Are vent crab behavioral preferences adaptations for habitat choice?

Low high-temperature tolerance of the hydrothermal vent crab Xenograpsus testudinatus: Thermal biochemistry, survival rate and histological analyses, and habitat investigations

Rising environmental temperatures increase stress on marine organisms, causing physiological damage or death in extreme cases. In this study, we examined the crab species Xenograpsus testudinatus from a shallow hydrothermal vent area in Kueishan Island, off northeast Taiwan. This crab species belongs to the monotypic family Xenograpsidae and is a well-known inhabitant of the shallow hydrothermal vents of Kueishan Island. The maximum temperature tolerance of X. testudinatus and the effects of temperature on this crab species remain unclear. Our laboratory experiments demonstrated the effects of elevated seawater temperatures on the survival and enzymatic function of X. testudinatus. Its mortality rate increased with the elevation of seawater temperature from 25 °C to 40 °C. Survival duration was short (<4 h) when the crabs were reared at 40 °C, and the lethal temperature (LT50; death within 7 days of exposure) was 34.74 °C. The response of six enzymes varied with temperature. Significant differences of enzyme activities were noted among various temperature groups (25 °C, 30 °C, 30 °C, and 40 °C) in the gill, but no significant difference was noted in the hepatopancreas. A univariate analysis indicated the temperature level and exposure duration as key factors driving the variations in enzyme activity. A field investigation on the temperature of the seabed where the crabs aggregated revealed that the preferred temperature was <28 °C. Therefore, X. testudinatus does not prefer elevated temperatures. An expected combined effects of temperature tolerance and acidic, sulfuric-rich vent water on X. testudinatus is warrant further investigation.

Pathological and biochemical effects of polyethylene microplastic exposure in hydrothermal vent crab, Xenograpsus testudinatus

Microplastic pollution significantly threatens marine ecosystems, including those with unique adaptations. This study evaluates the implications of polyethylene microplastics (PE-MPs) on the hydrothermal vent crab, Xenograpsus testudinatus. Crabs were exposed to varying fluorescent green polyethylene microspheres (FGPE) concentrations for 7 days. Histological analysis revealed severe damage to the hepatopancreas and gills at higher FGPE concentrations. Antioxidant enzyme activities (SOD, CAT, GR, GST, GPx, LPO) and gene expression (sod, cat) were assessed to evaluate oxidative stress responses. Results indicated significant upregulation of SOD and CAT activities at lower FGPE concentrations, suggesting an initial antioxidant response. However, GR and GST activities were inhibited at higher concentrations, and oxidative stress markers increased. These findings proved that PE-MPs induce molecular oxidative damage and lead to possible oxidative responses. Despite their resilience to extreme environments, HV crabs are vulnerable to pollutant-induced stressors, which pose ecological risks to species interactions and population dynamics within vent ecosystems.

Distribution and sexual dimorphism of the crab Xenograpsus testudinatus from the hydrothermal vent field of Kueishan Island, northeastern Taiwan

The sulphur-rich and acidic vent waters of a shallow hydrothermal vent field next to Kueishan Island in Taiwan provide a specific and generally toxic environment. Among only a few aquatic organisms able to survive there, the grapsoid crab Xenograpsus testudinatus is the dominant species with a high population density in the vent area. Here we study the gender-specific distribution, morphological traits, and relationship of wet weight vs. carapace width of this crab. A total of 1120 individuals including 831 male and 289 female (included 15 ovigerous) were examined during August and September in 2011 and May and September in 2012. Except in August 2011, there are no significant differences in the distribution of X. testudinatus in the hydrothermal vent area from the vent spout during most months. Among crabs, the weight of male (6.87 ± 2.90 g) was significantly heavier than that of females (4.17 ± 1.25 g) (p < 0.001, Student’s t-test). As for the wet weight of crabs, significant differences were noted in both the length of chela and the width of carapace between males and females. Sexual dimorphism of X. testudinatus is evident in three morphological traits. Pearson’s correlation showed a significant and positive correlation (p < 0.001) of wet weight, width of carapace and length of chela of the two sexes. Ovigerous crabs (shortest carapace width: 1.93 cm) were present in the specimen collected from August 2011 and May 2012. The ovigerous crabs were not found in the samples collected from September in both years 2011 and 2012, indicating that reproduction may have ceased during the period of sampling. The present results suggested that the reproductive period of X. testudinatus was before September. The distribution pattern and sexual dimorphism of X. testudinatus provided a better understanding of the idiobiology of this dominant metazoan in the hydrothermal vent area.

Reproductive adaptations of the hydrothermal vent crab Xenograpus testudinatus: An isotopic approach

The vent crab Xenograpsus testudinatus was firstly discovered in 2000 at the hydrothermal vent field off the coast of Kueishan Island. The present study attempts to understand the adaptive reproduction of this crab living in an extreme environment by examining its spatial and temporal distribution and isotopic signatures. The seasonal variation of the female-male ratio suggests that ovigerous females may migrate from beneath the vent orifice to the vent-periphery region to release their larvae to avoid the larvae contacting high toxic plumes, and then returns to the vent orifice habitat. We used variation of the isotopic crab signatures as indicators for this unique female migration. Our results showed that this vent crab evolved an adaptive modulation of reproductive behavior to successfully survive and propagate in an oceanic shallow hydrothermal vent field.