The study of bottom communities, according to the quantitative approach of Petersen, has not been attempted in the Gulf of Mexico or its coastal bays. It is possible to assemble information, in a general way, about certain characteristic bottom aggregations such as coral reefs, sponge grounds, and oyster beds, but the necessary data for comparing these areas with similar situations elsewhere do not exist. This gap in our knowledge has been due in part to our fragmentary information concerning the qualitative composition of the flora and fauna of the area. As many of the contributors of the sections on various animal groups in this volume have shown, our knowledge of the fauna is still far from satisfactory. It is, on the other hand, not without advantage that we have lagged behind other countries in quantitative bottom studies, since we may profit by example. Leaders in this field in recent years have been Russian fishery biologists whose work on biological productivity, biomass, and bottom communities in general has been summarized up to 1947 by Zenkevich (1947). Discussion of the methods and principles of this work are available in English (Brotskaja and Zenkevich, 1939; Zenkevich and Brotzky, 1939). It must be pointed out that such concepts of "productivity," "production," and "biomass" should be used with caution and only after careful consideration of the life spans and metabolic rates of the components of the community.

An example of a study of sponge grounds, in some ways comparable to those off western Florida, will be found in the paper by Chambost (1928). Communities of bay bottoms, including many closely related species living under similar conditions to those encountered along the northern Gulf coast, have been recently described by de Oliveira (1948, 1950). The literature on North Atlantic bottom communities has been reviewed by Jones (1950) who also discusses the concept of marine communities in general.

There is a certain amount of published misinformation, most of it well intentioned, concerning the occurrence of organisms which are considered important community dominants in the Gulf of Mexico. Andree (1920, v. 2, pl. 7), for example, presents a map indicating the occurrence of pearl oysters from Panama northward to Texas and eastward to Alabama. Literally, this is true; small specimens of Pinctada are frequently cast adrift on sargassum and reach these coasts, but there are no pearling grounds in the northern Gulf. At the same time, however, reef building oysters are not indicated west of the Atlantic side of Florida, and there is no indication of the Gulf of Mexico sponge grounds. This is comparable to the gaps in Bartholomew's (1911) Atlas of Zoogeography and Orton's (1937) peculiar omission of oysters from the northern Gulf coast. well-known map of coral reefs indicating reefs throughout the Gulf prepared many years ago by Joubin has appeared in many texts (including the standard Russian text on hydrography). From such information as this, Ekman (1935) originally classified the northern Gulf of Mexico as a tropical littoral region. In his new edition Ekman (1953) gives a more precise account, characterizing the northern part of the Gulf of Mexico as a "warm temperate" region, with species common to temperate eastern North America. A more detailed discussion of the biogeographical relationships of the northern Gulf of Mexico will be found in Hedgpeth (1953).

The

Of the various bottom communities in the Gulf of Mexico, several of them of prime economic importance, the most intensive work has been done on the oyster bottoms. Various surveys have been undertaken, mostly with the purpose of delineating the beds and determining the causes of decline. While most of the published surveys are out of date, they are useful in studying the changes, many of them the result of human interference with the environment, which have

occurred subsequently. The results of the most recent and intensive surveys are not generally available, although two minor papers from one of them have appeared (Norris 1953; Puffer and Emerson 1953).

Most of the study of coral reefs has been faunistic or geological; some minor contributions to the ecology of Gulf of Mexico coral reefs are to be found in the publications of the Tortugas Laboratory. The sponge grounds have been even less adequately studied from the ecological standpoint, although investigations are now in progress.

Gunter's work in Louisiana and Texas

Important marine ecological work, chiefly with motile invertebrates and bottom-feeding fishes, has been done by Gunter in the past 20 years on the coast of Louisiana and Texas. The general results can be divided into the following main categories:

1. A description of the relative abundance and species mass of the larger motile vertebrates and invertebrates.

2. Description of seasonal cycles and movements from Gulf to estuarine waters and return, and seasonal variations in abundance, correlated with temperature change.

3. Distributions of organisms as related to salinity.

This work probably gives as complete a view of the motile and free-swimming fauna of the bays and shallow Gulf as there is for any coast of this continent. While it describes the motile part of the bottom community it does not add to knowledge of the in-fauna of the bottoms. Life history notes on the various important fish and Crustacea with some data on food consumption were gathered as a side issue.

