Echinoderm responses to variation in salinity

Effects of hypo-osmotic stress on osmoregulation, antioxidant response, and energy metabolism in sea cucumber Holothuria moebii under desalination environment

With global climate changing, hypo-salinity events are increasing in frequency and duration because of continuous rainfall and freshwater inflow, which causes reduced cytosolic osmolarity and cellular stress responses in aquatic animals. Sea cucumbers are considered stenohaline because they lack osmoregulatory organs and are vulnerable to salinity fluctuations. In this study, we performed multiple biochemical assays, de novo transcriptomics, and widely targeted metabolomics to comprehensively explore the osmoregulatory mechanisms and physiological responses of sea cucumber Holothuria moebii to hypo-osmotic stress, which is a representative specie that is frequently exposed to hypo-saline intertidal zones. Our results found that H. moebii contracted their ambulacral feet and oral tentacles, and the coelomic fluid ion concentrations were reduced to be consistent with the environment. The microvilli of intestines and respiratory trees underwent degeneration, and the cytoplasm exhibited swelling and vacuolation. Moreover, the Na+, K+, and Cl- concentrations and Na+/K+-ATPase activity were significantly reduced under hypo-osmotic stress. The decrease in protein kinase A activity and increase in 5'-AMP level indicated a significant inhibition of the cAMP signaling pathway to regulate ion concentrations. And small intracellular organic molecules (amino acids, nucleotides and their derivatives) also play crucial roles in osmoregulation through oxidative deamination of glutamate, nucleotide catabolism, and nucleic acid synthesis. Moreover, lysosomes and peroxisomes removed oxidative damage, whereas antioxidant metabolites, such as N-acetyl amino acids and glutathione, were increased to resist oxidative stress. With prolonged hypo-osmotic stress, glycerophospholipid metabolism was enhanced to maintain membrane stability. Furthermore, acyl-CoA-binding protein activity was significantly inhibited, and only a small amount of acylcarnitine was significantly accumulated, which indicated a disruption in energy metabolism. PPAR signaling pathway and choline content were up-regulated to promote fatty acid metabolism under hypo-osmotic stress. Overall, our results provide new insights into the osmoregulatory mechanisms and physiological responses of sea cucumbers to hypo-osmotic stress.

The Chengjiang Biota inhabited a deltaic environment

The Chengjiang Biota is the earliest Phanerozoic soft-bodied fossil assemblage offering the most complete snapshot of Earth’s initial diversification, the Cambrian Explosion. Although palaeobiologic aspects of this biota are well understood, the precise sedimentary environment inhabited by this biota remains debated. Herein, we examine a non-weathered core from the Yu’anshan Formation including the interval preserving the Chengjiang Biota. Our data indicate that the succession was deposited as part of a delta influenced by storm floods (i.e., produced by upstream river floods resulting from ocean storms). Most Chengjiang animals lived in an oxygen and nutrient-rich delta front environment in which unstable salinity and high sedimentation rates were the main stressors. This unexpected finding allows for sophisticated ecological comparisons with other Burgess Shale-type deposits and emphasizes that the long-held view of Burgess Shale-type faunas as snapshots of stable distal shelf and slope communities needs to be revised based on recent sedimentologic advances.

The Chengjiang Biota is the earliest most diverse animal community from the Cambrian Explosion (~518 million years ago). This biota is shown to have colonized a delta, highlighting the importance of this shallow environment in recording early snapshots of life on Earth.

Infrequent Fluctuations in Temperature and Salinity May Enhance Feeding in Pisaster ochraceus (Asteroidea) but Not in Dendraster excentricus (Echinoidea) Larvae

AbstractIn recent years, low-salinity events characterized by high temperatures (18-23 °C) and low-salinity waters (20‰-22‰) have increased during late spring and summer, when many marine invertebrate larvae are developing. The present study examines the effects of low-salinity events on particle ingestion for larvae of two echinoderm species, the sea star Pisaster ochraceus and the sand dollar Dendraster excentricus. Larvae were exposed to high temperatures and low salinities for 24 hours, followed by feeding on the alga Isochrysis galbana in high or low salinity for another 10 minutes. Exposing Pisaster larvae to high temperatures and low salinities, followed by feeding in low salinity, did not impair ingestion rates. In fact, these larvae ingested particles at similar and sometimes higher rates than those in the controls. In sharp contrast, a 24-hour exposure to a high temperature and low salinity, followed by continued exposure to low salinity to feed, led to a decrease in the number of particles ingested by 8-arm Dendraster larvae. Larvae of both species captured very few particles when returned to 30‰ after a low-salinity event, indicating that continuous interruption of larval feeding by low-salinity events during development could be deleterious. Sand dollar larvae may have responded negatively to low-salinity events in our experiments because they are found in protected bays, where they may seldom experience these events.

