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Discovering Amazing Life in the Deep Sea

Steven Haddock


The deep sea is one of the few remaining under-explored regions of the world. The little we already know tells us that:

  • our ideas of life in these waters are mostly misconceptions
  • climate change is troublesome for sea creatures who live within a specific temperature range
  • space biology (exobiology) can learn from the marine organisms that don’t need light or oxygen to survive
  • different species adapted to deep sea life in unique ways over time

November 2010


The body color of the bloodybelly comb jelly (Lampocteis cruentiventer) ranges from purple to red and sometimes black. Like other comb jellies, it propels itself by moving paddle-like organs, called combs, which are composed of hundreds of cilia. Photo: George Matsumoto.

Why do you study life in the deepest regions of the ocean?

The deep sea is the world’s largest habitat.

Haddock: There is such an incredible variation of biology in these regions. The deep sea is the largest habitat on our planet, and yet it has not been investigated enough. For most people, it is still a mysterious place. The deep sea is an underexplored habitat that still contains a lot of fundamental diversity.

Over 70% of the ocean’s surface lies over such deep water that it never sees the light of day. People study coral reef diversity and insect diversity, but to me, the diversity of entirely different life forms—echinoderms [phylum Echinodermata] versus ctenophores [phylum Ctenophora], or starfish versus comb jellies—those very basic divisions of macroscopic organisms are really interesting. The deep ocean is a candy shop of questions and problems that I can explore. Also, the marine life at these depths offers solutions to a better life on earth as a whole.

What are some misconceptions about the deep sea?

Haddock: I think probably the number one misconception that dates back centuries is what really lives down there—the thought that there is very little life. The other misconception is related to pressure; many people believe that pressure is a terrible burden to bear for organisms in that habitat, even though they are totally adapted to that environment. Truly, that seems to be one of their least worries, in terms of just surviving.

Deep sea biodiversity is a mystery to many people.

People have strange ideas about the type of animals that live down there. An article in a popular science magazine depicted life in the deep, and how scientists are exploring deeper and deeper, yet, the story’s illustrations showed mostly air-breathing organisms. The article explained that sea turtles can dive to a thousand meters, whales can go this deep, elephant seals can go this deep, and so on. However, those animals are the most ephemeral of visitors to the deep sea. People do not seem to understand that there is this whole realm of life under the surface that is not made up of air-breathing vertebrates that we can see easily see when sailing on a boat on the water’s surface. The real diversity, and the real actual bulk of life down there, consists of many crustaceans to be sure, but there are also many more really fragile creatures like jellies and other species that people may feel are exotic and not widespread in the deep sea.

How vast is the understudied space of the deep?

Many species remain to be discovered.

The zones of the open ocean. Chart:

Haddock: People always seem to want a number about how much of the sea floor has been mapped out. In terms of where we have done actual investigating—for example, in areas where we have done a dive into the deep sea—the percentage of space we have covered is vanishingly small. On the other hand, there is probably not a lot of difference in species in waters 50 to 100 kilometers apart in most cases. In the deep sea, you will see many of the same organisms in both of those locations, which means you can study one area and have a sense of what you are going to find within a radius of that area. So, although we have actually visited only a very small portion—a percentage in the single digits, maybe—what we know depends on the organism. Biologically, in terms of just the species, depending on the group, we have probably identified approximately 80% of deep sea fish species. Then with some of the fragile things, such as the jellies, it is closer to 20%. Remember, though, we have only explored less than 5% of the entire ocean.

What do deep-sea organisms have to contend with that is different for organisms living high up in the water column?

Haddock: Well, let us start with light and temperature for examples. There is very little light once you get down below a few hundred meters, except for biologically generated light; and the temperature is on the cold side—a couple of degrees above freezing. Additionally, the food supply is small, and the pressures are high. On the flip side of that, though, it is a very stable environment, so an animal is not going to go through a 30° temperature fluctuation like it would in a tidal pool, or even at the surface.

Climate change affects deep sea creatures.

These organisms have evolved to accommodate some amount of transitory change, for example, getting caught in a current that takes them to areas with a few degrees of temperature change. If humans perturb that environment, the creatures may be less adapted to accommodate such changes. Climate change is a real issue here. So for example, if the temperature warms up just a little bit in the deep sea for too long a time, the organisms may be completely unequipped to deal with those changes. If we were to pump CO2 [carbon dioxide] down into the deep sea, as a way of sequestering [removing and storing carbon], then the resulting changes in pH could have a much more profound effect on the organisms because they are not used to living in such an environment.

Some organisms require minimal oxygen.

In terms of oxygen, oxygen levels are actually high at the water’s surface, and the levels drop to a minimum needed for life. In the Pacific, that’s usually around 700 meters down. The levels shift around that point and begin to increase. So, by 2,000-meters deep, you actually have plenty of oxygen, and it is really in that band of low oxygen—the oxygen-minimum zone—that there is a real deficiency of oxygen available to organisms. Some organisms have adapted to move in and out of that oxygen-minimum zone; for example, they may have evolved low metabolic rates, or they may have other ways to buffer and build up oxygen supplies. Oxygen would be one of the harder constraints for any organism.

