Once again, it is that time of year when Kodiak fifth-graders visit the Ocean Science Discovery Lab on Near Island to conduct a scientific experiment complete with predictions, measurements, data recording and analysis. The stars of the unit are sea stars, and we begin by talking about the biology of stars.
Later, we move on to sea star geometry and math, only to wrap it up in a discussion of statistics. If that sounds intimidating, don’t think so — it really is a fun hands-on lesson where kids measure how long the arms on their sea stars are and mark the time it takes a sea star to flip over when placed on its back.
I usually talk about where we find sea stars along the shore and at the bottom of the ocean. We observe the movement of the tube feet, which are used for crawling along and can also attach to the shells of a clam or mussel if a sea star wants to eat it.
Because tube feet are powered by hydraulics, it costs the sea star no energy to create a pull on both shells of a clam once its arms are wrapped around the victim and its tube feet attached. The sea star simply pumps some water out of the tube feet, which creates a pull on the clamshell. The clam, which has to use muscle power to hold the two parts of its shell together, eventually gets tired and has to let go.
When that happens, the sea star extrudes its stomach and digests the clam in its shell. The pre-digested mass is then sucked into the second stomach and absorbed. At this point I usually hear a few comments of “uugh” and “gross” and sometimes a “cool” or “weird”.
Their eating habits are not the only thing cool about sea stars. Last week, a girl asked me how sea stars make babies and how you can tell a “boy” from a “girl”.
In sea stars, you can’t really tell males from females unless you look at the gonads inside a fully-grown animal. Some sea star species have males and females, in some species young males turn into females when they get older, but in yet different species this process goes the other way and young females turn into males when they get older. Yet different sea stars are male and female at the same time (the term for this is “hermaphroditic”).
All sea stars hatch from eggs. Most are broadcast spawners, which means the eggs and sperm are released into the water. This release is carefully timed among many individuals so that the probability of an egg encountering viable sperm in the water is high. In some species, males actually sit on top of the females and wait for the egg release so that the release of their sperm is perfectly timed.
Fertilized eggs float in the plankton until a small larva hatches. This does not look anything like the adult and only has two little horns (bipinnaria larva) that could remotely resemble an arm of a sea star. Later, this larva changes into a brachiolaria larva, with so many little horns, lobes and arms that it resembles a kids’ alien fantasy more than the proper 5-armed form of a local sea star. Only when the larva settles to the bottom, does it change into a resemblance of the adult, if only a tiny one.
In most species, the older larval form and the fully formed sea stars have “pentaradial symmetry”. Penta means five, and radial symmetry is when life forms are oriented around a radial axis of symmetry rather than a bilateral one. What does this mean? Picture a sea star, sea urchin, or a sea anemone. In each organism there is a central axis around which the animal can rotate like the spokes of a wheel.
The term radial symmetry is only applied to organisms in biology and you will not hear about it in math or engineering books. On the other hand, in bilateral symmetry you picture for example a fish, there is one symmetry plane from its mouth to its tail and the only symmetry is that the left side looks like the right side (more or less).
Of course the whole discussion about symmetry assumes that each arm of the sea star is exactly the same length and shape and is thus completely hypothetical. In fact, sea star arms are not all the same length and if an unlucky sea star gets attacked or very agitated it may drop and arm and grow it back later.
You can find sea stars with one or more short and stumpy arms, which are in the process of growing back. These amazing animals have stunning powers of regeneration. The more I learn about them, the more amazed I am by how different this life form is from us, how many designs in nature preceded the human form, and how many solutions there are to the challenges of life!
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.