Comb jellies, scientifically known as Ctenophora, are a fascinating phylum of marine invertebrates found in oceans worldwide. The name Ctenophora comes from the Ancient Greek words "kteis" (comb) and "pherō" (to carry), referring to their distinctive comb-like rows of cilia. These gelatinous creatures exhibit a wide range of sizes and play a crucial role in marine ecosystems.
Adult ctenophores vary in size from a few millimeters to 1.5 meters (5 feet), depending on the species. Their bodies are primarily composed of a jelly-like substance, featuring two cell layers on the outside and another lining the internal cavity. Despite their delicate, gelatinous structure, fossil records suggest that ctenophores have existed since the early Cambrian period, approximately 525 million years ago.
The body of many ctenophore species exhibits radial symmetry, with the main axis extending from the mouth (oral pole) to the opposite end (aboral pole). However, they lack true mirror symmetry due to the presence of only two anal pores near the statocyst.
The outer layer, or epidermis, consists of several types of cells:
The internal cavity includes:
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The inner surface of the cavity is lined with an epithelium called the gastrodermis. The gastrodermis varies in different parts of the canal system, with the side nearest to the supplied organ composed of tall nutritive cells that store nutrients, germ cells that produce eggs or sperm, and photocytes that produce bioluminescence.
Ctenophores are characterized by eight comb rows, also called swimming-plates, which they use for swimming. These rows run from the oral pole to the aboral pole, spaced evenly around the body. Each "comb" consists of thousands of long cilia, up to 2 millimeters (0.08 inches) in length, arranged in a 9 + 3 pattern, unlike the 9 + 2 pattern found in conventional cilia and flagella. The extra filament is believed to provide support. These cilia beat in a coordinated manner, propelling the ctenophore through the water.
The mechanism by which ctenophores control their buoyancy is not fully understood. However, some species rely on osmotic pressure to adapt to water of different densities. Their body fluids are typically as concentrated as seawater. When they enter 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 the comb rows, pharynx, tentacles, and the aboral sensory complex. Nerve cells communicate via synaptic connections and a distinctive syncytium.
The aboral organ, located at the end opposite the mouth, is the primary sensory structure. It consists of a statocyst, a balance sensor containing a statolith (a tiny grain of calcium carbonate) supported by four bundles of cilia called "balancers." The statocyst is protected by a transparent dome of immobile cilia.
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The last common ancestor of ctenophores was hermaphroditic. While some species are simultaneous hermaphrodites, capable of producing both eggs and sperm at the same time, others are sequential hermaphrodites, in which eggs and sperm mature at different times. At least three species are known to have evolved separate sexes (dioecy).
The gonads are located in the internal canal network under the comb rows, and eggs and sperm are released through pores in the epidermis. Fertilization is generally external, except in platyctenids, which use internal fertilization. Development is direct, without a distinct larval form. Juveniles are typically planktonic and resemble miniature adult cydippids, gradually developing their adult body forms as they grow.
Ctenophores exhibit remarkable reproductive capabilities. They can produce eggs and sperm as long as they have sufficient food. When food is scarce, they cease reproduction and shrink in size. Upon restoration of the food supply, they regrow and resume reproduction. This allows them to rapidly increase their populations. Some 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.
Many ctenophores are capable of bioluminescence. Most species that live near the surface are colorless and transparent, but when disturbed, some species produce luminescent secretions. Bioluminescence is caused by calcium-activated proteins called photoproteins in cells named photocytes, usually confined to the meridional canals underlying the comb rows. Juveniles often luminesce more brightly than adults.
Almost all ctenophores are predators, with only one partially parasitic genus. If food is plentiful, they can consume up to ten times their own weight per day. While Beroe primarily preys on other ctenophores, other species feed on zooplankton, including mollusc and fish larvae, copepods, amphipods, and krill. Some members of the genus Haeckelia prey on jellyfish and incorporate their nematocysts (stinging cells) into their own tentacles.
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Ctenophores employ diverse prey capture techniques, including hanging motionless with tentacles as "webs," ambushing prey like jumping spiders, and dangling sticky droplets on fine threads like bolas spiders. Their remains break down quickly, making it difficult to identify them in the guts of predators, although the combs sometimes remain intact long enough to provide a clue.
Ctenophores play a significant role in marine ecosystems. They consume large quantities of copepods, which prey on phytoplankton that grow on the ocean surface, regulating phytoplankton populations. They also serve as a food source for fish, jellyfish, and leatherback sea turtles.
However, some ctenophore species can have detrimental impacts as invasive species. The American Atlantic coast comb jelly, Mnemiopsis leidyi, has become a notorious invader in the Black and Azov Seas. Introduced via ballast water, it rapidly propagated due to a lack of natural predators, decimating zooplankton populations and causing significant economic losses to the seafood industry. Similar introductions have been reported in the Caspian Sea and Baltic Sea, highlighting the dangers of ballast water transport.
