The Relationship between Shell Morphology and Microhabitat Flow in the Endemic Hawaiian Stream Limpet (Hihiwai), Neritina granosa (Prosobranchia: Neritidae)

Abstract

The Hawaiian stream limpet, Neritina granosa Sowerby, has three shell morphologies: conic (smooth, narrow shell), intermediate (rugose, narrow shell), and winged (flattened, rugose, and flared shell margin). We studied the relationship between shell morphology and water flow in a laboratory flume and in populations from Palauhulu Stream, Maui. Winged morphs represented 82% of the population at the mouth below the terminal waterfall. At sites above the falls, conic and intermediate morphs dominated. Limpets from the mouth had significantly lower shell-length/shell-width and body-weight/shellweight ratios and occurred in areas of lower benthic and surface velocities than upstream populations. Field determinations of velocities (measured with a thermistor-based microcurrent meter) around individual N. granosa in the field that were oriented parallel to flow demonstrated that conic and intermediate morphs experienced significantly less drag than winged morphs; there was no significant effect when shells were oriented perpendicular to flow. In a laboratory flume, conic and intermediate shells oriented parallel to flow exhibited significantly greater lift and less drag than a winged morph. There was no significant difference in lift and drag for conic and winged morphs in a perpendicular orientation. Because field orientation of the three shell morphs is unpredictable, we hypothesize that microhabitat flow has little or no effect on the phenotypic expression of shell morphology in N. granosa. We feel that the transition between winged and conic/intermediate morphs in upstream populations is restricted by bioenergetic constraints on the partitioning of energy between the competing demands of shell and tissue growth.