. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 202 PHILIP G. MURPHY 3 mm. FIGURE 3. Shells of (a) Collisclla digitalis from MIL; (b) C. austrodigitalis from MIL; (c) C. aitstrodit/ittilis from COR with sculpture similar to that of the holotype. Shells have been coated to accentuate sculpture. higher temperatures near the bay. Addicott (1966) presents the possibility that unusually warm episodes of the annual Davidson Current period off central Cali- fornia (Bolin and Abbott. 1963) may result in blooms of southern species with planktonic larvae in this area. Recent hydrog
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 202 PHILIP G. MURPHY 3 mm. FIGURE 3. Shells of (a) Collisclla digitalis from MIL; (b) C. austrodigitalis from MIL; (c) C. aitstrodit/ittilis from COR with sculpture similar to that of the holotype. Shells have been coated to accentuate sculpture. higher temperatures near the bay. Addicott (1966) presents the possibility that unusually warm episodes of the annual Davidson Current period off central Cali- fornia (Bolin and Abbott. 1963) may result in blooms of southern species with planktonic larvae in this area. Recent hydrographic data (Lasley, 1977) indicate that such a warming cycle is in progress. Therefore, numbers of C. austrodigitalis in the vicinity of Monterey Bay may, at present, be higher than usual. Allelic frequencies previously reported for C. digital is (Murphy. 1976) are in error because specimens of C. austrodigitalis were unwittingly included in the samples from the vicinitv of Monterev Bav. I )ISCUSSION Allopatric speciation seems to be the only possible mode of origin that can explain siblings such as Collisclla anstrodigitalis and C. digitalis that are restricted to the same habitats in the intertidal zone. However, the barriers that might have served to divide spatially the original species are not obvious. One possible chain of events, involving any of the late Cenozoic world wide climatic fluctuations is as follows. During warming trends warm-water species extend their ranges into higher latitudes, and during cooling intervals, as the polar- equatorial temperature gradient increases, they contract their ranges equatorward (Valentine, 1973). During episodes of range contraction populations might become isolated in suitably warm, higher latitude embayments (Addicott, 1966 considered this as a possible explanation for the presence of a warm-water, late Pleistocene fossil assemblage in northern California). Should this isolation last long enough, speciation would occur. Golikov
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
