as fast as an equal weight of shrimps, and that ctenophores can provide a good diet if there enough of them around. Beroids prey mainly on other ctenophores. Some jellyfish and turtles eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists, and may have a greater effect on populations than the specialist jelly-eaters. The larvae of some sea anemones are parasites on ctenophores, as are the larvae of some flatworms that parasitize fish when they reach adulthood.
Ecological impacts
Ctenophores may balance marine ecoystems by preventing an over-abundance of copepods from eating all the phytoplankton (planktonic plants), which are the dominant marine producers of organic matter from non-organic ingredients.
On the other hand in the late 1980s the North Atlantic ctenophore Mnemiopsis leidyi was accidentally introduced into the Black Sea and Sea of Azov via the bilges of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish. Mnemiopsis is well-equipped to invade new territories, as it can breed very rapidly and tolerate a wide range of water temperatures and salinities. The impact was increased by overfishing, and by eutrophication that gave the entire ecosystem a short-term boost, causing the Mnemiopsis population to increase even faster than normal – and above all by the absence of efficient predators on ctenophores. Mnemiopsis populations in those areas were eventually brought under control by the accidental introduction of the ctenophore-eating ctenophore Beroe, and by a cooling of the local climate from 1991 to 1993, which significantly slowed the animal's metabolism. However the abundance of plankton in the area seems unlikely to reach pre-Mnemiopsis levels.
In the late 1990s Mnemiopsis appeared in the Caspian Sea, probably via the canals that connect this to the Black Sea. Beroe arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis. Mnemiopsis also reached the eastern Mediterranean in the late 1990s and now appears to be thriving in the North Sea and Baltic Sea.
Classification
The number of known living ctenophore species is uncertain, since many of those named and formally decribed have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100 to 150 valid species that are not duplicates, and that at least another 25, mostly deep-sea forms, have been correctly recognized as distinct but not yet analyzed in enough detail to support a formal description and naming.
The traditional classification divides ctenophores into two classes, those with tentacles (Tentaculata) and those without (Nuda). The Nuda contains only one order (Beroida) and family (Beroidae), and two genera, Beroe (several species) and Neis (one species).
The Tentaculata are divided into the following eight orders:
- Cydippida, egg-shaped animals with long tentacles
- Lobata, with paired thick lobes
- Platyctenida, flattened animals that live on or near the sea-bed; most lack comnbs as adults
- Ganeshida, with a pair of small lobes round the mouth, but an extended pharynx like that of platyctenids
- Cambojiida
- Cryptolobiferida
- Thalassocalycida, with short tentacles and a jellyfish-like "umbrella"
- Cestida, ribbon-shaped and the largest ctenophores
Evolutionary history
Fossil record
Because of their soft, gelatinous bodies, ctenophores are are extremely rare as fossils and have been found only in lagerstätten, places where the environment was exceptionally suited to preservation of soft tissue. Until the mid-1990s only two specimens good enough for analysis were known, both from the early Devonian period. Three species were then found in the Burgess Shale and other Canadian rocks of similar age, about in the mid-Cambrian period. All three apparently lacked tentacles but had between 24 and 80 comb rows, far more than the 8 typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. Evidence from China a year later suggests that ctenophores were wide-spread in the Cambrian, but perhaps very different from modern species - for example one fossil's comb-rows were mounted on prominent vanes.
The early Cambrian sessile frond-like fossil Stromatoveris, from China's Chengjiang lagerstätte and dated to about , is very similar to Vendobionta of the preceding Ediacaran period. De-Gan Shu, Simon Conway Morris et al found on its branches what they considered rows of cilia , used for filter feeding. They suggested that Stromatoveriswas an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals which later became swimmers and changed the cilia from a feeding mechanism to a propulsion system.
Family tree
It is difficult to reconstruct the early stages in the evolutionary "family tree" of animals using only morphology (their shapes and structures), because the large differences between Porifera (sponges), Cnidaria (jellyfish, sea anemones, etc.) plus Ctenophora, Placozoa and Bilateria (all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics, which groups organisms according to similarities and differences in their biochemistry, usually in their DNA or RNA.
It is now generally thought that the Calcarea (sponges with calcium carbonate spicules) are more closely related to Cnidaria, Ctenophora (comb jellies) and Bilateria (all the more complex animals) than they are to the other groups of sponges. In 1866 it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called Coelenterata ("hollow guts"), because Cnidaria and Ctenophora both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881 it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer (mesoglea in Ctenophora, mesoderm in Bilateria). However more recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "Planulozoa" because it suggests that the earliest Bilateria were similar to the planula larvae of Cnidaria.
However in 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have striated muscle, which in bilaterians arises from the mesoderm. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians. A study in 2008 of 150 genes in 21 genera proposed a third hypothesis, that sponges and cnidarians are more closely related to each other than either is to ctenophores.
Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor. Instead he found that various cydippid families were more similar to members of other ctenophore orders than to other cydippids. He also suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like. A molecular phylogeny analysis in 2001, using 26 species including 4 recently-discovered ones, concluded that the cydippids are not monophyletic and that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were very small – so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps was lucky enough to survive the Cretaceous-Tertiary extinction while other lineages perished. When the analysis was broadened to include representatives of other phlya, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain.
Further reading
- R. S. K. Barnes, P. Calow, P. J. W. Olive, D. W. Golding, J. I. Spicer, The invertebrates – a synthesis, 3rd ed, Blackwell, 2001, ch. 3.4.3, p. 63, ISBN 0-632-04761-5
- R. C. Brusca, G. J. Brusca, Invertebrates, 2nd Ed, Sinauer Associates, 2003, ch. 9, p. 269, ISBN 0-87893-097-3
- J. Moore, An Introduction to the Invertebrates, Cambridge Univ. Press, 2001, ch. 5.4, p. 65, ISBN 0-521-77914-6
- W. Schäfer, Ctenophora, Rippenquallen, in W. Westheide and R. Rieger: Spezielle Zoologie Band 1, Gustav Fischer Verlag, Stuttgart 1996
- Bruno Wenzel, Glastiere des Meeres. Rippenquallen (Acnidaria), 1958, ISBN 3-7403-0189-9
- Mark Shasha, Night of the Moonjellies, 1992, Simon & Schuster, ISBN 0-6717-7565-0
External links
- – striking images, including a Beroe specimen attacking another ctenophore
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