Comb jellies, scientifically known as ctenophores (from the Ancient Greek words "kteis" meaning 'comb' and "pherō" meaning 'to carry'), are a phylum of marine invertebrates found in oceans worldwide. These fascinating creatures exhibit a range of intriguing characteristics, from their unique method of locomotion to their debated position in the animal kingdom.
Adult ctenophores vary in size, ranging from a few millimeters to as large as 1.5 meters (5 feet), depending on the species. Their bodies are primarily composed of a gelatinous mass, featuring two cell layers that sandwich a jelly-like middle layer, similar to cnidarians. In cnidarians and ctenophores this middle layer is called the mesoglea. More complex animals, in contrast, possess three main cell layers and lack an intermediate jelly-like layer.
While many species exhibit radial symmetry, with the main axis running from the mouth (oral pole) to the opposite end (aboral pole), comb jellies lack true mirror symmetry. This is due to the fact that only two of the canals near the statocyst terminate in anal pores. The outer layer, or epidermis, contains sensory cells, mucus-secreting cells for protection, and interstitial cells capable of transforming into other cell types. Specialized areas may contain colloblasts for prey capture or cells with multiple cilia for locomotion.
The internal cavity comprises a mouth (typically closable by muscles), a pharynx ("throat"), a stomach-like central area, and a system of internal canals. These canals branch through the mesoglea, supplying nutrients to active parts of the animal. The inner surface of the cavity is lined with the gastrodermis. The gastrodermis differs depending on its proximity to the organs it serves. The nearer side consists of tall nutritive cells that store nutrients in vacuoles, germ cells that produce eggs or sperm, and photocytes that generate bioluminescence. When prey is consumed, it is liquefied in the pharynx by enzymes and muscular contractions. Cilia then waft the resulting slurry through the canal system, where it is digested by the nutritive cells. Ciliary rosettes may aid in transporting nutrients to muscles within the mesoglea. The mechanisms by which ctenophores eliminate cellular waste products remain largely unknown.
The outer surface of comb jellies is characterized by eight comb rows, also known as swimming-plates, which facilitate swimming. These rows run from the oral pole to the aboral pole and are spaced relatively evenly around the body, though spacing patterns can vary. The "combs" (ctenes or comb plates) span each row and consist of thousands of exceptionally long cilia, reaching up to 2 millimeters (0.08 inches). Unlike conventional cilia, which have a 9 + 2 filament arrangement, comb jelly cilia exhibit a 9 + 3 pattern, with the extra filament believed to provide support. These cilia typically beat away from the mouth for propulsion but can reverse direction.
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The means by which ctenophores control their buoyancy is not fully understood. Some species adapt to varying water densities through osmotic pressure. Their body fluids are generally as concentrated as seawater. In less dense brackish water, ciliary rosettes may pump water into the mesoglea to maintain buoyancy.
Ctenophores lack a brain or central nervous system. Instead, they possess a subepidermal nerve net that forms a ring around the mouth and is densest near structures such as the comb rows, pharynx, tentacles, and the aboral sensory complex. Nerve cells communicate via synaptic connections and a distinctive syncytial nerve net. Cambrian fossils suggest that earlier species had more complex nervous systems with long nerves connected to a ring around the mouth. Additionally, there is a less organized mesogleal nerve net consisting of single neurites.
The most prominent sensory feature is the aboral organ, located at the end opposite the mouth. This organ is underlined by its own nerve net and features a statocyst, which functions as a balance sensor. The statocyst comprises a statolith, a tiny grain of calcium carbonate, supported by four bundles of cilia called "balancers" that detect its orientation. A transparent dome of long, immobile cilia protects the statocyst. A ctenophore's response to the statolith's position is determined by its "mood," reflecting the overall state of its nervous system, rather than an automatic attempt to keep the statolith evenly resting on the balancers.
Ctenophore nerve cells and nervous systems have unique biochemical properties.
The last common ancestor of ctenophores was hermaphroditic. Some species are simultaneous hermaphrodites, capable of producing both eggs and sperm at the same time, while others are sequential hermaphrodites, with eggs and sperm maturing at different times. Development is direct, without a distinct larval form. However, at least three species are known to have evolved separate sexes (dioecy): Ocyropsis crystallina, Ocyropsis maculata, and Bathocyroe fosteri.
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The gonads are situated within the internal canal network beneath the comb rows. Eggs and sperm are released through pores in the epidermis. Fertilization is generally external, with the exception of platyctenids, which utilize internal fertilization and retain eggs in brood chambers until hatching. Juveniles are typically planktonic and resemble miniature adult cydippids, gradually developing their adult forms as they mature. In the genus Beroe, juveniles have large mouths and lack tentacles and tentacle sheaths, similar to adults. Some groups, like the flat, bottom-dwelling platyctenids, exhibit larval-like juvenile behavior.
