Comb Jellies (Ctenophora)

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Ctenophores (pronounced TEEN-oh-fours) are predators commonly known as comb jellies that travels through the ocean in search of food. Though they resemble jellyfish, they are distinctly different creatures that propel themselves through water using cilia (short vibrating or moving hairlike structure) instead of tentacles. Ctenophora make up a phylum of marine invertebrates, that inhabit marine waters worldwide. The phylum includes seven orders, with over 200 species, which are biradially symmetrical (divided into equal parts, but only in two planes) and acoelomate (animal without a body cavities) and resemble cnidarians (jellyfish). [Source: CBS News, Animal Diversity Web (ADW)]

Ctenophores use groups of cilia (commonly referred to as "combs") for swimming , and they are the largest animals to swim with the help of cilia. Depending on the species, adult ctenophores range from a few millimeters (fractions of an inch to 1.5 meter (5 feet) in size. According to some scientists only 100 to 150 species have been validated, and possibly another 25 have not been fully described and named. The name "ctenophora" means "comb-bearing", from the Greek word meaning "comb" and the Greek suffix meaning "carrying".

Comb jellies are among — if not THE — world’s oldest animals. Despite their soft, gelatinous bodies, fossils thought to represent ctenophores appear in fossils dating as far back as the early Cambrian, about 525 million years ago. The position of the ctenophores in the "tree of life" has long been debated by scholars.

Stephanie Pappas wrote in Live Science: Ctenophores, or comb jellies, are strange jelly-like animals that ghost through the sea propelled by tiny hairs called cilia. They're an enigmatic bunch, with origins that stretch back approximately 540 million years, and no one is sure exactly when they diverged from the rest of the tree of life. The genetic and chemical basis of the ctenophore neural system is quite different from that seen in other animals. [Source: Stephanie Pappas, Live Science, May 16, 2023]

Among animal phyla, Ctenophores are more complex than sponges, about as complex as cnidarians (jellyfish, sea anemones, etc.), and less complex than bilaterians (which include almost all other animals). Almost all ctenophores are predators, taking prey ranging from microscopic larvae and rotifers to small crustaceans the adults. The exceptions are juveniles of two species, which live as parasites on the salps on which adults of their species feed.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio

Comb Jellies Characteristics

The bodies of Comb jellies consist of a mass of jelly, with a layer two cells thick 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. [Source: Wikipedia]

Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like basement membranes; muscles; nervous systems; and some have sensory organs. 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 sponges and 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, along with sponges. Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer. 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.

Cilia ("hairs") are comb jellies’ 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.

Comb Jellies Not Sponges the World’s Oldest Animals?

There has long been a debate on which animal came first — comb jellies (ctenophore) or sponges. Sponges spend most of their lives in one spot, filtering water through their pores to collect food particles. Many have argued that due to the sponge's primitive features, it came first. In May 2023, scientists at the University of California Berkeley announced in a study published in Nature that the world’s first animal likely were comb jellies. "The most recent common ancestor of all animals probably lived 600 or 700 million years ago. It's hard to know what they were like because they were soft-bodied animals and didn't leave a direct fossil record," said Daniel Rokhsar, a UC Berkeley professor and co-author of the study, in a statement. "But we can use comparisons across living animals to learn about our common ancestors." [Source: Simrin Singh, CBS News, Fri, May 19, 2023]

fossil of half-billion-year-old ctenophore, ctenophore)from

CBS News reported: This new research has determined that while sponges came early, they were likely second to ctenophores. In order to make that determination, scientists looked at the organization of genes in the chromosomes of the organisms. The chromosomes of the ctenophore look very different than the chromosomes of sponges, jellyfish and other invertebrates — alerting researchers that the ctenophore could have either come much earlier than the others, or much later. "At first, we couldn't tell if ctenophore chromosomes were different from those of other animals simply because they'd just changed a lot over hundreds of millions of years," Rokhsar explained in the news release. "Alternatively, they could be different because they branched off first, before all other animal lineages appeared. We needed to figure it out."

