GULF OF MEXICO MADREPORARIA' 



By F. G. Walton Smith, Marine Laboratory, University of Miami 



By reason of the ^reat differenee in llieir normal 

 habitat corals fall into two very distinct groups. 

 The hermatypic or reef corals are usually, but not 

 always, largo and massive or branching in form. 

 They are usually associated with other corals in 

 building considerable masses of living coral reef. 

 The deep-water or ahermatypic corals, on the 

 other hand, are usually small and solitarj' though 

 sometimes branching in form. 



Hermatypic corals grow in water up to 50 fath- 

 oms in depth but are only active in reef building 

 in depths to 25 fathoms. Most reef growth occurs 

 in less than 15 fathoms. Ahermatypic corals are 

 found mostly in deeper water from the edge of 

 the continental slope to the neighborhood of 3,000 

 fathoms. The majority live between 90 and 300 

 fathoms. The temperature range for reef corals 

 is approximately 19° C. to 36° C. (63° F. to 97° 

 F.) with an average minimum, however, of 22° C. 

 (72° F.). Ahermatypic corals live best within a 

 range from about 8° C. to 21° C. in the West 

 Indian region. 



The distribution of reef corals in the Gulf of 

 Mexico and the relation of the coral fauna of this 

 area to those of neighboring areas is dependent 

 upon the physiological requirements of corals. 

 These have been studied in detail and are dis- 

 cussed by Vaughan in a series of papers describing 

 experiments carried out at Dry Tortugas. 



The average optimum salinity for reef corals is 

 36 parts per thousand, although a range of 27 to 

 40 parts per thousand may be tolerated. Expo- 

 sure to air is also tolerated to a variable extent. 

 Species with more porous skeletons are consider- 

 ably more resistant to exposure. Strong light is 

 essential to vigorous growth. This is apparently 

 the result of the zooxanthellae which are normally 

 present in the tissues of reef corals. Corals are 

 carnivorous in habit. 



Reef corals do not, as a rule, withstand any 

 great amount of sediment and are accordingly 



' Contribution No. 106 from the Marine Laboratory, University of Miami. 

 259534 O— 5-I 20 



found where vigorous water circulation exists 

 Tiie branching corals grow more readily in com- 

 paratively still water than do the massive types. 

 A few species such as Porites furcata and Manicina 

 areolata may be found on muddy bottoms. 



Growth rates of corals at Dry Tortugas have 

 been measured by Vaughan. Non-porous species 

 grew at an average rate of 9.0 mm. in diameter 

 and 5.00 mm. in height per year. Porous species 

 increased at an annual rate of 40 mm. in diameter 

 and 25.0 mm. in height. Montastrea annularis, 

 a massive type, showed an annual increment in 

 weight of 54.8 percent, whereas, a branching coral, 

 Acropora palmata, increased 194.9 percent in 

 weight per annum. Both specimens were approxi- 

 mately 100 grams at the beginning of the experi- 

 ment. Growth of corals is greater at higher 

 temperatures. Since temperatures in the Gulf of 

 Mexico are generally close to the lower limit of 

 the range, reef growth is accordingly slower than 

 in the warmer seas. Larvae of species found in 

 the Gulf of Mexico have a planktonic life of be- 

 tween 1 and 3 weeks. 



Winter temperatures in the Gulf of Mexico are 

 close to the lower limit for vigorous reef growth. 

 There are therefore no strongly developed reefs 

 except for those of the Dry Tortugas, the Florida 

 Keys, and the Alacran and other reefs of the 

 Campechc Bank. Less vigorous reef develop- 

 ment is found at Veracruz and at a few other 

 places within the warmer, more souther!}' waters 

 of the Gulf (fig. 62). 



Scattered coral heads which fail to form true 

 reef structures are found elsewhere in the Gulf of 

 Mexico, particularly off the west coast of Florida. 

 Because of the amount of sediment present, they 

 are rarely found close inshore but usually at some 

 distance from the coast in more than 5 fathoms of 

 water. Scattered heads are also found in deeper 

 water in a line running from south of New Orleans 

 toward the Texas coast and in another line run- 

 ning southward parallel to the Texas coast. Sur- 



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