File:American malacological bulletin (1986) (17533709484).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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HARRY: GENERIC CONCEPT OF LIVING OYSTERS 159 characters, resulted in more exact definitions of taxa. Several new taxa were recognized, at the level of subgenus, genus, tribe and subfamily, to explain the relationships and diversity of oysters more exactly (Harry, 1985). Torigoe (1981), whose study was limited to the living oysters of Japan, independently found several new anatomical characters which are useful in systematics. He named one new subfamily, Crassostreinae, but no taxa at lower levels. From the standpoint of faunal distribution of the taxa, it soon became evident that every species of a given area belongs to a different genus or subgenus; or, by logical con- version of this proposition: a genus or subgenus is represented in a given area by only one species. This does not preclude the possibility of a species extending into more than one area, and indeed it implies that genera may do so. The principle will be more easily understood if we understand the meaning of the terms genus and area, as they are used here. In studying the distribution of shallow water benthic marine molluscs, six major regions are generally recognized (Fig. 1). Four are longitudinal, and these we may call pro- vinces: Eastern Atlantic, Western Atlantic, Eastern Pacific and Indo-Western Pacific. The two latitudinal regions, which we may call zones, are the Arctic and Antarctic. The natural boun- daries of these provinces and zones are formed by things which constitute distributional barriers, and they are of three kinds. The longitudinal barriers are alternating continental masses and broad areas of deep water. The two latitudinal zones are separated not only by great distance, but also by temperature gradients along the provinces. The provinces can be subdivided by regimes of light and temperature variation, and these might be exactly limited by the Arctic and Antarctic Circles and the Tropics of Cancer and Capricorn, except for the presence of major oceanic cur- rents. Around Antarctica the water moves in a single current, from west to east; it is uniformly cold, throughout the year. No comparable current serves as a barrier in the Arctic Ocean, where the shallow water region is along the northern shores of Eurasia and North America, and the ocean is separated from the others by a narrow passage into the Pacific and a broader one into the Atlantic Ocean. In
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Fig. 1. Map of the world, showing the latitudes bounding climatic zones (labeled: Arctic and Antarctic Circles, Tropics of Cancer and Capicorn), and the effect of major ocean currents in shifting the real thermal boundaries of those zones. Arrows on the lines indicating oceanic currents show direction of movement; continuity of those lines indicate temperature; the continuous part of each line representing the warmer part of a current, with the cooler part being dotted.

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