Between tide and wave marks: a unifying model of physical zonation on littoral shores

Thermal threshold and interspecific competition help explain intertidal hermit crab assemblages

Habitat heterogeneity promotes species diversity by providing a variety of abiotic and biotic conditions, whose impact on performance varies among species. Then, mobile species would be expected to move to areas whose conditions maximize their fitness. However, biotic pressures such as interspecific competition can push subordinate species into suboptimal areas, impeding this matching. The tropical hermit crab Clibanarius albidigitus occupies mostly upper intertidal sites where they can experience extreme environmental conditions. Meanwhile, its stronger agonistic competitor, Calcinus californiensis, mostly inhabits more moderate conditions at the mid intertidal. We estimated the avoidance threshold of the two hermit crab species to increasing water temperatures to help explain their intertidal distribution. We also compared the avoidance threshold of Cli. albidigitus to rising temperatures when presented alone and in the presence of chemical cues of its competitor to assess potential competitive niche exclusion. The avoidance threshold was measured in experimental tanks with a ramp that led from the water to an air-exposed platform; the threshold was defined as the temperature at which individuals emerged and remained air-exposed. Clibanarius albidigitus emerged at a higher temperature than its competitor, showing a higher thermal tolerance and potentially explaining its distribution in the upper intertidal. In the presence of Cal. californiensis, Cli. albidigitus emerged at lower temperature than when alone, likely as a strategy to reduce competition with stronger agonistic competitors, even at the cost of coping with harsh conditions. Our results support the hypothesis that competitive habitat exclusion contributes to explaining hermit crab assemblages.

Spatial variation and antecedent sea surface temperature conditions influence Hawaiian intertidal community structure

Global sea surface temperatures (SSTs) are increasing, and in Hawai'i, rates of ocean warming are projected to double by the end of the 21st century. However, current nearshore warming trends and their possible impacts on intertidal communities are not well understood. This study represents the first investigation into the possible effects of rising SST on intertidal algal and invertebrate communities across the Main Hawaiian Islands (MHI). By utilizing citizen-science data coupled with high-resolution, daily SST satellite measurements from 12 intertidal sites across the MHI from 2004-2019, the response of intertidal algal and invertebrate abundance and community diversity to changes in SST was investigated across multiple spatial scales. Results show high rates of SST warming (0.40°C Decade-1) over this study's timeframe, similar to predicted rates of warming for Hawai'i by the end of the 21st century. Changes in abundance and diversity in response to SST were variable among intertidal sites, but differences in antecedent SST among intertidal sites were significantly associated with community dissimilarity. In addition, a statistically significant positive relationship was found between SST and Simpson's diversity index, and a significant relationship was also found between SST and the abundance of six dominant taxa. For five of these six dominant taxa, antecedent SSTs over the 6-12 months preceding sampling were the most influential for describing changes to abundance. The increase in community diversity in response to higher SSTs was best explained by temperatures in the 10 months preceding sampling, and the resultant decreased abundance of dominant turf algae. These results highlight rapidly warming nearshore SSTs in Hawai'i and the longer-term effects of antecedent SSTs as significant drivers of change within Hawaiian intertidal communities. Therefore, we suggest that future research and management should consider the possibility of lagging effects of antecedent SST on intertidal communities in Hawai'i and elsewhere.

Preliminary Design and Testing of a Resetting Combination Anchor, Antenna, and Tether Mechanism for a Spherical Autonomous Underwater Vehicle

This article details the preliminary design and testing of a Resetting Anchor/Antenna Tether Mechanism (RAATM) for an autonomous underwater vehicle (AUV). The proposed mechanism is intended to enable an AUV to secure itself to the seabed, ascend, descend, transmit and receive signals via the tether, retract the anchor, and re-anchor again as required. The ability of an AUV to passively loiter on station for extended periods preserves power and may otherwise expand mission capabilities for a variety of underwater vehicles. If they are capable of communication through electromagnetic transmission, AUVs equipped with such technology may be utilized to form mobile networks that may, in turn, receive external communications from above the surface. Spherical AUV (SAUV) capabilities may be especially enhanced through the integration of the proposed mechanism. The RAATM was designed for integration with the Wreck Interior Exploration Vehicle (WIEVLE), a small SAUV designed for operations in entanglement-prone, extreme environments, but the RAATM may be used in any suitably-sized underwater vehicle capable of safely contacting the ocean floor. A prototype of the anchoring portion of the mechanism was constructed, and anchoring strength was tested repeatedly in three types of sediment, under varied configurations and loading angles, with promising results.

Integration of Bacterial Expansin on Agarolytic Complexes to Enhance the Degrading Activity of Red Algae by Control of Gelling Properties

Expansin act by loosening hydrogen bonds in densely packed polysaccharides. This work characterizes the biological functions of expansin in the gelling and degradation of algal polysaccharides. In this study, the bacterial expansin BpEX from Bacillus pumilus was fused with the dockerin module of a cellulosome system for assembly with agarolytic complexes. The assembly of chimeric expansin caused an indicative enhancement in agarase activity. The enzymatic activities on agar substrate and natural biomass were 3.7-fold and 3.3-fold higher respectively than that of agarase as a single enzyme. To validate the effect on the agar degradation, the regulation potential of parameters related to gel rheology by bacterial expansin was experimentally investigated to indicate that the bacterial expansin lowered the gelling temperature and viscosity of agar. Thus, these results demonstrated the possibility of advancing more efficient strategies for utilizing agar as oligo sugar source in the biorefinery field that uses marine biomass as feedstocks.