Short-Term Exposure to Storm-Like Scenario Microplastic and Salinity Conditions Does not Impact Adult Sea Urchin (Arbacia punctulata) Physiology

The effects of microplastic pollution on sea urchins has received little attention despite their ecological and economical importance. This is the first study to focus on adult sea urchins (Arbacia punctulata). These organisms were exposed to storm-like sediment resuspension of microplastic concentrations (9-μm polystyrene 25,000 spheres L^-1) combined with salinity reductions (salinity 25 vs. 33) associated with high precipitation. Urchins were exposed to these parameters for 24 h before assessing righting times and for 48 h before assessing oxygen consumption rates. No significant impacts on urchin physiology were observed showing resilience to short-term exposures of storm-like induced microplastics and salinity. No microplastic particles blocked the madreporite pores indicating the active removal of particles by cilia and pedicellariae. Gut tissue samples indicated consumption of microplastics. Studies on more species are urgently required to determine their responses to plastic pollution to inform management decision-making processes.

miR-10 involved in salinity-induced stress responses and targets TBC1D5 in the sea cucumber, Apostichopus japonicas

The sea cucumber is an economically important aquaculture species in China, where it encounter hypo-saline conditions caused by freshwater outflow from rivers and rainfall. MicroRNAs (miRNA) are small noncoding RNAs of about 22 nucleotides, which are crucial regulators of gene expression at the post-transcriptional level and are involved in a variety of physiological and pathophysiological processes. miR-10 is differentially expressed in salinity acclimation, and has a seed-region match with TBC1D5. The expression profiles of miR-10 and TBC1D5 indicate that miR-10 negatively regulates the expression of TBC1D5 in coelomocytes and sea cucumbers with a miR-10 agomir or antagomir. During salinity acclimation, up-regulation of miR-10 was induced after transfection in coelomocytes with a miR-10 inhibitor, while down-regulation of TBC1D5 was induced. The miR-10 expression maximum in coelomocytes appeared at 48 h post-transfection with a miR-10 inhibitor, was later than that of in sea cucumbers, which appeared 24 h after miR-10 antagomir injection. There was no longer a negative relationship between miR-10 and TBC1D5 expression in coelomocytes and sea cucumbers with miR-10 mimics or agomir during salinity acclimation. The miR-10 antagomir or agomir only affected sodium and NKA enzyme activities, and has little effect on other chloride and potassium ions. Our results demonstrate miR-10 directly regulates TBC1D5 by targeting its 3'-UTR, and that miR-10 suppression substantially increases TBC1D5 mRNA levels in vivo and in vitro. Furthermore, miR-10 and TBC1D5 fluctuating expression patterns after treatment with a miR-10 inhibitor or mimics during salinity acclimation may indicate that they are required for adaptation to salinity stress caused by environmental change. Especially, the miR-10 up-regulation in coelomocytes with miR-10 inhibitor during salinity acclimation indicated that they are required for adaptation to salinity stress caused by environmental change. We propose that miR-10 participates in a regulatory circuit that allows for rapid gene program transitions in response to osmotic stress.

Developmental Consequences of Temperature and Salinity Stress in the Sand Dollar Dendraster excentricus

Animals that reside, reproduce, and develop in nearshore habitats are often exposed to strong fluctuations in abiotic conditions, including temperature and salinity. We studied the developmental response of the sand dollar Dendraster excentricus to increased temperature and reduced salinity at levels comparable to those induced by summer freshwater input into the San Juan Archipelago, Washington. We observed that embryos exposed to temperature and salinity stress exhibited polyembryony (the splitting of one embryo into multiple independent individuals), and we subsequently tested the competency of twin and normal embryos to reach metamorphosis. We found that twin embryos generated from a single egg are each capable of reaching metamorphosis. To begin investigating the mechanisms underlying polyembryony, we tested whether osmotic stress caused swelling of the fertilization envelope, thus allowing embryos the physical space to produce multiples within a single envelope. We also tested whether reduced calcium levels in low-salinity seawater reduced cell-cell adhesion and allowed cells to separate and develop as multiple embryos within a fertilization envelope. However, neither osmotic stress nor reduced calcium levels alone appear sufficient to induce polyembryony. We hypothesize that changes in the properties of the hyaline layer that lies beneath the fertilization envelope facilitate polyembryony.

