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(swooshing) - [Narrator] Dive beneath the ocean waves and you'll find a bright, bustling city. Full of diverse residents, swimming, eating, and sharing homes together. At the California Academy of Sciences, our mission is to explore, explain, and sustain coral reefs, like this one in the Philippines. Each of the thousands of species that live here have their own unique role and place within the ecosystem. But it's not all just predator and prey, the relationship between these species are as diverse as the ecosystem itself. In this video, we'll investigate the following key questions. How do species interact on a coral reef? What are the effects of abiotic and biotic factors? And how do these interactions influence patterns of distribution and abundance within the ecosystem as a whole. Take for instance this bubble-tip anemone. Notice anything strange? Instead of avoiding its stinging tentacles, a fish seems to be relaxing in them. These two reef residents have formed what is known as a mutualistic relationship. In exchange for defending the anemone from predators like butterflyfish, the maroon clownfish receives shelter and a safe nest site, both benefit from their interaction. In fact, while the bubble-tip can live with up to 14 species of clownfish, this partnership is so important to the maroon clownfish that it can't survive without a bubble-tip anemone host. Let's take a closer look. Imagine you're a biologist exploring the relationship between bubble-tip anemones and maroon clownfish in the Philippines. You visit the same coral reef every year and sample only bubble-tips that host maroon clownfish. Look over your field notes now. What trends do you see? How are the populations of these two coral reef residents connected? Take a moment to pause the video and examine the table. Let's hear from academy scientist, Luiz Rocha. - When diving around coral reefs, we always find maroon clownfish living in pairs with their bubble-tip anemone host. You can see this 2:1 population ration in your field notes. Understanding this relationship allows us to make predictions about the population of maroon clownfish based on what we know about the population of bubble-tip anemones. - [Narrator] Now imagine you travel back to the same reef to continue your research. What trends do you notice over the following two years? (mild playful music) Given what you know about this mutualistic relationship, can you predict the effect this drop on bubble-tip anemones will have on maroon clownfish populations? Pause the video and make your prediction. - [Luiz] In this example, extreme temperatures led to decrease in anemones. And given what we learned about the population ratio, we can predict a similar drop in maroon clownfishes. It's important to keep in mind that this just a theoretical example. Natural coral reef systems are very complex and involve a wide variety of factors that need to be taken into account. - [Narrator] Exploring further, we find another example of cooperation among reef residents. This whitetip reef shark sits peacefully while a bluestreak wrasse swims around its head and mouth. Why so calm? The whitetip has come for a cleaning and the wrasse, which feeds on dead skin, is happy to provide it. Relationships like this one are so beneficial, the positive impacts spread across the entire coral reef system. Cleaner fish attract a wide diversity of fish to their cleaning stations. And as a result of their cleaning care, improve the overall health of the reef. Not all interactions on the coral reefs are win-win however. Take this saber-toothed blenny. It looks like a wrasse but instead of providing a beneficial cleaning, it usually takes a bite from its client's body. This parasitic relationship means one organism benefits while the other is harmed. Passing overhead, a slender fish clings to the side of a whale shark. Remoras like this one live directly on other fish. Feeding on discarded food without negatively impacting their host. This relationship benefits one without affecting the other. Which is why we call it commensal. On a coral reef, species interactions and the various symbiotic relationships that arise from them are fundamental to the functioning of the entire ecosystem. Understanding these interactions allows us to predict patterns of diversity and most importantly, come to a better understanding of these biologically rich cities under the sea.