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Taking advantage of the massive genome sequencing effort made on thermophilic prokaryotes, thermal adaptation has been extensively studied by analysing amino acid replacements and codon usage in these unicellular organisms. In most cases, adaptation to thermophily is associated with greater residue hydrophobicity and more charged residues. Both of these characteristics are positively correlated with the optimal growth temperature of prokaryotes. In contrast, little information has been collected on the molecular 'adaptive' strategy of thermophilic eukaryotes. The Pompeii worm A. pompejana, whose transcriptome has recently been sequenced, is currently considered as the most thermotolerant eukaryote on Earth, withstanding the greatest thermal and chemical ranges known. We investigated the amino-acid composition bias of ribosomal proteins in the Pompeii worm when compared to other lophotrochozoans and checked for putative adaptive changes during the course of evolution using codon-based Maximum likelihood analyses. We then provided a comparative analysis of codon usage and amino-acid replacements from a greater set of orthologous genes between the Pompeii worm and Paralvinella grasslei, one of its closest relatives living in a much cooler habitat. Analyses reveal that both species display the same high GC-biased codon usage and amino-acid patterns favoring both positively-charged residues and protein hydrophobicity. These patterns may be indicative of an ancestral adaptation to the deep sea and/or thermophily. In addition, the Pompeii worm displays a set of amino-acid change patterns that may explain its greater thermotolerance, with a significant increase in Tyr, Lys and Ala against Val, Met and Gly. Present results indicate that, together with a high content in charged residues, greater proportion of smaller aliphatic residues, and especially alanine, may be a different path for metazoans to face relatively 'high' temperatures and thus a novelty in thermophilic metazoans.

The shrimp Mirocaris fortunata is a hydrothermal vent species that is found at most vent-sites along the Mid-Atlantic Ridge. This endemic species is found across a hydrothermal gradient, with thermal conditions ranging from 2-9 degrees C in ambient seawater to fairly warm values of about 25 degrees C. We performed in vivo experiments on M. fortunata specimens originating from different sites and depths (850 m to 2300 m), both at atmospheric pressure and in pressurized aquaria, to characterise the upper thermal limits of this species. Atmospheric pressure results show that thermal physiology should be studied at each population's native pressure. At in situ pressure, shrimps from Menez Gwen (850 m depth) and Lucky Strike (1700 m depth) do not survive temperatures of 39 degrees C, and the 'loss of equilibrium' response suggests that their critical thermal maximum (Ctmax), is about 36+/-1 degrees C for both sites. This value is similar to those found for another vent shrimp, Rimicaris exoculata, which is thought to be a more temperature-resistant organism, so temperature resistance does not appear to be a crucial factor for explaining differences in distribution of shrimp species in a given vent site. Finally, the data for both vent shrimps are also comparable to those of other non-vent tropical caridean species.

The past 20 years have witnessed the publication of numerous studies on hemoglobins (Hbs) from deep-sea animals. Most of the animals studied were collected at deep-sea hydrothermal vents and cold seeps, both being environments where the physical-chemical conditions may be severely challenging for metazoans. These environments may be characterized by deep, chronic hypoxia and high concentrations of toxic compounds such as sulfide and heavy metals. Many species from these environments express Hbs, even though they belong to taxa that otherwise were characterised by the absence of respiratory pigments. Hbs from vent and seep invertebrates commonly exhibit high affinities for oxygen when compared to related species from normoxic, shallow-water environments, and marked pH-dependence. These high affinities permit uptake of oxygen from hypoxic waters and the strong Bohr effects favor its release in the metabolizing acidic organs.

Methanoaricia dendrobranchiata Blake (Polychaeta; Orbiniidae) occurs in large numbers in association with communities of the mussel Bathymodiolus childressi at hydrocarbon seeps on the Louisiana Slope of the Gulf of Mexico. Its microhabitat can be strongly hypoxic (oxygen is often undetectable) and sulfidic (sulfide concentrations can reach millimolar levels), which may seriously challenge aerobic metabolism. We describe a suite of adaptations to its low-oxygen environment. The worms are capable of regulating their rate of oxygen consumption down to partial pressures of approximately 870 Pa oxygen. This capability correlates with a large gill surface area, a small diffusion distance from sea water to blood, a very high hemoglobin oxygen-affinity (P50=27.8 Pa at 10 degrees C and pH 7.6) and a Bohr effect that is pronounced at high oxygen saturations. When fully saturated, the hemoglobin binds sufficient oxygen for only 31 min of aerobic metabolism. However, these polychaetes can withstand extended periods of anoxia both in the absence and presence of 1 mmol l(-1) sulfide (TL50=approx. 5.5 and 4 days, respectively).