Ctenophora (; ) or ctenophores (), commonly known as comb jellies, is a basal phylum of marine invertebrate animals notable for their unique aquatic locomotion using groups of cilia (commonly referred to as "combs"). They inhabit marine habitats worldwide, with 186 recognised living species.
Ctenophores are the largest animals to swim with cilia, with adult ctenophores range from a few millimeters to in size depending on the species. Their bodies consist of a mass of jelly (mesoglea) with a two-cell-thick layer on the outside, and another lining the internal cavity. The phylum has a wide range of body forms, including the egg-shaped cydippids with a pair of retractable tentacles that capture prey; the flat, generally combless platyctenids; and the large-mouthed beroids, which prey on other ctenophores.
Almost all ctenophores are predators, feeding on prey ranging from microscopic larvae and rotifers to the adults of small crustaceans; the exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed.
Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in Lagerstätten (well-preserved fossil beds) dating as far back as the early Cambrian, about 525 million years ago, though stem-group ctenophores that were attached by a stalk to the seafloor may extend back as far as the Ediacaran, around 550 million years ago. The position of the ctenophores in the "tree of life" has long been debated in molecular phylogenetics studies. Biologists proposed that ctenophores constitute the second-earliest branching animal lineage, with sponges being the sister-group to all other multicellular animals (Porifera sister hypothesis). Other biologists contend that ctenophores diverged earlier than sponges (Ctenophora sister hypothesis), which themselves appeared before the split between cnidarians and bilaterians. Pisani et al. reanalyzed the data and suggested that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species. However, follow up analysis by Whelan et al. (2017) yielded support for the 'Ctenophora sister' hypothesis; the issue remains a matter of taxonomic dispute. Schultz et al. (2023) found irreversible changes in synteny in the sister of the Ctenophora, the Myriazoa, consisting of the rest of the animals.
Etymology
The New Latin name is constructed , alluding to the rows of cilia that are distinctive features of animals in the phylum.
Distinguishing features
Ctenophores are distinguished from all other animals by having colloblasts, which are sticky and adhere to prey, although a few ctenophore species lack them.
Like cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians and ctenophores; more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic. Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer, and as a result some text books classify ctenophores as triploblastic, while others still regard them as diploblastic. The comb jellies have more than 80 different cell types, exceeding the numbers from other groups like placozoans, sponges, cnidarians, and some deep-branching bilaterians.
Ranging from about to in size, ctenophores are the largest non-colonial animals that use cilia as their main method of locomotion. Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes", stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below. The name "ctenophora" means "comb-bearing", from the Greek (stem-form ) meaning "comb" and the Greek suffix meaning "carrying".
+ Comparison with other major animal groups Sponges Cnidarians Ctenophores Bilateria Cnidocytes No Yes Only in some species microRNA Yes No Yes Hox genes No Yes No Yes Colloblasts No In most species No Digestive and circulatory organs No Yes Anal pores No Yes Mostly Yes Number of main cell layers Two, with jelly-like layer between them Debate about whether two or three Three No, except that Homoscleromorpha have basement membranes Yes: Inter-cell connections; basement membranes Sensory organs No Yes Eyes Larval forms have light sensing organs Yes No Yes Apical organ No Yes In species with primary ciliated larvae Cell abundance Many Few Outer layer cells can move inwards and change functions do not move or change Nervous system No Yes, simple Simple to complex Muscles None Mostly epitheliomuscular Mostly myoepithelial Mostly myocytes
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 some 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.
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-symmetry, although many have rotational symmetry. In other words, if the animal rotates in a half-circle it looks the same as when it started.
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 that they secrete. The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer 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 contains colloblasts along the surface of tentacles, 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 that 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 inner surface of the cavity is lined with an epithelium, the gastrodermis. The mouth and pharynx have both cilia and 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 surrounded by double whorls of cilia and connected to the mesoglea.
Feeding, excretion and respiration
When prey is swallowed, it is liquefied in the pharynx by enzymes and 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 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 usually eight comb rows, called swimming-plates, which are used for swimming. The rows are oriented to run from near the mouth (the "oral pole") to the opposite end (the "aboral pole"), and are spaced more or less evenly around the body, although spacing patterns vary by species and in most species the comb rows extend only part of the distance from the aboral pole towards the mouth. The "combs" (also called "ctenes" or "comb plates") run across each row, and each consists of thousands of unusually long cilia, up to . Unlike conventional cilia and flagella, which has a filament structure arranged in a 9 + 2 pattern, these cilia are arranged in a 9 + 3 pattern, where the extra compact filament is suspected to have a supporting function. 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 eating, unlike jellyfish.
It is uncertain how ctenophores control their buoyancy, but some species 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 may pump this into the mesoglea to maintain buoyancy. Conversely, if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea.
Nervous system and senses
Ctenophores have no brain or central nervous system, but have a subepidermal nerve net that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles and the sensory complex furthest from the mouth. Nerve cells communicate by two different methods; some of the neurons have synaptic connections, but those in the nerve net are highly distinctive by being fused into a syncytium. Fossils show that Cambrian species had a more complex nervous system, with long nerves which connected with a ring around the mouth. The only ctenophores with long nerves today is Euplokamis in the order Cydippida. Their nerve cells arise from the same progenitor cells as the colloblasts.
In addition, there is a less organized mesogleal nerve net consisting of single neurites. The largest single sensory feature is the aboral organ (at the opposite end from the mouth), which is underlined with its own nerve net.Did the ctenophore nervous system evolve independently? This organ's main component is a statocyst, a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead, its response is determined by the animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it often puts some comb rows into reverse, spinning the mouth towards the prey.
Reconstruction of the aboral organ in Mnemiopsis leidyi identified three multinucleate aboral nerve-net neurons, distinct from the body-wall subepithelial nerve net, that synapse onto balancer cells and form reciprocal connections across the statocyst with bridge cells. The balancer cells themselves lack presynaptic sites, and no synaptic path connects them to motor effectors such as comb-plate cells. This suggests a coordinating rather than sensory–motor role for the nerve net.