Jellyfish, with no central brain, shown to learn from past experience

Even without a central brain, jellyfish can learn from past experiences like humans, mice, and flies, scientists report for the first time on September 22 in the journal Current Biology. They trained Caribbean box jellyfish (Tripedalia cystophora) to learn to spot and dodge obstacles. The study challenges previous notions that advanced learning requires a centralized brain and sheds light on the evolutionary roots of learning and memory.

No bigger than a fingernail, these seemingly simple jellies have a complex visual system with 24 eyes embedded in their bell-like body. Living in mangrove swamps, the animal uses its vision to steer through murky waters and swerve around underwater tree roots to snare prey. Scientists demonstrated that the jellies could acquire the ability to avoid obstacles through associative learning, a process through which organisms form mental connections between sensory stimulations and behaviors.

"Learning is the pinnacle performance for nervous systems," says first author Jan Bielecki of Kiel University, Germany. To successfully teach jellyfish a new trick, he says "it's best to leverage its natural behaviors, something that makes sense to the animal, so it reaches its full potential."

The team dressed a round tank with gray and white stripes to simulate the jellyfish's natural habitat, with gray stripes mimicking mangrove roots that would appear distant. They observed the jellyfish in the tank for 7.5 minutes. Initially, the jelly swam close to these seemingly far stripes and bumped into them frequently. But by the end of the experiment, the jelly increased its average distance to the wall by about 50%, quadrupled the number of successful pivots to avoid collision and cut its contact with the wall by half. The findings suggest that jellyfish can learn from experience through visual and mechanical stimuli.

"If you want to understand complex structures, it's always good to start as simple as you can," says senior author Anders Garm of the University of Copenhagen, Denmark. "Looking at these relatively simple nervous systems in jellyfish, we have a much higher chance of understanding all the details and how it comes together to perform behaviors."

The researchers then sought to identify the underlying process of jellyfish's associative learning by isolating the animal's visual sensory centers called rhopalia. Each of these structures houses six eyes and generates pacemaker signals that govern the jellyfish's pulsing motion, which spikes in frequency when the animal swerves from obstacles.

The team showed the stationary rhopalium moving gray bars to mimic the animal's approach to objects. The structure did not respond to light gray bars, interpreting them as distant. However, after the researchers trained the rhopalium with weak electric stimulation when the bars approach, it started generating obstacle-dodging signals in response to the light gray bars. These electric stimulations mimicked the mechanical stimuli of a collision. The findings further showed that combining visual and mechanical stimuli is required for associative learning in jellyfish and that the rhopalium serves as a learning center.

Next, the team plans to dive deeper into the cellular interactions of jellyfish nervous systems to tease apart memory formation. They also plan to further understand how the mechanical sensor in the bell works to paint a complete picture of the animal's associative learning.

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Oldest known species of swimming jellyfish identified

Royal Ontario Museum (ROM) announces the oldest swimming jellyfish in the fossil record with the newly named Burgessomedusa phasmiformis. These findings are announced in the journal Proceedings of the Royal Society B.

Jellyfish belong to medusozoans, or animals producing medusae, and include today's box jellies, hydroids, stalked jellyfish and true jellyfish. Medusozoans are part of one of the oldest groups of animals to have existed, called Cnidaria, a group which also includes corals and sea anemones. Burgessomedusa unambiguously shows that large, swimming jellyfish with a typical saucer or bell-shaped body had already evolved more than 500 million years ago.

Burgessomedusa fossils are exceptionally well preserved at the Burgess Shale considering jellyfish are roughly 95% composed of water. ROM holds close to two hundred specimens from which remarkable details of internal anatomy and tentacles can be observed, with some specimens reaching more than 20 centimetres in length. These details enable classifying Burgessomedusa as amedusozoan. By comparison with modern jellyfish, Burgessomedusa would also have been capable of free-swimming and the presence of tentacles would have enabled capturing sizeable prey.

"Although jellyfish and their relatives are thought to be one of the earliest animal groups to have evolved, they have been remarkably hard to pin down in the Cambrian fossil record. This discovery leaves no doubt they were swimming about at that time," said co-author Joe Moysiuk, a Ph.D. candidate in Ecology & Evolutionary Biology at the University of Toronto, who is based at ROM.

