Over the last couple of weeks 103 Kodiak third-grade students have come to the Ocean Science Discovery Lab to learn about Ocean Food Webs. The sea lion eats the salmon, which eats the young herring, which eats the copepods, which eat the phytoplankton, which need sunlight and nutrients to grow. Another day the sea lion may catch the octopus, which ate the crab, which ate the sea star which ate the mussel which filtered the water for plankton, which, in the case of phytoplankton, needed sunlight and nutrients to grow.
Of course, the sea lion is not the only one who likes to eat the salmon or the herring, there are also the eagle and the bear. The kids usually mention the shark as another predator that would join the feeding frenzy and I can think of several others, including the seal, several sea birds, and many opportunists that are too slow to hunt a fast herring, but will happily eat it if it should happen to come too close.
On the low end of the size spectrum, one can watch the feeding dependencies even in a single drop of water under the microscope. There are one-celled protozoa, which eat tiny bacteria or one-celled algae like diatoms or dinoflagellates.
This used to be easier to explain when scientists still called algae plants and those one-celled predators animals. Then it was found out that these tiny organisms don’t hold to one type of diet, but some of them can produce their own food from sunlight or go and eat someone else, which by the old definition made them both a plant and an animal. The solution was to create a new group and allow any type of feeding style. This group was called the protista. However, strangely enough, seaweeds, which are different from plants, were also thrown into the protista group, so that it now features the tiniest microscopic dinoflagellates as well as the 300-foot long bull kelp. Go figure.
When the kids look at specimen of krill and copepods under the microscopes at the zooplankton station, I tell them that those animals are vegetarians and eat mainly diatoms, a group of pretty one-celled algae. That is only mostly true, because the copepods and krill will also take an opportunity and eat a protist or even the eggs or small offspring of other plankton organisms.
Among the numerous species of copepods there are several that are predators and specialize in hunting other copepods. Like the herring, copepods have many predators, which may be the reason they are relative to their size the fastest swimmers in the world. They have two speeds: When they beat their swimming legs they cruise along slowly. However, when danger approaches, a copepod can make a powerful stroke with its long first antennae and zip out of the way.
Who has an appetite for copepods? First, there are several predators in the plankton community including the arrow worm, the amphipods and those larger, predatory copepods. Then, almost any fish at some stage of its life cycle eats copepods. For herring it is the main food all their life.
If the copepods try to get away from all the hunters by going closer to the ground, they are now attacked by the filter feeders growing on the ocean bottom and filtering the plankton: sea anemones, coral, sponges, clams and mussels, tube worms and many more. Even the largest plankton feeders, baleen whales, at times eat copepods.
Some animals like copepods and herring have more connections to other species, while some have fewer. For example, only few animals eat sea stars. There are many ways animals try to protect themselves in this eat-and-get-eaten world. Sea slugs, for example, taste bad and many are poisonous. The slime star produces a lot of nasty slime when attacked, and a lot of tiny plankton organisms avoid the light and only come to the surface to feed at night, when the fish can’t see them as well.
One thing all of Kodiak’s third-graders know is that humans are an integral part of that ocean food web. Whenever the picture of the salmon or the king crab goes up on the board and I ask, “Who eats this animal?” Someone will volunteer, “I do.”
When we check and see how many of the organisms we talked about humans can eat, it becomes clear that the ocean food web is connected with us in many ways. Every creature has its place in this web of life, from the smallest protist to the largest baleen whale. We humans have the priviledge to share this amazing table with them.