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Eelgrass – a zoo of strange and small animals
By Russel Barsh and Madrona Murphy
Like true grasses on land, eelgrass responds to longer, warmer days by growing new leaves and sometimes forming small, inconspicuous flowers. Eelgrass competes for light with the mat of algae and diatoms [a single-celled algae] that floats above like green clouds. One native and one non-native species of eelgrass live side by side in our waters; the non-native “Japanese” species tolerates warmer, shallower water and expands eelgrass meadows towards shore. Both species are otherwise functionally identical, supporting a zoo of strange and small animals that rely on thickets of eelgrass for food and refuge.
If you run your finger up a blade of eelgrass, you may find something gelatinous and green with yellow racing stripes.
Shimmering like a melted jellybean, Taylor’s sea hare is actually a sea slug. It belongs to a primitive family – the Anaspideans – that begin life with a shell like other gastropods [a large class of mollusks] but gradually lose it. Sea hares graze on eelgrass, and the hard-shelled diatoms that accumulate on eelgrass. Diatoms have very hard shells made of silica, so Taylor’s sea hare has siliceous teeth lining its stomach wall to grind up its food. Newly hatched sea hares do not float around in the plankton but settle right in with the adults, so numbers can increase very fast in a suitable eelgrass patch.
Run a sieve or scoop net through the eelgrass and you will almost certainly find another group of tiny green animals, so clear that you can see through them: an entire menagerie of shrimp no bigger than common insects. Most are members of the genus Heptacarpus, which can change color to match their surroundings: green, red, brown, or even colorless. These diminutive scavengers scour the eelgrass for even tinier animals and plants, and in turn, they feed visiting fish like surfperch, smelt, and herring.
Camouflaged in the eelgrass and waving slowly in time with shifting currents, our only native seahorse and the bay pipefish, also feast on Heptacarpus shrimp. Gold, bronze, and emerald green in colors, which vary individually, the pipefish can grow to nearly two feet in length. Females implant their eggs in a tumescent, swollen brood patch on the belly of the male, who broods the eggs for up to six weeks, then shepherds and protects the juveniles. Bay pipefish can also make homes in shallow kelp. Pick one up and it may make a sharp snapping or popping sound. It wants to eat you! High-speed videos have shown that bay pipefish suck their prey into their tubular “beak” with almost explosive force.
Great blue herons are known to hunt bay pipefish in the eelgrass. They also hunt slippery lime-green pinpoint gunnels. This eel-like fish can attain a length of 18 inches, and like pipefish, are attracted to the abundance of shrimp and other small crustaceans in eelgrass. They lay their eggs in rocky reefs, and young gunnels can often be found hiding under rocks when the tide goes out. A neat trick: they are adapted to breathing air. The bright green, and sometimes rust red, color of penpoint gunnels is due to carotenoids, a family of chemical compounds ordinarily found only in plants. Gunnels do not appear to obtain their colors from plants, however; rather, they choose their homes in vegetation that matches their own birth colors.
You can see bay pipefish and sea hares in eelgrass meadows at the Indian Island Marine Health Observatory near Eastsound. Visit indianisland.info for summer research schedules.