GULF OF MEXICO 



207 



ii^grcgation of oysters covering entire bay bottoms 

 but an arrangement of ridges and patches of 

 oysters and dead sliells, "fragmented faeiations 

 of the Macoma-Mya biome," in the language of 

 Ch>ments and Slielford (1939). Since Mya is 

 absent from Gulf waters and Macoma sparsely 

 scattered, this terminology has little meaning. 

 To think of oyster reefs as isolated patches in 

 extensive clam beds is to overlook the influence 

 of oysters in changing the bottom of the bays and 

 the conditions of life for the clams. The clam 

 beds, where they may occur, might better be 

 considered as fragmented by the oyster reefs. 



The formation of oyster reefs was studied by 

 Grave (1905) who proposed a theory of the forma- 

 tion of oyster reefs transversely across bays. 

 This theory still remains the best explanation for 

 this characteristic placing of oyster reefs. See 

 figure 52 for a sampling of typical examples, 

 including some studied by Grave.' As may be 

 seen from the figure, not all reefs are transverse; 

 some are parallel to the main currents. 



The typical oyster reef on the Gulf coast is, in 

 cross section, a low mound with a high center, or 

 "hogback," which is occupied by loose dead shells 

 with the live oysters on the sloping shoulders. 

 These reefs occur on muddy bottoms widely dis- 

 tributed in bays of lower salinities and more or 

 less restricted to the upper ends of those bays 

 which are subject to the invasion of higher salini- 

 ties tlirough the passes from the Gulf during 

 periods of low rainfall and decreased run-off. A 

 natural reef is usually oval or spindle-shaped or 

 is a narrow bar extending from the shore. Al- 

 though reefs in Texas have been badly cut up in 

 recent years by artificial channels and mudshell 

 dredging so that the original pattern is now ob- 

 scured, the usual location of the reefs is such that 

 their long axes are at right angles to the prevailing 

 currents of the bays. Many of these reefs can be 

 studied in the various coastal charts, and details 

 of the more important oyster reefs of the Gulf 

 waters will be found in the old survey papers of 

 Gary (1906), GaltsofT (1931), Moore (1899, 1907, 

 1913a, 1913b), Moore and Danglade (1915). 

 Ecological accounts will be found in Pearse and 

 Wharton (1938), Archer (1947, 1948a, 1948b), 

 Puffer and Emerson (1953, pp. 164-173). 



' The biology of the oyster of the Oulf coast and the oyster reefs of the Gulf 

 of Mexico are discussed in detail in chapter XV of this book in articles by 

 Philip A. Butler, p. 479, and W. Armstrong Price, p. 491. 



Gulf coast oyster communities differ from those 

 of Chesapeake Bay and more northern waters in 

 lacking predacious starfish, and the Atlantic oyster 

 drill, Urosalpini, is replaced in the lower bays 

 of the Gulf by Thais. Other than this, the 

 communities are essentially like those of the 

 Atlantic coast. One of the peculiarities of distri- 

 bution within the oyster community or biocoenosis 

 is the apparent absence of the commensal (or at 

 times parasitic) crab. Pinnotheres ostreum, from 

 the northeastern part of the Gulf, although it has 

 been reported from Cameron, Louisiana, and is 

 not rare in Matagorda and Mesquite Bays in 

 Texas. 



There are some examples of marginal oyster 

 communities which are worthy of notice. In parts 

 of coastal Louisiana, especially in the vicinity of 

 Atchafalaya Bay and Marsh Island, oj'ster reefs 

 in the bays have been reduced by invasion of fresh 

 water, and salinity conditions suitable for the 

 development of reefs are found in the Gulf itself. 

 At the other extreme, a small oyster community 

 persists near Port Isabel where salinities are nearly 

 oceanic most of the year, and the epifauna is 

 characteristically marine (Hedgpeth 1953). 



Since the reefs south of Marsh Island were 

 mapped in 1906 by Cary, there seems to have been 

 little change in their extent, and they remain the 

 only extensive oyster reefs known in the Gulf of 

 Mexico proper. From time to time there have 

 been rumors of large reefs in offshore waters, but 

 these rumors seem to be kin to those of fabulous 

 lost mines which can never be found. 



Clam beds have been reported for various places, 

 but none have been studied. The low-salinity 

 Rangia forms extensive beds in Louisiana and 

 brackish lakes of Texas as far south as Green Lake. 

 Extensive worm communities probably exist, in 

 view of the great shrimp populations, but none 

 have been studied in detail. We have only re- 

 cently begun to learn which species of worms 

 occur (Hartman 1951). Beds of Spiochaetopterus 

 have been observed in Louisiana. The only study 

 of clam beds is that of Spaulding (1906) who 

 worked out the distribution of clams and scallops 

 in the Chandeleur Islands (fig. 53). 



Investigations of bottom communities in Texas 

 and Louisiana are now being conducted as part 

 of a study of the nearshore Recent sediments. 

 This project is sponsored by the American Petro- 

 leum Institute (Shepard and Moody, 1952). The 



