Editing Ctenophora (section)

== Description ==

[[File:Comb Jelly, Shedd Aquarium, Chicago.webmhd.webm|thumb|Comb jelly, [[Shedd Aquarium]], Chicago]]

For a phylum with relatively few species, ctenophores have a wide range of body plans.<ref name="RuppertBarnes2004Ctenophora"/> 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.<ref name="MillsNotesFromExpert" /> In addition, oceanic species do not preserve well,<ref name="MillsNotesFromExpert" /> and are known mainly from photographs and from observers' notes.<ref name="Horita2000LobatolampeaTetragona">{{cite journal | last=Horita |first=T. | date=March 2000
| title=An undescribed lobate ctenophore, ''Lobatolampea tetragona'' gen. nov. & spec. nov., representing a new family, from Japan | journal=Zoologische Mededelingen | volume=73 | issue=30| pages=457–464 | url=http://www.repository.naturalis.nl/document/44309 | access-date=2009-01-03 }}</ref> Hence most attention has until recently concentrated on three coastal [[genus|genera]] – ''[[Pleurobrachia]]'', ''[[Beroe (ctenophore)|Beroe]]'' and ''[[Mnemiopsis]]''.<ref name="MillsNotesFromExpert" /><ref name="Haddock204goldenAgeofGelata">{{cite journal |last1=Haddock |first1=Steven H. D. |author-link=Steven Haddock |title=A golden age of gelata: past and future research on planktonic ctenophores and cnidarians |journal=Hydrobiologia |date=November 2004 |volume=530-531 |issue=1–3 |pages=549–556 |doi=10.1007/s10750-004-2653-9 |s2cid=17105070 }}</ref> At least two textbooks base their descriptions of ctenophores on the [[Cydippida|cydippid]] ''Pleurobrachia''.<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /><ref name="RuppertBarnes2004Ctenophora" />

Since the body of many species is ''almost'' [[radial symmetry|radially symmetrical]], the main axis is [[Mouth|oral]] to [[aboral]] (from the mouth to the opposite end). However, since only two of the canals near the [[statocyst]] terminate in [[anus|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.<ref>{{cite journal |last1=Martindale |first1=M. Q. |last2=Henry |first2=J. Q. |title=Intracellular Fate Mapping in a Basal Metazoan, the Ctenophore ''Mnemiopsis leidyi'', Reveals the Origins of Mesoderm and the Existence of Indeterminate Cell Lineages |journal=Developmental Biology |volume=214 |issue=2|date=October 1999|pmid=10525332 |pages=243–257 |doi=10.1006/dbio.1999.9427 |doi-access=free}}</ref>

===Body layers ===

[[File:Ctenophore diagram - en.svg|thumb|alt=Anatomy of Cydippid Ctenophore|Anatomy of Cydippid Ctenophore]]

Like those of [[cnidaria]]ns, ([[jellyfish]], [[sea anemone]]s, etc.), ctenophores' bodies consist of a relatively thick, jelly-like [[mesoglea]] sandwiched between two [[epithelium|epithelia]], layers of [[cell (biology)|cells]] bound by inter-cell connections and by a fibrous [[basement membrane]] that they [[secrete]].<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /><ref name="RuppertBarnes2004Ctenophora"/> 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.<ref name="RuppertBarnes2004Ctenophora"/>

The outer layer of the [[Epidermis (zoology)|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 [[colloblast]]s 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 [[muscle]]s.<ref name="RuppertBarnes2004Ctenophora"/>

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 [[vacuole]]s (internal compartments), [[germ cell]]s 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.<ref name="RuppertBarnes2004Ctenophora"/>

=== Feeding, excretion and respiration ===

When prey is swallowed, it is liquefied in the [[pharynx]] by [[enzyme]]s 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 [[anus|anal]] pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth.<ref name="RuppertBarnes2004Ctenophora"/>

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.<ref name="RuppertBarnes2004Ctenophora"/>

=== 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,<ref name="Hinde2001CnidariaAndCtenophoraInAnderson" /> 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 {{convert|2|mm|in|2|sp=us}}. Unlike conventional cilia and flagella, which has a [[Axoneme|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.<ref>{{cite journal |pmc=2224992 |pmid=13681575 |volume=9 |issue=2 |title=The fine structure of the cilia from ctenophore swimming-plates |journal=The Journal of Biophysical and Biochemical Cytology |pages=383–94 |last1=Afzelius |first1=BA |doi=10.1083/jcb.9.2.383 |year=1961}}</ref> 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]].<ref name="RuppertBarnes2004Ctenophora" /> 

