GULF OF MEXICO 



183 



Caloglossa, and Bostrychia will generally be im- 

 portant, while much farther up, fresh-water genera 

 including, particularly, the peculiar Compsopoijon, 

 will dominate. It is notable, however, how far 

 upstream the adaptable genera mentioned may, 

 in the absence of pollution, be found. 



SANDY SHALLOWS AND "REEFS" OF SHELL AND 

 CORAL RUBBLE 



Shallow water among the keys combines the 

 advantages of shelter from storms with ample 

 water circulation, sunshine, and freedom from 

 fresh water and mud. Sometimes similar sandy 

 flats extend out far from the shore but are less 

 protected toward the sea than those among the 

 keys. Often the white coral sand is directly ex- 

 posed and is only partly covered by the vegeta- 

 tion which is chiefly limited by the number of 

 suitable objects on which algae may attach. Over 

 the soft sand bottom the marine vascular plants 

 may form vast "sea grass" meadows with Halo- 

 dule wrightii, Cymodocea manatorum Asch. (man- 

 atee grass), and especially Thalassia testudinum 

 (turtle grass) in shallow water, and with Halophila 

 baillonis Asch., at a depth of several meters, gen- 

 erally 5 or more, exceeded by H. engelmanni Asch., 

 which may go as deep as 75 meters, probably 

 much more. While little grows on the Halophila, 

 there may be a great variety of epiphj'tes on the 

 Thalassia and other shoal-water types. Most 

 ubiquitous of these are the thin encrusting species 

 of Fosliella. Also common are species of Spyridia, 

 Hypnea, small Laurencias and Chondrias, slippery 

 brown Aegira and Ectocarpus, and many others. 

 Nowhere is one feature of the subtropical and 

 tropical marine flora, the extreme etiolation of 

 the Rhodophyceae, more evident than on these 

 sandy shoals. The Chlorophyceae are just as 

 gi'een here as elsewhere, the Phaeophyceae as 

 brown, but the Rhodophyceae do not have much 

 need for and so do not develop their phycoerythrin 

 pigment, and their small proportion of green 

 chlorophyll is not conspicuous, so their aspect is 

 light and dull purplish to straw-colored. 



Apart from epiphytes, many other algae grow 

 in these "grass" beds. Mats of Amphiroa Jragil- 

 issima are common as are scattered plants of 

 Penicillus capitatus, P. pyriformis and P. dume- 

 tosus, Avrainvillea nigricans, Udotea flabellum, 

 Halimeda tridens, and H. monile. Others, like 

 the Caulerpas, rather tend to replace the "grass" 



beds as they have similar spreadmg rhizomes. 

 Wliile C. sertularioides and C. cupressoides may 

 be commonest, other species also occur, such as 

 C. paspaloides and particularly C. prolifera. 



In more open water where "grass" beds are 

 lacking, great beds of Halimeda opuntia (fig. 49-b) 

 occur, a plant of different habit from its congeners, 

 and, if large shells and coral fragments abound, 

 many species typical of coralline reefs also find 

 footing, particularly Laurencia obtusa (fig. 50-b), 

 Padina sanctae-crucis, Dictyota divaricata, CaUlerpa 

 racemosa, Batophora oerstedi, and Acetabularia 

 crenulata. The presence of a slight ridge or "reef" 

 of shell fragments may, for instance, determine 

 the presence of a striking local colony of Aceta- 

 bularia or of its relative, Neomeris annulata. 



ROCKY SHORES AND INSHORE REEFS 



The area under consideration is exceptionally 

 poor in tracts of rocky shoreline. The only por- 

 tion of shoreline with such a substratum of whose 

 algae we have any detailed account is that of 

 south Florida, particularly the Florida Keys, 

 where a certain amount of calcareous rock may 

 lie at the waters' edge, and where old coral forma- 

 tions in shallow water afford even better attach- 

 ments for algae. Equally promising stretches of 

 shoreline occur in Cuba and Mexico, but of their 

 productivity we have no detailed account. Ma- 

 rine algae do not flourish m tracts preempted by 

 vigorous, actively growing corals and gorgonians. 

 It is on these older reefs, on submerged structures 

 built of stone, and in lesser degree on concrete, 

 wood, and iron structures, that the vegetation 

 gives us our chief view of algae suited to these 

 stable anchorages. Through much of Florida and 

 all the north and northwestern sectors only these 

 artificial structures support what in a completely 

 natural state would only occur as lithophilic 

 colonies, and being relatively recent, small, and 

 scattered, the variety of plants they support is 

 far less than one would expect on the rocky shores 

 of West Indian islands. 



In dealing with so great a coastline, much quite 

 unknown as to vegetation, and considering all 

 seasons of the year, it is impossible to be specific 

 as to algal communities, though some suggestions 

 can be made. If circumstances permit, the upper 

 zone of algal growth will be one of marine Myx- 

 ophyceae in a black or olive crust, with below it 

 very usually a clear zone of Bostrychia tenella or 



