The one that looks like green intestines has fronds that are long, thin, hollow tubes, bright green, sometimes branching, sometimes just single strands. They look like small intestines or deflated balloons or green spaghetti. They are soft and slippery, filled with water and air. They grow in tangled mats, covering the rocks and the mud like a green blanket. It is the alga of the messy water, the one that turns up when things get out of balance. What makes it special is the opportunism. The green thread algae is a nutrient sponge. It thrives where nutrients are high, in estuaries, in harbours, in rock pools that collect runoff from the land. It grows fast, doubling its mass in a few days under ideal conditions. It is the weed of the coastal zone, the one that turns up whenever things get messy, the one that is a sign that something is wrong. The green thread algae is a green alga, a close relative of the sea lettuce. In fact, some scientists now place it in the same genus, Ulva. Its tubular, hollow fronds are a simple adaptation, more surface area for absorbing nutrients, less material to build. Under a hand lens, the fronds are beautiful, the surface covered in tiny cells. Biologically, the green thread algae reproduces by releasing spores from its fronds. It also reproduces by fragmentation, a piece broken off can grow into a whole new plant. This is why it can appear so quickly in a new area. To find green thread algae is to find a green mat in the rock pool. It looks like intestines, like deflated balloons, like green spaghetti. You can pick it up, feel its soft, slippery texture, watch the water drip from its tubes. It is the messy one, the opportunistic one, the one that turns up wherever nutrients get out of hand, the one that proves that excess has consequences.