🐡 Ocean ecosystems and food chains

Plankton are tiny organisms, some plant-like and some animal-like, that drift through ocean water rather than swimming strongly against currents, and despite being mostly invisible to the naked eye, they form the base of almost the entire ocean food chain. Plant-like plankton, called phytoplankton, use sunlight to produce their own food, similar to plants on land, and in doing so they also release an enormous amount of the oxygen that ends up in the air we breathe. Countless small ocean creatures eat plankton directly, and those creatures get eaten by slightly bigger ones, and so on up the food chain, meaning even giant whales and sharks ultimately depend on these nearly invisible organisms.

Many shark species sit near the very top of the ocean food chain as apex predators, meaning few other creatures regularly hunt a full-grown shark, which lets sharks help keep populations of the animals they eat in careful balance. By hunting weaker, sick, or slower prey animals more often than healthy ones, sharks help keep prey populations strong overall, since the healthiest animals are more likely to survive and reproduce. Removing top predators like sharks from an ocean ecosystem can actually cause serious, cascading problems further down the food chain, sometimes allowing certain prey species to grow far too numerous and damage their own habitat.

A kelp forest is a dense underwater habitat made of tall, fast-growing seaweed called kelp, which anchors itself to the rocky seafloor and grows upward toward sunlight, sometimes reaching many metres tall. Kelp forests provide shelter, food, and nursery space for an enormous variety of ocean creatures, from tiny fish hiding among the fronds to sea otters that wrap themselves in kelp to avoid drifting away while they sleep. Because kelp needs sunlight to grow, kelp forests only form in relatively shallow, clear coastal water, and they can grow back remarkably fast, some kelp species growing tens of centimetres in a single day under good conditions.

Fish swimming in a school constantly watch their close neighbours, reacting almost instantly to tiny changes in speed and direction, which lets an entire group shift together smoothly without any single fish leading or giving instructions. Special sensory organs along a fish's body, called a lateral line, let it feel subtle changes in water movement created by nearby fish, helping it match its neighbours' speed and direction almost automatically. Moving in a tight, coordinated school can help confuse predators, since a huge, shifting mass of fish is much harder to focus on and catch than a single fish swimming alone.