Grazing of the free-living filamentous brown seaweed, Pilayella littoralis, by the amphipod Gammarus tigrinis

Pilayella littoralis is a common brown alga with a cosmopolitan distribution. A distinct form of the alga is found free-living in Nahant Bay, Massachusetts. Characteristics that distinguish the free-living form include radial branching that results in a ball-like morphology, reproduction by fragmentation of filaments rather than by spores, and year-round persistence of the population. The amphipod Gammarus tigrinis is frequently found within the floating drifts of P. littoralis. We conducted gut content analyses to document that G. tigrinis consumes P. littoralis. Culture studies revealed that ingested P. littoralis could survive and grow from fecal pellets produced by G. tigrinis. In addition, inefficient grazing produces vegetative fragments (propagules) that can increase numbers within the free-living population. {delta}13C values for the amphipods and P. littoralis averaged -17.4 o/oo and -17.7 o/oo, respectively, suggesting that G. tigrinis acquires most of its carbon from P. littoralis.