It is uncertain how ctenophores control their buoyancy, but some species rely on [[osmotic pressure]] to adapt to water of different densities.<ref>{{cite journal |last1=Mills |first1=Claudia E. |title=Density is Altered in Hydromedusae and Ctenophores in Response to Changes in Salinity |journal=The Biological Bulletin |date=February 1984 |volume=166 |issue=1 |pages=206–215 |doi=10.2307/1541442 |jstor=1541442 |url=https://www.biodiversitylibrary.org/part/37655 }}</ref> Their body fluids are normally as [[concentration|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.<ref name="RuppertBarnes2004Ctenophora"/>

=== 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.<ref name="RuppertBarnes2004Ctenophora"/> Nerve cells communicate by two different methods; some of the neurons have [[synapses|synaptic connections]], but those in the nerve net are highly distinctive by being fused into a [[syncytium]].<ref name=Burkhardt2023>{{cite journal |last1=Burkhardt |first1=Pawel |last2=Colgren |first2=Jeffrey |last3=Medhus |first3=Astrid |last4=Digel |first4=Leonid |last5=Naumann |first5=Benjamin |last6=Soto-Angel |first6=Joan |last7=Nordmann |first7=Eva-Lena |last8=Sachkova |first8=Maria |last9=Kittelmann |first9=Maike |display-authors=6 |title=Syncytial nerve net in a ctenophore adds insights on the evolution of nervous systems |journal=[[Science (journal)|Science]] |date=20 April 2023 |volume=380 |issue=6642 |pages=293–297 |doi=10.1126/science.ade5645 |pmid=37079688 |bibcode=2023Sci...380..293B |s2cid=258239574 |hdl=11250/3149299 |hdl-access=free }}</ref> 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 ''[[Euplokamididae|Euplokamis]]'' in the order Cydippida.<ref>{{cite journal |last1=Parry |first1=Luke A. |last2=Lerosey-Aubril |first2=Rudy |last3=Weaver |first3=James C. |last4=Ortega-Hernández |first4=Javier |title=Cambrian comb jellies from Utah illuminate the early evolution of nervous and sensory systems in ctenophores |journal=iScience |date=2021 |volume=24 |issue=9 |page=102943 |doi=10.1016/j.isci.2021.102943 |pmid=34522849 |pmc=8426560 |bibcode=2021iSci...24j2943P |doi-access=free}}</ref> Their nerve cells arise from the same [[progenitor cell]]s as the colloblasts.<ref>{{cite journal |last1=Pennisi |first1=Elizabeth |author-link=Elizabeth Pennisi |title=The gluey tentacles of comb jellies may have revealed when nerve cells first evolved |journal=[[Science (journal)|Science]] |date=10 January 2019 |doi=10.1126/science.aaw6288 |s2cid=92852830 }}</ref>

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.<ref>[http://ryanlab.whitney.ufl.edu/pdfs/doi_10.1016_j.zool.2014.06.001.pdf Did the ctenophore nervous system evolve independently?]</ref> 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.<ref name="RuppertBarnes2004Ctenophora" />

The ciliated larvae in cnidarians and bilaterians appear to share an ancient and common origin.<ref>{{cite journal |last1=Marlow |first1=Heather |last2=Tosches |first2=Maria Antonietta |last3=Tomer |first3=Raju |last4=Steinmetz |first4=Patrick R. |last5=Lauri |first5=Antonella |last6=Larsson |first6=Tomas |last7=Arendt |first7=Detlev |date=29 January 2014 |title=Larval body patterning and apical organs are conserved in animal evolution |journal=[[BMC Biology]] |volume=12 |issue=1 |page=7 |doi=10.1186/1741-7007-12-7 |pmid=24476105 |pmc=3939940 |doi-access=free }}</ref> The larvae's apical organ is involved in the formation of the nervous system.<ref>{{cite journal |last1=Nielsen |first1=Claus |title=Larval nervous systems: true larval and precocious adult |journal=[[Journal of Experimental Biology]] |date=15 February 2015 |volume=218 |issue=4 |pages=629–636 |doi=10.1242/jeb.109603 |pmid=25696826 |s2cid=3151957 |doi-access=free }}</ref> The aboral organ of comb jellies is not homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin.<ref>{{cite journal |last1=Nielsen |first1=Claus |title=Early animal evolution: a morphologist's view |journal=Royal Society Open Science |date=July 2019 |volume=6 |issue=7 |page=190638 |doi=10.1098/rsos.190638 |pmid=31417759 |pmc=6689584 |bibcode=2019RSOS....690638N }}</ref>

