Coral Reefs of the Southern Gulf of Mexico

Chapter 6. Reef Biodiversity

Reef Biodiversity Kim Withers and John W. Tunnell Jr. Coral reefs are known to harbor the highest biological diversity (biodiversity) of any habitat within the sea. Over 2,000 marine species and almost 300 terrestrial species have been reported from the southern Gulf of Mexico coral reefs and their islands, respectively (Table 6.1 and 6.2). Since a list of this size is beyond the scope of this book, we refer the reader to www.gulfbase.org (Nipper et al. 2004) for complete species checklists that include references, distributions, and habitats. Few reefs anywhere in the world have a complete species list available for comparison, but regional comparisons for selected taxa are helpful in understanding regional biogeography. Generally speaking, for coral reef organisms, the number of species within a particular group decreases as one moves from the center to the periphery of the tropics. In the case of the Caribbean Sea, latitudinal gradients in species diversity (numbers of species) have been recognized as one makes the transition from Jamaica (center with high diversity) to the Bahamas and east coast of Florida (periphery with lower diversity). Likewise, moving westward into the semienclosed Gulf of Mexico, gradients exist in numbers of species from east to west and south to north. Temperature has been suggested as the single most important environmental factor governing these changes and limitations, but local factors, such as salinity, topography, depth, turbidity, and nutrients, among others, can also account for varying numbers of species or genera in any given locality. For example, both stony corals (scleractinians) and soft corals (gorgonians), demonstrate this distributional pattern. Horta-Puga et al. (chapter 8, this volume ) indicate that almost 70 zooxanthellate coral species are found in the Caribbean Sea, the Caribbean Biogeographic Subprovince (Veron 1995). Within the southern Gulf of Mexico 42 species have been recorded, and there appears to be a gradient from slightly more species on the Campeche Bank reefs to fewer on southern Veracruz reefs (Veracruz Reef System) and even fewer still on northern Veracruz reefs (Tuxpan Reef System). Notable Caribbean genera missing from the southern Gulf include Cladocora, Dendrogyra, Isophyllia, Isophyllastrea, and Solenastraea (Horta-Puga et al., chapter 8, this volume). Likewise, within the southern Gulf, the following species occur on Campeche Bank reefs but are absent from Veracruz reefs: Diploria labyrinthiformis, Eusmilia fastigiata, Madracis mirabilis, and Mycetophyllia aliciae. Among the milleporid hydrocorals , only Millepora alcicornis occurs throughout the southern Gulf, while M. complanata and M. squarrosa are found only on Campeche Bank reefs. Among the gorgonians, or soft corals, this gradient or geographic limitation is even more evident. Jordán-Dahlgren (2002), who studied the northeastern Yucatán–Caribbean Sea gorgonians for two decades, clearly demonstrated two distinct gradients in the Gulf of Mexico: (1) a westward-decreasing gradient in 6 A4339.indb 68 A4339.indb 68 7/6/07 11:19:09 AM 7/6/07 11:19:09 AM Table 6.1. Number of marine species found in each division (plants) or phylum (animals) on the coral reefs of the southern Gulf of Mexico (Plates 40–46). Number of Taxon Species Division Cyanophyta (Blue-green algae) 17 Division Bacillariophyta (Diatoms) 50 Division Chlorophyta (Green algae) 116 Division Phaeophyta (Brown algae) 50 Division Rhodophyta (Red algae) 165 Division Spermatophyta (Vascular plants, seagrasses) 5 Phylum Granuloreticulosa (Foraminiferans) 130 Phylum Ciliophora (Ciliates) 71 Phylum Porifera (Sponges) 38 Phylum Cnidaria (Corals, anemones, jellies) 102 Phylum Annelida (Segmented worms, polychaetes) 45 Phylum Mollusca (Seashells) 548 Chitons (7) Bivalves (134) Gastropods (Snails and conchs) (405) Cephalopods (Octopus) (2) Phylum Arthropoda 257 Ostracods (88) Decapods: Shrimp (63) Lobsters (3) Crabs (93) Phylum Sipunculida (Sipuniculids) 2 Phylum Echinodermata (Echinoderms) 47 Sea lilies (1) Starfish (5) Brittle stars (15) Sea urchins, etc. (14) Sea cucumbers (12) Phylum Hemichordata (Acorn worms) 1 Phylum Chordata (Chordates) 413 Subphylum Tunicata (Tunicates) (24) Subphylum Vertebrata (Vertebrates) (389) Class Elasmobranchiomorphi (Sharks, rays) (29) Class Osteichthyes (Bony fish) (347) Class Reptilia (Sea turtles) (4) Class Mammalia (Marine mammals) (9) Total Marine Species 2057 Notes: A full checklist of all species is at www.gulfbase.org (Nipper et al. 2004). Numbers in parentheses are subtotals for species of lower taxa and are included in the total for phyla. A4339.indb 69 A4339.indb 69 7/6/07 11:19:09 AM 7/6/07 11:19:09 AM 70 Withers and Tunnell gorgonian diversity and abundance from the northeastern Yucatán to the southwestern Gulf (Table 6.3...