The Distribution of Nephtys californiensis and N. caecoides on the Californian Coast
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Abstract
Two species of Nephtys, N. californiensis Hartman and N. caecoides Hartman, occur in the intertidal zone of sandy and muddy beaches on the Californian coast. The former occurs in clean sand, the latter in muddy substrata (Hartman 1944, 1950). These two worms therefore appear to occupy similar habitats to N. cirrosa Ehlers and N. hombergi Lamarck, respectively, on the British coast (Clark & Haderlie 1960, Clark, Alder & McIntyre 1962). The two British species, although they clearly have different substratum preferences, do not suffer a total ecological isolation, and some beaches are populated by both species. The fauna of the Californian coastline has not yet been sufficiently investigated for us to tell whether the ecological isolation of N. californiensis from N. caecoides is partial or total, nor if substratum differences account for the ecological separation of the two species, as a superficial knowledge of the sites at which each has been found suggests. We have attempted to answer these questions in the present survey and our approach has been the same as in our previous investigation in south and southwestern England and Wales (Clark & Haderlie 1960). A survey has been made of a number of sandy and muddy beaches along the coast of California and the ecological conditions prevailing on them related to the presence or absence of one or other species of Nephtys. This has been coupled with a more detailed investigation of the distribution of N. californiensis and N. caecoides on a few beaches where the conditions appear to be nearly optimal for one or other species. The detailed investigations of the beaches at Bodega Harbor, Dillon Beach and Tomales Bay were made during the summer of 1954. Other beaches north of San Francisco were examined at about the same time, but the greater part of the faunistic survey was completed during the autumn and winter of 1960-61. The general procedure adopted has been the same as that in the survey of beaches on the British coast and is described in a previous paper (Clark & Haderlie 1960). The only substantial modification has been that different series of screens have been used in the mechanical analysis of the soils. The difficulty of comparing substrata analysed in different ways can be overcome by considering the logarithmic cumulative distribution of grade fractions in the various samples.