The Problem of Physiological Species with Special Reference to Oysters and Oyster Drills

Abstract

The term physiological species, as used here, is meant to describe those groups of organisms which, on morphological grounds, fit presently accepted species designation but which are functionally different in certain respects one from the other. Many other terms have also been used in dealing with such groups of organisms including biological races, varieties and strains (see Bates, '40). Some examples from other fields will not only illustrate the idea but suggest the practical importance which may result from recognition of their existence. species of parasitic roundworm from the chimpanzee is morphologically indistinguishable from Ascaris lunbricoides found in man or in the pig, yet the strain found in the chimpanzee is specific for host and not infective for the other host species (Augustine, '39). The strains in man and the pig are also each specific for these hosts (Faust, '39). This discounts the importance of the pig as reservoir host in the control of human ascariasis. Anopheles maculipennis was described in 1818 by Meigen. It was over 100 years later, however, when Bates ('40) wrote this so-called species forms a group of more or less independent populations distinguished by various physiological and morphological characters. The physiological characters include (1) different tolerances of the larvae for sea water, (2) variations in adult sexual and feeding behavior, (3) sterility of F. hybrid males and (4) differences in ecological and geographical distribution (Bates and Hackett, '39). Some 20 latinized names have been proposed for these groups and Bates himself recognizes 6 species although the only important morphological differences are in egg pattern and structure, not in adult form. Yet the recognition of these types has solved many vexing problems of the epidemiology of malaria in Europe. The races A and B of Drosophila pseudoobscura are another example. These 'races' have distinctive geographical distributions that, however, broadly overlap without the formation of hybrids in nature. The ecological preferences are somewhat different; pronounced, though incomplete, sexual isolation is observed; the F1 hybrid males are absolutely sterile; and the viability of the backcross products is decreased (Dobzhansky, '41). In short, these races are distinct species by any criterion except the absence of morphological differentials (Mather and Dobzhansky, '39). That similar examples exist among inshore marine invertebrates is especially plausible to those who know the striking morphological and physiological differences between oysters of the edulis and virginica, types still classified in the same genus despite these great differences (Orton '28; McLean '41). The presence of physiological species could also explain Mayer's observations in 1914 on the different effects of temperature on bell rhythm in Aurelia aurita from Halifax and from Tortugas. Furthermore, Runnstr6m ('29, '36), who has devoted much thought and effort to this problem in European waters, has concluded physiological races of several marine invertebrates exist in the Eastern Atlantic. Without attempting to enter the discussion of what species is, Dobzhansky's ('35) dynamic definition seems most useful in this connection. He defines species as that stage of evolutionary process,