Amazing Nature

A hermit crab is seen outside its snail shell. (Switgard Duesterloh photo)

We all know what an ecosystem is. We use the term all the time. But how is the term defined? How big or small is an ecosystem? Where are the boundaries? What are the factors in an ecosystem and how are they connected? These are questions that Kodiak seventh-grade students are contemplating during our new Marine Ecosystem lesson.

The definition of an ecosystem uses terms like “a network of interactions” or a “set of relationships” between living organisms and their non-living environment. There is no mention of size or limits.

When you try to describe a specific ecosystem, it seems as if it starts slipping from your grip like a glob of oobleck, because a specific ecosystem does not exist until you define exactly what network of interactions or set of relationships you will describe. You can define the ecosystem in the dirt under your fingernail or the ecosystem planet earth.

Kodiak students are given three choices of ecosystems to study: Open ocean, intertidal, and kelp forest. The first surrounds a set of animals living in the open ocean plankton community, spending their lives adrift in the currents and interacting with other organisms in various ways.

Some plankton animals, for example the very common copepods, feed on tiny algae in the plankton community, which are called phytoplankton. When the copepods poop, that becomes food for the bacteria in the plankton, which turn the remaining nutrients back into a form, which the phytoplankton can use for their growth. Students look at copepods and other plankton animals under the microscope.

There are arrow worms in the samples, which feed exclusively on copepods, and amphipods, which will eat anything they can get their mandibles (mouth parts) on. Also connected to this ecosystem are plankton-feeding fish and whales, and filter feeding organisms like sponges and sea anemones.

Many of these connections that the students are looking for are food chains and food webs, but not all. Another ecosystem students can choose to study is the intertidal community and specifically the interaction between various snails and hermit crabs. Hermit crabs are active little crabs, which live in empty snail shells.

Snails, on the other hand, are slow, soft-bodied animals with shells that grow as the animal ages. If you look closely at a snail shell and run your finger along the opening of the shell, you can see parallel lines that follow the same shape as the opening and keep going further and further up the revolutions of the turret.

If you let yourself be impressed by the design and variety of snail shells, you can appreciate why hermit crabs would be so fond of them!

In truth, hermit crabs have a problem that requires them to find a snail shell to slip into: their hind end is soft. Other crustaceans, shrimp and lobster for example, have developed armor for their whole body. Crabs fold their hind end under their chest and grow a shield to keep it protected. A hermit crab outside of its shell has a curled wormlike and soft hind end that makes every seagull and fish think of snack time.

Therefore, hermit crabs have always lived near snails and used their houses for protection.

However, most of the little hermits you find on our beaches are not capable of killing the snails to use their houses. Only if an unlucky snail was eaten by a sea star or some other predator can a hermit move into the empty house.

When the hermit grows up and outgrows the snail shell, it needs to go out and find a bigger house. These are available toward the water line in the form of the larger dogwhelk shells.

Dogwhelks are fewer than the small periwinkles, and competition is fierce. Hermit crabs fight endless battles over their real estate. The winners in the survival game have yet another challenge ahead of them. The next larger snails, the hairy tritons, live in the subtidal zone farther down in the ocean.

Those large mansions are even harder to find and only few hermit crabs are lucky enough to secure a large shell and grow to the size of those hermits you can watch at the touch tank on Near Island. To find the big ones, you need either a very low tide or a wetsuit!

At the end of the class period, we discuss connections between the different ecosystems, and how people or a change in global ocean temperature may influence any of them.

When I leave the classroom, I don’t know if the students will only remember how the hermit crab felt when it scuttled over their hands or that the krill under the microscope had huge eyes. I hope that they understand that all these animals’ lives are connected and that we are all part of the same huge and amazing marine ecosystem.

Switgard Duesterloh, Ph.D., is an assistant professor of natural sciences at Kodiak College. She operates the Kodiak Ocean Science Discovery Lab and teaches ocean science to students throughout the Kodiak Island Borough School District.

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