Marine biologist Craig McClain is assistant director of science at the National Evolutionary Synthesis Center (NESCent). He studies the relative sizes of different organisms, and at 6 foot 2 has a hard time fitting into most submarines.

Q - How many species live in the deep sea?

People used to think the deep sea was a lifeless void. Edward Forbes in the late 1800’s proposed this ‘azoic hypothesis’ for the deep sea, based on samples from the Aegean Sea. But now we know that area has relatively low densities of organisms compared to other deep-sea areas. Later surveys found odd  deep-sea species attached to sounding lines and dredges, but abyssal life was still thought to be rare.Then a 1968 survey off New England demonstrated that the deep sea contained more species in the same amount of area than the coast. Indeed, the number was so high it compared to the diversity found in tropical ocean systems. More effective sampling gear developed since then has indicated that deep sea diversity is probably even higher than the 1968 study estimated, even rivaling that of tropical rain forests! If you were to count all the species in a single square meter of mud in the deep sea you would likely find more than 300. A mid-1990s study proposed that there may be more than 10 million species in the deep sea. Of course, deep-sea scientists still quibble about the number and we may never know what the actual number is. But it is safe to say that the number is very high.

Q - How do deep-sea organisms deal with high pressure, cold temperatures, and no light?

They adapt, often in spectacular fashion. We routinely find life thriving at 1100 times the atmospheric pressure you feel at sea level and at temperatures as low as -1 Celsius, below freezing.Deep sea creatures have developed a different chemical structure for the fats that comprise the cell membrane so that the membranes remain permeable even under these tremendous pressures. We also find their proteins contain more hydrogen and disulfide bonds that increase rigidity and minimize changes in shape due to pressure. The deep sea also has anoxic/hypoxic regions (i.e. zero or little oxygen) that require alterations to proteins to increase oxygen binding efficiency and transport.Extreme temperature gradients experienced at hydrothermal vents present unique challenges. For example, hydrothermal vent worms such as Alvinella can experience 22 degrees C near the gills and 60+ degrees C near the body trunk. Hydrothermal vents and methane seeps also place animals in close proximity to potentially toxic levels of chemicals. The most abundant of these is sulfide that many vent/seep organisms require for their symbionts, affiliated organisms that help them survive.In the absence of ambient light, many organisms also make their own light, called bioluminescence, to attract prey, fend off predators, or give a ‘come hither’ look to a potential mate.

Q - Why haven't deep-sea animals changed very much, when everything else seems like it has?

There are some "living fossils" around, to be sure, but much of the deep-sea fauna appears not to be ancient forms. Our current understanding of the deep sea is that much of its fauna died out in the mid-Cenozoic Era and was replaced by shallow-water immigrants.During the period of 30-40 million years ago, bottom temperatures throughout much of the deep sea decreased by up to 10 C at the same time as an ocean-wide event of really low oxygen. This mass extinction was followed by a colonization of species coming from shallow water. The best guesses are that the shallow-water immigrants came from the polar regions, the Mediterranean Ocean, and other regions where the water column is relatively the same temperature or has multiple shallow-water areas.Deep-sea snails for example, are proposed to have only recently (30 million years) immigrated to the abyss from multiple coastal centers. Some deep-sea organisms do appear to be ancient, but the evidence is for a migration of many organisms going deeper. This suggests that many new forms have developed in a relatively short geologic timespan.

Q-  Looking at your publications, you seem to be obsessed with size. What can we infer from that?

In the biological world, size is more than a novelty.  How an organism relates to the world around it is determined by its size, and understanding what influences size is understanding the diversity of life itself. (Or perhaps I am just trying to figure out why I am freakishly large myself.)On islands and in the oceans, we've found that large organisms evolve toward smaller sizes and small organisms toward larger sizes, both heading toward a medium size. This pattern occurs with such frequency on islands that it’s often referred to as the “island rule.” As shallow-water species colonized the deep, small species evolved larger and large species evolved smaller, both heading toward a size that is just right. It's interesting that the Earth's largest environment, the oceans, and its smallest, the islands, operate under similar processes. But the fact that the two environments share so little in common was a bonus that has allowed us to refine and eliminate hypotheses.Despite all its area, what the deep sea lacks is food. The lack of sunlight precludes plants, meaning that for the majority of organisms, the food chain starts with plankton, dead organisms, and other organic debris that falls from the ocean surface above. But less than 5% of the total food available at the surface reaches the deep, leading to an extremely food-limited environment. On islands, less food occurs because the small island area supports fewer plants at the base of the food chain. In both cases, animals need to be efficient and creative about acquiring food. There is not enough food in the deep ocean to support a whole population of giants. On the other hand, tiny animals cannot travel long distances looking for food and do not have the body volume to store surplus food when it does become available. Thus, conditions in the deep sea favor a medium-sized organism, causing some species to evolve toward giantism and others toward miniaturization to reach this optimum size.Bathynomus giganteus, a deep sea pill-bug the size of a large shoe, is an extreme example of gigantism among crustaceans, making it something like the Komodo Dragon among island lizards. These giant isopods have developed larger in response to food limitation. They are scavengers, utilizing a variety of food sources. When you put a dead fish on the seafloor, swarms of them attack in a few hours. Their great size gives them the ability to  traverse distances quicker than smaller relatives, getting them to the spoils first. Then their large size allows them to store a great quantity of fat, giving them the potential to survive up to two months without food, in an aquarium at least.Often a single observation or event stimulates a question you spend the rest of your life trying to answer, like when I encountered Bathynomus during my first submarine dive as a graduate student. It is for these questions that I became a scientist. Perhaps, my next question is: why is the giant squid giant?  Or perhaps more interesting: why isn't the giant squid larger?

Q - Okay then, why aren't giant squids and blue whales bigger?

Typically people ask why giant squids and blue whales are so big, but by far the more interesting question is why aren’t they bigger?  No one knows for sure.  Perhaps given another few million years of evolution, super-sized whales and squids will roam the oceans terrorizing smaller animals.On the other hand, maybe they are not bigger because they don't need to be. Their size is adequate to fend off predators or ensure they don’t have any. They can be large enough to eat whatever they want, or exploit some novel way of interacting with the surrounding ocean.Or maybe whales and squids cannot evolve larger sizes because of physical constraints to their maximum size. For example, maybe blue whales cannot find enough krill to eat to sustain anything beyond their present size. Consider also that a blue whale’s heart is already the size of a Volkswagen Beetle. That heart must circulate blood through more than 80 feet of whale and get oxygen to every cell in a 100-ton animal! With every additional branch of an artery, blood vessel or capillary comes resistance, so the heart has to work harder to pump. Engineers designing municipal water supplies have decided that the efficient way to move water from the reservoir to the farthest outlying homes isn't to add a larger and larger central pump. Rather, they add satellite pumping stations along the way. Maybe a larger whale would require more than one heart!

Next Month:

Charmaine Royal of the Institute for Genome Sciences & Policy tackles tough questions about genetic testing to determine one's ancestry or racial identity. Post your questions via email.