The "smoking gun" for researchers was when they compared the chromosomes of ctenophores to non-animals. "When the team compared the chromosomes of these diverse animals and non-animals, they found that ctenophores and non-animals shared particular gene-chromosome combinations, while the chromosomes of sponges and other animals were rearranged in a distinctly different manner," the news release said. According to researchers, the new insight is valuable to learning about the basic functions of all animals and humans today, such as how we eat, move and sense our surrounding environment.

In December 2013, sponges lost their crown as the world’s oldest animals when scientists from the University of Miami announced in the journal Science that a species comb jelly known as sea walnuts and sea gooseberries represented the oldest branch of the animal family tree based on DNA research. Associated Press reported: All animals evolved from a single ancestor and scientists want to know more about how that happened. More than half a billion years ago, the first split in the tree separated one lineage from all other animals. Traditionally, scientists have thought it was sponges. [Source: Malcolm Ritter, Associated Press, December 12, 2013]

“The evidence in favor of comb jellies comes from deciphering the first complete genetic code from a member of this group. Scientists were finally able to compare the full DNA codes from all the earliest branches. The genome of a sea walnut, a plankton-eating creature native to the western Atlantic Ocean, was reported online Thursday in the journal Science by Andreas Baxevanis of the National Human Genome Research Institute with co-authors there and elsewhere. The work supports some earlier indications that comb jellies were the first to branch off.

Sorting out the early branching of the tree could help scientists learn what the ancestor of all animals was like. But despite decades of study and the traditional view favoring sponges, there is plenty of disagreement about which early branch came first. The question is "devilishly difficult" to answer, and the new paper is probably not the last word, said Antonis Rokas of Vanderbilt University, who did not participate in the new work. "The results need to be taken seriously," he said, but "I'm pretty sure there will be other studies that suggest something else."

Strange Nervous System of Comb Jellies

The nervous system of comb jellies is like nothing ever seen before. Stephanie Pappas wrote in Live Science: Instead of relying on gaps between nerve cells called synapses for communication, at least part of the ctenophore nervous system is fused. "We haven't actually seen this in any other animal before," study co-author Maike Kittelmann, a cell and developmental biologist at Oxford Brookes University in the U.K., told Live Science. "It means that there are other ways that neurons can connect to each other." [Source: Stephanie Pappas, Live Science, May 16, 2023]

The discovery raises questions about how all nervous systems evolved and adds fuel to a long-standing debate about how comb jellies are related to the rest of the animal kingdom. Many scientists thought that the nervous system in animals evolved only once, at some point after sponges broke off from the rest of the animal kingdom, as sponges do not have a nervous system. But some scientists think ctenophores diverged from other animals early and evolved their own nervous system separately.

Comb jellies don't have brains, but have a weblike system of neurons known as the nerve net. It's within this nerve net that researchers found the fused neurons. The strange fused arrangement could hint that these systems evolved independently, Kittlemann said. But it's still an open question. "We don’t really know for sure," she said.

The new research, published April 20, 2023 in the journal Science, looks at ctenophores in an early developmental stage, when they're just a few days old. At this stage, ctenophores can move around freely and even reproduce, but they're not full adults. (Depending on species, ctenophores have life spans between about a month and several years.) The vast majority of nerve cells in animals communicate via synapses, which are gaps between cells. To "talk," neurons release chemicals called neurotransmitters across these gaps. But the new study found that within the ctenophore nerve net, the cells are fused and their membranes connected so that the path from cell body to cell body is continuous. This structure is called a syncytium. "There are some other animals which show fused neurons but not to that extreme, where you have a whole nerve net," study co-author Pawel Burkhardt, who studies the evolutionary origin of neurons and synapses at Norway's University of Bergen, told Live Science.

The discovery raises a whole bevy of new questions, Burkhardt said, from how this fused network develops to how it functions. The same cells that are fused together also make connections to other nerve cells via synapses, and other parts of the ctenophore nervous system use synapses, too.

