San Salvador’s geographic location, geologic origin, and climate have definitely influenced the marine flora and fauna of the island. The distribution and diversity of the marine fauna living around the coral reefs were very impressive as we circled the island over the week. It was very interesting to observe the distribution of the marine plants and animals between the coral reef communities and sea grass beds.
About 150 million years ago, calcium carbonate precipitated and accumulated in warm, shallow ocean that formed the Bahamian limestone bank above the Atlantic crust (Sealey, 2006). Presently known as the Bucket Theory, ocean channels cut through the limestone and erosion formed separate islands over geologic time (Sealey, 2006). San Salvador Island (and other Bahamian islands) subsided due to the weight of limestone on top of the crust (Sealey, 2006). Coral reefs began to form millions of years ago, attaching to oolitic limestone substrate in warm, shallow ocean near the shore.
Wind and ocean currents of the Coriolis Effect as well as geologic time allowed for the diversification of the coral species seen today. The currents create a circle around the North Atlantic Ocean, the lowest latitude current heading toward the Caribbean Sea from northern Africa. This strong Easterly current has a large effect on the coral reef distribution of San Salvador. The east shore of the island receives much higher wave energy than the west shore and so corals are more likely to settle on substrates on the western side of the island. The pattern of this coral reef distribution was noticed because the East side of the island was rarely snorkeled; the first morning, we went to Pigeon Creek, where little to no reefs were found. The reefs on the northern and southern shores of the island, although exposed to high wave energy from the east, had an abundance of corals particularly of the species that combatted the waves. For example, on Gaulin Reef and in French Bay, there seemed to be more of the Acropora and Gorgonia species, perhaps because they have adaptations to deal with the rough waves. Acropora species grew quite thick “branches” that seemed to have resistance against the constant thrashing of the waves. Soft corals, like Gorgonia ventalina, were abundant in these reefs because their structure allowed for the constant changing flow of the waves. Overfishing and White Band Disease of the 1970s extinguished coral species such as Agaricia agaricites, Madracis mirabilis, and Acropora palmata (Aronson, 2006). Those natural phenomena have less impacted other coral species such as Colpophyllia natans, Montastraea annularis (Aronson, 2006). Porites and Diploria species were quite prevalent during the snorkeling excursions maybe because their “boulder-like” structures are similar to C. natans and M. annularis which were less affected by the historical events. As coral was a prime and diversified organism of these reefs, there was also much diversity of fish species cohabiting in the reefs.
The many species of coral growth provided both shelter and food for the many species of tropical fish around San Salvador. From Gramma loreto to Acanthurus coeruleus to Sparisoma viride, they each coexisted together within the crevices of the coral reefs while simultaneously competing for algae to eat and shelter to hide from predators. Juvenile fish were skittish with much larger organisms; Sphyraena barracuda were often found surrounding the reefs looking for food during the day. Some fish species were more active at night, in particular, Holocentrus at Dump Reef probably because their red iridescence made them more vulnerable to predators when the Sun was shining.
It is the most parsimonious explanation that the many tropical fish and marine fauna of San Salvador arrived via ocean currents since swimming is their primary activity and Coriolis currents nearly led to the island. The closest ecosystems en route to San Salvador are the Caribbean islands and the northern coast of South America so close comparison of the marine fauna of San Salvador and the Caribbean islands would show similar species. Possibly island-hopping to San Salvador from the Caribbean reefs was due to high population density or seasonal migration pattern. The fauna of San Salvador and the Caribbean islands may also be similar because the herbivores were following their food source flowing with the Antilles current. Sea grasses and algae most likely colonized the seashores of San Salvador by means ocean currents carrying them from the Caribbean and South America, probably making little taxonomic diversity between the islands.
At Graham’s Harbour, Pigeon Creek, and Dump Reef, Thalassia testudinum and Syringodium filiforme sea grass beds were the most abundant producers. These beds provide both shelter and food for many marine animals. Chelonia mydas “Green Sea Turtle” was seen in these areas because it consumes Thalassia “turtle grass”. Also, echinoderms, mollusks, and arthropods, such as sea stars, clams, and crustacea have been known to hide in and feed on sea grasses (Diehl, 1988).
