Direct relationship between osmotic and ionic conforming behavior and tissue water regulatory capacity in echinoids

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.

Behavioural and physiological impacts of low salinity on the sea urchin Echinus esculentus

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.

Tetrodotoxin/Saxitoxin Accumulation Profile in the Euryhaline Marine Pufferfish Chelonodontops patoca

Marine Takifugu pufferfish, which naturally possess tetrodotoxins (TTXs), selectively take up and accumulate TTXs, whereas freshwater Pao pufferfish, which naturally possess saxitoxins (STXs), selectively take up and accumulate STXs. To further clarify the TTXs/STXs selectivity in pufferfish, we conducted a TTX/STX administration experiment using Chelonodontops patoca, a euryhaline marine pufferfish possessing both TTXs and STXs. Forty nontoxic cultured individuals of C. patoca were divided into a seawater group (SW, acclimated/reared at 33‱ salinity; n = 20) and a brackish water group (BW, acclimated/reared at 8‱ salinity; n = 20). An aqueous TTX/STX mixture was intrarectally administered (both at 7.5 nmol/fish), and five individuals/group were analyzed after 1-48 h. Instrumental toxin analyses revealed that both TTX and STX were taken up, transferred, and retained, but more STX than TTX was retained in both groups. TTX gradually decreased and eventually became almost undetectable in the intestinal tissue, while STX was retained at ~5-10% of the dose level, and only STX showed transient transfer in the liver. The BW group showed a faster decrease/disappearance of TTX, greater STX retention in the intestine, and greater STX transient transfer to the liver. Thus, C. patoca appears to more easily accumulate STXs than TTXs, especially under hypoosmotic conditions.

Mass mortality of collector urchins Tripneustes gratilla in Hawai`i

As grazers, sea urchins are keystone species in tropical marine ecosystems, and their loss can have important ecological ramifications. Die-offs of urchins are frequently described, but their causes are often unclear, in part because systematic examinations of animal tissues at gross and microscopic level are not done. In some areas, urchins are being employed to control invasive marine algae. Here, we describe the pathology of a mortality event in Tripneustes gratilla in Hawai`i where urchins were translocated to control invasive algae. Although we did not determine the cause of the mortality event, our investigation indicates that animals died from inflammation of the test and epidermal ulceration, followed by inability to maintain coelomic fluid volume, colonization of coelomic fluid by opportunists (diatom, algae), and inappetence. Parasites, bacteria, fungi, and viruses were not evident as a primary cause of death. Pathology was suggestive of a toxin or other environmental cause such as lack of food, possibilities that could be pursued in future investigations. These findings highlight the need for caution and additional tools to better assess health when translocating marine invertebrates to ensure maximal biosecurity.

Will climate changes enhance the impacts of e-waste in aquatic systems?

The increase of the worlds' population is being accompanied by the exponential growth in waste of electrical and electronic equipment (e-waste) generation as a result of the rapid technological implementations. The inappropriate processing and disposal of this e-waste, containing rare-earth elements (REEs) such as gadolinium (Gd), may enhance its occurrence in the environment. In particular, the presence of Gd in marine systems may lead to environmental risks which are still unknown, especially considering foreseen climate modifications such as water salinity shifts due to extreme weather events. Within this context, the present study intended to assess the combined effects of Gd at variable salinities. For that, biochemical modifications were assessed in mussels, Mytilus galloprovincialis, exposed to Gd (0 and 10 μg/L) and different salinity levels (20, 30 and 40), acting individually and in combination. A decrease in salinity, induced an array of biochemical effects associated to hypotonic stress in non-contaminated and contaminated mussels, including metabolism, antioxidant and biotransformation defenses activation. Moreover, in Gd-contaminated organisms, the increase in salinity was responsible for a significant reduction of metabolic and defense mechanisms, possibly associated with a mussels' physiological response to the stress caused by the combination of both factors. In particular, Gd caused cellular damage at all salinities, but mussels adopted different strategies under each salinity to limit the extent of oxidative stress. That is, an increase in metabolism was associated to hypotonic stress and Gd exposure, an activation of defense enzymes was revealed at the control salinity (30) and a decrease in metabolism and non-activation of defenses, associated with a possible physiological defense trait, was evidenced at the highest salinity. The different strategies adopted highlight the need to investigate the risk of emerging contaminants such as REEs at present and forecasted climate change scenarios, thus providing a more realistic environmental risk assessment.

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.

Effects of seawater chemistry (Mg2+/Ca2+ ratio) and diet on the skeletal Mg/Ca ratio in the common sea urchin Paracentrotus lividus

It has been argued that concentration of major metallic ions such as Mg²⁺ and Ca²⁺ plays a role in determining the composition of the echinoderm skeleton. Consequently, in several studies Mg/Ca ratio from modern and fossil echinoderm ossicles was used as a proxy of secular Mg²⁺/Ca²⁺ changes of Phanerozoic seawater. However, although significant progress has been made in understanding biomineralization of echinoderms, it is still largely unknown what are the sources and physiological pathways of major ions that contribute to skeleton formation. Herein we tested the effects of modifications of ambient seawater Mg²⁺/Ca²⁺ ratio (which is typically ∼5) and Mg-enrichment of the diet on the Mg/Ca ratio in regenerating spines of sea urchin Paracentrotus lividus under experimental conditions. We found that sea urchins cultured in seawater with Mg²⁺/Ca²⁺ ratio decreased to ∼1.9 produced a skeleton with also decreased Mg/Ca ratio. However, the skeleton of specimens fed on a Mg-enriched diet showed significantly higher Mg/Ca ratio. This suggests that the seawater is an important but not the only source of ions that contributes to the Mg/Ca ratio of the skeleton. Consequently, the reliability of geochemical models that link directly seawater chemistry with the Mg/Ca ratio of the skeleton should be reevaluated.

