ABSTRACT

The shallow, brackish-water environment of San Antonio Bay, Texas, supports a benthic foraminiferal fauna whose major constituents are widespread around the margin of the Gulf of Mexico, the southern Atlantic Coast of the U. S., the West Indies, and in low latitudes along the Atlantic and Pacific Coasts of South America. Several species of Ammotium, Ammonia, and Elphidium have been recognized by most authors as the dominant taxa in these estuaries. A scanning electron microscope (SEM) analysis of the exterior test morphology in five species from San Antonio Bay (Ammonia parkinsoniana, Elphidium gunteri, E. galvestonense, Palmerinella palmerae, and Ammotium salsum) reveals that two distinct phenotypes are present within each species. Each phenotype of a given pair is linked to the other by transitional phenotypes whose taxobases vary clinally. The distribution of each member of a phenotypic pair is directly correlated with the distribution of salinity and temperature in the bay. Thus, the paired phenotypes are ecophenotypes. Smaller, thinly calcified ecophenotypes having fewer chambers characterize environments that are near optimum for the respective calcareous species; the agglutinant species A. salsum is small, thin, and made of fine grains, in near optimum environments. Larger, thickly calcified ecophenotypes having more numerous chambers are characteristic of environments that approach minimum tolerances of each calcareous species; A. salsum becomes larger, more inflated, and composed of larger grains in near-minimum environments. Field and laboratory evidence demonstrates that this paired ecophenotypy is caused by contrasting results of delayed reproductive maturation in minimum environments, versus accelerated maturation in optimum environments. Longer growth periods produce larger, thickly calcified tests; shorter growth periods produce smaller, thinly calcified tests. The phenomenon of paired ecophenotypy, though rarely mentioned, has persisted in low-latitude estuaries since at least the early Miocene, as demonstrated by a review of published records. Recognition of this characteristic among the cited and other species (living and fossil) will clarify much of the taxonomic and ecologic confusion that has arisen from close morphologic similarity among estuarine and near-shore marine phenotypes. It also will help to provide more accurate paleoecological interpretation and correlation of marginal marine strata.

"Failure to take into consideration the particular characteristics of a group and the nature and degree of its variation may result in the artificial separation of many 'morphological' species on the basis of minor phenotypic variations, even when the population at a given locality or stratigraphic level contains the complete series of gradations between two or more of these. As these individuals represent minor portions of a continuous population, regardless of the method of reproduction, they represent a single biological species, for which subdivision is unwarranted."

(Helen Tappan, 1976, P. 304)