Exotic macroalga Gracilaria vermiculophylla provides superior nursery habitat for native blue crab in Chesapeake Bay

Abstract

ABSTRACT: Exotic species often reduce the abundance or diversity of species in marine ecosys-tems, but some exotics may benefit native species, such as when habitat is enhanced. In Chesa-peake Bay, the exotic macroalga Gracilaria vermiculophylla (Rhodophyta) has flourished and dis-persed widely, yet the consequences for native species diversity and abundance are not wellknown. We experimentally examined the capacity of the structurally complex G. vermiculophylla to provide nursery habitat for the blue crab Callinectes sapidus in Chesapeake Bay, where nativeeelgrass nursery habitat has dwindled. We also examined ontogenetic shifts in survival acrossalternative nursery habitats. In field surveys, juvenile density was similar in macroalgae and eel-grass, but lower in unvegetated mud habitat. In field experiments, juvenile survival was positivelyrelated to crab size in mud but negatively in eelgrass, confirming the paradigm of a predation-induced ontogenetic shift from seagrass to unvegetated habitat. In contrast, irrespective of crabsize, survival was higher in macroalgae than in either native habitat. Thus, exotic habitat-formingmacroalgae can compensate for severe declines in seagrass nurseries, and facilitate the emer-gence of a novel ecosystem.KEY WORDS: Exotic species · Ontogenetic habitat shift · Predation · Prey size refuge · Novelecosystem · Emerging ecosystem · Macroalgae · Seagrass