Reference: /?/<>/. Bull.. 152: 3 (.0-3 72. (June, 1977) A GASTROPOD COLOR POLYMORPHISM: ONE ADAPTIVE STRATEGY OF PHENOTYPIC VARIATION T K. ELAINE HOAGLANDi 1 Department nf Iliolcyy, Harrurd University, Cambridge, Massachusetts 02138 Most species of the genus Crepidula possess very few discrete, multistate traits, but rather show subtle gradations in characters. There are actually very few characters in which polymorphism can be observed, because ornamentation is lacking in most species, and shell shape is a plastic character, strongly affected by the shape of the substrate. However, Crepidula conrc.ro does show distinct, nonintergrading color differences within single populations. The external colors of many species of Crepidula are often obscured by algal growth, as well as by a shaggy periostracum. C. conrc.ro, however, has a thin periostracum of the same hue as the shell. Being small and able to move, at least in the male phase, it does not become encrusted by advancing colonies of epibionts (bryozoans, algae, tunicates) as do other species. It is photopositive and prefers the upper surfaces of substrates (personal observation), hence its color is visible to other organisms. The limpet-like C. conrc.va requires smooth, hard substrates in intertidal to shallow subtidal waters. Such substrates especially the inner surfaces of other shells, polished slate or quartz stones, glass, or, if nothing else exists, eel grass blades exist in spatially heterogeneous patches. They are also impermanent in time but not usually within the generation time of individual C. convc.va. The most common color of the shell of this northern Atlantic species is pur-plish-grey, with a rich, dark brown interior. Darker rays of pigment extend radially from the apex but are visible only at the shell margin where the background color is less intense. A small proportion of pale tan or yellow shells, with reddish-brown rays, coexists with the darker specimens. An even smaller number of light brown specimens with uneven pigmentation (streaks of yellow mixing with the brown) is usually present. Eranz and Hendler (1970) noted these color differ-ences, along with the fact that there were more pale shells living on cultch (clam shell debris) than on gastropod shells. Color differences are discrete, with scoring relatively uncomplicated by ambiguous intermediates. Color polymorphism in C. convc.va is of interest because it is a rare occurrence in the genus. Analyses of shell color, substrate color, and relative fitness of indi-viduals of each color on each substrate were performed to investigate how such a polymorphism is maintained in this species. C. convex a-was compared with one other species which shows discrete color classes, and with species lacking color polymorphism to investigate why it exists in this but not most other species of Crepidula. 1 Present address: Department of Biology, Lehigh University, Bethlehem, Pennsylvania 18015. 360
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