by Shelby Wyzykowski
For Charles Darwin, all sorts of species—from birds and large land animals to flowers and tiny invertebrates—captured his interest and encouraged him to explore the great diversity of life. After years of observation and research, he published his famous book On the Origin of Species in 1859. In it, he presented his revolutionary and controversial theory of natural selection, which is also commonly referred to as “survival of the fittest.” His theory suggested that individuals of a species are more likely to survive when they inherit traits from their parents that are best suited for their specific environment. Essentially, beneficial adaptations give an organism the greatest chance to live and carry on its genetic line. This well-known theory is in part rooted in Darwin’s early experiences with and on the ocean. In 1831, he embarked on a five-year journey on the HMS Beagle, serving as their on-board naturalist. As the crew surveyed and mapped the South American coastline, Darwin marveled at the wonder and beauty of the sea, observing and collecting surface plankton as well as theorizing how coral reefs form. Unfortunately, with no photography and limited technology, studying ocean life was difficult even in shallow water. So, in Darwin’s time, little if anything was known about life far beneath the waves. But if he were alive now, Darwin would no doubt delight in all of the incredible underwater discoveries that have been made by modern-day science. And he would more than likely be awestruck by the many amazing adaptations that sea animals employ to survive.
Aquatic Adaptations: Antarctica
When one thinks of an environment in which adaptation is of the utmost necessity, Antarctica may be the first spot that comes to mind. The Southern Ocean, which encircles Antarctica, is an unforgiving and inhospitable place to live. Rotating currents almost completely isolate these waters from the rest of the Earth’s much warmer seas. This keeps temperatures low…it can drop to 28.6 degrees Fahrenheit in the winter! To combat the cold, Antarctic icefish produce and carry special antifreeze proteins in their blood and body fluids. These proteins bind to ice crystals, dividing their crystalline structures and therefore inhibiting crystal growth. Without this antifreeze, microscopic ice crystals would form in their bodies, severing nerves and damaging tissues to a deadly degree. It’s an incredible adaptation, but it did not happen quickly. About 25 million years ago, the Southern Ocean, flowing around the isolated Antarctic continent, began to cool. Aquatic life in this area had to evolve the special antifreeze proteins, find some other way to adapt to the cold, or go extinct. Today, thanks to their special cold-water adaptation, icefish make up more than 90 percent of all fish species in the Antarctic!
Aquatic Adaptations: Mariana Trench
But Antarctica is not the only harsh environment that demands extreme adaptations. You’d be hard-pressed to find living conditions that are more punishing and severe than in the Mariana Trench. Located in the western Pacific, it is considered to be the deepest part of the ocean anywhere on Earth. Near the trench’s bottom, the lunar-like landscape is pitch-black, and the pressure of the freezing cold waters would instantly kill any land animal. But, amazingly, sea animals have found remarkable ways to thrive.
In most places in the trench, the temperatures are between 34 and 39 degrees Fahrenheit. This extreme cold would not be good for most animals’ bodies because it would damage their cell membranes. These membranes are of a fatty consistency and must stay liquid to function properly. The Mariana Trench’s frigid temperatures would make the fat in a land creature’s cell membranes solid like butter. But deep-sea animals have evolved in a unique way that enables them to avoid such a chilly catastrophe. They have lots of unsaturated fats in their membranes, and these kinds of fats remain liquid at low temperatures and keep their membranes loose and intact.
Besides the bone-chilling temperatures, these aquatic creatures must contend with the pulverizing pressure. Extreme pressure can have a devastating effect on a body’s proteins (these are the molecules that do much of the work in a cell). To keep their proteins healthy and working well, sea life collect tiny organic molecules called piezolytes in their cells. These piezolytes prevent water from distorting and damaging the proteins. The deeper in the ocean an animal lives, the more piezolytes they need to have in their cells. One type of piezolyte, called TMAO (Trimethlyamine-oxide), gives fish their “fishy” taste and smell. Since TMAO increases with depth, being “fishier” is crucial for survival in the deep-ocean environment!
But food is also crucial for the survival of any organism; how is it possible to hunt in a world of darkness? Sea life have found many ways to deal with the lack of light. The stout blacksmelt, for example, has giant eyes that can capture the faintest glimmer of fleeting prey. The tripod fish has such unreliable vision that it mainly relies on sensors in its pectoral fins to detect the movement of a potential meal. And the anglerfish actually emits its own light by a process known as bioluminescence. The light from their built-in “headlight” will actually attract the prey to them!
Aquatic Adaptations Near the Ocean’s Surface
Marine life that live a bit closer to the ocean’s surface have also developed ingenious ways to search for food. The Great White Shark could very well be thought of as the bloodhound of the sea. Its sense of smell is so good that it can detect one drop of blood in ten billion drops of water! But, if the prey is close enough, it need not spill one drop of blood for the Great White to detect its presence. This is because these sharks are experts in electroreception, which is the ability to detect weak electric fields in water. Unlike in air, the ability to conduct electricity in water is extremely easy. This scientific fact allows many underwater species, including Great Whites, to sense the weak electrical fields of biological sources (such as their prey). These sharks are known to react to charges of one millionth of a volt (for reference, a tiny AA battery has a mere 1.5 volts of stored energy). This acute sensitivity to electrical fields can be traced to electroreceptors in the shark’s skin. Pore openings peppered over its head receive minute electrical signals from the water and channel these signals into tubes of highly-conductive gel. Each tube ends in a bulb known as an ampulla of Lorenzini. Sensory nerves are activated in the ampulla and send the message to the shark’s brain. Their electrosensitivity is so precise that they can detect prey hiding in the sand bottom!
With such an extraordinary adaptation, Great Whites can be a formidable and terrifying predator. But sometimes even the hunter can become the hunted. If a Great White is foolish enough to go after a sick or young Bottlenose Dolphin, they might find themselves biting off more than they can chew. Living in groups called pods, dolphins have tightly-knit family groups with complex social structures. They actually have their own cultures and display positive cultural behaviors such as compassion and cooperation. So when one member of a pod is targeted as prey, the others will come to its defense and work in a coordinated effort to combat the Great White. They’ll surround the shark and attack it relentlessly. Some use their sturdy, bony snouts like battering rams and slam into the shark’s underbelly and gills, causing massive internal injuries. If the shark is lucky enough, it can make a quick escape, but pods have been known to actually kill sharks. These incidents involving selflessness and cooperation have also crossed the species barrier from time to time when pods of altruistic dolphins have come to the rescue of humans in distress. There have been many reported cases of dolphins encircling and protecting swimmers as they work to successfully fend off a shark’s persistent advances.
The altruistic and cooperative behaviors of dolphins are adaptations that exemplify the true meaning of Charles Darwin’s theory of natural selection. Believing that compassion was the key to evolutionary success, Darwin was in fact frustrated with the way many readers misinterpreted the phrase “survival of the fittest” (a term that he himself did not even coin…biologist Herbert Spencer did so in 1864). This phrase implies the use of selfishness, ruthlessness, and callousness to ensure survival. There’s certainly no denying that these actions have definitely played a part in evolution and in the realities of life. But Darwin chose to believe that sympathy, benevolence, and cooperation played even greater roles in the survival, flourishing, and evolution of a species. In the end, it’s the positive adaptive traits that determine as well as define the overall success of life on Earth.
Shelby Wyzykowski is a Gallery Experience Presenter in CMNH’s Life Long Learning Department. Museum staff, volunteers, and interns are encouraged to blog about their unique experiences and knowledge gained from working at the museum.
Carnegie Museum of Natural History Blog Citation InformationBlog author: Wyzykowski, Shelby
Publication date: August 12, 2021