The interesting thing about the oxygen-minimum zone is that it is generated biologically; you would not have that zone if it were not for organisms and bacteria that are living there and respiring oxygen out of the environment; but of course, we would not have any oxygen on the planet without such organisms, either. The discovery in the 1970s that life can exist in the ocean’s depths without much or no oxygen or light encouraged scientists who study biology in places beyond Earth. There may be life on other planets where liquid water may exist.

Did organisms adapt to the harsh conditions in the same way?


Half-naked Hatchetfish, Argyropelecus hemigymnus, chasing a crustacean. Photo: Francesco Costa.

Transparent bodies are common in deep waters.

Haddock: Whether the adaptation is the same or different, it depends on what water depth and what species we are talking about. There is a lot of convergence in the deep sea, so transparency is one thing we have observed that is common among the organisms. We have also seen larger body sizes, sort of flabby body masses, soft muscular structures, and large mouths—especially in the cases of organisms that have sit-and-wait strategies in their feeding habits. These organisms will have a convergent set of features that make them look like deep-sea versions of other organisms. Fish are an easy example to think of—there are fish with either really big eyes or really small eyes, depending on how they use sight to survive. I think the genetic underpinnings of such adaptations, however, all arrived independently in evolutionary time. I do not believe one event allowed a whole range of organisms to go into the deep sea. Organisms have ventured at different points in evolutionary time into other levels of the water column [the open water of the ocean between the surface and the seabed]. Some moved from benthic habitats [lowest level] to higher levels or from shallow water near coastlines into deeper water in the open ocean.

What are the challenges you face when observing these creatures?

Scientists cannot keep live specimens for long.

Haddock: For deep-sea studies, we use submarines—either manned or remotely operated vehicles (ROVs) that are controlled from the ship that allow you to observe with video. You can monitor the physical environment with sensors and then you can also sample organisms with a variety of samplers that we have come up with—from fairly sophisticated and expensive to fairly simple (like hardware store repurposing of devices for collecting). We do not do as much manipulation in situ because it is hard to set up an experiment on site, although in some cases, it is possible, but especially where I work, in the water column, you do not have a point of reference as you would have if you had an installation on the bottom. In the water column, we will go in, collect organisms, and then bring them back to the ship to study in the lab; but we cannot keep them very long in the lab (in a seawater room) in most cases. We will do as much as we can in the first hours or days that we have the organisms, and then we have to rely on the DNA and the RNA to tell us about their features, such as bioluminescence. Those researchers that work on respiration or physiology will look at the enzymes as a way to figure out what is going on.

Have new species been discovered with new technology?


The deep-sea worm, the green bomber (Swima bombiviridis) with bioluminescent “bombs” was selected as one of the “Top Ten Species” for 2009. Its luminescent “bombs” are attached just below the head. Photo: Steve Haddock.

Green bombers throw bioluminescent grenades for defense.

Haddock: We have found numerous new species—from all kinds of different taxa. Actually, there is a whole group of worms that arrived from the bottom (about 2600 meters deep) in their evolutionary history and started living higher in the water column—independent from other polychaetes [phylum Polychaeta consists of segmented worms], swimming around in the water column. This whole group has these little bioluminescent hand grenades attached to the sides of their bodies that they release into the water for protection. We call them the green-bomber worms (Swima bombiviridis). So the green bomber is a newly discovered species, but we also discovered its entire polycheate lineage.

Additionally, we found some new siphonophores, which are these elongate cnidarians [jellyfish are example cnidarians]. Some of the newer ones that we found have unique bioluminescent structures that they use to attract prey. There are many new or undescribed comb jellies, which are so fragile that even with ROVs, it is sometimes difficult to sample them. Some of the interesting ones of those do not fit into any of the known species’ families, so we had to adjust their classification or category. That’s one reason I call on students to consider a career in deep sea biology—there remains so much to discover, and the discoveries are fascinating.

Why did so many deep-sea organisms develop bioluminescence?

Bioluminescence is a light-emitting molecule.

Haddock: Bioluminescence is this ability to create light, to create visible light based on chemical reactions that are happening inside your cells, inside specialized tissue called photophores. Bioluminescence occurs primarily in salty water, with a few exceptions in fresh water [a bacterium] and on land [some fungi and insects]. In oceans, it is found in everything from bacteria through single-celled protozoans, jellyfish, and shrimp and all the way across the range of diversity, and we think in many ways that it has evolved independently in all these different groups. There is actually a little bit of advantage, some starter material that circulates in the food chain in the form of luciferins. The light-emitting molecule that a lot of these diverse organisms use is actually the same between many different groups, and so we think that one of those groups, or a couple of those groups are producing the molecule, and the other ones are just getting it from their diet.