Juvenile ctenophores of some species can produce small quantities of eggs and sperm even when significantly smaller than adults. Adults continue to produce eggs and sperm as long as they have sufficient food. When food becomes scarce, they cease reproduction and shrink in size, resuming normal size and reproduction when the food supply improves. These adaptations enable ctenophores to rapidly increase their populations.
Members of the Lobata and Cydippida exhibit a reproductive form called dissogeny, characterized by two sexually mature stages: first as larvae, and later as juveniles and adults. During their larval stage, they periodically release gametes. After their initial reproductive period, they do not produce more gametes until later in life.
Many ctenophores, especially those near the surface, are colorless and almost transparent. However, some species, such as Bathyctena chuni, Euplokamis stationis, and Eurhamphaea vexilligera, emit luminescent secretions (ink) when disturbed, with wavelengths similar to those of their bodies. Juveniles tend to luminesce more brightly relative to their body size than adults, whose luminescence is diffused.
Bioluminescence in ctenophores is caused by the activation of calcium-activated proteins called photoproteins within photocytes, often located in the meridional canals underlying the comb rows. The genome of Mnemiopsis leidyi contains ten genes encoding photoproteins. The moving cilia refract the light produced, creating a magical bioluminescent effect.
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Almost all ctenophores are predators, with only one partially parasitic genus and no vegetarian species. When food is abundant, they can consume up to ten times their weight per day. While Beroe primarily preys on other ctenophores, other species consume zooplankton, including microscopic organisms, mollusc and fish larvae, and small crustaceans such as copepods, amphipods, and 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 exhibit diverse prey capture techniques, comparable to spiders. Some hang motionless, using their tentacles as "webs," while others are ambush predators similar to jumping spiders. Some dangle a sticky droplet at the end of a fine thread, like bolas spiders.
Identifying ctenophore remains in the guts of potential predators can be challenging due to their rapid breakdown. However, the combs sometimes remain intact long enough to provide clues. Chum salmon digest ctenophores 20 times faster than shrimp, suggesting that ctenophores can provide a good diet if available in sufficient quantities. Jellyfish and turtles consume large quantities of ctenophores, and jellyfish may temporarily decimate ctenophore populations. Because ctenophore and jellyfish populations often experience significant seasonal fluctuations, most fish that prey on them are generalists, potentially having a greater impact on populations than specialist jelly-eaters.
In the late 1990s, Mnemiopsis appeared in the Caspian Sea. Shortly after, Beroe ovata arrived, which is expected to reduce but not eliminate the impact of Mnemiopsis. The unintentional introduction of Mnemiopsis leidyi into the Caspian Sea had a catastrophic effect on the entire ecosystem. Several years later, it was introduced into the Baltic Sea, devastating anchovy fisheries. More recent introductions into other parts of Europe have caused severe hardships in local fisheries. Large populations of voracious comb jellies significantly reduce the volume of fish eggs and larvae and also diminish other planktonic forms that these developing animals require for food. Populations of fishes and dolphin have crashed and the resulting loss of harvestable fishes has caused a severe decline in the fisheries industry.
Mnemiopsis leidyi, also known as the sea walnut, is endemic to the east coasts of North and South America, ranging from the Canadian Maritime Provinces to the southern tip of South America. It has become an invasive species in the Black Sea, Azov, Aegean, and Marmara Seas, as well as the western coast of Sweden and the southern and northern Baltic Sea. This species typically inhabits coastal saltwater habitats in bays and estuaries but can tolerate a wide range of salinity (3-39%), temperature (4.0-31.0°C / 39.2-87.8°F), and water quality conditions. It is often found in brackish water with low oxygen content and high pollution levels. Occasionally, it can be found in open ocean waters far from land.
The vertical cross-section of Mnemiopsis leidyi is bell-shaped, with the oral lobes forming the rim of the bell. The mouth is positioned where the bell clapper would be. Visible internal structures include the gonads and the digestive system. Externally, the animal has eight longitudinal rows of cilia that divide the body into eight symmetrical shapes, enabling slow movement through the water. Two fine, filamentous lobes on either side of the mouth are used for feeding and can be retracted into the body. The translucent, almost colorless body often presents a color show, with the moving cilia refracting ambient light into rainbow colors and bright fluorescent stripes visible on the body. The sea walnut typically ranges from 100-120 mm (3.9-4.7 inches) in length, although larger specimens have been reported from the Caspian and Black Seas.
Mnemiopsis leidyi is a voracious carnivore and a major predator of edible zooplankton, consuming up to 10 times its weight per day. It prefers a broad diet of zooplankton, including eggs and larval forms of invertebrates and fishes, juvenile fish, copepods, sea jellies, and even other ctenophores. It feeds by pumping water into its body cavity, trapping small prey on adhesive cells (colloblasts) found on the tentacles and the inside surface of the two lobes. The food is then transferred to the mouth for ingestion. Larger prey is captured by swimming with outstretched lobes, then snapping them closed to trap the meal. Mnemiopsis leidyi never feels full.