It's not clear, Burkhardt said, why comb jellies use two different methods of communication between their nerve cells. One possibility is that the fused nervous system has some advantage for tissue repair and healing, Leslie Babonis, an evolutionary biologist at Cornell University who was not involved in the new study, told Live Science. Ctenophores are capable of regenerating an entirely new animal from a small chunk of flesh. "Maybe this is one of the secrets to their incredible ability for regeneration," Babonis said.

The research team only looked at one species of ctenophore — Mnemiopsis leidyi — in one developmental stage, so they now plan to find out whether other species have fused neural networks and whether this fusion persists through the animal's whole lifespan.This could help answer questions about the evolution of the nervous system and whether it arose once, twice or more times. If many ctenophores have unique fused nervous systems, this could lend credence to the hypothesis that ctenophores evolved their nervous system separately from other animals. But it's also possible that all animal nervous systems still share a common origin, and ctenophores evolved the fusion later, the researchers said.

Only a handful of lineages in the animal kingdom have had their nervous systems closely studied, Leonid Moroz, a biologist at the Whitney Laboratory for Marine Biosciences at the University of Florida, told Live Science. Moroz If the nervous system is a poem, Moroz said, ctenophores use a different alphabet from the rest of the animal kingdom to write theirs. He argues that these jellies evolved their nervous system independently, and that other understudied animals may have done the same. Unraveling this diversity could lead to a deeper understanding of how neurological disorders arise.

Comb Jelly That Sometimes Has An Anus and Sometimes Doesn't.

The warty comb jelly sometimes has an anus and sometimes doesn’t. Mindy Weisberger wrote in Live Science: An anus is a gateway for solid-waste removal from an animal's digestive system; in most animals, the anus is reliably found in one location all the time. But Mnemiopsis leidyi, a jellyfish relative that is also known as a warty comb jelly or sea walnut, is not "most animals." M. leidyi's anus isn't fixed in place on its gelatinous body. Instead of a permanent opening, a so-called anal pore appears when the jelly needs to defecate and then disappears immediately afterward, leaving unblemished skin behind, according to a study The findings were published online February 22, 2019 in the journal Invertebrate Biology.. [Source: Mindy Weisberger, Live Science, March 8, 2019]

Unlike such close relatives as sponges and jellyfish, ctenophores — especially their bodily functions — are poorly understood, Sidney Tamm, a researcher with the Marine Biological Laboratory in Woods Hole, Massachusetts, wrote in the study. Prior studies had concluded that M. leidyi had a permanent anus. But when Tamm used video microscopy to closely examine M. leidyi larvae and adults, he discovered that their anuses were intermittent, and that the jellies' defecation took place through an opening "which appears and disappears" in a regular rhythm, Tamm reported. After M. leidyi gulps down prey, the meal travels through a six-part digestive system. Eventually, the food ends up in a central stomach that feeds into canals for pooping, which dead-end at the body's surface as lobes, Tamm wrote in the study.

Tamm observed that when a jelly was ready to defecate, the shape of its stomach would change — narrowing into a rectangular box — and its anal canals would widen. Two minutes later, the esophagus "crumpled," preventing more food from entering the stomach. Lobes at the ends of paired anal canals filled with waste particles and began to swell, with one lobe protruding significantly. As that lobe reached "maximum volume," a pore opened and released a stream of poo as particles and clumps, Tamm reported. But before the pore opened, the skin of that lobe appeared "uniformly smooth," and there was no sign that the pore had opened there before. Then, when all the waste had been released, "the pore closed completely and disappeared," Tamm wrote. From start to finish, the entire process lasted from 2 to 3 minutes in M. leidyi larvae and juveniles measuring up to 0.8 inches (2 centimeters) long, and 4 to 6 minutes in adults with body lengths between 1.2 and 2 inches (3 and 5 cm). M. leidyi is to date the only known animal with a "now-you-see-it-now-you-don't" anal pore.