Sepioteuthis lessoniana, one of many bioluminescent squid. Komodo National Park.

From an evolutionary perspective, the organisms have bioluminescence available in the food chain and since it exists, the animal puts it to some use. Organisms use other enzymes and develop the ability to use that bioluminescence for multiple functions. Squid for example, use it along their lower surface to erase their silhouette; it is like a cloaking device. However, they will also have bright photophores on their eyes or on their tentacles that are thought to be used for warning flashes or to stun prey. They use these bright flashes, but at the same time, they are also able to produce a steady glow that is more appropriate for camouflage.

What happens to organisms that don’t get luciferins in their diet?

The diet of some fish makes them luminescent.

Haddock: There is a fish called the midshipman fish [Porichthys notatus], for example, and in Southern California, or in the lower latitude waters, it feeds on a ostracods [Vargula spp.], which is a little crustacean. The populations of midshipman fish in that area that eat those crustaceans are able to make bioluminescence themselves because their food contains luciferins, and they are able to camouflage and hide their shadow.

Conversely, there are populations up near Washington state, and up in the Northern Pacific Ocean, where that bioluminescent prey item doesn’t occur, so those populations of midshipman fish are not bioluminescent. Yet, if you take these same fish to places where they can eat ostracods, they turn bioluminescent right away. Genetically the non-bioluminescent fish still have the genes responsible for bioluminescence, and even the photophores, but they do not have the fuel, or luciferins, to power that system.

These midshipman fish in the northern Pacific are presumably at a disadvantage. It is difficult to measure that effect in our scope of operation, but they do not have one of the weapons that others of that same species would have. A similar thing actually happens in jellyfish. We did some experiments on jellies at the Monterey Bay Aquarium, California, that were not getting the luciferins in their diet. As soon as we gave them food with luciferins, they became bioluminescent.

Do the organisms use bioluminescence for purposes other than defense?

Bioluminescent flashes attract mates.

Haddock: A few organisms do. It is one of those things that are not impossible but very difficult to observe. In the case of deep-sea fish, for example, they will have species-specific patterns of photophores and even sexually dimorphic patterns of photophores, and so the inference is that if they turn on their light, a potential mate could recognize the light. How do you observe that in the natural environments, though? There is more evidence for species-specific patterning than there is for it actually being used during mating.

There is one really well studied example of Caribbean ostracods. They are little—they are about the size of a mustard seed, a millimeter or a couple millimeters in diameter. The ostracods emerge from the reef at sunset. As they swim through the water, they send out little puffs of luminescence at timed intervals. Each species has a different pattern: some organisms swim up slowly and send out a puff every so often; some will swim horizontally or diagonally across the reef and send a puff; some will send a really fast train of bioluminescence; and some will swim down. Then the opposite sex, lurking in the reef, will see the right signal, and emerge from the reef to produce a small mating cloud. I got to see this on a cruise in 2009; we were doing some evening and dusk dives. We saw light flashes—blip, blip, blip, blip. They were very much like firefly signals. Species are very attuned to a pattern of light, and that is really the best example we have in the ocean.

There are some polychaetes that have lunar [tied to the moon’s phases] swarming displays that are used for reproduction. Polychaetes are annelid worms and include mostly marine worms such as the lugworm. They have fleshy, paired appendages tipped with bristles. But it is not as clear with these organisms as it is in the case of the ostracods that have a very specific sexual purpose for bioluminescence.

Why do you say that the fragile survive best in the deep sea?

Being fragile can be advantageous in the deep sea.

Haddock: I was alluding to the fact that fragile organisms have an advantage in the deep sea. Large, muscular organisms, such as big fish, generally cannot survive the extreme environment. Fragile creatures, such as jellies, do not invest their energy to become a big burly tuna, for example, or to swim at the surface. An organism can be this gelatinous, flaccid, really fragile form down in the deep sea and be a superb survivor, devoting its resources to other things besides having big, hard body parts. Fragile beings are not the only ones that will survive, but it is not necessarily a disadvantage to be fragile.

Steve Haddock is a research scientist at the Monterey Bay Aquarium Research Institute, and he is an adjunct associate professor in the Ocean Sciences Department at the University of California, Santa Cruz. His work focuses on bioluminescence, biodiversity, and ecology of deep-sea and open-ocean ctenophores, siphonophores, radiolarians, and medusae (also known as gelatinous zooplankton). In addition to assembling phylogenies for these groups, he is interested in cloning novel photoproteins and fluorescent proteins from them. He is involved in the site, which encourages citizen scientists to report on found species. Steve’s educational background includes a B.S. from Harvey Mudd College and a Ph.D. from the University of California, Santa Barbara. Haddock’s new book, with co-author Casey Dunn, Practical Computing for Biologists, was released by Sinauer Associates in November 2010.

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