Figure 57. Profile of marine habitats off the Florida Keys 

 (adapted from Enos 1977). 



The former of the two (high- 

 velocity hardbottom) generally 

 occurs within channels and passes 

 between keys where strong tidal cur- 

 rents and low sedimentation maintain 

 an exposed hardrock substrate, e.g.. 

 Knight Key Channel between Vaca and 

 Pigeon Keys. These areas are typi- 

 cally quite shallow ( 1 to 5 m or 3 

 to 16 ft) and, because of the place- 

 ment in tidal channels, are exposed 

 to a wide range of water quality 

 conditions (temperature, salinity, 

 turbidity). On this limestone-pave- 

 ment substrate a soft coral ( Alcyo - 

 narian ) and sponge community usually 

 dominates, commonly covered with 

 less than 3 cm (1 in) of calcareous 

 sediment (Zischke 1973). Voss and 

 Voss (1955) designate this region 

 the Alcyonaria zone in their study 

 of Soldier Key. Zischke (1973) 

 expanded the habitat name to the 

 Alcyonaria-sponge community to 

 signify the numerous poriferas, the 

 most prominent of which is the large 

 loggerhead sponge Spheciospongia 

 vesparia . Other sponges commonly 

 observed include Spongia qraminea , 

 three species of I rcinia , and 

 Halicondria melanadocia . Typical 

 soft corals (alcyonarians) include 

 Briareum asbestium , Eunicea knighti , 

 Pteroqorqia anceps , Corqonia flabel - 

 lum , and Pseudopteroqorqia acerosa . 

 The stony corals (scleractinian 

 corals) Porites jurcata , Flavia 

 fraqum , and Siderastrea siderea are 

 also common to this community (Kis- 

 sling 1965, Zischke 1973). Communi- 

 ties of small invertebrates reside 



among the sponges and soft corals; 

 these usually include annelid worms, 

 brittlestars, and a variety of small 

 crustaceans, e.g., snapping shrimp. 



The Alcyonaria-sponge zone is 

 generally an area of low primary 

 productivity with only a few algal 

 species including the green algae 

 Acetabularia crenulata , Halimeda 

 sp., Penicillus sp., and Valonia 

 ventricosa ; the brown algae Dictyota 

 dichotoma ; and the red algae Champia 

 parvula , Chondria sp., Eucheuma 

 isiforme , Goniolithon sp . , Gracila - 

 ria sp., Heterosiphonia qibbesii , 

 Laurencia poitei , and Lithothamnium 

 sp. (Zischke 1973). Most of the 

 community's nutrients are imported 

 from adjacent grass beds (as detri- 

 tal material) or from plankton. The 

 stinging fire-coral Millepora alci- 

 cornis is commonly found on the dead 

 skeltons of the alcyonarians, and 

 the brittlestar Astrophyton murica- 

 tum is often found entwined in the 

 branches of sponges. Irregularities 

 in the rock face or the presence of 

 coral fragments provide a shelter 

 for the long-spined black sea urchin 

 Diadema antillarum , many species of 

 mollusks, and the Florida spiny lob- 

 ster Panlirus arqus (Voss and Voss 

 1955, Enos 1977). 



The distinquishing characteris- 

 tic of the two nearshore rock-bottom 

 habitats is circulation. In the 



tidal channel habitat previously 

 discussed, high velocity currents 

 dominate the area's hydrology. In 



189 



