348 



THE GORGONACEiE AS A FACTOR 



average truly representative of the condition throughout the reefs within 

 this group of islands. Squares 16 to 18 were on the outer, exposed side of 

 a long reef which slopes rapidly into the deep water of the Rebecca Shoal 

 channel, and is the most exposed location where collections were made in 

 shallow water. Squares 19 and 20 were on a very shallow protected reef 

 where wave action and currents are at their minimum for this region. 

 Altogether, the conditions represented by the areas chosen for these collec- 

 tions fairly represent those under which the Gorgonians are growing on all 

 the reefs about the Tortugas groups. Since all of the species included in 

 these determinations occur on the reefs in depths from just below low-tide 

 mark to 6 fathoms, the fact that the collections were made in shallow water 

 does not introduce any unusual condition which could invalidate the results 

 secured. 



Table 2. — Weight of Gorgonians taken from a square yard on the crests of shallow reefs about 

 Dry Tortugas, and the amount of spicules for each square. 



Square. 



No. 



3 

 4 



5 

 6 



7 

 8 



9 

 10 

 11 

 12 

 13 

 14 

 15 

 16 



17 

 18 



19 



20 



Location of square. 



Weight of 



Gorgonian 



colonies. 



Shallow reef west of light-house on Loggerhead Key 



Outer shallow reef northwest from light-house 



West Reef opposite light-house wharf 



West Reef opposite north end of Loggerhead Key 



West Reef north from laboratory wharf 



West Reef north by east from Loggerhead Key 



East Reef southeast from light-house, sandy bottom with scattered coral heads. 



East Reef south from light-house, rough bottom 



East Reef south by west from light-house, near shore 



East Reef, very shallow portion southeast from light-house, on north side of reef. 



East Reef, central part of shallow area 



East Reef, south side of shallow area 



East Reef northeast from light-house; in I fathom 



East Reef, east from laboratory 



Reef southwest of Bird Key 



Outer edge of Bush Key reef, at north end 



Outer edge of Bush Key reef, near entrance to 3-foot channel 



Outer edge Bush Key reef, southeast from Fort Jefferson 



On shallow reef northwest from Fort Jefferson 



On same reef, west from Fort Jefferson 



lbs. 



9.25 



16 

 7 

 4 



11 



8 



7^ 

 SO 

 25 

 75 

 5° 



25 .00 

 3.2; 

 5.00 

 3.00 

 4-75 

 7-5° 



20.50 

 2.00 

 1.50 

 5.00 

 7.50 

 3-75 

 4-25 

 6.25 



Average weight of spicules from the 20 square yards. 



Weight 



of 

 spicules. 



lbs. 

 2.514 

 4.92 

 1.875 

 1. 19 

 2.89 



2-45 

 6.94 

 0.84 



1 -5° 

 0.91 



i-43 

 2.25 

 '4.10 

 0.56 

 0.45 

 1.22 

 1.875 

 1. 125 

 1. 180 

 1 .650 



. 1225 



'The Gorgonian fauna of this square was made up entirely of the two species Gorgonia ftabellum andC. acerosa, 

 so that the weight of spicules in proportion to the weight of the mass of colonies is decidedly lower than the average 

 for other squares. 



The very marked differences in abundance of the Gorgonian fauna seen 

 when square 7 is compared with square 15 may be found over any large reef 

 area. In some particular locations, comparatively large areas, several hun- 

 dred yards in extent, are found where the Gorgonians are more abundant 

 than on square 7, although every reef has areas where nothing but bare lime- 

 stone or coral sand occurs. As a general feature of most of the shallow reefs 

 within the Tortugas group, the Gorgonians are, however, the most prominent 

 feature of the topography, usually outnumbering all other organisms of any 

 noticeable size. The locations of the squares mentioned above are shown 

 on the map (plate 100). 



