Can tolerances of multiple stressors and calculated safety margins in fiddler crabs predict responses to extreme environmental conditions resulting from climate change?

Variation in mangrove species diversity across gradients of climate-change-induced environmental conditions and hydrological restoration

Increasing drought, elevated temperatures, and salinization are significant challenges to reestablishing species in mangrove restoration areas. In this study, we assessed how the diversity of two key mangrove faunal groups, molluscs and brachyuran crustaceans (hereafter referred to as crabs), varies across a gradient of disturbed, restored, and natural (undisturbed) mangroves. We also explored what are the environmental factors driving these variations in ten sites across the southern Gulf of Mexico, one of the global regions with the largest mangrove coverage. A total of 15 species were recorded (10 mollusks and 5 crabs), with higher abundance in natural (612 individuals) than in restored (554 individuals) or degraded (98 individuals) sites. Community structure analyses revealed that certain species were restricted to specific restoration conditions. For example, the crab Minuca vocator was found only in restored sites, while the mollusc Vitta virginea was exclusive to natural sites. In contrast, species like the crab Minuca rapax were present across all site types. Salinity emerged as the primary environmental factor influencing community structure, with disturbed sites exhibiting significantly higher salinity levels than restored and natural sites. All sites were classified as hypersaline, presenting challenges for species that cannot tolerate such conditions. This study provides a valuable baseline for understanding the ecological conditions that influence on the success of mangrove restoration, offering insights on the effects of environmental factors driving species diversity in this ecosystem.

The impact of settleable atmospheric particulate on the energy metabolism, biochemical processes, and behavior of a sentinel mangrove crab

We use the sentinel mangrove crab, Minuca rapax, as a model to investigate the effects of metallic settleable particulate matter (SePM) on wetland. Multiple levels of energetic responses, including (i) metabolic rate and energy budget, (ii) oxidative stress, and (iii) behavioral response by righting time, were assessed as well as the metal and metalloid content in crabs exposed to 0, 0.1 and 1 g.L-1 of SePM, under emerged and submerged conditions over five days, simulating the rigors of the intertidal habitat. Al, Fe, Mn, Cr, and Y exhibited a concentration-dependent increase. Metal concentrations were higher in submerged crabs due to the continuous ingestion of SePM and direct exposure through gills. Exposure concentration up to 1 g.L-1 decreased metabolic rate and enzymatic activities, reduced assimilation efficiency and energy for maintenance, and induces a slower response to righting time, probably by metal effects on nervous system and energy deficits. In conclusion, SePM exposure affects the redox status and physiology of M. rapax depending on he submersion regime and SePM concentration. The disruption to the energy budget and the lethargic behavior in M. rapax exposed to SePM implies potential ecological alterations in the mangrove ecosystem with unknown consequences for the local population.

Application of ecotoxicological tools to evaluate the quality status of mangroves under restoration in the Yucatán Peninsula, Mexico

Ecotoxicological tools, namely biomarkers and bioassays, may provide insights on the ecological quality status of mangroves under restoration. We investigated how 1) physicochemical parameters and water bioassays using Artemia franciscana; and 2) quantification of sublethal (osmoregulatory capacity, biochemical, and oxidative stress) and individual biomarkers (density, length-weight relationship [LWR], parasitic prevalence) in the sentinel fiddler crab Minuca rapax, can improve restoration indicators in mangroves from the Yucatán Peninsula, Southern Gulf of Mexico. We showed that water quality was improved with restoration, but still presented toxicity. Regarding sublethal biomarkers, M rapax from restored areas lower osmotic regulatory capacity, higher oxidative stress, and showed lipid peroxidation. As to the individual biomarkers, the density, LWR, and the prevalence of parasites in M. rapax was higher in restored areas. The use of bioassays/biomarkers were useful as early warning indicators to better assess the health of mangroves under restoration.

Tolerance and behavioral responses of crabs in disturbed mangroves during a heatwave event

We assessed the tolerance, safety margins, and behavioral responses to extreme conditions of the mangrove fiddler crab Leptuca speciosa during a heatwave event (May of 2022), in the Yucatán Peninsula, Mexico. In the field, L. speciosa demonstrated aggregation behavior, congregating in areas that were above the water level to escape the extreme water conditions. In the laboratory, we determined that the upper critical thermal limits (UT99) ranged from 40.2 °C and 42 °C. For salinity, the lethal concentration was LC99 = 39 psu. Our study showed that L. speciosa, one of the most conspicuous and resilient inhabitants of mangroves, had no safety margin and low tolerance to the climatic conditions as measured in the heatwave, and displayed a protective behavior. Considering that the frequency and intensity of heatwaves have been predicted to increase in the next few years, the combined multiple stressors effect may increase the vulnerability of mangrove organisms.

Trophic transfer mechanisms of potentially toxic elements from sediment and plant leaves (Rhizophora mangle) to fiddler crabs (Minuca rapax) ()

To assess "bottom-up" to "top-down" trophic transfer, we analyze As, Cd, Co, Cr, Cu, Hg, Ni, Se, Zn, Fe, and Mn from two sediment chemical fractions (exchangeable and organic-bound), red mangrove (Rhizophora mangle) leaves, and fiddler crab (M. rapax) soft tissues from Isla del Carmen, Yucatán Peninsula. Both mechanisms were observed indictive that R. mangle and M. rapax indeed bioaccumulated the toxic elements from the different matrices with the latter being a macro-concentrator only for Cu and Zn. Although the modified Geo-accumulation factor (combined exchangeable and organic matter fractions) suggested that the studied sites are practically "uncontaminated", Hg is the only toxic element to be having a "moderately to strongly" impact. Data shows how M. rapax had progressively bioaccumulated Hg, but no biomagnification could be corroborated given that the fiddler crab behaved as a de-concentrator.