Growth dynamics and genetic variation in attached and free-living populations of the filamentous brown seaweed, Pilayella littoralis
We used common garden growth experiments to study genetic variation among geographic isolates (Greenland, Massachusetts, and Connecticut, USA) of the filamentous brown seaweed Pilayella littoralis, including the free-living form unique to Nahant Bay, Massachusetts. Ecotypic variation for temperature growth maximum was demonstrated for a west Greenland isolate (10{degrees} C versus 15{degrees} C for other attached isolates) and between Nahant Bay attached (narrower phenotypic plasticity) and free-living forms (broader phenotypic plasticity) of the species. Morphological and reproductive characteristics of attached and free-living isolates remained distinctive under identical culture conditions after four years. The attached forms were characteristically cabled, twisted, and clumped; unilocular reproductive cells were common and plurilocular reproductive cells were present. The free-living form was characteristically loosely branched and ball-like; only vegetative reproduction occurred, with a few unilocular reproductive cells observed in one experiment. Free-living and attached isolates cultured using no water movement and turbulent conditions to mimic surf and surge conditions did not develop forms that resembled each other after eight months. We additionally used starch gel electrophoresis to study genetic variability in attached and free-living forms of P. littoralis from Nahant Bay. Free-living and attached populations were not different at the isozyme level because a limited number of isozymes were resolved (six out of 39 enzymes tested). One isozyme (PGI) was polymorphic, with two alleles present. The two alleles shared in the attached and free-living populations suggest that the free-living form is not one large identical clone. For attached and free-living P. littoralis, both transplant and growth studies in the laboratory provide convincing evidence of ecotypic differentiation.