The introduction of Mnemiopsis leidyi into the Caspian Sea had a catastrophic effect on the entire ecosystem. Several years later Beroe ovata arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis there.
This species generally prefers coastal saltwater habitats in bays and estuarine locations; however, it is tolerant of a wide range of salinity, 3% to 39%; temperature, 4.0o-31.0o C (39.2o-87.8o F); and water quality conditions. It is frequently found in brackish water that is low in oxygen content and high in pollution. It can also be occasionally found in the open ocean waters long distances from land.
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 where it devastated the 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 diminished other planktonic forms that these developing animals require for food. Populations of fishes and dolphin have crashed. The resulting loss of harvestable fishes has caused a severe decline in the fisheries industry.
Given this species’ wide tolerance of both water temperature and ocean salinity levels, climate change is unlikely to impact this species.
Early classification systems grouped ctenophores with cnidarians (jellyfish, sea anemones, and corals) into a single phylum called Coelenterata due to morphological similarities. However, ctenophores differ from cnidarians in several key aspects, including the depth of their cell layers.
The traditional classification divides ctenophores into two classes: Tentaculata (those with tentacles) and Nuda (those without tentacles). Fossils thought to represent ctenophores have been found in Lagerstätten dating back to the early Cambrian period.
Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in Lagerstätten, places where the environment was exceptionally suited to the preservation of soft tissue. 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. Other researchers have argued that the placement of Ctenophora as sister to all other animals is a statistical anomaly caused by the high rate of evolution in ctenophore genomes, and that Porifera is the earliest-diverging animal taxon instead (a "sponge sister" topology). They also have extremely high rates of mitochondrial evolution, and the smallest known RNA/protein content of the mtDNA genome in animals.
The deep sea presents a challenging environment with no light, freezing temperatures, and immense pressure. Ctenophores that inhabit these depths have developed remarkable adaptations to survive.
University of California San Diego Assistant Professor of Chemistry and Biochemistry Itay Budin teamed up with researchers from around the country to study the cell membranes of ctenophores (“comb jellies”) and found they had unique lipid structures that allow them to live under intense pressure.
The researchers call this adaptation “homeocurvature” because the curve-forming shape of the lipids has adapted to the ctenophores’ unique habitat. In the deep sea, the cone-shaped lipids have evolved into 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.
Ongoing research continues to uncover new aspects of ctenophore biology, ecology, and evolution.
MBARI researchers have learned that gelatinous animals like Beroe have a large impact on deep-sea food webs. Our archive of nearly 28,000 hours of deep-sea video contains hundreds of observations of deep-sea animals feeding.
Comb jellies are also a popular attraction for bioluminescence kayaking tours, particularly during the cooler months.
During the months of April and early May, that neon blue-green glimmer in front of your eyes comes from jellyfish-looking creatures. Technically, comb jellies are not jellyfish so you don’t have to worry about getting stung. Millions of these tiny, sparkly jellies come alive when you touch the water during our winter and spring bioluminescence tours.
They are Not Jellyfish Although they look like jellyfish, comb jellies belong to a completely different phylum called Ctenophora (pronounced teen-oh-for-ah); true jellyfish fall under Cnidaria. These delicate beauties don’t have stingers, so even when you scoop one up during your paddle out bioluminescent kayaking Cocoa Beach, you don’t have to worry about getting zapped. They are completely harmless, just soft, squishy, and kind of hypnotic to watch.
Most glowing marine creatures, like dinoflagellates, light up due to chemical reactions involving a molecule called luciferin and an enzyme called luciferase. But with comb jellies, the spark is triggered by physical movement or disturbance. When you scoop one gently in your palm near Haulover Canal or paddle through a group of them near Cocoa Beach, they light up because the stimulation activates light-producing cells known as photocytes. Some species show flashes of green, while others blue or purple light depending on the angle and water clarity. The best part is that the shimmer is delayed by a split second, so it feels like the light is following your movement.
According to genetic studies, comb jellies may have evolved even before sponges, which were once thought to be the most primitive animals. That’s right; the next time you are out bioluminescent kayaking Merritt Island, know that you are floating alongside a species that’s been around for over 500 million years. Half a billion! Think about that for a second. These multicellular creatures are basically living fossils. And despite that ancient lineage, they have developed surprisingly complex traits, like a nerve net for sensing the environment and muscle cells for movement, even though they don’t have a centralized brain or heart.
One of the biggest misconceptions visitors have is assuming that bioluminescence in Florida is best during the summer. That’s true if you are looking for the luminous plankton (dinoflagellates). But if your trip to Cocoa Beach or Titusville falls between November and May, you are actually in peak comb jelly season. They prefer cooler, stable water temperatures in the 60°F to 70°F range. In April and early May, the Indian River Lagoon still holds the ideal conditions to support large blooms of comb jellies. Also, unlike plankton, you can hold a jelly in your hand, examine its shimmering cilia, and feel like you are part of the ecosystem.
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