CORAL ON THE BEACH
Last week I told you about the white, rocklike fragments of calcareous bryozoa I find washed up on the beach near Hotel Reef. On my beach walk this week mostly I found chunks of coral. It's shown above.
You might be interested in comparing the general form of that with the calcareous bryozoa found the week earlier, shown at http://www.backyardnature.net/yucatan/bryozoa1.jpg.
Googling on the hotel's computer I found out that most coral species are either soft corals, which look rather spongy, or the hard, or calcareous, ones, such as the rocklike item in my hand.
You can see that the surface of the chunk in my hand is conspicuously ornamented with many shallow craters inside each of which appear calcareous, nipplelike structures with slender arms radiating from them. These craters and nipples began making some sense when I viewed the coral-anatomy page at http://www.undersea.com.au/corals/coral_structure.htm.
When the chunk of coral in my hand had been part of a living community of coral organisms, each crater had been occupied by a coral polyp -- the individual coral animal. Coral polyps are much simpler affairs than the bryozoa zooids described last week, but they do have tiny tentacles, like an octopus. The arms, or septa, radiating from the nipple (columella) serve as low partitions which help keep the tentacles in place and provide sheltered slots into which they can withdraw when they're disturbed. When a crater is occupied by a living coral polyp its tentacles usher food into the mouth at the crater's center. The stomach resides below the mouth. There's a more detailed description of polyp anatomy, but without illustrations, at http://www.uvi.edu/coral.reefer/anatomy.htm.
I mentioned last week that often the limestone of central Kentucky and Tennessee, as well as much of North America, is full of fossil bryozoa. The same is true of fossil coral. More than once in geological history large areas of North America were submerged beneath the waters of shallow tropical seas, and bryozoa and coral were abundant in those seas. However, over the eons both the land and the seas sometimes rose and sometimes fell, and the continents drifted across the globe as climates changed and changed again.
So now fossils of tropical marine organisms outcrop on mountaintops, the oceans are filling with ground-down mountains as they always have, and here I am thinking about it all on a Yucatan beach.