Ctenophore nerve cells and nervous system have distinctive biochemistry. They lack the genes and enzymes required to manufacture neurotransmitters like [[serotonin]], [[dopamine]], [[Biological functions of nitric oxide|nitric oxide]], [[octopamine]], [[noradrenaline]], and others, seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing.<ref>{{cite web |first=Douglas |last=Fox |title=Aliens in our midst |url=https://aeon.co/essays/what-the-ctenophore-says-about-the-evolution-of-intelligence |date=1 August 2017 |work=[[Aeon (digital magazine)|Aeon]] |access-date=1 August 2017}}</ref> Monofunctional [[catalase]] (CAT), one of the three major families of antioxidant enzymes that target [[hydrogen peroxide]], an important signaling molecule for synaptic and neuronal activity, is also absent, most likely due to gene loss.<ref>{{Cite journal |last1=Hewitt |first1=Olivia H. |last2=Degnan |first2=Sandie M. |date=2023-02-13 |title=Antioxidant enzymes that target hydrogen peroxide are conserved across the animal kingdom, from sponges to mammals |journal=Scientific Reports |volume=13 |issue=1 |page=2510 |doi=10.1038/s41598-023-29304-6 |pmid=36781921 |pmc=9925728 |bibcode=2023NatSR..13.2510H |s2cid=256811787}}</ref> They use [[L-glutamate]] as a [[neurotransmitter]], and have a distinctively high number of ionotropic glutamate receptors and genes for glutamate synthesis and transport.<ref name="Norekian">{{cite journal |last1=Norekian |first1=Tigran P. |last2=Moroz |first2=Leonid L. |title=Neural system and receptor diversity in the ctenophore Beroe abyssicola |journal=[[Journal of Comparative Neurology]] |date=15 August 2019 |volume=527 |issue=12 |pages=1986–2008 |doi=10.1002/cne.24633 |pmid=30632608 |doi-access=free }}</ref> The genomic content of the nervous system is the smallest of any animal, and could represent the minimum genetic requirements for a functional nervous system.<ref>{{cite book |doi=10.1093/acprof:oso/9780199682201.003.0006 |chapter=Ctenophora |title=Structure and Evolution of Invertebrate Nervous Systems |year=2015 |last1=Simmons |first1=David K. |last2=Martindale |first2=Mark Q. |pages=48–55 |publisher=[[Oxford University Press]] |isbn=978-0-19-968220-1 }}</ref> The presence of directly fused neurons without synapses suggests that ctenophores might form a sister group to other metazoans, having developed a nervous system independently.<ref name="Burkhardt2023"/> If so, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans.<ref name="Jékely2015">{{cite journal |last1=Jákely |first1=Gáspár |last2=Paps |first2=Jordi |last3=Nielsen |first3=Claus |year=2015 |title=The phylogenetic position of ctenophores and the origin(s) of nervous systems |journal=[[EvoDevo]] |volume=6 |issue=1 |page=1 |doi=10.1186/2041-9139-6-1 |pmid=25905000 |pmc=4406211 |doi-access=free }}</ref>

===Reproduction and development===

[[File:Juvenile Bolinopsis ctenophore.jpg|thumb|Cydippid larva of ''Bolinopsis'' sp., a few millimetres long]]

Adults of most species can regenerate tissues that are damaged or removed,<ref>{{cite journal
|last=Martindale |first=M. Q. |title=The ontogeny and maintenance of adult symmetry properties in the ctenophore, ''Mnemiopsis mccradyi''|journal=Developmental Biology |date=December 1986|volume=118 |issue=2 |pages=556–576 |pmid=2878844|doi=10.1016/0012-1606(86)90026-6}}</ref> although only platyctenids reproduce by [[cloning]], splitting off from the edges of their flat bodies fragments that develop into new individuals.<ref name="RuppertBarnes2004Ctenophora" /> Lab research on ''Mnemiopsis leidyi'' also show that when two individuals have parts of their bodies removed, they are able to fuse together, including their nervous and digestive systems, even when the two individuals are genetically different; a phenomenon so far only found in comb jellies.<ref>[https://www.cell.com/current-biology/abstract/S0960-9822(24)01023-6 Rapid physiological integration of fused ctenophores]</ref>