This study, identifying Burgessomedusa, is based on fossil specimens discovered at the Burgess Shale and mostly found in the late 1980s and 1990s under former ROM Curator of Invertebrate Palaeontology Desmond Collins. They show that the Cambrian food chain was far more complex than previously thought, and that predation was not limited to large swimming arthropods like Anomalocaris (see field image showing Burgessomedusa and Anomalocaris preserved on the same rock surface).

"Finding such incredibly delicate animals preserved in rock layers on top of these mountains is such a wonderous discovery. Burgessomedusa adds to the complexity of Cambrian foodwebs, and like Anomalocaris which lived in the same environment, these jellyfish were efficient swimming predators," said co-author, Dr. Jean-Bernard Caron, ROM's Richard Ivey Curator of Invertebrate Palaeontology. "This adds yet another remarkable lineage of animals that the Burgess Shale has preserved chronicling the evolution of life on Earth."

Cnidarians have complex life cycles with one or two body forms, a vase-shaped body, called a polyp, and in medusozoans, a bell or saucer-shaped body, called a medusa or jellyfish, which can be free-swimming or not. While fossilized polyps are known in ca. 560-million-year-old rocks, the origin of the free-swimming medusa or jellyfish is not well understood. Fossils of any type of jellyfish are extremely rare. As a consequence, their evolutionary history is based on microscopic fossilized larval stages and the results of molecular studies from living species (modelling of divergence times of DNA sequences). Though some fossils of comb-jellies have also been found at the Burgess Shale and in other Cambrian deposits, and may superficially resemble medusozoan jellyfish from the phylum Cnidaria, comb-jellies are actually from a quite separate phylum of animals called Ctenophora. Previous reports of Cambrian swimming jellyfish are reinterpreted as ctenophores.

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Scientists investigate the evolution of animal developmental mechanisms, show how some of Earth's earliest animals evolved

Lacking bones, brains, and even a complete gut, the body plans of simple animals like sea anemones appear to have little in common with humans and their vertebrate kin. Nevertheless, new research from Investigator Matt Gibson, Ph.D., at the Stowers Institute for Medical Research shows that appearances can be deceiving, and that a common genetic toolkit can be deployed in different ways to drive embryological development to produce very different adult body plans.

It is well established that sea anemones, corals, and their jellyfish relatives shared a common ancestor with humans that plied the Earth's ancient oceans over 600 million years ago. A new study from the Gibson Lab, published in Current Biology on June 13, 2023, illuminates the genetic basis for body plan development in the starlet sea anemone, Nematostella vectensis. This new knowledge paints a vivid picture of how some of the earliest animals on earth progressed from egg to embryo to adult.

"Studying the developmental genetics of Nematostella is sort of like taking a time machine into the very distant past," said Gibson. "Our work allows us to ask what life looked like long ago -- hundreds of millions of years before the dinosaurs. How did ancient animals develop from egg to adult, and to what extent have the genetic mechanisms that guide embryonic development endured across millennia?"

Most contemporary animals, from insects to vertebrates, develop by forming a head-to-tail series of segments that assume distinct identities depending on their position. Within a given segment, there is a further axis of polarity that informs cells whether they are at the front or back of the segment. Collectively, this is referred to as segment polarization.

Shuonan He, Ph.D., a former predoctoral researcher from the Gibson Lab, uncovered genes involved during development of the sea anemone, Nematostella vectensis, that guide the formation of segments and others that direct segment polarity programs strikingly similar to organisms higher up the evolutionary tree of life, including humans.

"The significance is that the genetic instructions underlying the construction of extremely different animal body plans, for example, a sea anemone and a human, are incredibly similar," said Gibson. "The genetic logic is largely the same."

This new study builds upon a 2018 study published in Science from the Gibson Lab that showed that sea anemones have an internal bilateral symmetry early in development with eight radial segments. The study demonstrated that Hox genes -- master development genes that are crucial for human development -- act to delineate boundaries between segments and likely had an ancient role in segment construction.