Comb Jellies Cannibalize Their Own Babies For Food

Every year Baltic Sea jellies have babies just to eat them, a new study suggests. Brandon Specktor wrote in Live Science: In late summer, thousands of invasive jellyfish-like creatures in the Baltic Sea begin eating their children. According to a study published May 7, 2020 in the journal Communications Biology, cannibalism may simply be a fact of life for jellies living in nutrient-poor waters outside their natural habitats, providing adults a few extra weeks of energy after they've decimated local prey populations. "In some ways, the whole jelly population is acting as a single organism, with the younger groups supporting the adults through times of nutrient stress," study co-author Thomas Larsen, of the Max Planck Institute for the Science of Human History in Jena, Germany, said in a statement. "Overall, it enables jellies to persist through extreme events and low food periods, colonizing farther than climate systems and other conditions would usually allow." [Source: Brandon Specktor, Live Science, published May 8, 2020

Ctenophora (Comb jelly): Mertensia ovum

For their new research, the authors studied a population of warty comb jelly (Mnemiopsis leidyi — a jellyfish relative also known as a "sea walnut") living in Kiel Fjord, a long inlet of the Baltic Sea near northeastern Germany. Comb jellies are native to the western Atlantic Ocean near North and South America, but were introduced to the comparatively cold waters of the Baltic in the 1980s. These invaders of the Baltic Sea face much longer periods of low food availability than their Atlantic cousins, but have nevertheless managed to thrive, the researchers wrote. Part of their success in the Baltic is owed to the comb jelly's "bloom-and-bust" reproductive cycle. In late summer, Baltic comb jellies enter a period of rapid reproduction, or a "bloom," when each jelly releases hundreds of eggs and sperm cells into the water simultaneously, increasing local populations by the thousands. (M. leidyi is hermaphroditic, meaning it can fertilize its own eggs.)

This new army of larvae is ready to feast, devouring all the prey the itty-bitty jellies can find — mostly tiny zooplankton, a cornerstone prey item in aquatic food webs — eating until there's literally nothing left. That's the bloom; then comes the "bust." Soon after the local prey population collapses, so too does the larval jelly population, which starves to death. These bloom-and-bust events have huge downstream effects on the local food web, decimating the prey that other sea creatures depend upon and causing significant population drops for local fish that can't compete with the invasive jellies, a 2007 study in the journal Aquatic Invasions found. Strangely, though, even after the larval jellies and their prey die off, adult comb jellies continue to thrive through the ensuing famine. How?

According to the authors of the new study, the reason is cannibalism. While sampling the population of Baltic comb jellies and their prey in August and September 2008 (the period right before and after that year's jelly bloom), the researchers discovered an adult jelly with two baby jellies captured inside its auricles — the gelatinous sacs that direct prey toward a jelly's mouth.

The grisly scene (captured in the photograph above) helped explain the team's findings that adult jellies continued to grow for weeks after their prey and larvae died off. But to prove there was indeed cannibalism afoot, the researchers needed to confirm that the adult jellies were actually getting a nutrient boost from these captured babies.

So, in September 2016, the team incubated adult and larval jellies together in a lab. Within 36 hours, the adults devoured any juveniles in their tank. A subsequent analysis showed that the cannibalized larvae accounted for about 4% of the total carbon and 2.5% of the total nitrogen content in each adult's body, giving the cannibal adults significantly higher nutrient concentrations than jellies in the non-cannibal control group, which ate only tiny crustaceans called copepods. "To our knowledge, we have presented the first unequivocal evidence that adult M. leidyi cannibalize their own larvae," the researchers concluded in the study. "Since larvae cannot survive winters in the species' northernmost habitats, our study suggests that the primary purpose of M. leidyi larvae is to gather and store energy and nutrients for adults." Simply put: After a jelly bloom devours the local prey, adults begin to eat their babies. This post-bloom cannibal feast appears to give adults the nutrient boost they need to continue growing for several more weeks amid food-scarce conditions, the researchers said, allowing them to become ever more competitive predators in the cold Baltic waters.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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