Ctenophore

Comb jellies
"Ctenophorae" from Ernst Haeckel's Kunstformen der Natur, 1904
"Ctenophorae" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Ctenophora
Eschscholtz, 1829
Classes

Tentaculata
Nuda

The phylum Ctenophora (pronounced /tɨˈnɒfərə/), commonly known as comb jellies, is a phylum that includes the sea gooseberry (Pleurobrachia pileus) and Venus' girdle (Cestum veneris). Classically grouped with Cnidaria (jellyfish) in the Coelenterata infrakingdom, ctenophores have recently been identified as the most basal known lineage of animals.[1]

Despite their appearance, they are zoologically not jellyfish, not least because they lack the characteristic cnidocytes (stinging cells) but have connective tissues and a nervous system. There are close to 150 described species of ctenophora spread throughout the world's oceans, from shallow estuarine waters to the deep sea. Although there are a few benthic species, most are members of the gelatinous zooplankton and form a considerable proportion of the entire plankton biomass worldwide. A few species, such as the sea gooseberry, native to the North Sea, have reached such high populations that they clog fishermen's nets, while of other species only a few examples are known. The fragile makeup of ctenophores makes research into their way of life extremely difficult; for this reason data on their lifespan are not available, but it is known that ctenophores begin to reproduce at an early age and so can be assumed to have a short generation cycle.

The word ctenophore (pronounced /ˈtɛnəfɔər/ or /ˈtiːnəfɔər/, without the c) comes from Greek, kteno-, kteis, "comb" and -phore, meaning "bearer". It comes via the New Latin ctenophorus in the 19th century.

Contents

Anatomy and morphology

Body

Light refracting and diffracting along the comb rows of a Mertensia ovum. Specimen is damaged and asymmetrical, with right lower portion of body regenerating from previous damage. The body of this species is commonly about 3 cm long.

Ctenophores are generally colorless, but some species have red, orange, golden, or even black pigment. The most common species are often only a few centimetres long. The largest ctenophores are the Venus' girdles Cestum veneris, which can reach up to one and a half meters.

The rainbow-light effect seen in daylight on ctenophores is not bioluminescence but merely diffraction of ambient light by the beating comb rows. In addition to this diffraction, most species of ctenophores are also bioluminescent[2], but this (usually bluish) light is rarely seen under normal circumstances, and never by daylight; one species, Eurhamphaea vexilligera, produces a reddish-brown ink which is bioluminescent, glowing blue in the dark, perhaps to dissuade predators. Species that live in deep waters, such as the Tortugas red, are brightly-coloured when brought to the surface, although usually with pigments that absorb blue light, making them appear dark at depth, so they effectively disappear in the dark waters they inhabit.

Ctenophores have an interesting form of symmetry, with many bilateral components, but a few asymmetrical structures such as the anal pores near the statocyst and sometimes the proportions of their auricles (ciliated lobe-like structures).

Ctenophores are diploblastic (having only two body layers). The body consists of two transparent cell layers, which make up its outer skin (ectoderm) and inner skin (gastroderm). The ectoderm, made up of two cell layers, is mostly covered by a protective layer of mucus, excreted by special glands. The gastroderm surrounds a cavity which serves as a stomach and is only accessible by the mouth opening, connected by a long, narrow gullet. Captured quarry is pre-digested in the gullet by strong enzymes and fully decomposed in the stomach. There is no separate exit from the stomach apart from two 'anal pores', which despite their name appear to be only moderately used for excretion, so indigestible waste is principally expelled via the mouth.

The space between the inner and outer skin is taken up by the mesoglea, a thick, transparent, jelly-like layer made from collagen and connective tissue, pervaded by numerous small canals, which are used for transport and storage of nutrients. The position of the canals varies from species to species, but they mostly run directly underneath the tissues that they serve. The extracellular net of structural protein is kept upright by special cells similar to amoebas.

The mesogloea may also play a role in the lift of the creatures. Cilia found in the canals of the digestive system may serve to pump water in or out of the mesogloea, when osmotic water pressure changes, perhaps because the creature has swum out of saline sea water into coastal brackish water. Ctenophores do not possess a specific circulatory system, neither do they have any organs for breathing; gas exchange and the excretion of waste products of cell metabolism such as ammonia occur over the body's entire surface through simple diffusion. The body is pervaded by a simple net of neurons without a 'brain'. These nerves are concentrated around the mouth, tentacles, 'combs' and statocysts and are connected with the muscular cells found in the mesogloea and the inner cellular layer of the ectoderm.

