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.

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.

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.

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.