Meet Posidonia oceanica, a true marine flowering plant. It is endemic to the Mediterranean Sea and is an important part of near shore ecosystems there. Our European LJ friends may recognize the little ball of dead leaves in the photos as a common scene on the Mediterranean beaches. Wave action rolls the dead sea grass (not really a grass, of course) leaves and other detritus into these forms that litter the beaches. Reminds me of a tribble.
Of course, since it is a flowering plant that spends its life under water, one interesting consequence of this is that the flowers are pollinated by wave action. The fruits produced by this plant are free-floating and frequently called the olive of the sea because of the similarity in shape.
And from Wikipedia:
"In 2006 a huge clonal colony of Posidonia oceanica was discovered south of the island of Ibiza. At 8 km across and possibly up to 100,000 years of age, it may be one of the largest and oldest clonal colonies on Earth." (That info might be suspect, as that news item seems to have come from here and I couldn't locate any peer-reviewed articles in scientific journals to back up that claim. Maybe they just haven't published yet. Take with a grain of salt and call me in the morning.)
Because of its ecological importance, it has also been called the "Lungs of the Mediterranean Sea". I'm beginning to wonder what it isn't called.
More information and photos can be seen here at UBC's Botany Photo of the Day.