John Booker

Program Director at SeaCamp /Newfound Harbor Marine Institute on Big Pine Key, FL.

Hardbottom communities are dominated by algae, sponges, gorgonian corals and ahermatypic stony corals. All of these species attach themselves firmly to the bedrock.

Hardbottom communities are often sub-divided into two types. Nearshore restricted hardbottom communities are located in restricted embayments, and are characterized by little water movement and a corresponding low sedimental turbidity. Nearshore high-velocity hardbottom communities are located in tidal channels among islands and in seaward regions between islands and patch reefs. These communities generally have strong currents that sweep sediments from the hard bottom.

The nearshore restricted hardbottom community is dominated by epilithic and drift algae. Epilithic algae attach directly to the limestone substrate. Species of epilithic algae common to this community are sea bottles (Ventricaria ventricosa), mermaid’s wine glass (Acetabularia crenulata), star algae (Anadyomene stellata), green bubble algae (Dictyospaeria cavernosa) and squirrel tail algae (Dasycladus vermicularis). Drift algae are usually formerly attached species which have broken free due to waves and currents. Species of brown algae commonly found include forked tumbleweeds (Dictyota spp.) and several species of sargassum. Red algae represented are Laurence’s weed (Laurencia papillosum and L. poitei), and spiny seaweed (Acanthophora spicifera and Eucheuma isiforme). The coralline red algae Neogoniolithon strictum is also an important species in this community.

The nearshore high-velocity hardbottom community may be further sub-divided. Around islands, high velocity communities are often dominated by sponges of the class Demospongiae. All demospongiae have leuconoid (complex) body forms, and many types of growth patterns are exhibited. Sponges are stiffened by spicules, synthesized from calcium carbonate, silicon dioxide, a spongin protein, or combinations of all of these.

The most common sponge of this community is the loggerhead sponge (Speciospongia vesparia). Loggerhead sponges are easily identified by the characteristic grouping of osculae (dark holes) on the upper surface. Other common sponges in this area are the vase sponge (Ircinia compana), stinker sponge (Ircinia fasciculata), candle sponges (Verongia longissima and V. fistularis), green sponge (Haliclona viridis), sprawling sponge (Neopetrosia longleyi) and chicken liver sponge (Chondrilla nucula). Fire sponge (Tedania ignis) is easily identified by its orange color and can be irritating to the skin when touched. In addition, several species of commercially valuable sponges are found in this community.

The second type of high-velocity hardbottom community is found seaward of the Keys. Here colonial gorgonian corals are the dominant species. Gorgonians, often referred to as octocorals, are easily identified by their bush, fan, rod or whiplike appearance and their flexibility. This flexible nature is due to an inner axial skeletal core composed of gorgonin, a fibrous protein which is surrounded by an outer living crust containing calcium carbonate spicules. When touched, the polyps retract, leaving only tiny slits, dimples, or holes within the crusty, outer living tissue. These corals are able to survive in waters with high sediment loading because any particles landing on the polyps will be dislodged as the coral sways in the currents.

Typical gorgonian corals in the waters of Monroe County include the common sea fan (Gorgonia ventalina), knobby candelabra (Eunicea mammosa and E. calyculata), double forked sea rod (Plexaurella dichotoma), gray sea rod (P. grisea), dry sea plume (Pseudopterogorgia acerosa), slimy sea plume (P. americana) and spiny candelabra (Muricea muricata). As these corals grow, they often orient themselves perpendicular to the local current flow to feed efficiently, and thus give information about typical current patterns in the area.

Ahermatypic stony corals are found within the hardbottom communities. These corals, unlike those found in deeper water, have evolved a mucous system that can withstand the high suspended sediment levels found in shallow waters. Some of the most conspicuous include smooth starlet coral (Siderastrea radians), golfball coral (Favia fragum), bifurcate finger coral (Porites divaricata), mustard hill coral (P. astreoides), rose coral (Manicina aereolata), elliptical star coral (Dichocoenia stokesii), knobby brain coral (Diploria clivosa) and smooth brain coral (D. strigosa). The hydroid Millepora alcicornis (encrusting fire coral) is often found on dead gorgonians.

The hardbottom habitat supports many other plant and animal species. Other inhabitants of this community include several species of rhizophytic algae, ahermatypic corals, anemones, mollusks such as conchs and tulip snails, spider and stone crabs, sea stars, sea cucumbers and tunicates. Juvenile spiny lobsters are often found in crevices. Many species of fish forage in these areas including grunt, snapper, grouper and barracuda.

The hardbottom community also contributes ecological niches for other species. Many fish seek cover in the complex growth. Gorgonians also are a source of attachment points for tunicates, anemones, mollusks, barnacles, bryozoans and worms. The flamingo tongue (Cyphoma gibbosum) is often found preying on colonies of sea fans.

Hardbottom sponges also serve as a habitat for many species. Sponges are eaten by relatively few fishes, so animals that live within sponges are protected. Currents of water flowing within the sponge bring in a steady supply of food. Snapping shrimp are the most common inhabitants of sponges, and are responsible for the familiar "clicking" sounds often heard when snorkeling in these habitats. Other sponge-inhabiting invertebrates include mantis shrimp (stomatopods), cleaning shrimp, brittle stars, crabs, barnacles, clams, polychaetes, amphipods, tunicates and flatworms. In addition, several species of fish are known to inhabit sponges.

Hardbottom sponge species also contribute to the development of sediments. Sponges filter suspended sediments, and produce sediments of their own. Their spicules eventually become part of the sediments in the surrounding community.

The hardbottom community grows some products of commercial value. Sponges are collected directly from the bottom, dried, and sold for cosmetic and medicinal purposes. The most important sponges for these purposes are the sheepwool, yellow and grass sponges. This industry was severely impacted in the early 1900s when a blight wiped out large populations of commercially valuable species. Gorgonian corals have some value in compounds used in medicines.