Assessment of treatment-specific tethering survival bias for the juvenile blue crab Callinectes sapidus in a simulated salt marsh.

The blue crab (Callinectes sapidus) is ecologically and economically important in Chesapeake Bay. Nursery habitats, such as seagrass beds, disproportionately contribute individuals to the adult segment of populations. Salt marshes dominated by smooth cordgrass Spartina alterniflora are intertidal nursery habitats which may serve as a refuge from predation for juvenile blue crabs. However, the effects of various characteristics of salt marshes on nursery metrics, such as survival, have not been quantified. Comparisons of juvenile survival between salt marshes and other habitats often employ tethering to assess survival. Although experimental bias when tethering juvenile prey is well recognized, the potential for habitat-specific bias in salt marshes has not been experimentally tested. Using short-term mesocosm predation experiments, we tested if tethering in simulated salt marsh habitats produces a habitat-specific bias. Juvenile crabs were tethered or un-tethered and randomly allocated to mesocosms at varying simulated shoot densities and unstructured sand. Tethering reduced survival, and its effect was not habitat specific, irrespective of shoot density, as evidenced by a non-significant interaction effect between tethering treatment and habitat. Thus, tethering juvenile blue crabs in salt marsh habitat did not produce treatment-specific bias relative to unvegetated habitat across a range of shoot densities; survival of tethered and un-tethered crabs was positively related to shoot density. These findings indicate that tethering is a useful method for assessing survival in salt marshes, as with other nursery habitats including seagrass beds, algae and unstructured sand.

Long-term persistence of structured habitats: seagrass meadows as enduring hotspots of biodiversity and faunal stability.

Ecological studies indicate that structurally complex habitats support elevated biodiversity, stability and resilience. The long-term persistence of structured habitats and their importance in maintaining biodiverse hotspots remain underexplored. We combined geohistorical data (dead mollusc assemblages, 'DA') and contemporary surveys (live mollusc assemblages, 'LA') to assess the persistence of local seagrass habitats over multi-centennial timescales and to evaluate whether they acted as long-term drivers of biodiversity, stability and resilience of associated fauna. We sampled structured seagrass meadows and open sandy bottoms along Florida's Gulf Coast. Results indicated that: (i) LA composition differed significantly between the two habitat types, (ii) LA from seagrass sites were characterized by significantly elevated local biodiversity and significantly higher spatial stability, (iii) DA composition differed significantly between the two habitat types, and (iv) fidelity between LA and DA was significantly greater for seagrass habitats. Contemporary results support the hypotheses that local biodiversity and spatial stability of marine benthos are both elevated in structured seagrass habitats. Geohistorical results suggest that structured habitats persist as local hotspots of elevated biodiversity and faunal stability over centennial-to-millennial timescales; indicating that habitat degradation and concomitant loss within structurally complex marine systems is a key driver of declining biodiversity and resilience.