characteristic of this zone is 

 caused, in part, by the discoloring 

 agents of the blue-green algae 

 Entophysalis seasta (granulosa) and 

 other blue-green and green algae. 

 Usually little or no flowering vege- 

 tation exists and the only marine 

 macroalgae of regular occurrence in 

 the black zone form a moss-like 

 growth primarily in hollows. The 

 growth is dominated by the red algae 

 Bostrychia binderi and B. tenella , 

 accompanied by several other smaller 

 forms. Many of the snails charac- 

 teristic of the gray zone also are 

 common in the black. Several 



( Littorina ziczac , Nodilittorina 



tuberculata and Nerita versicolor ) 

 tend to reach their maxima in the 

 black zone. The elongate snail 



Betillaria minima occurs in great 

 numbers here, particularly in shal- 

 low pools and depressions. Other 

 common fauna include the isopod 

 Liqia baudiniana and the crab 

 Pachygrapus transversus. 



The most seaward of the upper 

 platform subenvironments is the yel- 

 low zone. This comprises the main 

 part of the midlittoral zone on the 

 Florida Keys rocky shorelines. The 

 yellow zone represents the only 

 truly intertidal region of the upper 

 platform; it is inhabited by a 

 considerable number of fully marine 

 organisms with a greater species 

 diversity than observed in the main- 

 land zones. Normal spring tides 

 submerge or at least wet the entire 

 yellow zone which includes: (1) the 

 marginal region of the platform top; 

 and (2) the actual edge of the plat- 

 form and its seaward face down to 

 the beginning of the lower platform. 

 The platform-edge's profile may vary 

 from a gradual slope to a precipi- 

 tous or overhanging scarp. In gen- 

 eral, the yellow zone consists of 

 two distinct communities which cor- 

 respond to the platform's regions: 

 (1) marginal regional-upper yellow 



zone; and (2) seaward face-lower 

 yellow zone. The upper yellow zone 

 fauna consists primarily of the 

 small barnacle Chthamalus stellatus , 

 or the larger Tetraclita squamosa . 

 The red algae moss Bostrychia sp., 

 is the primary flora of the zone. 

 The nerite snails, Nerita tessellata 

 and N. versicolor also extend into 

 the upper yellow zone, occasionally 

 in great numbers. The lower yellow 

 zone is well marked by the presence 

 of two dominant colonial organisms: 

 (1) a green algae, Valonia ocellata , 

 which forms nodulated sheets or mas- 

 ses of small bubble-like vesicles; 

 and (2) a sedentary vermetid gastro- 

 pod, Spiroqlyphus annulatus , which 

 produces sheets or masses of narrow 

 tubes. Occasionally a third subzone 

 of reddish brown algal 'velvet' 

 overlaps the two previous subzones. 

 The velvet mat is composed of sev- 

 eral algae including Gelidiella 

 acerosa , Centroceras clavulatum , 

 Cladophoropsis membranacea , Cera- 

 mium subtile , Herposiphonia sceun - 

 da , and Polysiphonia howei . Scat- 

 tered throughout the lower yellow 

 zone are two whelks Thais rustica 

 and Cantharius tinctus . Fauna com- 

 mon to the entire yellow zone in- 

 clude the small finely ribbed mussel 

 Mytilus exustus , a small dark-edged 

 oyster Isognomon alatus , the large 

 chiton Acanthopleura granulata , the 

 most frequently encountered crab 

 Pachyqrapsus transversus , the lim- 

 pets Siphonaria pectinata and S. 

 alternata , and a small black slug 

 Onchidium floridanum (Stephenson 

 and Stephenson 1950). 



The lower platform, when pre- 

 sent, is fully exposed to the air at 

 low water of major spring tides and 

 partly exposed or awash at lesser 

 spring tides. A distinctive feature 

 of this platform is the occurrence, 

 often quite extensive, of a low 

 yellowish-green carpet dominated by 

 the red alga Laurencia papillosa . 



184 



