By Paul Belz
Few things beat an afternoon with animals that have no backbones! Damon Tighe, an invertebrate enthusiast who lives near Lake Merritt, guided a group of explorers on a sunny September afternoon. The Lake’s different concentrations of salt and of dissolved oxygen give homes to creatures who have come here from all over the world. The Lake is brackish, meaning that freshwater that flows from streams in the hills & mixes with saltwater from the Bay. The salt content in the water makes it uninhabitable for dragonfly nymphs, crayfish, and other freshwater invertebrates.
Damon shared creatures he’d collected and placed in tubs of water. “Because we’re so close to the Port of Oakland, the majority of organisms you’re going to see today are not from California,” he said. Native mussels do thrive in large clumps on the Lake’s docks, where the oxygen concentration is high. Damon shared a local yellow shorecrab (Hemigrapsus oregonensis) and described sea hares, which are large sea slugs. “They can be as big as a rugby ball, or a small rat,” he said. These interesting creatures have been seen in big numbers this summer near the floodgate that separates the Lake from the Bay.
Oriental shrimp (Palaemon macrodactylus) thrive in low oxygen regions along the Lake’s edges. “If you come through at night with a UV light, you’ll find them, Damon said. “They fluoresce under the light, kind of like scorpions.”
“But less harmful,” an young girl commented.
“And probably more tasty,” Damon laughed. He described eastern mud snails (Ilyanassa obsoleta) from the Carolina oyster beds that also live in this harsh habitat. They crawl over the bodies of creatures that have died, and feast on them.
Dead man’s finger is a sponge (Halichondria bowerbanki) that looks like a yellow, shriveled up hand when it grows large. Sponges, the world’s simplest multicellular animals, spend their lives attached to rocks or other hard surfaces. A sponge doesn’t have real tissues or organs; rather it is a collection of cells working together to survive. Collar cells move their whip like flagellums to make water flow into the sponge’s many pores, and gather bits of food on their sticky surfaces. Other cells move the food through the pores so all parts of the animal gets to eat. “You can put a sponge in a blender and turn it on,” Damon said. The cells given time will gather back together to form a sponge again,”
A segmented worm (Typosyllis nipponica) slid quietly through the tub of water. At a certain point, this animal’s tail changes from green to an orange color, breaks off, develops eye spots, and swims away. These epitokes are sacks of gametes, or sperm and eggs. “They’ll swim and gather together and start popping,” Damon said. They release their sex cells and produce their young this way. People in Hawaii love to eat epitokes of some of their native polychaetes
Damon shared two oyster drill snails (Urosalpinx cinerea)whose ancestors came from the Carolina oyster beds. The pair had settled on a mussel, and seemed to be about to mate. Later, they would drill into the mussel and use their proboscises to feast on it. The group later found drill snails’ egg cases, which one hiker said looked like passionfruit seeds.
Sea squirts, or tunicates, are some of the strangest of the Lake’s critters. These animals have notochords, bundles of nerves that function as primitive spines when they are in their young, free swimming stage. As they age, these filter feeders settle to rocks, and their notochords wither away. Now they absorb and release water, eating the small living things they can grab. These odd animals are ancestors to all vertebrates, including humans. Damon touched one and showed the group how it squirts water, getting its name from this activity.
Damon mentioned that the many invertebrates in the Lake form the basis of its food web. Gulls can easily feast on these dark shelled mussels when hot temperatures and low tides cause them to dry out. Yellow finned gobies and other fish eat the spineless ones, and become prey to the Lake’s birds. “We saw a cormorant eating a pipefish, a relative of seahorses, when we got here,” Damon said. Birds also feast directly on invertebrates. The Lake’s complex food web explains why so many birds make it their home.
Bat rays and leopard sharks, which eat mussels and clams, sometimes come into the Lake from the Bay. The otter that was spotted once has not been seen again but it could return. Damon hoped that more Bay invertebrate and other animals would enter the Lake as the connections between the two water bodies are improved. Keep watching the Lake, you never know what weird treasures you’ll find!