The southeastern Florida continental shelf is part of an extensive subsiding carbonate platform that includes the Florida peninsula and west Florida shelf. The shallow Florida Reef Tract appears to end gradually, ranging from abundant to scattered low-relief rubble or hardground separated by extensive sediment. Hard substrates become smaller and more isolated with increasing depth.
In the northern Strait, sediment substrates dominate to 253 m, where extensive outcrops of the western margin of the Miami Terrace begin to appear. The seafloor becomes chiefly hard substrate in 295 m. However, from about 120 to 150 m, and again in about 160 to 185 m (sometimes to ~200 m), the sediment bottom is interrupted by areas of sometimes abundant rubble and occasionally larger rocks. From these depths to 295 m, sediment bottoms are flat, smooth or weakly bioturbated muddy sand characterized by scattered small low mounds, depressions and small tubes (perhaps polychaete), alternating with areas of inactive rippled sediment usually elevated ~10-30 cm above adjacent bioturbated areas, and sometimes recognizable as low sand waves.
In the northwestern Strait, surveys off Fort Lauderdale found that, beyond the outer reef, sediments exhibit small pockmark burrows, and broad low rubble mounds possibly produced by the tilefish, Malacanthus plumieri. Craters, sometimes >30 cm across, first appear in around 100 m and continue in some places to 160 m, but their excavators are unknown. Rubble and rocks in ~120-140 m support numerous bright red octocorals, perhaps Leptogorgia sp., accompanied by numerous small unidentified solitary corals. Fishes include scorpionfishes (Scorpaenidae) and sea robins (Triglidae). Below ~150 m, epifaunal organisms include anemones (Actinoscyphia sp. and possibly Actinauge longicornis). The latter anemone anchors to both hard bottoms and to sediment by surrounding a bolus of mud with its pedal disk. Larger rocks support thecate hydroids, other anemones, sabellid polychaetes and an often dense turf of small (1-2 cm tall) arborescent organisms, perhaps agglutinated astrorhizacean foraminiferans. Tufts up to 3 cm tall on sediment substrates are either bushy growths or stalks with a cluster of fine radiating filaments arising from the upper half of the stalk—perhaps worm tubes with epifauna or attached sponge spicules.
Echinoderms, crustaceans and anthozoans dominate the sediment megafauna west of the Miami Terrace, although many of the same mobile taxa also occur on low-relief hardbottoms. Species that first appear in 100-200 m and continue to at least 270-295 m include burrowing anemones (Ceriantharia), the sea stars Coronaster briareus and Sclerasterias contorta, the squat lobster Munida iris, hermit crabs, the swimming crab Bathynectes longispina, the Jonah crab Cancer borealis, the spider crab Rochinia crassa, the blind torpedo ray Benthobatis marcida, the flatfish Citharichthys arctifrons, and armored sea robins Peristedion sp.
Species that first appear on sediments below about 200 m include the asteroid Astropecten sp., the urchins Gracilechinus sp., Araeosoma sp., Brissopsis spp. and the pencil urchin Cidaris rugosa; the greeneye Chlorophthalmus agassizi, tilefishes Caulolatilus microps and Lopholatilus chamaeleonticeps and unidentified Rajidae. Ophiuroids are locally abundant chiefly below 260 m, and mostly isolated sea pens (Pennatula sp. or Ptilosarcus sp.)appear in >275 m.