Comb jellies, also known as ctenophores, are fascinating marine invertebrates found in oceans worldwide. Often mistaken for jellyfish, these gelatinous creatures possess unique characteristics and play a vital role in marine ecosystems. This article dives into the captivating world of comb jellies, exploring their biology, behavior, and ecological significance.
Comb jellies are transparent, gelatinous invertebrates that drift through the waters of our global ocean. They belong to the phylum Ctenophora (pronounced "TEE-no-fores"), which literally means "comb-bearer" in Ancient Greek, referring to their distinctive comb-like rows of cilia. There are between 100 and 150 known species of comb jellies, the best known of which are found close to shore. Comb jellies comprise the phylum Ctenophora. Depending on the species, adult ctenophores range from a few millimeters to 1.5 m (5 ft) in size. Their bodies consist of a mass of jelly, with a layer two cells thick on the outside, and another lining the internal cavity.
Comb jellies are colorful, simple invertebrates that are part of the family Ctenophora. Their combs are giant fused cilia cells that allow the animals to swim, and they also scatter light like a prism and present a rainbow of colors. These animals have two major cell layers, the external epidermis and internal gastrodermis; in between these cell layers is the mesoderm that is what gives the animals their gelatinous appearance.
Even though comb jellies look a bit like jellyfish, they are not closely related. Early writers combined ctenophores with cnidarians into a single phylum called Coelenterata on account of morphological similarities between the two groups. Like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. Jellyfish have stinging cells called nematocysts. Comb jellies do not have these. Instead, they have special sticky cells called colloblasts. These cells help them catch small prey.
Comb jellies get their name from eight rows of tiny hairs called cilia. These cilia look like the teeth of a comb and help them swim. 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 2 millimeters (0.08 in). 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.
Read also: Choosing the Right Straightening Tool
Comb jellies have been found to have no intestines, lungs or stomach. Instead, oxygen and nutrients are passed directly through gastrodermis or even through the epidermis. 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. Little is known about how ctenophores get rid of waste products produced by the cells. Recently, scientists discovered that comb jellies remove waste not through their mouths like similar invertebrate species, but instead release the indigestible byproducts of waste into the water through pores at the end of their bodies.
Comb jellies are 95% water. This enables them to float around without any bones or muscle to weigh them down! The mesoderm is mostly water and acts as a soft skeleton as it contains muscle cells, proteins, and nerve cells.
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. The largest single sensory feature is the aboral organ (at the opposite end from the mouth), which is underlined with its own nerve net. 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.
There are three groups of comb jellies, each of which is known for a distinct appearance:
We find comb jellies in all areas of the world’s ocean, and especially along the eastern coasts of the Americas. Most species prefer warmer temperate, tropical, or subtropical waters and live near the ocean’s surface. Mnemiopsis leidyi Size: Up to 6 cm Habitat: Marine to brackish waters worldwide. In 2013 Mnemiopsis was recorded in lake Birket Qarun, and in 2014 in lake El Rayan II, both near Faiyum in Egypt, where they were accidentally introduced by the transport of fish (mullet) fry. Though many species prefer brackish waters like estuaries and coastal lagoons in open connection with the sea, this was the first record from an inland environment. Both lakes are saline, with Birket Qarun being hypersaline, and shows that some ctenophores can establish themselves in saline limnic environments without connection to the ocean.
Read also: The Comb Knife: A Hidden Weapon
Comb jellies are carnivorous and opportunistic, feeding on whatever passes them by. They find food by detecting chemical changes in the water around them These chemicals tell that comb jelly that there is something edible nearby. By following the potency of these chemicals until they get stronger, the comb jelly can hunt down its next meal. Using sticky cells (colloblasts) that are lined on their tentacles, they can capture prey and then move the victim to the main body for digestion. Their diets vary based on where in the ocean they live, but it is most often a diet of zooplankton, larvae, and sometimes small fish or other organisms. If food is plentiful, they can eat ten times their own weight per day. Members of the genus Haeckelia prey on jellyfish and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblasts. While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplankton (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepods, amphipods, and even krill.
Comb jellies use different feeding strategies based on the species and its habitat, but most are known as passive feeders. Ctenophores have been compared to spiders in their wide range of techniques for capturing prey - some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid jumping spiders, and some dangle a sticky droplet at the end of a fine thread, as bolas spiders do.
Comb jellies are cannibals Comb jellies will eat other comb jellies that are larger than themselves. They do this by biting off chunks of them with special cilia structures in their mouths. However, they first prefer to eat plankton, zooplankton, crustaceans and small fish.
Many types of comb jelly are naturally bioluminescent The protocytes that create the blue and green light they project can be found underneath the rows of cilia that help them to move around. The moving cilia then refract this light, creating a more potent and magical bioluminescent effect. 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.
Their transparency means that comb jellies are great at camouflaging, one of their best defenses against potential predators. Some also produce a red pigment which makes it easier for them to hide in darkness. Most ctenophores that live near the surface are mostly colorless and almost transparent. When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions (ink) that luminesce at much the same wavelengths as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies.
Read also: Benefits of Fin Combs
Comb jellies are simple organisms and have an equally simple lifecycle. The last common ancestor (LCA) of the ctenophores was hermaphroditic. Most species of comb jelly have been found to be hermaphroditic. This means that they can act as both males and females, making it easier for them to reproduce. 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. They release both sperm and eggs into the water every single day and rely on nature for them to make a match - either with gametes from another comb jelly or even their own. 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. The gonads 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, but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch.
Development of the fertilized eggs is direct; there is no distinctive larval form. Juveniles of all groups are generally planktonic, 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. 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. 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 constant food supply is essential for comb jellies to release sperm and eggs every day. If they don’t get enough nutrition, they will shrink in size and halt their internal production of gametes until they can eat properly again.
According to the Smithsonian National Museum of Natural History, the comb jelly is at least 500 million years old! 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. 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 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.
Comb jellies are an important part of the marine food web, serving as both predators and prey. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists and may have a greater effect on populations than specialist jelly-eaters. Natural predators of comb jellies include sea turtles, fishes, marine mammals, and even other comb jellies. Some jellyfish and turtles eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Chum salmon, Oncorhynchus keta, digest ctenophores 20 times as fast as an equal weight of shrimps; ctenophores can provide the fish with a good diet if there are enough of them around.
Comb jellies tend to be abundant when ocean conditions are unfavorable for other species; when their populations are out of balance, we can infer that the quality of the ocean is diminishing. Comb jellies are more successful than other species because of their tolerance to wide ranges of salinity, temperature, and other environmental factors. Threats to comb jellies include chemical pollution, ocean acidification, and climate change.
In the late 1990s Mnemiopsis appeared in the Caspian Sea. Beroe ovata arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis there.
Since they look so similar to jellyfish, one of the most common questions that we get asked is whether or not a comb jelly can sting you. Fortunately not! They do not possess stinging cells, so they can be safely caught.
Comb jellies offer a unique opportunity for scientific research, particularly in the fields of evolutionary biology and neuroscience.