Capacity of tissue water regulation is impaired in an osmoconformer living in impacted estuaries?

Estuarine osmoconformes rely on their ability to perform tissue and cell water regulation to cope with daily osmotic challenges that occur in the estuary. In addition, these animals currently must deal with pollutants present in the estuarine environment, which can disturb their capacity of water regulation. We collected the mangrove oyster Crassostrea rhizophorae in two tropical estuaries in the Northeast region of Brazil with different degrees of human interference: the Paraíba Estuary (impacted) and the Mamanguape Estuary (preserved). Tissue water content was analyzed after exposure to salinities 12, 24 and 36 for 24 h. Gill cell volume regulation was analyzed in vitro upon hypo- and hyper-osmotic conditions. We also analyzed gill MXR (multi-xenobiotic resistance) mechanism, as reference of environmental pollution. Gill and muscle of oysters from two sites of Paraíba Estuary, and from one site of Mamanguape Estuary were not able to maintain tissue water content upon hypo- and hyper-osmotic conditions. Gill cells of oyster from the same sites exhibited swelling followed by regulatory volume decrease upon hypo-osmotic condition. Gill MXR activity was increased in oysters from these sites. The best tissue and cell water regulation, and the lowest MXR activity, was found in oyster from downstream of Mamanguape Estuary, our reference site and the one most preserved. Tissue and cell water regulation proved to be a sensitive parameter to environmental pollution and could be considered as biomarker of aquatic contamination.

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.

Acid-base physiology response to ocean acidification of two ecologically and economically important holothuroids from contrasting habitats, Holothuria scabra and Holothuria parva

Sea cucumbers are dominant invertebrates in several ecosystems such as coral reefs, seagrass meadows and mangroves. As bioturbators, they have an important ecological role in making available calcium carbonate and nutrients to the rest of the community. However, due to their commercial value, they face overexploitation in the natural environment. On top of that, occurring ocean acidification could impact these organisms, considered sensitive as echinoderms are osmoconformers, high-magnesium calcite producers and have a low metabolism. As a first investigation of the impact of ocean acidification on sea cucumbers, we tested the impact of short-term (6 to 12 days) exposure to ocean acidification (seawater pH 7.7 and 7.4) on two sea cucumbers collected in SW Madagascar, Holothuria scabra, a high commercial value species living in the seagrass meadows, and H. parva, inhabiting the mangroves. The former lives in a habitat with moderate fluctuations of seawater chemistry (driven by day-night differences) while the second lives in a highly variable intertidal environment. In both species, pH of the coelomic fluid was significantly negatively affected by reduced seawater pH, with a pronounced extracellular acidosis in individuals maintained at pH 7.7 and 7.4. This acidosis was due to an increased dissolved inorganic carbon content and pCO2 of the coelomic fluid, indicating a limited diffusion of the CO2 towards the external medium. However, respiration and ammonium excretion rates were not affected. No evidence of accumulation of bicarbonate was observed to buffer the coelomic fluid pH. If this acidosis stays uncompensated for when facing long-term exposure, other processes could be affected in both species, eventually leading to impacts on their ecological role.

Echinoderm responses to variation in salinity

Although Echinodermata is one of the only stenohaline phyla in the animal kingdom, several species show remarkable abilities to acclimate and survive in euryhaline habitats. The last comprehensive review of this topic was over 25 years ago and much work has been published since. These recent studies expand the field reports of species living in hyposaline environments and detail experimental research on the responses, physiological range, and limits of echinoderms to salinity challenges. I provide a brief review of the historical concepts and measures of salinity and relate this overview to the physiological and ecological studies on echinoderms. Many marine biologists are not aware that chemical oceanographers advocate abandoning today's commonly used measure of salinity, 'PSU', in favour of absolute salinity (SA)-a return to the ppt (‰) metric. The literature survey reveals only one euryhaline-tolerant species in the Southern Hemisphere (there are 42 in the North) and more euryhaline species in the geologically older, brackish seas. The green sea urchin, Strongylocentrotus droebachiensis, is one of the most tolerant echinoids to hyposalinity. Different source populations have varying levels of acclimation and tolerance to hyposalinity. Experiments show that green urchins previously unexposed to hyposalinity experience a clear decrease in growth rates; however, this adverse effect is short lived. Green urchins already acclimated to hyposalinity can endure intense and repeated bouts and grow at the same rate of urchins not exposed. Promising future work on the physiological and cellular mechanisms of hyposalinity acclimation includes comparative studies of the role of heat shock proteins in the response to changing salinities.