Statocysts

Undescribed deep-sea species, sometimes called "Tortugas red," with a pair of trailing tentacles and clearly visible side-branches (tentilla). The body of this robust ctenophore may be 10-20 cm long.

The statocyst is a specialised system of the ctenophore that is a balancing organ and also controls movement. It can be found on the end of the body opposite the oral opening and is formed by a collection of a few hundred calcareous cells balanced on four horizontal groups of serpentine flagella, known as the statolith. As outside influences cause the ctenophore to change its position, the statolith puts more pressure on one of the four flagella groups than on the other three. This sensation is transmitted to the ectoderm, which is propagated along eight long "comb rows" (ctenes). The ctenes are formed from rows of cilia, which coalesce with one another in groups of hundreds and form ctenes or comb plates about 2-5 millimetres long. By erecting these ctenes in succession, the ctenophore can use them as an oar, which, when the eight ctenes are properly synchronised, allow it to propel itself through the water. A ciliary group of the statocyst is needed for every quadrant and controls two ctenes as a pacemaker. The rhythm is carried automatically, and the signal is propagated mechanically, and not by nerve impulses.

Whether gravity acting on the statocyst raises or lowers the stroke frequency depends on the "disposition" or geotaxis of the ctenophore; the ctenophore can alter the beat frequency of different comb rows to either swim upward or downward in the water column. The "disposition" of the ctenophore is determined by sensations handled by the nerve net, in association with the ambient light levels.

Tentacles

Many species have two opposing retractable tentacles emerging somewhere near the midpoint of the body, which are used to catch prey. From these central tentacles branch additional filaments called tentilla, which unlike in Cnidaria do not contain stinging cells, but colloblasts or "lasso cells". These cells burst open when prey comes in contact with the tentacle. Sticky threads released from each of the colloblasts will then capture the food. The colloblasts, like the tentacles, are regularly fully regenerated.

Not all varieties rely mainly on tentacles. Some like Beroe engulf gelatinous prey directly, and others instead use their muscular mouth lobes to catch food, with oral tentacles serving a secondary entangling function.

Regeneration

Ctenophores are capable of extraordinary regeneration; even if half of the creature is destroyed, often the remaining half can rebuild itself. The same is true of single organs such as the statoliths, which can be regenerated even after being completely lost.

Movement

Most ctenophores drift largely with the current, although they swim some of the time, sometimes quite rapidly, by means of the coordinated strokes of cilia on their comb plates. Nevertheless, as with other planktonic organisms, it is the movement of ocean currents, tides, and waves that determine where they go on a large scale. They are the largest animals to use cilia for movement and some can reach speeds of about five centimetres a second. A possible evolutionary advantage is that constant strokes do not cause vibrations that would alert prey or predators.

Most ctenophores swim using coordinated ciliary beating of the eight comb rows. In addition, a few species flap their oral lobes in a gesture reminiscent of a "frog kick", giving these species a much more effective swim sometimes used for escape, while others (the Venus' girdles) move by undulating their elongate body; another group of species (the platyctenes) creep like flatworms.

Distribution

As an invasive species

Although ctenophores are generally hardly noticeable and their influence on an ecosystem is ostensibly very low, they can still do significant damage when they occur in non-native waters. The North Atlantic species Mnemiopsis leidyi was brought by ships' ballast water into the Black Sea and spread rapidly. Within ten years the anchovy fishing industry around the sea had collapsed, as the newly introduced species fed on the same plankton as the anchovy larvae. The biomass of ctenophora in the Black Sea reached a million tons at the highest point of its development.

Through the similarly sudden appearance in 1997 of another ctenophore, Beroe ovata, which feeds on Mnemiopsis leidyi, the balance was somewhat restored; since then the Black Sea has been occupied by both foreign species. The same scenario with the same species has now begun to be played out in the Caspian Sea, and M. leidyi was also reported from the North Sea in 2006.

The same scenario is now awaited in the Baltic Sea, where Finnish scientists have found that Mnemiopsis gardeni have survived the winter and spread very quickly. A recent expedition found over 600 Mnemiopsis ctenophores per cubic meter in the greater depths of the Baltic Sea.[3]

Ecology and life history

Habitat

All ctenophora live in the sea, where they live in depths of up to four kilometres. There are no freshwater ctenophores. As plankton they are largely subject to movement of ocean currents, although various species are particular to certain habitats. They can be found in abundance in the tropics and to both poles.

