Osmoregulation by juvenile brown-banded bamboo sharks, Chiloscyllium punctatum, in hypo- and hyper-saline waters

Physiologic responses of Cownose Rays following freshwater dips for treatment of capsalid monogeneans

OBJECTIVE

Two Cownose Rays Rhinoptera bonasus were presented for management of a severe capsalid monogenean infection Benedeniella posterocolpa in a mixed species habitat at an aquarium.

METHODS

A series of freshwater (FW) dips were elected to mitigate parasite resistance to praziquantel due to the endemic monogenean population in the system. A pretreatment blood sample was opportunistically obtained, and subsequent samples were processed due to development of clinical signs.

RESULT

While the first FW dip was tolerated well and did not induce abnormal behavior, the second led to one individual presenting with tachypnea, pallor, and lethargy. Marked hematological abnormalities requiring medical intervention occurred in both individuals after the second dip. After treatment with elasmobranch Ringer's fluid therapy, sodium bicarbonate, prednisolone acetate, and vitamin E/selenium in the more severely affected ray, and elasmobranch Ringer's alone in the second ray, hematologic derangements and symptoms resolved. Hemoconcentration, hyperproteinemia, hyperlactatemia, and hyperglycemia were attributed to a stress response. A selective loss of urea after exposure to FW resulted in decreased plasma osmolality.

CONCLUSION

Given the severity of the stress response and associated complications observed, hyposalinity treatments should be utilized with caution in this species. If this therapy is employed, the clinician is advised to be prepared to administer supportive care coinciding with the FW dip if necessary.

Tissue metal concentrations and antioxidant enzyme activity in western north Atlantic white sharks (Carcharodon carcharias)

Anthropogenic practices have increased metal contamination in marine ecosystems. Most sharks have long lifespans, occupy an important ecological position at the top of marine food webs, and can accumulate metals. However, reference levels of metal contaminants in the tissues of sharks, particularly, apex predators such as the white shark (Carcharodon carcharias), are lacking. In this study, concentrations of copper (Cu), cadmium (Cd), nickel (Ni), lead (Pb), silver (Ag), and zinc (Zn) were measured in the muscle tissue of white (n = 42) and tiger (Galeocerdo cuvier; n = 3) sharks. Metal exposure in various species, including sharks, has been correlated with increased oxidative stress. Therefore, the main objectives of this study were to assess metal accumulation and antioxidant enzyme activity (superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx)) in the muscle tissue of the population of white sharks and tiger sharks inhabiting the Western North Atlantic. The measured parameters were qualitatively compared between species. The small sample size of tiger sharks (collected from only one site) limited statistical analyses, therefore, white sharks were the primary focus of this study. Differences in tissue metal (Cu, Cd, Ni, and Zn) concentrations and antioxidant enzyme activities were detected based on collection site, with significant positive correlations between Cd and enzymes, SOD and CAT, and Zn and enzymes, SOD and GPx in C. carcharias. Differences in Ni concentration were detected based on sex, with females having higher Ni levels. Additionally, plasma osmolality was not correlated with tissue metal concentrations; however, osmolality decreased with increasing length in C. carcharias. This study is the first to report baseline levels of Cu, Zn, Cd, Ni, Ag, and Pb in muscle of North Atlantic white sharks and provides new insights into oxidative stress responses of these sensitive species to metal contaminants.

Physiological responses in Nile tilapia (Oreochromis niloticus) induced by combined stress of environmental salinity and triphenyltin

Triphenyltin (TPT) has attracted considerable attention owing to its vitality, bioaccumulation, and lurking damage. TPT widely exists in complex salinity areas such as estuaries and coastal regions. However, there are few studies on the toxicological behavior of TPT under different salinity. In the study, juvenile Nile tilapia (Oreochromis niloticus) were utilized as model animals to investigate the effects of environmental relevant TPT exposure on the osmoregulation and energy metabolism in gill under different salinity. The results showed that salinity and TPT single or combined exposure affected the morphology of the gill tissue. After TPT exposure, Na⁺-K⁺-ATPase (NKA) activity significantly decreased at 0 ppt, while NKA and Ca²⁺-Mg²⁺-ATPase (CMA) activities significantly increased at 15 ppt. In addition, significantly higher succinate dehydrogenase (SDH) and lactate dehydrogenase (LDH) activities were found in the control fish compared to the TPT-exposed ones at 15 ppt. Quantitative real-time PCR results showed that TPT exposure affected the expression of osmoregulation and energy metabolism-related genes under different salinity. Overall, TPT exposure interfered with osmoregulation and energy metabolism under different salinity. The study will provide reference data for assessing the toxicity of organotin compounds in complex-salinity areas.

