While not a marine plant or macro algae, I thought my fellow plant geeks might find it interesting as it appears to be a Sargassum mimic. I found it today while walking on the beach near St. Agustine FL, clinging to a clump of Sargassum that had washed up. I have a nasty habit of inspecting clumps of plant material caught in the surf and today it paid off. Unfortantly, I was not prepared to take pictures of marine life. So, I had to make due with a ziploc bag and some silty water. Which resulted in less than perfect pictures. But I hope you enjoy anyway.
Jay
Jay
There is a lot of Sargassum washing up from New Smryna all the way down to Ft. Pierce at the moment. The crazy thing is that I find a lot of black mangrove, turtle grass leaves and other plant material in the wrack with it. As if the contents of the lagoon are flowing out and wayyy down the shoreline in the Atlantic. Its been interesting. 