The most well-known species live as plankton in the ocean layers near the surface. However, as they are largely transparent, extremely fragile and rarely grow longer than a few centimetres, they are unknown to most people. On the coast Pleurobrachia species (called sea gooseberries) are encountered most frequently by beachgoers. Bolinopsis, Mnemiopsis and the tentacle-less Beroe can also be found fairly frequently.

About 35 species are not planktonic, but live either on the bottom or attached to other organisms including seaweed, starfish, sponges and other invertebrates.[4] These species are ordered in the taxon of Platyctenida, due to their flattened forms which more closely resemble slugs or flatworms than jellyfish.

The ctenophore Mertensia ovum is one of the most predominant members of plankton in arctic waters.

Community Ecology

A ctenophore (Beroe sp.) looking for food. The open mouth is on the left. This animal is 3-6 cm long.

Ctenophora are predators which use their tentacles to catch plankton, larvae, worms, crustaceans, cnidaria, other ctenophora, and sometimes small fish. When their tentacles are loaded with food, they can be retracted and wiped off. The food is then carried into the stomach either by mucus or inner cilia. The species of the genus Haeckelia feed almost exclusively on cnidaria, but do not digest their cnidocytes; instead they build them into their own tentacles as 'kleptocnidae'. This 'theft' baffled zoologists for a long time as they falsely assumed ctenophora were also capable of forming cnidocytes. Parasitism has only been observed in a single genus, Lampea, which is parasitic on salps when too small to engulf them entirely.

Among the species that prey on ctenophora are jellyfish, sea turtles, various fish such as chum salmon, mackerels and lumpfish, seabirds and other ctenophora. They are often infested with parasitic crustaceans of the group Amphipoda, and may also have internal parasitic trematodes. Some also carry parasitic dinoflagellates of the genus Oodinium on their comb rows.[5]

Life History

Cydippid larva of Bolinopsis sp., a few mm long.

Ctenophora reproduce sexually. Some species of the order Platyctenida are also able to reproduce asexually. Almost all ctenophores are hermaphroditic, or monoecious, possessing both male and female reproductive organs, which lie directly under the 'combs' near the small channels of the mesogloea. In the few cases where it has been carefully examined, these hermaphroditic species seem to be self-fertile, that is, the eggs of an individual can be activated and fertilized by sperm produced by the same animal. Lobate ctenophores in the warm-water genus Ocyropsis are unusual in having separate-sexed individuals. In almost all species, spawning is triggered by changes in outside lighting conditions, and the gametes are discharged into the surrounding water through small openings in the ectoderm, called gonopores, where external fertilisation takes place. Internal fertilization is unusual; the platyctene Tjalfiella tristoma, like some other platyctenes, is viviparous; that is, the young grow in an internal brood chamber until they swim away as a planktonic stage before settling again on the bottom.

Certain species of ctenophores, like Beroe ovata, have a special method of preventing polyspermy. After several sperm pronuclei have entered the egg, the egg pronucleus goes through a process where it migrates around the cell and finally chooses which sperm pronucleus it wants to fuse with, rejecting others because of signals indicating close relationship or lack of fitness.

After the fertilised eggs have divided twice, the ctenophore's later radial body symmetry has already been set. They develop into a free-floating cydippid state, which looks very similar between all ctenophora and sometimes is labeled as a larva, although in many cases this already represents a miniature version of what the creature will grow up to be. Among some groups such as lobates and platyctenids, the cydippid and adult forms do differentiate morphologically, so that the 'larva' label is more appropriate.

Etymology and Taxonomic history

The soft bodies of ctenophores, which have no hard parts whatsoever, makes fossilisation generally very improbable, meaning that the phylogeny of ctenophoran fossils is very sparsely documented. The sole fossil records, of Archaeocydippida hunsrueckiana and Paleoctenophora brasseli, date from the Devonian Period; enough details remained in the fine-grained schist of Hunsrück to make identification possible. It is disputed whether the species Maotianoascus octonarius, known from the Chengjiang Fauna of the lower Cambrian Period, is a member of the ctenophore phylum, while three species, Ctenorhabdotus capulus, Fasciculus vesanus and Xanioascus canadensis, are known from the Cambrian Burgess Shale.

