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So! Continuing our discussions of environmental adaptation, today I want to focus on a species of fish, called the Notothenioids. A bit of a mouthful, I know.
Scientists have documented over 90 species of Notothenioids, in both deep and shallow waters. If you’re getting creeped out by the powerpoint slide, don’t worry, they really only live around Antarctica. Most of them are pretty small. However, a few species can weigh up to 150 kilograms.
These fish can be identified by their huge eyes, insulated with a layer of thick transparent protective tissue. This kind of tissue protects the fluid in their eyeballs from freezing in the incredibly cold saltwater. Remember, saltwater has a lower freezing point than freshwater, so moisture in an animal’s tissue would be particularly vulnerable to freezing and causing cell death.
So while the cold ocean of Antarctica could freeze and kill most fish, the Notothenioids end up thriving in these icy waters. They actually make up nearly 95% of all fish species in the southern ocean around that cold continent. That kind of thing is a stark contrast to tropical oceans, where the biodiversity is extremely high. When you think of a tropical reef, for instance, you probably imagine a plethora of different types of fish and sea creatures living together and preying on each other. The average coral reef supports over four thousand types of fish, sponges, crustaceans, and others. To have one species of fish in an entire section of ocean? Pretty impressive. Yes! Amy?
How, er, when did the Notothenioids end up taking over the southern ocean?
I’m glad you asked. That was actually my next point of interest. So, about thirty million years ago, the water in that area was a lot warmer than it is today. Way back then, South America and Antarctica were actually connected. The air from around the equator could travel south to heat up the chilly Antarctic waters. The warm tropical waters could also flow southwards, bringing the rich biodiversity along with it.
Because Antarctica’s waters were relatively warm back then, it could support a lot of different types of animals. We confirmed this by looking at the fossil record, even finding out that 90 or so of the Notothenioids even existed back then.
Historians think that somewhere between five and fourteen million years ago, two huge changes occurred. First of all, a chance mutation allowed the Notothenioids to develop a special protein that now flows through their body. Umm this special protein is actually a type of antifreeze. It works by bonding to an ice crystal that formed inside their flesh, preventing it from growing any larger.
So! At the time, the waters they swam in were still decently warm, and the protein didn’t do much for their overall survival rates. Still, somewhere around this time period, there are records of a colossal shift that moved around the Earth’s tectonic plates. This movement is now known as Continental drift. It pushed Antarctica away from ..umm.. South America and down toward the chilly southern end of the earth. This caused a current to swirl up and form, encircling the cooling continent with a rush of cold water. This cold water prevented warmer tides from intermingling. It eventually dropped to the sub-zero icy landscape we know today
As you may have guessed, the tropical fish didn’t do so well in the sudden drop of environmental temperatures, and many species went extinct. Luckily for the Notothenioids, they had ahandy dandy gene mutation that let them produce this antifreeze protein. Now the only type of fish that could survive the cold waters, it had virtually no competition for food or resources and went wild.
So! As its population increased dramatically, they migrated into different parts of oceans. Over time they developed into different physical adaptations to survive in their particular environment. Eventually they split off into subspecies, mutated a bit more, had a bit of geologic morphology going on, very cool stuff. We call this kind of physical differentiation a process of Adaptive Radiation. It really only happens when a species rapidly changes, and ends up with quite a few new species to fill empty niches that either didn’t exist before, or weren’t available.
So now we have about 90 species of Notothenioids, kicking it in the southern ocean.