Why Comb Jellies Don’t Crack Under Pressure

by Olivia Sacco
figures by Fiona Qu

Big exam coming up? About to give a presentation? Cooking a holiday meal for your entire family? We’ve all felt it: pressure. Throughout our lives, we must deal with high-pressure situations, but thankfully, they’re temporary. However, this is not the case for many deep-sea organisms, which constantly live under pressure. At the deepest point of the ocean, the pressure of 36,200 feet of seawater is greater than the weight of an elephant on every square inch of your body. How could life possibly survive down there? In June 2024, researchers from across the country published findings that help explain how some deep-sea animals have mastered the art of dealing with pressure.

The study focused on comb jellies, or ctenophores (Figure 1), because, throughout evolution, different ctenophore lineages have adapted to live in various ocean depths. Therefore, by comparing comb jellies used to different ocean depths, the scientists could illuminate how the animals evolved to adapt to their highly pressurized environments. The team of marine biologists and biochemists collected 66 animals across 17 different species from varying ocean depths to figure out how these animals survive in the deep sea.

**Figure 1.** Ctenophora, or comb jellies, are found in sea waters all around the world. They are typically oval-shaped and have eight comb-like structures that they use to move around. Pictured are warty comb jellied from the Aquarium of the Pacific. *Image by Benoît Prieur, CC0, via Wikimedia Commons*

The researchers had reason to suspect that the key to this mystery was in the comb jellies’ phospholipids, a certain type of lipid or “fat” molecule. Phospholipids are composed of two main parts: a head and a tail (Figure 2A). The heads are hydrophilic, or water-loving, and the tails are hydrophobic, or water-disliking. You might remember learning about phospholipids from biology class – they make up the cell membrane. Within a cell’s membrane, phospholipids self-organize into two orderly layers, so that the water-disliking tails are shielded from the watery cellular environment (Figure 2B). In this way, phospholipids effectively make up a wall that helps create the shape of the cell and distinguish between the cell’s inside and outside.

**Figure 2. (A)** Phospholipids are composed of two major parts: a hydrophilic or water-loving “head” and a hydrophobic or water-disliking “tail”. **(B)** Phospholipids come together to form a lipid bilayer, which makes up our cell membranes. This is important for distinguishing between the inside and outside of a cell.

Phospholipid tails are long and flexible, allowing for movement and malleability of the cell membrane. The tails can adopt different structures based on the characteristics of their surroundings (Figure 3). In normal conditions, for example, the phospholipid tails are flexible, like the streamers you hang up at a party, so that they can flow freely. This is called the fluid phase. Yet at colder temperatures or higher pressures, phospholipid tails become less flexible and shift to the gel phase, like streamers made up of construction paper – more stiff and rigid. Therefore, the structure of phospholipid tails is highly affected by changes in the environment, especially temperature and pressure. Additionally, it is known that when deep sea animals are brought to shallower waters, where the pressure is relatively lower, their tissues disintegrate, and their membrane structure becomes compromised. These pieces of background knowledge caused the research team to hypothesize that something about the phospholipids of deep-sea organisms allows them to survive down there.

**Figure 3.** At normal, or ambient, temperatures and pressures, phospholipid tails are fluid and free-moving (left). When the temperature drops or pressure increases, phospholipids take on a different phase and are more “gel-like” (right).

So, how do phospholipids react to pressures different than they’re used to? The researchers extracted phospholipids from animals accustomed to varying depths (and therefore pressures) and then used x-rays to examine the phospholipid shape under changing pressures. As expected, all the phospholipids transitioned from the fluid phase to the stiffer gel phase when they experienced higher pressures. However, when phospholipids from organisms from the deepest parts of the ocean were brought to lower pressures (which correspond to shallower waters), the phospholipids formed a different phase altogether: the “hexagonal inverted phase”. Normally in water, the water-loving phospholipid heads are oriented toward the water and the water-disliking tails are oriented inward away from the water in what is known as the lamellar phase (Figure 4A). In the hexagonal inverted phase, however, the opposite occurs: the tails face outwards and the heads face inwards (Figure 4B).

**Figure 4. (A)** In water, phospholipids are typically oriented with their water-loving heads towards the outside and their water-hating tails on the inside, protected from the water. **(B)** In the hexagonal inverted phase, on the other hand, phospholipid tails are oriented outwards while their heads come together. Known as plasmalogens, these phospholipids are “curvier” and can form the hexagonal inverted phase.

What is it about the deep-sea adapted comb jelly phospholipids that cause them to adopt this unusual arrangement in lower pressures? Could the types of phospholipids be different? Deep-sea animals did, in fact, contain significantly more of a certain type of phospholipid, called a plasmalogen. Plasmalogens made up a whopping ¾ of the deep-sea comb jelly phospholipids. Plasmalogens are curvier than other types of phospholipids (Figure 4), and while these curvier phospholipids would not be suitable for cell membranes in normal pressures, the researchers proposed that plasmalogens actually function properly under higher pressure, which would “push” them into the appropriate orientations.

But can we tell if that theory is indeed the reason behind the high-pressure adaptation of the deep-sea comb jellies? To get stronger proof, the scientists engineered simple bacterial cells to contain high levels of plasmalogens and then tested whether these engineered organisms could withstand higher pressures. Excitingly, the engineered bacteria with plasmalogen-rich membranes could indeed survive significant amounts of pressure that they normally wouldn’ be able to. This experiment was solid evidence that plasmalogens are a key adaptation of deep-sea organisms that allow them to thrive in high pressure environments (Fig 5).

**Figure 5.** Deep-sea comb jellies’ curvy phospholipids can maintain a healthy membrane structure at deep pressures, but as they rise to shallower waters and lower pressures, their membranes fall apart (left). The opposite is seen in comb jellies that live in shallower waters; when they are brought down to deeper, higher pressure environments, their membranes become too rigid (right).

As exciting as that finding is for marine biologists and biochemists, you might wonder, why should we care? One reason pertains to human curiosity: the deep sea encompasses over 90% of Earth’s habitable volume but remains largely unexplored. Therefore, the vast majority of life forms that exist there are still yet to be known. The research presented in this article, enabled by an interdisciplinary team and advanced technologies, is just the beginning of our deep dive into knowledge about the deep sea and the many organisms that live in it.

Additionally, plasmalogens are not specific to deep-sea animals. In fact, they are part of human cell membranes and are enriched in our hearts and brains. Deterioration of these special phospholipids is linked to diseases like Alzheimer’s disease. Understanding how these phospholipids function and have been adapted to perform certain processes over evolutionary time can meaningfully advance our understanding of what plasmalogens do in our own bodies, in both health and in disease.

Olivia Sacco is a second-year PhD student studying biology at Harvard.

Fiona Qu is a PhD candidate in the Systems, Synthetic, and Quantitative Biology program at Harvard.

Figure 1 and featured image by

For More Information:

You can find the full scientific article about how comb jellies survive under pressure here!
To learn more about deep sea animals and their creative solutions to the tricky high-pressure life, check out this website. It’s more than just their phospholipids!
For more information about phospholipids, check out this review article.