Increases in Arctic temperatures have accelerated melting of the Greenland icesheet, exposing intertidal organisms, such as the blue mussel Mytilus edulis, to high air temperatures and low salinities in summer. However, the interaction of these combined stressors is poorly described at the transcriptional level. Comparing expression profiles of M. edulis from experimentally warmed (30 °C and 33 °C) animals kept at control (23‰) and low salinities (15‰) revealed a significant lack of enrichment for Gene Ontology terms (GO), indicating that similar processes were active under all conditions. However, there was a progressive increase in the abundance of upregulated genes as each stressor was applied, with synergistic increases at 33 °C and 15‰, suggesting combined stressors push the animal towards their tolerance thresholds. Further analyses comparing the effects of salinity alone (23‰, 15‰ and 5‰) showed high expression of stress and osmoregulatory marker genes at the lowest salinity, implying that the cell is carrying out intracellular osmoregulation to maintain the cytosol as hyperosmotic. Identification of aquaporins and vacuolar-type ATPase transcripts suggested the cell may use fluid-filled cavities to excrete excess intracellular water, as previously identified in embryonic freshwater mussels. These results indicate that M. edulis has considerable resilience to heat stress and highly efficient mechanisms to acclimatise to lowered salinity in a changing world.

Black corals (Anthozoa: Antipatharia) are an ecologically and culturally important group of deep-sea cnidarians. However, as the majority of species inhabit depths >50 m, they are relatively understudied. The inaccessibility of well-preserved tissue for species of interest has limited the scope of molecular analysis, and as a result only a small number of antipatharian mitochondrial genomes have been published. Using next generation sequencing, 18 complete and five partial antipatharian mitochondrial genomes were assembled, increasing the number of complete mitochondrial genomes to 22. This includes species from six antipatharian families, four of which were previously unrepresented, enabling the first family-level, full mitochondrial gene analysis over the whole order. The circular mitogenomes ranged in size from 17,681 to 21,669 bp with the large range in size due to the addition of an intron in COX1 in some species and size variation of intergenic regions. All mitogenomes contained the genes standard to all hexacoral mitogenomes (13 protein coding genes, two rRNAs and two tRNAs). The only difference in gene content is the presence of the COX1 intron in five families. The most variable mitochondrial gene is ND4 which may have implications for future barcoding studies. Phylogenetic analysis confirms that Leiopathidae is sister to all other families. Families Antipathidae, Cladopathidae and Schizopathidae are polyphyletic, supporting previous studies that call for a taxonomic revision.

Reduced seawater salinity as a result of freshwater input can exert a major influence on the ecophysiology of benthic marine invertebrates, such as echinoderms. While numerous experimental studies have explored the physiological and behavioural effects of short-term, acute exposure to low salinity in echinoids, surprisingly few have investigated the consequences of chronic exposure, or compared the two. In this study, the European sea urchin, Echinus esculentus, was exposed to low salinity over the short term (11‰, 16‰, 21‰, 26‰ and 31‰ for 24 h) and longer term (21, 26 and 31‰ for 25 days). Over the short term, oxygen consumption, activity coefficient and coelomic fluid osmolality were directly correlated with reduced salinity, with 100% survival at ≥21‰ and 0% at ≤16‰. Over the longer term at 21‰ (25 days), oxygen consumption was significantly higher, feeding was significantly reduced and activity coefficient values were significantly lower than at control salinity (31‰). At 26‰, all metrics were comparable to the control by the end of the experiment, suggesting acclimation. Furthermore, beneficial functional resistance (righting ability and metabolic capacity) to acute low salinity was observed at 26‰. Osmolality values were slightly hyperosmotic to the external seawater at all acclimation salinities, while coelomocyte composition and concentration were unaffected by chronic low salinity. Overall, E. esculentus demonstrate phenotypic plasticity that enables acclimation to reduced salinity around 26‰; however, 21‰ represents a lower acclimation threshold, potentially limiting its distribution in coastal areas prone to high freshwater input.