Cryptic species diversity and reproductive isolation among sympatric lineages of Strongylocentrotus sea urchins in the northwest Atlantic

Distinguishing between intra- and inter-specific variation in genetic studies is critical to understanding evolution because the mechanisms driving change among populations are expected to be different than those that shape reproductive isolation among lineages. Genetic studies of north Atlantic green sea urchins Strongylocentrotus droebachiensis (Müller, 1776) have detected significant population substructure and asymmetric gene flow from Europe to Atlantic Canada and interspecific hybridization between S. droebachiensis and Strongylocentrotus pallidus (Sars, 1871). However, combined with patterns of divergence at mtDNA sequences, morphological divergence at gamete traits suggests that the European and North American lineages of S. droebachiensis may be cryptic species. Here, we use a combination of cytochrome c oxidase subunit I ( COI) sequences and single nucleotide polymorphisms (SNPs) to test for cryptic species within Strongylocentrotus sea urchins and hybrids between S. droebachiensis and S. pallidus populations. We detect striking patterns of habitat and reproductive isolation between two S. droebachiensis lineages, with offshore deep-water collections consisting of S. pallidus in addition to a cryptic lineage sharing genetic similarity with previously published sequences from eastern Atlantic S. droebachiensis. We detected only limited hybridization among all three lineages of sea urchins, suggesting that shared genetic differences previously reported may be a result of historical introgression or incomplete lineage sorting.

Early time course of variation in coelomic fluid ionic concentrations in sea urchins abruptly exposed to hypo- and hyper-osmotic salinity challenges: Role of size and cross-section area of test holes

Echinoderms are restricted to the marine environment and are osmoconformer invertebrates. However, some species live in unstable environments. Especially those species, and those of larger body size, tend to show variable, albeit transient, ionic gradients between their coelomic fluid and external seawater. In order to further examine how sea urchin size relates to apparent ionic permeability of their body wall/epithelia, specimens of Echinometra lucunter, Lytechinus variegatus, Paracentrotus gaimardi, and Arbacia lixula-A. lixula of two distinct populations, Rio de Janeiro and Santa Catarina-were abruptly transferred from 35 psu to either 25 or 45 psu. Sodium, chloride, magnesium, and potassium concentrations were assayed in their coelomic fluids after 0, 1, 2, and 3 hr of exposure. Relative area of putative permeable (i.e., cross section areas of soft tissues, or test holes) surfaces (PPS) was estimated in empty tests as the sum of the peristomial area (oral hole in the empty test) and the total cross-section area of ambulacral holes, divided by the total volume (TV) of the test. L. variegatus and E. lucunter, the largest species, had PPS/TV values similar to that of the much smaller P. gaimardi. A. lixula was the "most putatively-permeable and conformer" among them all, especially urchins from the Santa Catarina population. Internal ionic levels equilibrated faster with external water in 45 than in 25, and differences among ions were observed. Body size is relevant, among many other factors, to aid conformers such as sea urchins to dwell in intertidal unstable habitats.

Environmental Induction of Polyembryony in Echinoid Echinoderms

Polyembryony, or the production of multiple offspring from a single zygote, is a widespread phenomenon in the animal kingdom. Various types of polyembryony have been described in arthropods, bryozoans, chordates, cnidarians, echinoderms, and platyhelminthes. We describe the induction of polyembryony in embryos of the sand dollar Echinarachnius parma and the pencil urchin Eucidaris tribuloides in response to elevated temperature and reduced salinity. Data on the environmental variation in temperature and salinity that normally occurs during the spawning season, combined with the range of laboratory conditions over which polyembryony was induced, suggest that polyembryony may occur frequently in these species under natural conditions. We tested an additional two species of echinoids for similar responses, but found little evidence for polyembryony in the green urchin Strongylocentrotus droebachiensis or the variegated urchin Lytechinus variegatus, suggesting that polyembryony is not a universal response of echinoids to fluctuations in temperature and salinity. The unexpected developmental changes that we observed in response to present-day fluctuations in temperature and salinity suggest that ongoing and future environmental shifts may drive substantial changes in marine invertebrate developmental patterns, and that these changes will be different across taxa.

The skeleton of postmetamorphic echinoderms in a changing world

Available evidence on the impact of acidification and its interaction with warming on the skeleton of postmetamorphic (juvenile and adult) echinoderms is reviewed. Data are available on sea urchins, starfish, and brittle stars in 33 studies. Skeleton growth of juveniles of all sea urchin species studied so far is affected from pH 7.8 to 7.6 in seawater, values that are expected to be reached during the 21st century. Growth in adult sea urchins (six species studied) is apparently only marginally affected at seawater pH relevant to this century. The interacting effect of temperature differed according to studies. Juvenile starfish as well as adults seem to be either not impacted or even boosted by acidification. Brittle stars show moderate effects at pH below or equal to 7.4. Dissolution of the body wall skeleton is unlikely to be a major threat to sea urchins. Spines, however, due to their exposed position, are more prone to this threat, but their regeneration abilities can probably ensure their maintenance, although this could have an energetic cost and induce changes in resource allocation. No information is available on skeleton dissolution in starfish, and the situation in brittle stars needs further assessment. Very preliminary evidence indicates that mechanical properties in sea urchins could be affected. So, although the impact of ocean acidification on the skeleton of echinoderms has been considered as a major threat from the first studies, we need a better understanding of the induced changes, in particular the functional consequences of growth modifications and dissolution related to mechanical properties. It is suggested to focus studies on these aspects.