The team's latest finding explores how segments form and what accounts for differences in their identities. Using spatial transcriptomics, or the differences in gene expression between segments, the team discovered hundreds of new segment-specific genes. These include two crucial genes that encode transcription factors that govern segment polarization under the control of Hox genes and are required for the proper placement of sea anemone muscles.

The astonishing diversity of organisms on Earth can be compared to the assembly of Legos. "Whether you construct a dinosaur, a sea anemone, or a human, many of the core genetic building blocks are largely the same despite drastically different animal forms," said Gibson.

This is the first time that scientists have evidence of a molecular basis for segment polarization in a pre-bilaterian animal. While extensively studied in bilateral species like fruit flies and humans, the idea that cnidarian animals possess segmentation was unexpected. Now, the team has evidence that these segments are also polarized.

"This provides further evidence that investigating a broad diversity of animals can have direct implications for understanding general principles, including those which apply to human biology," said Gibson. "Going one step further, by understanding the logic of sea anemone development and comparing it to what we see in vertebrates, we can also extrapolate back in time to understand how animals likely developed hundreds of millions of years ago."

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Jellyfish's stinging cells hold clues to biodiversity

The cnidocytes -- or stinging cells -- that are characteristic of sea anemones, hydrae, corals and jellyfish, and make us careful of our feet while wading in the ocean, are also an excellent model for understanding the emergence of new cell types, according to new Cornell research.

In new research published in the Proceedings of the National Academy of Sciences on May 2, Leslie Babonis, assistant professor of ecology and evolutionary biology in the College of Arts and Sciences, showed that these stinging cells evolved by repurposing a neuron inherited from a pre-cnidarian ancestor.

"These surprising results demonstrate how new genes acquire new functions to drive the evolution of biodiversity," Babonis said. "They suggest that co-option of ancestral cell types was an important source for new cell functions during the early evolution of animals."

Understanding how specialized cell types, such as stinging cells, come to be is one of the key challenges in evolutionary biology, Babonis said. For nearly a century, it's been known that cnidocytes developed from a pool of stem cells that also gives rise to neurons (brain cells), but up to now, no one knew how those stem cells decide to make either a neuron or a cnidocyte. Understanding this process in living cnidarians can reveal clues about now cnidocytes evolved in the first place, Babonis said.

Cnidocytes ("cnidos is Greek for "stinging nettle"), common to species in the diverse phylum Cnidaria, can launch a toxic barb or blob or enable cnidarians to stun prey or deter invaders. Cnidarians are the only animals that have cnidocytes, but lots of animals have neurons, Babonis said. So she and her colleagues at the University of Florida's Whitney Lab for Marine Bioscience studied cnidarians -- specifically sea anemones -- to understand how a neuron could be reprogrammed to make a new cell.

"One of the unique features of cnidocytes is that they all have an explosive organelle (a little pocket inside the cell) that contains the harpoon that shoots out to sting you," Babonis said. "These harpoons are made of a protein that is also found only in cnidarians, so cnidocytes seem to be one of the clearest examples of how the origin of a new gene (that encodes a unique protein) could drive the evolution of a new cell type."

Using functional genomics in the starlet sea anemone, Nematostella vectensis, the researchers showed that cnidocytes develop by turning off the expression of a neuropeptide, RFamide, in a subset of developing neurons and repurposing those cells as cnidocytes. Moreover, the researchers showed that a single cnidarian-specific regulatory gene is responsible both for turning off the neural function of those cells and turning on the cnidocyte-specific traits.

Neurons and cnidocytes are similar in form, Babonis said; both are secretory cells capable of ejecting something out of the cell. Neurons secrete neuropeptides -- proteins that rapidly communicate information to other cells. Cnidocytes secrete poison-laced harpoons.

"There is a single gene that acts like a light switch -- when it's on, you get a cnidocyte, when it's off you get a neuron," Babonis said. "It's a pretty simple logic for controlling cell identity."

This is the first study to show that this logic is in place in a cnidarian, Babonis said, so this feature was likely to regulate how cells became different from each other in the earliest multicellular animals.

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