Climate change is causing increased coastal freshening in Antarctica, leading to reduced salinity. For Antarctica’s endemic echinoderms, adapted to the stable polar environment, the impact of rapid reductions in coastal salinity on physiology and behaviour is currently unknown. Six common Antarctic echinoderms (the sea urchin Sterechinus neumayeri ; the sea star Odontaster validus ; the brittle star Ophionotus victoriae ; and three sea cucumbers Cucumaria georgiana , Echinopsolus charcoti and Heterocucumis steineni ), were directly transferred from ambient salinity (34.5‰) to a range of salinity dilutions (29–9‰) for 24 h. All species showed reduced activity and the establishment of a temporary osmotic gradient between coelomic fluid and external seawater. Most species exhibited a depression in oxygen consumption across tolerated salinities; however, at very low salinities that later resulted in mortality, oxygen consumption increased to levels comparable to those at ambient. Low salinity tolerance varied substantially between species, with O. victoriae being the least tolerant (24 h LC ₅₀ (lethal for 50% of animals) = 19.9‰) while E. charcoti and C. georgiana demonstrated the greatest tolerance (24 h LC ₅₀ = 11.5‰). These findings demonstrate the species-specific response of Antarctica’s endemic echinoderms to short-term hypoosmotic salinity events, providing valuable insight into this phylum’s ability to respond to an underreported impact of climate change.

Coastal freshening in the Southern Ocean is expected to increase under projected climate scenarios. As a major environmental stressor, prolonged reduced salinity could pose a significant challenge to Antarctica's endemic echinoderms. Acclimatising to low salinity may be crucial for their continued survival as climate change accelerates, yet little is currently known about their capacity to do so. The sea star Odontaster validus and sea urchin Sterechinus neumayeri, two of the most ecologically important and abundant echinoderms of the shallow Antarctic seas, were exposed to reduced salinities (29 ‰ and 24 ‰) for at least 71 days after a stepwise dilution from 34.5 ‰. Feeding, faecal production (S. neumayeri only) and activity coefficient were significantly impacted at 24 ‰ and did not recover to control levels in either species. Oxygen consumption remained similar to control levels (34.5 ‰) across both treatments and species until day 85, when a significant increase was observed in S. neumayeri at 24 ‰. Coelomic fluid osmolality was near isosmotic with external salinities in both species, while coelomocyte composition and concentration were unaffected by reduced salinities (S. neumayeri only). Both species demonstrated the capacity to tolerate lower salinities that may be expected with climate change, with successful acclimation demonstrated at 29 ‰. Although survival rates were high at 24 ‰, significant reductions in mass and the failure of metrics to return to control levels suggest that long-term survival at 24 ‰ is unlikely, potentially impacting Antarctic food-web dynamics and ecological interactions.

For osmoconforming organisms, prolonged exposure to reduced salinity requires an adjustment to intracellular osmolyte levels to ensure an osmotic balance is maintained between the cell and external sea water. However, osmolytes-low-molecular-mass micromolecules-may also serve overlapping roles in freeze avoidance, desiccation resistance, and protein stabilisation. In Antarctic species living at or below 0 °C, multiple environmental stressors likely shape species-specific osmolyte profiles. Yet, the osmolytes utilised in osmotic acclimation and the broader micromolecular profile of Antarctic marine organisms remain poorly characterised. This study employed gas chromatography-mass spectrometry (GC-MS) to analyse the organic osmolyte composition of two endemic Antarctic echinoderms, the sea star Odontaster validus and sea urchin, Sterechinus neumayeri, following long-term acclimation (>12 weeks) to reduced salinity levels (29 ‰ and 24 ‰). Significant reductions in total tissue organic solute (osmolyte) concentrations after low salinity exposure indicated active cell volume regulation to reduce intracellular osmotic pressure. The osmolyte metabolic profiles of these Antarctic species appeared distinct from those of temperate echinoderms and other marine osmoconformers, suggesting a specialised adaptive response. Notably, the use of branched-chain amino acids (valine, leucine, and isoleucine) in cell volume regulation in O. validus, and to a lesser extent in S. neumayeri, suggests a micromolecular adaptation tailored to the extreme cold of the Antarctic environment.