If tidepools are living in the sea’s Manhattan, and bays are the ocean’s suburbs, what’s the open sea? Well, it’s a lot like rural living. Wide-open space abounds, real estate is cheap, but finding a job can be tough. Like a farmer trying to raise a cash crop, you’re at the mercy of things you can’t control. And like small towns, the open sea has a tight-knit community all its own.
The Great Wide Open
The open sea is just what it sounds like. Big swaths of ocean. Real estate is cheap and there’s lots of it. But there’s more than meets the eye. Just like every field may look the same to a cross-country driver crossing the Great Plains, it doesn’t mean they are all the same beneath the surface.
Currents Are Everything
If the oceans are like the Great Plains, the currents are the soil. Just as the ability the soil to bear crops aren’t readily apparent to the driver on the interstate, the ocean’s richness often isn’t apparent from the surface. In tidepools, coral reefs, and estuaries, the source of nutrients is obvious: coral, nutrients washed down rivers, and the intersection of land and sea. In the great wide open, it’s the invisible action of ocean currents. The good living is at the edges of the continental shelves, the Gulf Stream, the Antarctic Convergence where polar and temperate water meet, and the eastern Pacific’s upwelling of nutrient-rich water deep sea water, and similar places. The rest of the ocean is much less inviting.
A View to a Krill
Like a farmer whose food comes from out the back door, the food chain in the open ocean is very short. Plankton gathers the sun’s energy, and is eaten by krill, a tiny crustacean. Blue whales, the largest creature in Earth’s history, eat krill, going from one of the smallest to the largest creatures on Earth in only three steps. Fish, penguins, and other critters chow down on krill as well. Because they migrate from the sea surface at night to depths during the day, krill provide food for both the surface and subsurface food chains. Krill is to the sea what corn is to Iowa.
At the Mercy of the Weather
The ocean isn’t a stable place. Like farmers can lose a whole crop to an early freeze, ocean currents can shift. Changes in land weather that stop the northwest wind on the west coast of North America can break the chain of events that leads to coastal upwelling, pulling the floor out of the eastern Pacific’s food chain an leading to the starvation of fish and seabirds. El Nino and La Nina change ocean currents. Warm weather created an oceanic “blob” of low-oxygen waters in 2015. Oceanographers also worry about the effects of meltwater from Greenland’s glaciers on the Gulf Stream as climate warms. It’s an uncertain world.
Tough To Make a Living
Even when conditions are good, making a living off the land is hard work. The same is true in the open sea. There’s lots of space, and your food could be anywhere in that wide open space, which means that you’ll have to range far and wide to find it. If you’re a fish roaming the seas looking for snacks, that burns a lot of energy, which means you have to find more food, which means you have to look even more. The margin of error is wafer-thin—a detour a few miles in the wrong direction could put you in trouble.
Drifting From Town To Town
Since travel is costly and food is scarce, drifting has its advantages. You’ll have no say in where you go—but since you may not know where food is anyway, it’s a reasonable strategy. Jellyfish are most famous for simply drifting with the current and the wind (one is even called a By-The Wind Sailor) hoping to bump into something to eat. This saves the energy of active locomotion.
But if you’re not going to drift, you better be fast. You’ll have lots of miles to cover and you’ll want to be fast enough to beat all the competitors to the food when you do see it. There’s no benefit to being an ambush predator when there’s nothing to hide under or behind. It’s a speed game, as a look at the shapes of fish like barracuda and tuna will tell you.
The Ocean’s Rumor Mill
Like a small town where news spreads quickly, word of mouth spreads very quickly through the open sea. It spreads through sound or electric impulses, which can travel a long way underwater. That’s why the top-of-the-food chain predators in the open sea—sharks, porpoises, dolphins and other toothed whales—pack some kind of echolocation or electrical field sensing.
Good Schools for Your Own Good
If you are a prey fish, nobody can ambush you. But the flip side is that if a predator does find you, there’s nowhere to hide either. You have to be either faster or you need safety in numbers. Schooling provides two forms of protection. One is a lot of pairs of eyes to continually scan for the approach of a predator. The other is simply to reduce the odds of you being the one who gets eaten. Schooling provides other benefits as well: more eyes to spot potential food and a social group that puts potential mates close at hand. Swimming in schools may also provide some hydrodynamic efficiency over long distances for small fish. Like the close social bonds that keep rural towns together, the school’s rules enforce conformity. And maybe it’s a way for fish to feel not so alone in the ocean’s wide-open spaces.