The one that looks like it is wearing floaties has fronds that are made of chains of small, round, air-filled beads, like a string of green or brown pearls. Each bead is a hollow, watertight chamber, filled with air. The beads are connected end to end, forming branching chains that can be up to 20 centimetres long. The whole plant looks like a piece of seaweed that has swallowed a bunch of marbles. It is the alga of the bubbly surface, the one that floats on the tide. What makes it special is the bubbles. The air-filled beads keep the bubble weed afloat, lifting its fronds up toward the light. They also act as shock absorbers, protecting the plant from the pounding of the waves. When the tide goes out, the beads deflate slightly, and the plant collapses into a wet, squishy mat on the rocks. When the tide comes back in, the beads reinflate, and the plant floats back up. It is the alga of the tide cycle, the one that changes shape with the water. The bubble weed is a master of the intertidal zone. It can survive hours of exposure to the sun and the air at low tide, its beads shrivelling but not dying. When the tide returns, it rehydrates within minutes, the beads swelling back to their full, bubbly size. It is a survivor of the edge, the place where the land meets the sea, the one that has learned to hold water in its beads. Biologically, the bubble weed is a brown alga. It reproduces by releasing spores from specialised structures on its beads. The spores are released during high tide, carried away by the current, and settle on nearby rocks to grow into new plants. To walk across a rock platform at low tide is to walk on bubble weed. The beads squelch under your feet, popping like bubble wrap. It is strange, wonderful, and slightly addictive. The bubble weed is the seaweed that looks like it is wearing floaties, the bubbly one, the squishy one, the one that pops when you step on it.