This species is a free-spawning simultaneous hermaphrodite capable of self-fertilization. Spawning occurs during summer months and varies with habitat conditions. Internal fertilization can occur, but eggs and sperm are commonly broadcast into the water column where fertilization takes place. The eggs produce a fast-growing larva that is fully developed in 20 hours. On hatching, the larvae are 0.3-0.4 mm (0.12-0.16 inches) long. Sexual maturity is rapid, with some specimens beginning to produce eggs in as little as two weeks after hatching. In the Caspian Sea, spawning occurs at night, with 2,000-3,000 eggs produced per day, depending on food availability. These planktonic animals are subject to movement by currents and wind and wave action. The bands of cilia provide some mobility, primarily relating to vertical position in the water column.
Mnemiopsis leidyi can tolerate a broad range of water temperature, salinity, and pollution. It has several natural predators, including some species of fishes, sea jellies, and other ctenophores, but natural population control is minimal. Given this species’ wide tolerance of both water temperature and ocean salinity levels, climate change is unlikely to impact this species. The ballast water of ships unintentionally introduced Mnemiopsis leidyi into the Black Sea and adjacent seas in 1982. In 1999 it appeared in the Caspian Sea, introduced in the ballast water of oil tankers; in 2006 in waters on the western coast of Sweden and the southern Baltic Sea; and in 2007, the northern Baltic Sea.
The number of known living ctenophore species is uncertain due to many named and formally described species turning out to be identical to species known under other scientific names. Early writers grouped ctenophores with cnidarians into a single phylum called Coelenterata based on morphological similarities. Like cnidarians, ctenophore bodies consist of a jelly-like mass with one cell layer on the outside and another lining the internal cavity. However, ctenophore layers are two cells deep, while cnidarian layers are only a single cell deep.
The traditional classification divides ctenophores into two classes: Tentaculata (those with tentacles) and Nuda (those without).
Despite their fragile, gelatinous bodies, fossils thought to represent ctenophores have been found in Lagerstätten dating back to the early Cambrian, about 515 million years ago. These fossils appear to lack tentacles but have many more comb-rows than modern forms. Ctenophores are extremely rare as fossils due to their soft bodies. Fossils interpreted as ctenophores have been found only in Lagerstätten, where the environment was exceptionally suited to soft tissue preservation. Until the mid-1990s, only two specimens good enough for analysis were known, both members of the crown group, from the early Devonian (Emsian) period. Three additional putative species were then found in the Burgess Shale and other Canadian rocks of similar age, about 505 million years ago in the mid-Cambrian period. All three lacked tentacles but had between 24-80 comb rows, far more than the eight typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. The Ediacaran Eoandromeda could putatively represent a comb jelly. It has eightfold symmetry, with eight spiral arms resembling the comblike rows of a ctenophore. The early Cambrian sessile frond-like fossil Stromatoveris, from China's Chengjiang lagerstätte and dated to about 515 million years ago, 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. 520 million-year-old Cambrian fossils also from Chengjiang in China show a now wholly extinct class of ctenophore, named "Scleroctenophora", that had a complex internal skeleton with long spines. The skeleton also supported eight soft-bodied flaps, which could have been used for swimming and possibly feeding.
The phylogenetic relationship of ctenophores to the rest of Metazoa is very important to our understanding of the early evolution of animals and the origin of multicellularity. It has been the focus of debate for many years. Some biologists propose 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. Follow up analysis by Whelan et al. (2017) yielded further support for the 'Ctenophora sister' hypothesis; the issue remains a matter of taxonomic dispute.
The deep ocean poses extreme challenges to life, including the absence of light, freezing temperatures, and immense pressure. Animals inhabiting these depths have evolved unique adaptations to survive in these harsh conditions.
Cell membranes, composed of thin sheets of lipids and proteins, must maintain specific properties for cells to function correctly. Research has revealed that ctenophores have developed unique lipid structures to compensate for the intense pressure, a phenomenon termed "homeocurvature." Unlike adaptations to cold, the lipid structures that compensate for pressure are distinct. In the deep sea, cone-shaped lipids have exaggerated cone shapes. The pressure of the ocean counteracts the exaggeration so the lipid shape is normal, but only at these extreme pressures. The molecules with an exaggerated cone shape are a type of phospholipid called plasmalogens. Plasmalogens are abundant in human brains and their declining abundance often accompanies diminishing brain function and even neurodegenerative disease like Alzheimer’s.
Comb jellies are a common sight on Florida bioluminescent kayaking tours. Their shimmering, rainbow effect, produced by the diffraction of light by their comb-like plates, captivates tour guests. These stunning, oval-shaped animals propel themselves through the water using these comb-like plates.
Despite their resemblance to jellyfish, comb jellies cannot sting. They lack stinging cells (nematocysts) and instead possess sticky cells called colloblasts for prey capture.