Oyster Pond, a hypersaline lake, had a large distribution of two species of halophytic mangroves, Rhizophora mangle and Avicennia germinans. Conocarpus erectus “buttonwood” is another species of mangrove which was observed more inland during the walk to Oyster Pond. Rhizophora disperses with bobber-like seeds with a rudder-like root and a sail-like shoot, floating around until it settles in an area with the correct conditions (Odum, 1982). This dispersal adaptation seems to be an effective way of traveling to San Salvador via the North Atlantic current from northern Africa, a long distance in salt water; it also possesses an adaptation to exclude salt from its water uptake (Odum, 1982). Rhizophora was found at the edge of Oyster Pond due to its prop roots adaptation whereas the distribution of Avicennia was on the shore around Oyster Pond. Avicennia possesses pneumatophores to assist with respiration and they excrete salt through their leaves (Odum, 1982). The Rhizophora mangals host a diversified array of species such as Acetabularia calyculus, a green alga attached to its prop roots, and Pinctada longisquamosa “Atlantic pearl oysters” habituating in Oyster Pond and must have high salinity adaptations.
Algae were easily the most common marine flora around the seashores of San Salvador. At every snorkeling locale, any or all combinations of Chlorophyta, Rhodophyta, and Phaeophyta algae were noticed. Algae reproduce through a wide variety of methods and do not have true roots (Littler, 1989), attributing to their ability to survive almost anywhere and wide dispersal. Algae are primary producers of this marine ecosystem; many marine fauna such as snails, crustaceans, sea urchins, and fish feed on many different types of alga (Littler, 1989). Some algae contain chemicals that discourage herbivory, like Stypopodium zonale, a brown alga that reflects many colors along separated zones of its stalk, was plentiful around the reefs (Littler, 1989).
Generally the distribution of sea grass beds and coral reefs seemed to be segregated as if there is some form of competition between the two major individuals: sea grasses and corals. I found that sea grasses could grow in deeper water than corals can; however, sea grasses seem more apt to flow with the higher wave energy at the surface than corals. In addition, while snorkeling French Bay, there was relatively high turbidity from the high wave energy crashing on the coral reefs growing near the surface, and possibly suffocating corals as sand settles. Perhaps there is compromise involved with coral growth height between sunlight and wave energy; or maybe only certain species of coral that can deal with the high wave energy at the surface. All in all, this general observation and interpretation seemed to me to be anti-productive for corals.
In conclusion, the diversity of the marine flora and fauna of San Salvador seemed stunningly high; however the diversity between the Caribbean islands and San Salvador seemed low due to the currents of the Coriolis Effect carrying the species from the Caribbean. Throughout history, the marine species of San Salvador could have only increased in diversity since the island’s formation; time being an important variable for habitat colonization.
Aronson, Richard B. and William F. Precht. 2006. Conservation, precaution, and Caribbean reefs. Coral Reefs. Volume 25. 10 pp.
Diehl, F. A., D. Mellon, Jr., R. H. Garrett, and N. Elliott. 1988. A Field Guide to the Invertebrates of San Salvador. Bahamian Field Station, San Salvador, Bahamas. 105 pp. (out of print).
Littler, D. S., M. M. Littler, K. E. Bucher, and J. N. Norris. 1989. Marine Plants of the Caribbean. Smithsonian Institution. Airlife Publishing, Shrewsbury. England. 263 pp. (out of print)
Odum, W.E. 1982. The Ecology of the Mangroves of South Florida; A Community Profile. US Fish and Wildlife Service. Office of Biololgical Services, Washington, DC. 144 pp.
Sealey, N. 2006. Bahamian landscapes. 3rd Ed. Media Publishing, Nassau, Bahamas. Introduction (pp. viii-ix); Chapter 1 (pp. 1-13).