MEPS 282:115-128 (2004)  -  doi:10.3354/meps282115

Variation in blade morphology of the kelp Eisenia arborea: incipient speciation due to local water motion?

Loretta M. Roberson1,3,* James A. Coyer2

1Hopkins Marine Station, Stanford University, Pacific Grove, California 93950, USA 2Department of Marine Biology, University of Groningen, Kerklaan 30, PO Box 14, 9750 AA Haren, The Netherlands 3Present address: Institute of Neurobiology and Department of Biology, University of Puerto Rico, PO Box 23360, San Juan, Puerto Rico 00931-3360, USA

ABSTRACT: The southern sea palm kelp Eisenia arborea produces wide, bullate (bumpy) blades in low-flow areas, whereas in adjacent high-flow areas blades are flat and narrow. Here we determine if morphological differences in these 2 closely associated populations are correlated with physical factors in the environment, and whether this response is a genetically fixed or plastic trait. Both phenotypes were subjected to morphometric analysis, field and laboratory transplant experiments, and genetic analysis (M13 DNA fingerprinting). Physical variables (water motion, temperature, and light) were monitored simultaneously in the field. Results indicate that the 2 populations, separated by <10 m, are genetically distinct and transplants in the field and lab remained similar to individuals from the original collection location, regardless of the imposed flow environment. Water motion varied significantly between the 2 populations and was the single physical variable that correlated with morphology. Based on bump heights and water velocities in the field, bullate blades could increase turbulent transport of nutrients 4-fold that of flat blades. The thicker blades and large holdfasts of the flat morph may impart higher survivorship under high-flow conditions. This suggests water motion selects for 2 distinct morphotypes of E. arborea that represent adaptations to enhance fitness under different nutrient availability and drag force regimes. Thus, blade morphology is not a plastic response to the local environment but a genetically fixed trait and may be indicative of nascent speciation in the face of strong selection by water motion over spatial scales smaller than observed previously in kelps.

KEY WORDS: Multilocus DNA fingerprinting · Population differentiation · Phenotypic plasticity · Water motion · Santa Catalina Island · Laminariales

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