[Journal entry for Geiger Key Guided Mangrove Eco-Tour, November 29, 2025]

November 29, 2025. In the morning, Dallas, Lisa, Mellissa and I participated in a Guided Mangrove Eco-Tour that Lisa had arranged through Geiger Key Paddle Hut (5 Geiger Rd, Key West, Florida, 305-849-6901). We drove to the Atlantic side of Geiger Key, an island about ten miles east along Route 1 from Key West. Our guide, Kristin Horning (305-305-5172) assigned Dallas and me to an Ocean Kayak Malibu Two XL Tandem Kayak (as we had requested), and Lisa and Mellissa each to single-person Malibu Two’s. These are all sturdy plastic boats of a sit-on-top design that are propelled with a standard paddle (as contrasted to the wing paddles Dallas and I are used to). After a bit of instruction, we spent a few minutes paddling circles in the water just off the dock. Dallas and I noticed oysters clinging to a concrete sea wall. We then set out, padding in the open water between – and narrow channels within – several smallish Mangrove islands in the general vicinity of Saddlehill, Pelican and Bird Keys. Kristin explained that these islands formed about six thousand years ago, after sea level rose to flood this part of Florida, as water stored in the great glaciers of the Ice Age melted and returned to the sea. Coincidentally, Columbia University paleo-climatologist Dorothy Peteet and I had been discussing this very subject while on an intercampus bus about two weeks ago. Sea level was down about four hundred feet during the Last Glacial Maximum. The ancient Florida shoreline was at least ten miles south of the Keys and wrapped north about one hundred miles west of the current west coast of Florida. The Florida peninsula has at least twice the land area that is has now. We wondered what kinds of rivers crossed this now-flooded land and whether they drained into the sea anywhere near the present-day Keys. Kristin went on to explain that, initially, the seas flooded only sterile limestone pavements, but vegetation fertilized by dust blown in from Africa soon began to grow, forming the Mangrove islands that we now were seeing around us. We paddled along the shore, following an artificially-deepened boating channel with a stiff current that did not favor us. We spotted a Great White Heron standing in a clump of Mangroves; it took to flight as we passed. We then crossed an open expanse of water and dodged into a narrow channel in the one of the Mangrove islands. Kristen explained that these are all Red Mangroves, identifiable by their arching aerial prop roots. The Black and White Mangroves are less salt tolerant and are upland plants. She caught a Mangrove Tree Crab and showed it to us before releasing it, explaining that this small crustacean lives in the Mangrove canopy and primarily eats its leaves. Dallas asked about nodules on the mangrove branches overhead and Kristin said they were caused by a fungus that did not seem to harm the trees. We made our way along the length of the channel, sometimes paddling, sometimes pulling ourselves along by grabbing branches. We spotted fish – with lengths of an inch up to about a foot – swimming between the roots, as well as several species of sponge clinging to them. Dallas thought a purple species especially attractive. For the rest of the tour, we alternated between open stretches of water and channels within the Mangroves. Dallas and I had noticed that most of the channels were close to the outer edges of the islands. Kristen said that these channels formed after storms piled Mangrove debris near the island’s periphery, which subsequently decayed and fertilized new growth. In all likelihood, these channels would fill in, though a century might pass before they are gone. Some of the open expanses were meadows of sea grass, which Kristen said were important nurseries for fish and other marine life. Mellissa spotted a small shark swimming across one, and later, Kristen a barracuda, but I was looking elsewhere and missed them. Other open stretches of water were full of Cassiopeia “upside-down” jellyfish. They are shaped like snowflakes, mostly yellow or tan in color, and from an inch to up to six inches across. Most sat on the muddy bottom, pulsating slightly, but a few floated in the water column. Perhaps we inadvertently had knocked the latter from their roosts with our paddles. Dallas noticed that some had black tentacles. Kristen said that these were a morph of the same species. Kristen explained that the species assumed its inverted position as an adaptation to sustain photosynthetic algae, which colonize their tentacles and provide them some nutrition. They also catch prey with their tentacles, which have toxins (though not ones that have serious effects on most people). Dallas and I discussed the degree to which Cassiopeia and Sea Anemones represent convergent evolution to an inverted position associated with symbiotic algea, but this was not a subject we knew much about, except that while both are in the phylum Cnidaria, they are in different classes. Subsequently, I have researched the matter; the two classes diverged from a common ancestor in the Neoproterozoic, at least six hundred fifty million years ago, and the similar adaptations are often cited as archetypal convergent evolution. Our paddle back was a little easier as the current was favorable. Dallas and I spotted what we thought were foot-long Loggerhead Sponges on the edge of the boat channel (but Kristen was not nearby and did not verify). As we were leaving the parking lot, we ran into Susan Cabanas, a family friend who lives on Geiger Key. 2:15.