Lying along the southern edge of the Florida Peninsula, the Pourtalès Terrace forms a narrow, gently curved triangle that parallels the Florida Keys for 213 km, from southern Key Largo to just west of the Marquesas Keys between Key West and the Dry Tortugas. Geologically, it is continuous with the Miami Terrace to the north, but the connecting portion is buried under thick sediments. The Terrace covers 3,429 km2 and reaches its greatest width, 32 km, south-southeast of Vaca Key, where the apex of the triangle actually lies closer to Cay Sal Bank in the Bahamas than to the Florida Keys. The Terrace platform begins at a depth of ~180 m and slopes gently to 450 m, where the Pourtalès Escarpment slopes steeply to the deep floor of the southern Strait. Count Louis Francois de Pourtalès discovered the Terrace in 1867 during a survey aboard the U.S. Coast Survey steamer Bibb for a telegraph cable between Key West and Havana, Cuba. Alexander Agassiz named it the Pourtalès Platform in honor of its discoverer, and Jordan and Stewart renamed it the Pourtalès Terrace in 1961.
The Terrace surface is an extensive hardbottom habitat of Neogene limestones characterized by complex karst-like topography. A chain of sinkholes runs for about 100 km along the southwest margin; the largest is the Jordan Sinkhole, a pair of steep-walled depressions in 350 m that are as much as 260 m deep. The eastern Terrace features a band of numerous high-relief knolls and ridges that reach up to 91 m above the surrounding Terrace floor and include several popular fishing sites (e.g., The Islamorada Hump). Above 300 m, the exposed bedrock is Miocene, whereas below 350 m, it is highly phosphatized Eocene biocalcarenite. Mio-Pliocene cobbles and vertebrate bones (including those of extinct dugongs) lie scattered on top of the bedrock and accumulate in sinkholes. The strong Florida Current prevents more recent sediments from accumulating as more than a thin veneer.
Seafloor biological research on the Terrace was long based chiefly on a few submersible dives and a moderate number of trawls and dredges, so the pattern of assemblages was not well known. Since 2010, however, NOAA Fisheries has carried out a series of ROV expeditions to investigate how the recently established Marine Protected Areas on the Terrace have affected bottom assemblages and populations of commercially important fishes. The dominant skeleton-building corals on the Terrace are lace corals of the hydrozoan family Stylasteridae, which may form vast multi-colored meadows. However, in September 2011, one of these expeditions discovered in 490-510 m the first deep coral mound on the Terrace dominated by the stony coral Lophelia pertusa, which is the dominant deep reef builder elsewhere in the Strait (and in much of the rest of the world's oceans as well). Terrace stylasterids include Distichopora foliacea, Pliobothrus echinatus, Stylaster erubescens, S. filogranus, and S. miniatus. Other abundant attached organisms include a wide variety of sponges (Leiodermatium sp., Phakellia sp., Spongosorites sp., Echinodictyon sp., Auletta sp., Oceanapia sp., Geodia sp., Hymedesmia spp., Corallistes sp., Pachastrellidae, Petrosiidae and hexactinellids), octocorals (e.g., Plumarella pourtalesi, Placogorgia mirabilis, Muriceides sp., Paramuricea multispina and Thesea parviflora), antipatharian black corals (e.g., Leiopathes sp., Antipathes rigida) and an unidentified pterobranch hemichordate. Mobile invertebrates include spider crabs and the urchins Coelopleurus floridanus and Calocidaris micans. Interestingly, several trawl hauls in the 1960s recovered hundreds of small feather stars, Coccometra hagenii (Antedonidae), which are found almost nowhere else. However, none were seen during the recent ROV expeditions. Instead, Comatonia cristata was the most common feather star. Fishes include snowy and warsaw grouper (Epinephelus niveatus and E. nigritus); yellowfin, apricot and roughtongue bass (Anthias nicholsi, Plectranthias garrupelus, Holanthias martinicensis); red barbier (Hemanthias vivanus), (all Serranidae), blackbelly rosefish (Helicolenus dactylopterus), roughy (Hoplostethus sp.), Darwin’s slimehead (Gephyroberyx darwini),deepbody boarfish (Antigonia capros), blueline tilefish (Caulolatilus microps) and scorpionfishes (Scorpaenidae).
Although the results of the recent ROV dives have not been analyzed in detail, results of an earlier submersible dive in the Jordan Sinkhole give an idea of zonation with depth. Along the lower wall (457-546 m), the rock is almost entirely encrusted with sponges (Siphonodictyon sp.), hydroids, octocorals (P. pourtalesi, Swiftia spp.), ascidians, and serpulid worms. Rocky ridges nearer the top (384-335 m) support stylasterids, octocorals (Keratoisis flexibilis, Paramuricea multispina), hexactinellid sponges (Farrea sp.), the fan sponge Phakellia sp. and a few antipatharians. Fishes include roughies (3 species of Trachichthyidae), H. dactylopterus, Laemonema sp. and conger eel (Conger oceanicus).