The work in Louisiana was carried on for 2 years from 1931 to 1933. It consisted of the analysis of populations of bottom fishes over a salinity gradient from almost fresh water in the upper part of Barataria Bay to 3 miles offshore in the open Gulf of Mexico. It was during this initial work that Gunter became interested in relative numbers of species mass which he emphasized in later papers. In view of the impossibility of obtaining knowledge of total quantities of the species involved, the relative numbers data of the animals taken in the various environments seem to be about as quantitative as can be obtained. This

was used in estimating the relative species mass. Any gear used gives a somewhat distorted view of the actual populations, and the various advantages and disadvantages of the trawl were considered. The results of this work were given in Gunter (1936, 1938a, b). The seasonal variations in abundance of the whole fish population and of various species alone are given. The seasonal cycle of movement in and out of the bays, giving somewhat regular arrivals and departures of some species, was described. The predominant family of fishes was found to be Sciaenidae, followed by Otolithidae and Engraulidae. Several abundant species of Sciaenidae were led by the croaker, Micropogon undulatus. Certain comparisons between the shallow water fish fauna of the Louisiana and Texas coasts were made later (Gunter 1945). This work was done under the auspices of Shrimp Investigations of the United States Bureau of Fisheries, and during this time large catches of shrimp were made. The data have not been published, but it can be said that the motile fauna living close to the bottom in the Louisiana bays and shallow Gulf consists chiefly of the fishes described by Gunter, the peneid shrimp, Penaeus setiferus and Penaeus aztecus, and the blue crab, Callinectes sapidus.

During the years 1941 to 1943 Gunter set up a series of stations covering a transect from almost pure fresh water (salinity 2.1 parts per thousand) to 5 miles offshore in the Gulf of Mexico and 5 miles down the Gulf beach. This was on the Texas coast and ran through Copano Bay and Aransas Bay and out into the Gulf, a distance of 40 nautical miles. Thirty-two stations were covered by trawl hauls, trammel nets, beach seines, and fine-mesh net hauls on the beach and shores. The original plan was to carry the work on for 2 years, but it was carried out for a year and a half, and only 1 full year without spotty collections was obtained. Even so, it covered bay and estuarine waters and the connecting sea as extensively as has been done in this country.

Gunter (1945, 1950) was considerably impressed with the fact that the bays serve as nursery grounds for large numbers of organisms. For instance, many fishes such as the croaker, Micropogon undulatus, the redfish, Sciaenops ocellata, the mullet, Mugil cephalus, and several others spawn in the Gulf and grow up in the bays. The same holds true for the blue crab, Callinectes

sapidus, and the two shrimp, Penaeus setiferus and P. aztecus, all important components of the fauna. This system of bay waters forms a rim along the whole northern Gulf coast. Although certain species such as the cyprinodont fishes, the common oyster, and the various species of palaemonid shrimp remain in the bays and are found nowhere else, the bays are not faunistically isolated from the shallow Gulf, despite narrow connections through the passes, but form a system with it. The dominant life of the region has perforce become adapted to this estuarine-sea water system and moves back and forth within it during the life cycle.

Gunter also emphasized the importance of two gradients connected or correlated with salinity. One, an ostensible relation between salinity and size, depends on the fact that most motile animals move out from shallower waters as they grow larger and go toward or to the sea. This migration is accelerated by the onset of cool weather in the fall when large movements from the bays to the Gulf take place amounting almost to a general exodus for some species.

The other gradient is a decline in the number of species as the salinity falls. As he pointed out, the bay fauna is marine, and although practically all species can live in high salinities they only tolerate varying degrees of low salinity, and thus the numbers of species present become less as the salinity falls along the gradient. The difference in numbers of species in the Gulf and Copano Bay is particularly striking in winter. At that season the fresher, shallower, and thus cooler waters of Copano Bay are dominated by only four or five motile species. The importance of these general phenomena to paleoecological studies was specially mentioned (Gunter 1947).

