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.

It’s best to snorkel these rocks at high or mid tide.  This will give you a little more room to dodge the rocks as you get moved around by the surge.

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.

The large coral boulders sit on the sand and attract fish.  I am not sure if they are a natural outcropping or placed here for beach sand retention.  Note the sand waves that indicate vigorous water motion.

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.

You can just discern the nest, a dark pink-purplish area just above the female Sergeant Major (bold vertical stripes with yellow across the back) next to the male (blue).  The female first lays the eggs on the rock then the male passes over and fertilizes them.  Some Caesar Grunts (Haemulon carbonarium) wait further under the rock–maybe hoping for a caviar snack or maybe just looking.

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.

Surface view of one of the boulders.  Pink encrustation (under #1) is Reef Cement, a red algae (Rhodophyta).  Just to the left and below #2 is a Rock Boring Urchin (Echinometra lucunter).  Above #3 is some Star Coral and Below #4 is a Sea Anemone.  Close-ups of these are in the next 3 photos.

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.

Three Rock Boring Urchins have excavated shelter for daytime resting.  They come out at night to feed.  You can also see a few Hydrozoan colonies.  The one in front of the urchin in the upper right shows the bright white polyps well.  Each polyp has a ring of tentacles around a central mouth.  The colony grows by budding.

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.

Star Coral with a Hydrozoan colony in the background.  Corals, Hydrozoans, jellyfish, and sea anemones belong to the Cnidaria which are characterized by tentacles containing stinging cells (cnidoblasts).

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.

This is probably a Light Bulb Anemone which has not yet been officially named.  These are small anemones–this one is about 2″ across.  The base is attached securely in a crack in the rock and the anemone can retract into it if things get wild.

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.

Rock Boring Sponge adds a splash of color amid fuzzy brown algae and Hydrozoans.

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 .

Sea Fans, Sea Whips, and Sea Rods are often mistaken for plants but they are actually soft coral colonies.  Instead of a calcium carbonate skeleton, soft corals make a central support of finger-nail like material then surround the polyps with a matrix that allows the polyps to retract for protection.

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.

Fire coral has a characteristic color and fine texture.  The small holes within the fire coral and seen throughout the rocks house Worm Snails–a snail that stays put in a long tube within rock or coral.

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

The Dwarf Christmas Tree Worm (Spirobranchus sp.) grows within several types of coral but I often see many of them in Fire Coral.  The worm makes a hard tube where the “wormy” part of the body is protected.  The blue “Christmas Tree” is the gills and feeding apparatus of the worm.
Here is a close up of the gills and you can also see, right below the gills, the operculum, a trap door that shuts the entrance to the tube when the worm retracts its gills into it. You can also see the small pores that the Hydrozoan tentacles come out of when they expand.
Christmas Tree Worms are brightly colored.  Here’s a green one in some Star Coral.  The coral polyps are expanded in this photo and you can clearly see the ring of tentacles and mouth of each polyp.

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.

Orange Cup Coral has been introduced but is not invasive.  It is well established in the Atlantic but is not common and tends to grow small colonies in holes and caves.  Some of the corals in this photo have the tentacles retracted.  Hydrozoans and another type of Coraline Algae are also in this photo.

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

One thought on “Life On The Rocks”

  1. Always fascinating. I came back to this post today to read again about the Sargeant Majors. I saw loads of them today. Then a very aggressive blue one was guarding his hole coming at me teeth bared! Really great to be able to check back. Now I know he was guarding the eggs. Sadly my picture wasn’t good enough to see the eggs.

    I hope you do an index by species etc. I love coming back to your site.

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