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Biodiversity in the Deep Sea


Written By Dr. Steven Katona, Managing Director, Ocean Health Index

Dr. Bruce Robison may have the best job in the world! As Research Division Chair at the Monterey Bay Aquarium Research Institute (MBARI), he has spent much of his career exploring deep water aboard more than a dozen submersible vehicles as well as developing remotely operated vehicles (ROVs) that can photograph and collect animals at great depth. Most of them have never been seen alive by anyone else on Earth and many are entirely new to science. For thousands of years people believed that nothing at all could live in such dark, cold, unforgiving conditions. They were wrong. Just like astrophysicists look outward seeking to find and understand the mysterious 'dark energy' and ‘dark matter’ that make up most of the universe, Robison and his MBARI colleagues are discovering our planet's 'dark biodiversity,' the huge quantity and variety of animals that would forever remain invisible without their pioneering quest.  You wouldn’t want to miss the Vampire Squid from Hell (Vampyroteuthis infernalis) so take this dive into the deep. 

Biodiversity in the Deep Sea

I’m sitting in a comfy chair watching a high-definition video monitor, but instead of the Simpsons I’m watching a squid wrestle with a deep-sea owlfish that it’s trying to eat. And instead of my living room, I’m aboard the research vessel Western Flyer, using a robotic sub to explore a submarine canyon deep in Monterey Bay. Sitting to my left is Knute, the pilot who flies the remotely operated vehicle (ROV) that we are using on today’s mission. To my right is Susan, who controls the video logging and data management systems. Deep below us the ROV soars through the inky blackness of a world that few people ever see.

Aulococtena is a ctenophore or comb jelly. This is an undescribed species from deep in the Monterey Canyon. 

Credit: © 2002 MBARI

The mystery mollusc has no name yet and is in the process of being described by MBARI scientists. It is a new species, a new genus, and a new family of molluscs that lives 2,000 meters deep in the Monterey Canyon.

Credit: © 2005 MBARI

Let's jump right in. The largest living space on Earth is the vast volume of water between the sea surface and the deep seafloor. Not surprisingly, this enormous global habitat is home to the largest ecosystems on our planet. 

Colobonema is a pretty little medusa that drops its tentacles, like a lizard dropping its tail, when threatened by a predator. 

Credit: © 2000 MBARI

Living conditions in the deep water column are very different from what we humans are accustomed to. It is uniformly dark except for a narrow, daytime fringe of light near the surface. Also, the deeper you go the colder it gets and the greater the pressure from the weight of the water above.

Crossota is a medusa that feeds in sediments on the seafloor, then it swims up into the water column to digest its meal and to relocate by riding currents. 

Credit: © 2002 MBARI
Description: Crossota

But the biggest difference between that world and ours may be that while gravity ties us to surfaces, midwater animals move freely in all three dimensions.  Adapting to this unique habitat has led to some wonderfully strange animals that, despite their obscurity to us, are among the most abundant species on Earth.

Anoplogaster is the scientific name of this fangtooth fish. Some of its teeth are so long that they don't fit within its mouth. 

Credit: © 2008 MBARI

The way that we study deep pelagic animals changed drastically when we began using manned and robotically-operated undersea vehicles instead of nets. Likewise, our understanding of deep-sea ecology was radically improved by that technological transition.

We've never seen anything like these animals, but they have never seen anything like our ROV and especially its bright lights. The sun never shines down here. It is pitch black save for a miniscule trickle of photons down to about 1,000 meters and sprinkling of small flashes made by jellies, squids, fish and other animals to find mates, attract prey, startle predators---or for purposes still unknown. So despite the perpetual darkness, eyes---including some very strange ones indeed--- can still be important.

Macropinna is also known as a barreleye.  The green spheres within its transparent head are the lenses of its tubular eyes. The upward looking eyes silhouette the prey against dim down-welling surface light.

Credit: © 2004 MBARI

One of the first things we learned is that deep pelagic communities are far richer and more complex that we had ever imagined. In particular, gelatinous animals often dominate the scene. Jellies of all shapes and sizes occur throughout the deep water column. Among them are colonial animals called siphonophores, some of which reach lengths of over 40 meters. Other siphs are no bigger than your little finger but all of them are predators and all of them are comprised of specialized individuals who play specific roles in the life of the colony, just like in ant or bee colonies. 

Apolemia is a siphonophore, a long chain of specialized individuals, and a paradox because it behaves like a single organism. Some siphonophores reach lengths of 40 meters, this one is about 3 meters long.

Credit: © 2005 MBARI

Physophora is a small siphonophore, about 6 centimeters long, that feeds on small crustaceans that it traps with stinging cells in its tentacles.

Credit: © 2005 MBARI

Another important discovery was that deep pelagic communities are quite variable, and do not exhibit the unchanging monotony of sameness that had always been supposed. Indeed, their species composition and relative abundance patterns (both are measures of biodiversity) frequently change; sometimes on a seasonal basis, sometimes for reasons we cannot yet figure out. For example, the Humboldt squid had been only an occasional visitor to Monterey Bay until 2002 when it moved in with a vengeance, clobbering the populations of commercially valuable hake and market squid. They stuck around in abundance for about 8 years, and then mysteriously, they all but disappeared.

Dosidicus is the Humboldt squid that became an invader along the California coast. Changing conditions like warmer water and reduced oxygen, along with overfishing of the tuna that provided competition and predation, are believed to be the causes of its range expansion. 

Credit: © 2003 MBARI

Monterey Bay, where my team works, is probably the best-studied patch of deep ocean in the world. Yet even with sustained exploration we continue to find new life forms, new ecological relationships, and new patterns of change. Given the diversity and complexity of the deep pelagic system we study locally, it is staggering to contemplate what remains to be discovered out there in the rest of the World Ocean. Surely the deep pelagic is the mother lode of Earth’s animal biodiversity. There can be no question about its critical importance to the fundamental structure and functioning of the planet’s biosphere.

Vampyroteuthis is called the vampire squid because that's how it looked to the first person to see one. They aren't vampires and they're not really squid. Instead they are "living fossils," modern-day representatives little changed from their ancestors who lived during the age of the dinosaurs. 

Credit: © 2005 MBARI

One of the principal challenges for protecting deep-sea biodiversity is constructing a baseline; because you need a starting place in order to measure changes and to predict consequences. Building an ocean-scale baseline for the deep pelagic will take generations of scientific effort, but if we can use our work in Monterey Bay to provide a “reference community”, maybe we can apply what we learn here to the deep ocean in general – at least it’s a start. Then we can design marine protected areas and conservation programs that include the whole ecosystem, not just the top and bottom.

Meanwhile, we’ll keep exploring and shining a light into the deep. 

Visit the MBARI YouTube channel to see videos of Vampyroteuthis, Macropinna  and other denizens of the deep; and follow our expeditions on the MBARI website. Maybe they will inspire some of you toward careers in deep-sea biology, as there are lifetimes of exploration ahead!