Ctenophores Aren’t Just Beautiful. They’re Biological Wonders.

Around 700 million years ago, a group of simple, gelatinous organisms resembling glowing blobs split from other animals, forming what may be the earliest branch of the animal family tree. These organisms are now known as ctenophores, or comb jellies, and there are nearly 200 species alive today. Unlike jellyfish, ctenophores are unrelated to them and live in diverse environments, from deep-sea trenches to warm coastal waters. Their survival over hundreds of millions of years is not just due to their beauty or glowing appearance but also because of their unique genetic makeup. Scientists study ctenophores to understand fundamental biological questions, such as how the first nervous systems evolved and how bioluminescence—a phenomenon where organisms produce their own light—works. Marine biologist Steven Haddock, who studies ctenophores at the Monterey Bay Aquarium Research Institute, emphasizes that these organisms are valuable for investigating how life adapts to extreme conditions like high pressure or low light.

For over a century, scientists believed that sponges (Porifera) were the first animals to branch off from the rest of the animal kingdom. However, recent research suggests that ctenophores, not sponges, may hold this position. In 2023, a study analyzing the organization of chromosomes provided strong evidence supporting the idea that ctenophores are the sister group to all other animals. This finding challenges long-held assumptions, as sponges lack muscles and neurons, while ctenophores possess both. Evolutionary biologist Pawel Burkhardt from the University of Bergen notes that this discovery forces scientists to reconsider what the earliest animals might have looked like. The debate is not fully settled, but the evidence points toward ctenophores as a key group for understanding the origins of animal life.

Ctenophores possess a unique nervous system that differs from those of other animals. Unlike typical nervous systems, which consist of networks of neurons connected by synapses, ctenophores have a nerve net where neurons are linked by continuous cytoplasm without synapses. Burkhardt’s lab discovered this unusual structure in the comb jelly Mnemiopsis leidyi, identifying 17 unique cell types in the aboral organ—a structure that helps the organism sense light, pressure, and gravity. This finding suggests that nervous systems may have evolved twice in the animal kingdom: once in ctenophores and separately in jellyfish and other animals. The aboral organ is tightly integrated with a continuous network of fused neurons, providing insights into how the first brains might have formed.

Genome regulation is essential for determining when and where genes are activated, enabling cells to specialize and form complex tissues. A key process in this regulation involves the physical folding of DNA into loops, which allows genes to be turned on or off without requiring new genetic sequences. Researchers found that the comb jelly Mnemiopsis leidyi has over 4,000 DNA loops in its genome, which is only 100 million base pairs long—compared to 3 billion in the human genome. This discovery suggests that DNA looping, a process critical for multicellular life, may have evolved 150 million years earlier than previously thought. Evolutionary biologist Arnau Sebé-Pedrós from the Center for Genomic Regulation in Barcelona highlights that these loops are vital for cell-type specialization and building complex tissues.

Bioluminescence, the ability of organisms to produce their own light, is a widespread phenomenon in the ocean. Ctenophores, being distant relatives of other glowing multicellular organisms, provide clues about how and why bioluminescence evolved. Marine biologist Steven Haddock’s team found that non-glowing ctenophore species lack genes suspected to be involved in synthesizing coelenterazine, a light-emitting chemical. Subsequent research identified the full-length gene in bioluminescent ctenophores. Coelenterazine is one of the most abundant light-emitting molecules in the ocean, found in copepods, shrimp, squid, and mollusks. Haddock suggests that these organisms may have co-opted a precursor gene, modifying it for a different purpose, which explains its independent appearance across the tree of life.

Deep-sea ctenophores possess a special type of lipid called plasmalogen, which contains only one oxygen molecule instead of the usual two. These lipids are also found in human brains and play a crucial role in allowing ctenophores to withstand extreme hydrostatic pressure in the deep sea. Biophysicist Itay Budin from the University of California, San Diego, studied the comb jelly Bathocyroe aff. fosteri and found that when brought to the surface, the sudden release from pressure caused the plasmalogens to expand, splitting cell membranes and disintegrating the organism. This research highlights the importance of plasmalogens in both deep-sea life and human health, as deficiencies in these lipids are linked to neurodegenerative diseases like dementia. Plasmalogens may help neurons fire by enabling cell membranes to fuse and break rapidly.