Non-uniform consumption of a novel, non-native seaweed by native consumers

Introduced seaweeds can alter the structure and productivity of marine food webs, especially when they lack top-down control by native herbivores. However, relatively little is known about the role of consumption of introduced seaweeds by native herbivores, and the potential role of seaweed nutrient content to mediate local herbivore consumption. In southeastern USA estuaries, the introduced red seaweed, Gracilaria vermiculophylla, has transformed unvegetated intertidal mudflats into a patchwork of non-native seaweed beds. We used a series of laboratory feeding assays to assess how invertebrate and fish species on the Georgia coast utilize G. vermiculophylla as a novel food resource. Because G. vermiculophylla readily absorbs nutrients, we also tested the role of nutrient enrichment on its consumption. We found that G. vermiculophylla was not significantly consumed by the mud snail Ilyanassa obsoleta nor the mud crab Eurypanopeus despressus, but it was rapidly eaten and even preferred over the native seaweed Ulva lactuca by adult pinfish Lagodon rhomboides. Nutrient enrichment of G. vermiculophylla did not affect consumption rates by the amphipod Ampithoe valida, but did double consumption rates by pinfish over unenriched seaweed. The differential responses of native consumers highlight the importance of evaluating multiple species when investigating introduced species' impacts on recipient communities. Given that consumer identity and nutrient content of the seaweed mediated the consumption of the non-native seaweed, site-specific patterns of consumer populations and environmental conditions could lead to patchy abundance and impacts of the non-native seaweed.