Life in the World's Oceans

So much of what we take for granted about our world (from our body's access to and use of nutrients, to our planet's liquid oceans, to the ice floating in your glass of soda) is a direct cause of the structure and polarity of H2O. Learn how those specific properties make water the essential ingredient for life as we know it.

No matter where you live, your climate, weather, and even available foods are determined to a great extent by ocean circulation. The uneven heating of the Earth by the Sun and the Coriolis effect result in vast circulation cells of air above the Earth, the movement of huge water masses in the oceans, and resultant "hot spots" of marine life.

How and where did life begin on Earth? The existence of both photosynthetic and chemosynthetic food chains points to the possibility that life could have originated through two different paths. While many questions remain unanswered, two things seem certain: Life began in the oceans, and bacteria are the most successful organisms on the planet.

Beach organisms exist with the constantly changing winds, waves, and tides (sometimes underwater, sometimes fully exposed to the air). Life in estuaries, where rivers meet the oceans, face constant fluctuations in environmental salinity. And hard corals are continually pummeled by wave action. Yet each of these physically challenging environments can be diverse and fecund ecosystems.

Phytoplankton form the base of almost all marine food chains, including that of the blue whale, the largest animal known to have ever existed. But far below the penetration of sunlight, a very different and only recently discovered food web relies solely on the chemosynthetic ability of bacteria.

When we think of healthy marine ecosystems, we should be thinking about phytoplankton. In many ways, we owe our existence to these diatoms, dinoflagellates, green algae, cyanobacteria, and others. Not only do scientists believe they are the ancestors of terrestrial plants, but phytoplankton continues to produce about half of all the oxygen available in our atmosphere today.

The vast majority of animals on our planet are the gloriously diverse invertebrates. From microscopic organisms to the crab with a three-meter leg span, marine invertebrates exhibit enormous variety in form and function. They include sessile and mobile organisms, free-living and parasitic. They live at the surface and within the ocean floor sediments.

Mammals are certainly represented in ocean life, but which species should be identified as "marine" when considering ocean productivity? The extremely complex marine food webs maintain long-term stability, even as they undergo natural perturbations over time. But when Homo sapiens enters as an apex predator, productivity can deteriorate, and systems can even collapse.

Through 550 million years of evolution, fish have developed a wide variety of adaptations to the unique demands of living in a watery and mostly dark world. Learn how gills, swim bladders, bioluminescence, chemosensory glands, echolocation, and electrolocation have allowed fish to succeed in almost every type of ocean environment. Which fish are our ancestors? You might be surprised.

While humans have been fishing for hundreds of centuries, we have only recently had a significant impact on marine food webs. Industrialization has led to problems with by-catch and overexploitation of resources. Today, we are creating trophic cascades with long-term impacts we do not yet understand.

Fish certainly have good reason to fear these top-of-their-game predators, with their multiple rows of teeth and ability to detect electrical current better than any other animal. But while four species have been known to assault humans with no provocation, almost 99 percent of the many hundred shark species would rather swim away from us than attack.

While the reptilian evolution of the amniotic egg allowed animals to move completely from the sea onto land, some reptiles retained strong marine ties. These include sea turtles and sea birds whose wide variety of adaptations allow for drinking saltwater, remaining underwater for long periods, and flying great distances using very little energy.

Marine mammals did not evolve from marine species. Rather, they evolved from land mammals who found a plethora of "suddenly" open ecological niches when the dinosaurs became extinct. Today's marine mammals might resemble each other because convergent evolution has led to similar adaptation. But best as scientists can tell, they have five separate lineages and no single common ancestor.

Through tens of millions of years, evolution has resulted in a fascinating array of marine mammal adaptations. With the ability to process thousands of gallons of water each day or dive to a depth of almost three kilometers, and with numerous methods of locomotion or extraordinary social behaviors, these whales, porpoises, phocids, and more can thrive in varied environments around the globe.

If you've ever jumped into frigid water, you quickly realize humans are definitely not adapted to life in the sea. What are we missing? In a word, it's blubber. In fact, blubber is such a successful insulator that marine mammals have evolved internal and external means for getting rid of all that heat, possibly even including planetary migrations.

For all practical purposes, terrestrial mammals live on a plane. Marine mammals, on the other hand, navigate a more viscous, three-dimensional environment with all its opportunities and challenges. We understand their propulsion mechanisms fairly well. But how do they control their buoyancy to position themselves in the water column? We don't yet have the answers.

Not surprisingly, deep-diving marine mammals have evolved a physiology very different than our own. Adaptations including those related to blood chemistry, the location of stored oxygen, a variable heart rate, and articulated rib cages support the ability to go deep and stay long. But what about rising back up to the surface? How do they avoid getting "the bends"?

Sound travels much better in water than in air. In fact, low-frequency waves, such as those produced by certain whales, can travel through water uninterrupted for hundreds or even thousands of kilometers, allowing the animals to be "in touch" with their group over vast distances. But what happens when human-generated sound gets in the way?

Two things are clear: Almost all marine food webs are based on microscopic photosynthesizers, and only a small fraction of the energy available at any trophic level becomes available to the next level. Adaptations such as baleen, ventral pleats, and unique tooth morphology allows these large animals to meet their energy needs.

With plastic and nylon lines and nets becoming common in the last century, by-catch became an even greater problem for the marine mammals. When the media picked up the story in the mid-1960s, the public became engaged, and the Marine Mammal Protection Act was passed in 1972. But whale entanglement remains a problem, and some argue that even whaling was far less cruel.

Semi-aquatic marine mammals exhibit behaviors quite different than those who live fully in the water. In the former, an entire female community in one geographic area can come into estrus simultaneously and needs relatively few males to reproduce. In the latter, reproduction appears to be one of the driving forces of whale songs that can be heard over thousands of kilometers.

From individual whales that corral their confused prey to highly coordinated bubble-net feeding and aunts who "babysit," marine mammals have developed an extraordinary variety of social and hunting behaviors developed over millions of years. If the energy expenditure does not support the goal of passing on genetic material, natural selection will eventually drop the adaptation.

With 60 million years of evolution on their side, marine mammals have adapted to the widest possible variety of marine ecological niches. Some live only in rivers or lakes, others only in waters over the continental shelves, and some in the open ocean. A few are even adapted to live at the poles.

Within their own species, marine mammals have developed sophisticated communication. In captivity, we know they can be trained to learn rules, which indicates higher cognitive function. And even in the wild, we have documented some extraordinary instances of learning and cultural transmission of information. But is their intelligence comparable to our own?

Are marine mammals to be exploited as a resource? Or are they intelligent creatures to be revered with an almost religious admiration? Your answer might depend to some extent on your country and culture of origin. Our relationship with these impressive animals continues to evolve as we increase our understanding of their biology, cognition, and sociality.