The [[Most recent common ancestor|last common ancestor (LCA)]] of the ctenophores was [[hermaphrodite|hermaphroditic]].<ref name="Sasson">{{cite journal |last1=Sasson |first1=Daniel A. |last2=Ryan |first2=Joseph F. |title=A reconstruction of sexual modes throughout animal evolution |journal=BMC Evolutionary Biology |date=December 2017 |volume=17 |issue=1 |page=242 |doi=10.1186/s12862-017-1071-3 |pmid=29207942 |pmc=5717846 |doi-access=free |bibcode=2017BMCEE..17..242S }}</ref> Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. There is no [[metamorphosis]].<ref>{{cite journal |doi=10.1073/pnas.2122052119 |title=Ctenophores are direct developers that reproduce continuously beginning very early after hatching |year=2022 |last1=Edgar |first1=Allison |last2=Ponciano |first2=José Miguel |last3=Martindale |first3=Mark Q. |journal=Proceedings of the National Academy of Sciences of the USA |volume=119 |issue=18 |pages=e2122052119 |doi-access=free |pmid=35476523 |pmc=9170174 |bibcode=2022PNAS..11922052E }}</ref> At least three species are known to have evolved separate sexes ([[dioecy]]); ''Ocyropsis crystallina'' and ''Ocyropsis maculata'' in the genus [[Ocyropsis]] and ''Bathocyroe fosteri'' in the genus [[Bathocyroe]].<ref name="Harbison">{{cite journal |last1=Harbison |first1=G. R. |last2=Miller |first2=R. L. |title=Not all ctenophores are hermaphrodites. Studies on the systematics, distribution, sexuality and development of two species of Ocyropsis |journal=[[Marine Biology (journal)|Marine Biology]] |date=February 1986 |volume=90 |issue=3 |pages=413–424 |doi=10.1007/bf00428565 |bibcode=1986MarBi..90..413H |s2cid=83954780 }}</ref> The [[gonad]]s 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 generally [[external fertilization|external]], but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus ''[[Mnemiopsis]]'',<ref name="RuppertBarnes2004Ctenophora"/> and most hermaphroditic species are presumed to be self-fertile.<ref name="MillsNotesFromExpert"/>

Development of the fertilized eggs is direct; there is no distinctive larval form. Juveniles of all groups are generally [[plankton]]ic, and most species resemble miniature adult cydippids, gradually developing their adult body forms as they grow. In the genus ''Beroe'', however, the juveniles have large mouths and, like the adults, lack both tentacles and tentacle sheaths. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae. They live among the plankton and thus occupy a different [[ecological niche]] from their parents, only attaining the adult form by a more radical [[ontogeny]]<ref name="RuppertBarnes2004Ctenophora"/> after dropping to the sea-floor.<ref name="MillsNotesFromExpert"/>

At least in some species, 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 as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features enable ctenophores to increase their populations very quickly.<ref name="MillsNotesFromExpert" /> Members of the Lobata and Cydippida have a reproduction form called dissogeny; two sexually mature stages, first as larva and later as juveniles and adults. During their time as larvae they release gametes periodically. After their first reproductive period is over they do not produce more gametes until later. A population of ''Mertensia ovum'' in the central [[Baltic Sea]] have become [[Neoteny|paedogenetic]], and consist solely of sexually mature larvae less than 1.6&nbsp;mm.<ref>{{cite journal|pmc=4971632 |pmid=27489613 |doi=10.1186/s13227-016-0051-9 |volume=7 |title=Developmental expression of 'germline'- and 'sex determination'-related genes in the ctenophore ''Mnemiopsis leidyi'' |year=2016 |journal=[[EvoDevo]] |page=17 |last1=Reitzel |first1=A. M. |last2=Pang |first2=K. |last3=Martindale |first3=M. Q. |doi-access=free }}</ref><ref>{{cite journal|pmc=3440961 |pmid=22535640 |doi=10.1098/rsbl.2012.0163 |volume=8 |issue=5 |title=Ctenophore population recruits entirely through larval reproduction in the central Baltic Sea |year=2012 |journal=Biology Letters |pages=809–12 |last1=Jaspers |first1=C. |last2=Haraldsson |first2=M. |last3=Bolte |first3=S. |last4=Reusch |first4=T. B. |last5=Thygesen |first5=U. H. |last6=Kiørboe |first6=T.}}</ref>

In ''Mnemiopsis leidyi'', nitric oxide (NO) signaling is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms.<ref>{{cite journal|doi=10.3389/fnins.2023.1125433 |volume=17 |title=Nitric oxide signaling in ctenophores |year=2023 |journal=Front. Neurosci. |page=1125433 |last1=Moroz |first1=Leonid |last2=Mukherjee |first2=Krishanu |last3=Romanova |first3=Daria |pmid=37034176 |pmc=10073611 |doi-access=free }}</ref> The mature form of the same species is also able to revert back to the cydippid stage when triggered by environmental stressors.<ref>[https://www.zmescience.com/science/news-science/this-benjamin-button-like-jellyfish-can-age-in-reverse-from-adult-to-juvenile/ This Benjamin Button-like Jellyfish Can Age in Reverse, From Adult to Juvenile]</ref>

