Sickle-leaved Cymodocea

It is a flowering plant, and that is the strange part.

Seagrasses are not algae. They are land plants that went back to the sea about a hundred million years ago, and they took the whole terrestrial kit with them: roots, an internal plumbing system, rhizomes, leaves with veins, and flowers. There are only around sixty species of them in the world, which is a very short list for an entire way of life, and every one of them is descended from plants that had already spent hundreds of millions of years learning to live out of water.

Going back meant undoing much of that. Pollen had to work underwater, gas exchange had to happen through leaves in a medium holding a fraction of the oxygen air does, and the whole plant had to stay anchored in sediment that moves. What they got in return was an empty niche the size of every shallow sheltered coast on the planet.

This species is the stiff one: woody stems standing off the bottom with a crown of curved sickle-shaped leaves at the top, in dense stands in shallow lagoons. Sea urchins graze it heavily, and in places where urchin numbers have risen the stands go bare.