Since ecological studies by Gunter did not include the sessile, poorly motile or burrowing forms in the area they are incomplete. Nevertheless, they go a long way toward describing the communities of the shallow Gulf. Readers interested in details should consult the original papers. As for the deeper water communities, virtually nothing is known except the results of sporadic dredging stations by the Blake and Albatross more than 50 years ago. There has been no attempt to list the findings of these dredgings by stations, a difficult task of reassembling in view of the scattered publication of reports on

the various animal groups. Hence, our knowledge of the deep-water life of the Gulf of Mexico is still that of Agassiz' Three Cruises of the Blake.

Investigations of recent facies

A relatively new development is the study of assemblages of living (and dead) organisms as potential fossil assemblages. In such studies, groupings or facies correlated with environmental conditions are emphasized. Such facies may be the same as a natural community (especially that of the oyster reefs), or they may have no particular relationship to the communities in which they occur especially if they include such remains as mollusk shells and coral fragments carried there by physical forces. In his study of molluscanforaminiferan assemblages in San Antonio and Aransas Bays, Texas, Ladd (1951) recognizes a series of facies roughly corresponding to the salinity gradient: bay head, inter-reef, reef, polyhaline bay, passes, open gulf (near- and offshore), beaches, and highly saline lagoon. A similar series, based exclusively on foraminifera is recognized in the same region by Parker, Phleger, and Peirson (1953); river, marsh, bay, beach and open gulf. The distribution of various foraminifera along several transects in the northern Gulf from Florida to Texas in relation to sedimentary facies is discussed by Lowman (1949).

Community terminology

The matter of terminology and classification. of marine communities, in general, is not settled. The ambitious attempt of Clements and Shelford (1939) to classify the biota of the North Atlantic into various biomes and their component associations has served principally to emphasize that the criteria of terrestrial biomes have but limited application to the marine environment. The term biome was accepted somewhat uncritically by Jones (1950) who classified the North Atlantic bottom communities into various hard and soft bottom "biomes." It is suggested in the recent monumental treatise of Allee et al. (1949), that self-sustaining communities within the sea are difficult to recognize, and that biomes, as defined on land, do not exist: "The major marine community despite its great regional biotic variation, is so lacking in effective barriers to dispersal, is so much subject to slow continuous circulation of its medium and exhibits so much interdepend

ence of its components from region to region and area to area, that it may be regarded as a single biome type." This is probably an extreme view, but in view of our ignorance concerning the bottom communities of the Gulf of Mexico, we cannot examine this question in detail here.

It is necessary to emphasize, however, that the complex character of marine communities cannot be simplified by terminology. The dual character of that community which includes the various species of shrimp is an example. Both estuarine and neritic bottom communities are part of this complex which may be best regarded as elements in a major ecosystem transgressing the various environments in both space and time (Hedgpeth 1953). For the purposes of discussion, several "major bottom communities" are recognized (fig. 51). The four major communities recognized are: the oyster bottoms, the shrimp grounds, the coral

reefs and patches, and the sponge grounds. The sponge grounds occupy roughly the same area as the coral patches west of Florida and may, on further study, be considered a subcommunity of the coral grounds. Segregating these by physiographic or environmental requirements, we have in the euryhaline-bay environment the oyster community (and other communities); the shrimping grounds fall in the estuarine and neritic environment; while the coral and sponge communities are stenohaline-neritic. These are also working labels, simply describing as briefly as possible the conditions in which the communities are found.

THE OYSTER COMMUNITY Foremost among the communities of bay waters is the oyster community. This is not a continuous 85

80°

aggregation of oysters covering entire bay bottoms but an arrangement of ridges and patches of oysters and dead shells, "fragmented faciations. of the Macoma-Mya biome," in the language of Clements and Shelford (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 formation 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.

A

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 distributed 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 salinities through the passes from the Gulf during periods of low rainfall and decreased run-off. natural reef is usually oval or spindle-shaped or is a narrow bar extending from the shore. Although reefs in Texas have been badly cut up in recent years by artificial channels and mudshell dredging so that the original pattern is now obscured, 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 Cary (1906), Galtsoff (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).

1 The biology of the oyster of the Gulf 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, Urosalpinx, 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 distribution 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, oyster 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 recently 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 Petroleum Institute (Shepard and Moody, 1952). The