Striped shore crabs working the summer tide

The rocks along the Tiburon shoreline are wet from the last tide and warming fast in the summer sun. Where the water pulled back, the intertidal zone is exposed: barnacle-crusted granite, mats of green algae, and the dark crevices where striped shore crabs spend the hours when the sea is away from them.

Striped shore crabs are flat, quick, and about the size of a closed fist. Their carapace is mottled olive and brown with thin pale lines running across it, which makes them nearly invisible against wet rock until they move. And they do move, constantly, during low tide. They forage along the intertidal margin, turning over small stones, scraping algae, picking apart anything soft enough to eat: dead fish, invertebrates, plant matter, whatever the tide left behind. They are not specialists. They eat what is available, and in summer, with water temperatures rising in the bay and biological productivity near its peak, there is a great deal available. The crabs are most active during the hours after a low tide retreats, working the exposed zone before the water returns. This is not a choice so much as a constraint: they breathe through gills and must stay wet, so they work close to the water's edge and retreat into crevices or beneath rocks when conditions dry out too much or when something larger arrives.

That something larger is often a black-crowned night heron, which forages along these same rocks in the low-light hours of early morning and evening. The herons stand still for long intervals, then strike downward with the bill in a single fast movement. Crabs are a regular part of what they take. Brown pelicans work the open water just offshore, diving for fish, but they occasionally land on exposed rocks and will take a crab given the chance. The competition for intertidal prey during summer is real and layered: multiple species reading the same tidal schedule, converging on the same narrow strip of rock at the same time. The crabs are not passive in this. They are fast enough to escape under cover if they have a moment's warning, and their flattened bodies let them press into gaps that a bird's bill cannot follow.

The red-tailed hawk is less directly tied to the intertidal zone, but it is present in the hills and open slopes above Tiburon and hunts the transition zones between habitat types. A hawk working the edge where rocky shore meets scrub is watching for movement. A crab caught in the open on a flat rock, away from cover, is exposed in the same way a vole is exposed crossing a gap in the grass. Red-tailed hawks are generalist predators; they take what presents itself. Whether one takes a shore crab opportunistically or simply passes overhead is a matter of timing and distance, but the pressure from above is part of why these crabs stay close to cover even when foraging.

The broader intertidal community here includes species the crabs interact with more continuously. Barnacles and mussels are prey for the crabs when small enough to manage. The algae the crabs graze supports its own chain of smaller invertebrates. Striped shore crabs also turn up as prey for larger fish during high tide, when the water covers the rocks and the crabs move into the shallows. Their role shifts depending on the hour and the water level: forager during low tide, prey during high.

Summer is when the crabs are breeding. Females carry egg masses beneath their abdomens, held against the body by the curled tail flap. You can see this if you find a female under a rock: a dense, spongy cluster of orange eggs pressed against her underside. She will stay in cover more consistently while brooding, limiting her foraging time. The larvae, when they hatch, are planktonic and will spend weeks drifting before settling back into the intertidal zone.

Look down at the rocks nearest the water's edge. If the tide has pulled back in the last hour, watch for movement near the line where wet rock meets dry. The crabs are there, working the margin, pulling back when a shadow crosses the stone.