What lies hidden, or half hidden, along Aquatic Park’s shorelines? This page introduces some denizens of the not-so-deep — including many that don’t look like animals! Please join us identifying and exploring: Email f5creeks@gmail.com or report on your own using the free iNaturalist citizen-science app.
Pink and gold sponges, and fiery orange sea squirts flourish with red seaweed on a few shallow, shaded bottoms in Aquatic Park’s main lagoon.
Little here is native to California. Worldwide, shipping is mixing and homogenizing creatures that come with ships trade. San Francisco Bay may be the world’s most invaded estuary.
A welter of rivals: Many kinds of animals, fixed in place and filtering food from the water, compete for space and light on docks, rocks, rotting wood, and seaweeds. They can overgrow and kill each other — but also serve as a place for the larvae of competitors to settle.
A yellow sponge growing over dead-man’s fingers, a seaweed common in Aquatic Park. Sponges have a skeleton, usually made from silica, and canals or chambers for moving water from which they capture food. But they have no distinct organs.
The blob: The bud-like orange dots are colonial sea-squirts, individual animals living massed together, protected by a jelly-like membrane. The colony has blood vessels and a central chamber for wastes. Each animal has two siphons. One sucks in water and food. The other excretes wastes and propagules.
They form crusts over whatever surface they find — seaweeds, rocks, wood, or other animals.
Segmented or polychaete worms, often called bristleworms or lugworms, make up much of the mass of animals in the lagoons and are vital food for birds and fish. They live mostly hidden, so you seldom see them. They are much more varied than our familiar round-bodied worms (think earthworms). Their segmented bodies may sport rows of paddle-like “feet” down their sides, feather-like tentacles, varied shapes, and bright colors. They have sensitive, complex eyes and other sense organs we lack, as well as elaborate mouth parts that may be able to extend and sieze prey. They range from microscopic to over a foot long, with more types than scientists can identify.
Lugworms live solitary lives in bottom mud, hidden deep in tunnels. Males leave puddles of sperm on the mud (far left). Some washes to female burrows. Females’ protruding egg sacs (center) look like jellyfish — but tethered to the burrow.
Aquatic Park’s animals with shells — crustaceans — range microscopic to formidable. As with worms and other creatures, they come from all over the world, and many are hidden. Here are a few:
The European Green Crab (left), a serious predator on fish and other shellfish, can be distinguished from native crabs in the lagoons by the “teeth” on the front of its shell, five on each side of the eyes. Shrimp-like amphipods (right) have thin shells on legs and body segments. Hard to see unless magnified, they graze, scavenge, hunt, and are vital food for other animals.
Snails — above — have just one shell. Their scientific name, “gastropod,” means “stomach-foot.” Most in Aquatic Park’s lagoons are tiny grazers on algae, like the one above left. Probably most common is the Japanese bubble snail, above center. Light shines through its fragile shell, so it hardly looks like a snail at all. Jelly-like cases holding its golden eggs, above right, are even more common. Alas, this snail carries a parasite that causes swimmers’ itch.
Bivalves — below — have two shells that open and close. Most start life as small swimming larvae but are fixed in place once they settle. They close the two halves of their shells for protection or when there is no water, and open them to feed on small plants or animals, including dead fragments. This filter feeding can make water significantly cleaner and clearer.
Mussels boost water quality, filtering out small particles as they feed. Look for their large, dark, somewhat rough shells anchored to rubble by threads. To feed, they open their shells like a book and filter water through their gills. Local mussels are a welter of species and hybrids, often called the “blue mussel complex.”
Tiny Asian date mussels also thrive in the lagoon, usually unseen. In dense algae they can float to the top, perhaps seeking oxygen. Their shells are somewhat smooth and shiny, with wavy patterns.
Clams live with shells buried in mud, feeding by extending siphons that draw in water and food particles. This keeps them more hidden than mussels, so you see them mostly as shells opened by birds. Most in Aquatic Park were brought to the Bay Area for growing or accidentally. Soft-shelled clams, left, can be identified by the big projection that anchors the muscle that closed their shells. Japanese littleneck clams, right, have prominent ridges in two directions.
Small fish seem to rise, flash, and jump especially near the lagoons’ inflow pipes. Often, they don’t seem to be hunted by birds or fleeing larger fish. Are they newcomers? Trying to escape? Hunting even smaller prey? Breeding?
Topsmelt (a silverside, not a true smelt) is one common species. But there are more!
Help learn more by making observations with the free citizen-science app iNaturalist.org!