===Colors and bioluminescence===

[[File:LightRefractsOf comb-rows of ctenophore Mertensia ovum.jpg|thumb|Light [[diffraction|diffracting]] along the comb rows of a ''[[Mertensia ovum]]'', left tentacle deployed, right tentacle retracted]]

Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red"<ref name="MillsCtenoList">{{cite web |url=http://faculty.washington.edu/cemills/Ctenolist.html |access-date=2009-02-10 |title=Phylum Ctenophora: list of all valid scientific names |last=Mills |first=C. E. |date=May 2007}}</ref> (see illustration here), which has not yet been formally described.<ref name="MillsNotesFromExpert" /> Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these host organisms.<ref name="MillsNotesFromExpert"/> The gut of the deep-sea genus ''[[Bathocyroe]]'' is red, which hides the [[bioluminescence]] of [[copepod]]s it has swallowed.<ref name="Haddock2007ComparativeFeeding"/>

The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by [[bioluminescence]] but by the [[diffraction|scattering of light]] as the combs move.<ref name="MillsNotesFromExpert" /><ref>{{cite journal |last1=Welch |first1=Victoria |last2=Vigneron |first2=Jean Pol |last3=Lousse |first3=Virginie |last4=Parker |first4=Andrew |title=Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora) |journal=Physical Review E |date=14 April 2006 |volume=73 |issue=4 |page=041916 |doi=10.1103/PhysRevE.73.041916 |pmid=16711845 |bibcode=2006PhRvE..73d1916W }}</ref><!-- e pagination --> Most species are also bioluminescent, but the light is usually blue or green and can only be seen in darkness.<ref name="MillsNotesFromExpert" /> However some significant groups, including all known platyctenids and the cydippid [[genus]] ''[[Pleurobrachia]]'', are incapable of bioluminescence.<ref>{{cite journal |last1=Haddock |first1=Steven H.D. |last2=Case |first2=J.F. |date=December 1995 |title=Not all ctenophores are bioluminescent: Pleurobrachia |journal=[[Biological Bulletin]] |volume=189 |issue=3 |pages=356–362 |doi=10.2307/1542153 |pmid=29244577 |jstor=1542153 |url=https://www.biodiversitylibrary.org/part/19935 }}</ref>

When some species, including ''[[Bathyctena chuni]]'', ''[[Euplokamis stationis]]'' and ''[[Eurhamphaea vexilligera]]'', are disturbed, they produce secretions (ink) that luminesce at much the same [[wavelength]]s as their bodies. Juveniles will 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.<ref>{{cite journal |last1=Haddock |first1=Steven H.D. |last2=Case |first2=J.F. |date=8 April 1999 |title=Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: Ctenophores, medusae and siphonophores |journal=[[Marine Biology (journal)|Marine Biology]] |volume=133 |issue=3 |pages=571–582 |doi=10.1007/s002270050497 |bibcode=1999MarBi.133..571H |s2cid=14523078 }}</ref>

In ctenophores, bioluminescence is caused by the activation of calcium-activated proteins named [[photoproteins]] in cells called [[photocytes]], which are often confined to the meridional canals that underlie the eight comb rows. In the genome of ''[[Mnemiopsis leidyi]]'' ten genes encode photoproteins. These genes are co-expressed with [[opsin]] genes in the developing photocytes of ''Mnemiopsis leidyi'', raising the possibility that light production and light detection may be working together in these animals.<ref name=SchnitzlerPang2012>{{cite journal |last1=Schnitzler |first1=Christine E. |last2=Pang |first2=Kevin |last3=Powers |first3=Meghan L. |last4=Reitzel |first4=Adam M. |last5=Ryan |first5=Joseph F. |last6=Simmons |first6=David |last7=Tada |first7=Takashi |last8=Park |first8=Morgan |last9=Gupta |first9=Jyoti |last10=Brooks |first10=Shelise Y. |last11=Blakesley |first11=Robert W. |last12=Yokoyama |first12=Shozo |last13=Haddock |first13=Steven H.D. |last14=Martindale |first14=Mark Q. |last15=Baxevanis |first15=Andreas D. |display-authors=6 |year=2012 |title=Genomic organization, evolution, and expression of photoprotein and opsin genes in Mnemiopsis leidyi: a new view of ctenophore photocytes |journal=[[BMC Biology]] |volume=10 |issue=1 |page=107 |doi=10.1186/1741-7007-10-107 |doi-access=free |pmid=23259493 |pmc=3570280 }}</ref>