Life On The Rocks

Alice Town, Bimini, Bahamas—N25° 43.446′ W79° 18.031′ Depth: 0-6ft.  If you walk the beach you will see small outcroppings of boulders. This outcropping is near the beach bars to the left as you face the ocean. The rocks are in the surf zone so there can be heavy surge even in settled weather. An interesting quick snorkel while spending the day at the beach.

The intertidal zone is an environment that requires organisms to hang on tightly or seek shelter within, under or between the rocks. Waves crash, moving sand can be abrasive or smothering but the rushing water also replenishes oxygen and food. Periodic exposure to air and temperature changes further limit inhabitants to those who can take that kind of treatment so it is best to snorkel rocks that have part of them deep enough to always be covered by water.

The first thing you notice is that there are a bunch of fish here. Fish like to hang around any type of substrate in the water. Food can be found on the rocks; crevices and holes provide protection and/or territory. The fish you see here are typical of shallow rocks and reefs. The fish with the vertical stripes are Sergeant Majors (Abudefduf saxatilis). Most of the other fish in the picture are Chubs (the gray oval fish), juvenile grunts and snappers.

When I see a dark bluish Sergeant Major there is usually a nest close by. The dark individual is a breeding male who aggressively guards and cares for the eggs until they hatch. This one was spawning with a couple of females. The dark pink eggs are attached to the rock to the left of the fish. They are very tiny and thousands of them form a nest about a foot or so wide. When the eggs hatch, the larval fish spend some time as plankton then come down to the reef as juveniles.

Waves and sand scouring crops the growth on these rocks. On closer inspection, though, presumed “bare” places are encrusted with pink Reef Cement (#1). This is a Coraline Algae that deposits calcium carbonate (the same material that shell and coral are made from). Many marine algaes can deposit calcium carbonate by manipulating the pH of the surrounding water.

Spiny Rock-Boring Urchins (Echinometra lucunter) (#2) stick their spines out of depressions in the rock. Sea urchins are related to sea stars and, like sea stars, have a central mouth on the underside. The sea urchin mouth has 5 teeth that scrape algae off the rocks when feeding.

Small, low colonies of Star Coral (#3) are scattered around the rocks. Many different hard corals are named some-kind-of Star Coral since the calcium carbonate cups that make up the skeleton of each coral polyp often look like stars. This one could be a Madracis sp. or a Porites sp. I would need a little closer picture of the individual coral cups to be sure which one. Algae living in the coral tissue makes the calcium carbonate that builds the coral reef. The bushy growth with bright white polyps behind the star coral is a Hydrozoan colony which is a related to the corals.

A fuzzy brown circular patch attracted my attention and I found it to be an sea anemone (#4). Sea anemones are a type of large polyp that do not make a hard skeleton like corals do. They have a central mouth ringed by stinging tentacles that paralyze food and guide the food to the mouth. Waste is ejected from the mouth after the anemone is done with its meal.

Many creatures can exploit the protection between rocks and in cracks. The Red Boring Sponge (Cliona delitrix) actually burrows into the rock and pokes out here and there. The large round openings are the oscula which eject the water that has been filtered through the sponge’s body.

A small Spiny Sea Fan (Muricea muricata) is a soft coral that grows in a short fan-like colony. The brown polyps are partially extended from the calyxes which end in several spines .

Fire Coral (Millepora sp.) is not a true coral but a Hydrozoan that grows a calcium carbonate skeleton instead of a bushy growth. It is called Fire Coral because it stings like fire. Any of the several species of Fire Coral can be encrusting so it was impossible to tell which species it is.

Down between two rocks I found this Fire Coral which had a Dwarf Christmas Tree Worm living on it.

Under a rock, but within view by someone from the surface, a splash of bright orange turned out to be Orange Cup Coral (Tubastraea coccinea). This is an introduced species that probably came in from the Pacific Ocean on a ship’s hull. The large orange polyps look like Anemones but each polyp has a hard cup.

I have learned that the smallest, most common place can be interesting if you look carefully.