Convergent Evolution of Armor: Thermal Resistance in Deep-Sea Hydrothermal Vent Crustaceans

Abstract

Organisms occupy diverse ecological niches worldwide, each with characteristics finely evolved for their environments. Crustaceans residing in deep-sea hydrothermal vents, recognized as one of Earth's extreme environments, may have adapted to withstand severe conditions, including elevated temperatures and pressure. This study compares the exoskeletons of two vent crustaceans (bythograeid crab Austinograea sp. and squat lobster Munidopsis lauensis) with four coastal species (Asian paddle crabs, blue crab, hermit crab, and mantis shrimp) to identify traits influenced by vent environments. The goal was to identify distinctive exoskeletal characteristics commonly observed in vent crustaceans, resulting from their exposure to severe abiotic factors, including elevated temperatures and pressures, found in vent environments. Results show that the exoskeletons of vent crustaceans demonstrated significantly enhanced thermal stability compared to coastal species. These vent crustaceans consistently featured exoskeletons characterized by a reduced proportion of volatile components, such as water, and an increased proportion of CaCO₃, compared with coastal crustaceans. Furthermore, vent crustaceans lacked carotenoid pigments that had low heat resistance. However, no apparent differences were observed in the mechanical properties. Our findings suggest that the similar composition of exoskeletons in vent crustaceans evolved convergently to withstand high temperatures.

Keywords: crustacean; exoskeleton; extremophile; thermal stability.

Figure 1

Research was conducted on species (a), sampling sites (b), and phylogenetic relationships (c). The phylogenetic relationship was estimated using two specimens of each species based on BI and ML analysis using the combined data set (2101 bp) of mtCOI, mt16S rRNA, 18S rRNA, and H3 genes. The number at each node represents the bootstrap values from ML analysis, and the posterior probabilities from BI. The scale bar indicates phylogenetic distance of 0.03 nucleotide substitutions per site.

Figure 2

Mechanical properties and thickness ratio of each layer of the exoskeleton. (a) hardness, (b) reduced modulus, and (c) thickness ratio; mean ± SE; the significant difference is indicated by an asterisk (*).

Figure 3

Crustacean exoskeleton (endocuticle layer) compound analysis graph through Raman analysis.

Figure 4

Thermal stability analysis results. (a) TGA curve of crustaceans and each combusting substance on each range. Weight loss (%) for each temperature range: (b) Range 1, Range 2, and Range 3; mean ± SE; the significant difference is indicated by an asterisk (*).