Early classification

Sailors have observed ctenophores since ancient times. However, the first recorded sighting only came in 1671, made by a ship's doctor. The Swedish taxonomist Carl von Linné classified them with other 'primitive' invertebrates such as sea sponges (Porifera) or Cnidaria as 'zoophytes' ("animal plants"), alluding to the passive, "plant-like" character of the creatures. The French zoologist Georges Cuvier supported this classification. Only in the 19th century was ctenophora recognised as a standalone taxon.

Historical phylum

Bathocyroe fosteri a common but fragile deep-sea lobate, oriented mouth down

The initial classification of ctenophora has been disputed. According to cladistics, currently the leading ordering method, ctenophora are more closely related to the reflectively symmetrical Bilateria than Cnidaria. The fact that they have two opposing tentacles, breaking their radial symmetry and making them reflectively symmetrical, supports this, although certain structures give them a rotational or biradial symmetry. They differ from Cnidaria in their possession of true muscle tissue, sticky colloblasts in place of cnidocytes, and their 'combs'. Another important sign of ctenophore's relationship with Bilateria is the form of their spermatozoa. These consist in both groups of a single, large acrosome and a subacrosomic perforation disc. Cnidarian spermatozoa, in contrast, possess several acrosomic vesicles.

For this reason the 'classical' grouping of Coelenterata stands opposite the alternative taxon of Acrosomata:

Alternative 1: Coelenterata

Alternative 2: Acrosomata

In addition it has been suggested that ctenophora have a close relationship with flatworms, due to the similarities between flatworms and the flattened ctenophora of the order Platyctenida are one of the justifications for this. Some zoologists consider this resemblance superficial, and not indicative of a close relationship.

Current status

In 2008, a phylogenomic study of 150 genes in 21 genera[1] placed ctenophores as the sister group to all other animals included in the analysis, including sponges. The divergence of ctenophores from other animals prior to sponges would indicate that the most recent common ancestor of living animals was more complex than previously believed and that sponges are secondarily simplified (or that ctenophores evolved a nervous system and tissues independently of all other animals). The placement of ctenophores in this position had not been recovered in any previous analyses of smaller datasets. The authors of the study stated that this new finding should be treated provisionally despite its strong support, at least until further data from other animals is available (particularly from more sponges and the placozoan Trichoplax). It is also possible that the finding is an artifact due to the long phylogenetic stem leading to living ctenophores, a possibility that requires more detailed phylogenetic analyses in addition to more data to rule out.

Classification

Currently about 100-150 species are known (many old descriptions are difficult to justify with known species, so it is hard to tell how many constitute unique species)[4], which are traditionally split into the classes of Tentaculata (also known as Tentaculifera) and Nuda (also known as Atentaculata).

Due to the continued uncertainty over the ordering of ctenophora it is currently unclear whether the above divisions correctly reflect the actual phylogeny of the taxon. Molecular genetic studies indicate that cydipidda is a polyphyletic group, i.e. it does not include all the descendents of their common ancestor, and so the overall classification of the group needs to be revised.

The following diagram shows the putative phylogeny of ctenophora on the basis of morphologic and molecular genetic data (RNA):

Ctenophora
|--Cydippida (Mertensiidae family)
|--
    |--Platyctenida
    |--
        |--Cydippida (Pleurobrachidae family)
        |--
        |   |--Nuda Beroida
        |   |--Cydippida (Haeckeliidae family)
        |
        |--
            |--Lobata
            |--Cestida
            |--Thalassocalycida

The above details are however still in doubt. For the time being the phylogeny of ctenophora must be regarded as unsettled.

Bibliography

References

  1. ↑ 1.0 1.1 Dunn et al. 2008. "Broad phylogenomic sampling improves resolution of the animal tree of life". Nature 06614.
  2. ↑ Haddock, S.H.D.; Case, J.F. (1995). "Not All Ctenophores Are Bioluminescent: Pleurobrachia". Biological Bulletin 189: 356-362. 
  3. ↑ Internytt, 2007-08-27
  4. ↑ 4.0 4.1 Mills, C.E. (March 1998 to present). "Phylum Ctenophora: list of all valid species names" (in English). Retrieved on 2008-09-02.
  5. ↑ Mills, C.E.; McLean, N. (1991). "Ectoparasitism by a dinoflagellate (Dinoflagellata: Oodinidae) on 5 ctenophores (Ctenophora) and a hydromedusa (Cnidaria)". Diseases of Aquatic Organisms 10: 211-216. 

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