Humeral loads during swimming and walking in turtles: implications for morphological change during aquatic reinvasions.

During evolutionary reinvasions of water by terrestrial vertebrates, ancestrally tubular limb bones often flatten to form flippers. Differences in skeletal loading between land and water might have facilitated such changes. In turtles, femoral shear strains are significantly lower during swimming than during walking, potentially allowing a release from loads favoring tubular shafts. However, flipper-like morphology in specialized tetrapod swimmers is most accentuated in the forelimbs. To test whether the forelimbs of turtles also experience reduced torsional loading in water, we compared strains on the humerus of river cooters (Pseudemys concinna) between swimming and terrestrial walking. We found that humeral shear strains are also lower during swimming than during terrestrial walking; however, this appears to relate to a reduction in overall strain magnitude, rather than a specific reduction in twisting. These results indicate that shear strains show similar reductions between swimming and walking for forelimb and hindlimb, but these reductions are produced through different mechanisms.

Runnels mitigate marsh drowning in microtidal salt marshes.

As a symptom of accelerated sea level rise and historic impacts to tidal hydrology from agricultural and mosquito control activities, coastal marshes in the Northeastern U.S. are experiencing conversion to open water through edge loss, widening and headward erosion of tidal channels, and the formation and expansion of interior ponds. These interior ponds often form in high elevation marsh, confounding the notion applied in predictive modeling that salt marshes convert to open water when elevation falls below a critical surface inundation threshold. The installation of tidal channel extension features, or runnels, is a technique that has been implemented to reduce water levels and permit vegetation reestablishment in drowning coastal marshes, although there are limited data available to recommend its advisability. We report on 5 years of vegetation and hydrologic monitoring of two locations where a total of 600-m of shallow (0.15-0.30-m in diameter and depth) runnels were installed in 2015 and 2016 to enhance drainage, in the Pettaquamscutt River Estuary, in southern Rhode Island, United States. Results from this Before-After Control-Impact (BACI) designed study found that runnel installation successfully promoted plant recolonization, although runnels did not consistently promote increases in high marsh species presence or diversity. Runnels reduced the groundwater table (by 0.07-0.12 m), and at one location, the groundwater table experienced a 2-fold increase in the fraction of the in-channel tidal range that was observed in the marsh water table. We suggest that restoration of tidal hydrology through runnel installation holds promise as a tool to encourage revegetation and extend the lifespan of drowning coastal marshes where interior ponds are expanding. In addition, our study highlights the importance of considering the rising groundwater table as an important factor in marsh drowning due to expanding interior ponds found on the marsh platform.