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

Abstract

Exotic species often reduce the abundance or diversity of species in marine ecosystems, but some exotics may benefit native species, such as when habitat is enhanced. In Chesapeake Bay, the exotic macroalga Gracilaria vermiculophylla (Rhodophyta) has flourished and dispersed widely, yet the consequences for native species diversity and abundance are not well known. 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 native eelgrass nursery habitat has dwindled. We also examined ontogenetic shifts in survival across alternative nursery habitats. In field surveys, juvenile density was similar in macroalgae and eelgrass, but lower in unvegetated mud habitat. In field experiments, juvenile survival was positively related to crab size in mud but negatively in eelgrass, confirming the paradigm of a predationinduced ontogenetic shift from seagrass to unvegetated habitat. In contrast, irrespective of crab size, survival was higher in macroalgae than in either native habitat. Thus, exotic habitat-forming macroalgae can compensate for severe declines in seagrass nurseries, and facilitate the emergence of a novel ecosystem.

Mangrove expansion into temperate marshes alters habitat quality for recruiting Callinectes spp.

“…Panels were immediately redeployed. The panels were deployed in May 2014 and monitored Once recruitment be gan in late summer, monitoring continued until early No vember; at which time water temperatures fell, abundance returned to pre-recruitment levels, and crabs reached 15−20 mm carapace width (CW), when secondary dispersal is expected (Lipcius et al 2007, Johnston & Lipcius 2012. We used the resultant 10 weeks of cohort occurrence data to examine differences in recruitment dynamics, especially arrival and persistence, by habitat type (Fig.…”

“…Patchy recruitment has the potential to reduce settlement-based population attrition if recruits preferentially settle in habitats that provide higher probability of survival (Dahlgren & Eggleston 2000, Halpin 2000, Johnston & Lipcius 2012. The evolution of transient, mobile settler stages (e.g.…”

“…These patterns of Callinectes habitat use suggest that recruits respond most positively to non-structural attributes provided by Rhizophora and to structure formed by Spartina. Callinectes often distinguish between habitats upon initial settlement (Welch et al 1997, Moksnes & Heck 2006, Johnston & Lipcius 2012; however, settlement patterns are not always indicative of later population distribution, which is more readily attributed to juvenile survival and secondary dispersal (Heck et al 2001, Moksnes 2002, Moksnes & Heck 2006. Thus, recruit persistence was expected to vary by habitat according to quality.…”

“…(1) Mortality would lead to decreasing recruit abundance over time. (2) Redistribution could also lead to decreased occurrence, but secondary dispersal is unlikely to be confounding because the sampled crabs did not exceed 20 mm CW, the expected size at secondary dispersal (Lipcius et al 2007, Pardo et al 2007, Johnston & Lipcius 2012. (3) It is possible that recruit attrition was due in part to less effective capture of large or fast-swimming individuals with the panel retrieval method; however, finfish and swimming crabs ranging in size from 0.5 to 100 mm were regularly captured.…”

“…Accordingly, inhabitant fauna have declined less markedly where one structurally complex habitat is replaced by another (e.g. seagrass to macroalgae; Johnston & Lipcius 2012). With an increasing recognition of wide-spread shifts in species distributions, there is a growing need to understand how habitat use and inhabitant population dynamics differ between shifting foundation species.…”