Acute Stress in Lesser-Spotted Catshark (Scyliorhinus canicula Linnaeus, 1758) Promotes Amino Acid Catabolism and Osmoregulatory Imbalances

Simple Summary

In catsharks (Scyliorhinus canicula), air exposure induces amino acid catabolism altogether with osmoregulatory imbalances. This study describes a novel NHE isoform being expressed in gills that may be involved in ammonium excretion.

Abstract

Acute-stress situations in vertebrates induce a series of physiological responses to cope with the event. While common secondary stress responses include increased catabolism and osmoregulatory imbalances, specific processes depend on the taxa. In this sense, these processes are still largely unknown in ancient vertebrates such as marine elasmobranchs. Thus, we challenged the lesser spotted catshark (Scyliorhinus canicula) to 18 min of air exposure, and monitored their recovery after 0, 5, and 24 h. This study describes amino acid turnover in the liver, white muscle, gills, and rectal gland, and plasma parameters related to energy metabolism and osmoregulatory imbalances. Catsharks rely on white muscle amino acid catabolism to face the energy demand imposed by the stressor, producing NH₄⁺. While some plasma ions (K⁺, Cl⁻ and Ca²⁺) increased in concentration after 18 min of air exposure, returning to basal values after 5 h of recovery, Na⁺ increased after just 5 h of recovery, coinciding with a decrease in plasma NH₄⁺. These changes were accompanied by increased activity of a branchial amiloride-sensitive ATPase. Therefore, we hypothesize that this enzyme may be a Na⁺/H⁺ exchanger (NHE) related to NH₄⁺ excretion. The action of an omeprazole-sensitive ATPase, putatively associated to a H⁺/K⁺-ATPase (HKA), is also affected by these allostatic processes. Some complementary experiments were carried out to delve a little deeper into the possible branchial enzymes sensitive to amiloride, including in vivo and ex vivo approaches, and partial sequencing of a nhe1 in the gills. This study describes the possible presence of an HKA enzyme in the rectal gland, as well as a NHE in the gills, highlighting the importance of understanding the relationship between acute stress and osmoregulation in elasmobranchs.

Effect of dendritic organ ligation on striped eel catfish Plotosus lineatus osmoregulation

Unique amongst the teleost, Plotosidae catfish possess a dendritic organ (DO) as a purported salt secreting organ, whereas other marine teleosts rely on their gill ionocytes for active NaCl excretion. To address the role of the DO in ionregulation, ligation experiments were conducted in brackish water (BW) 3‰ and seawater (SW) 34‰ acclimated Plotosus lineatus and compared to sham operated fish. Ligation in SW resulted in an osmoregulatory impairment in blood (elevated ions and hematocrit) and muscle (dehydration). However, SW ligation did not elicit compensatory changes in gill or kidney Na⁺/K⁺-ATPase (NKA) activity and/or protein expression while a decrease in anterior intestine and increased in posterior intestine were observed but this was not reflected at the protein level. Following ligation in SW, protein levels of carbonic anhydrase (CA) and V-ATPase B subunit (VHAB) were higher in kidney but either lower (CA) or unchanged (VHAB) in other tissues. Taken together, the osmotic disturbance in ligated SW fish indicates the central role of the DO in salt secretion and the absence of a compensatory response from the gill.

Osmoregulation in the Plotosidae Catfish: Role of the Salt Secreting Dendritic Organ

Unlike other marine teleosts, the Plotosidae catfishes reportedly have an extra-branchial salt secreting dendritic organ (DO). Salinity acclimation [brackishwater (BW) 3aaa, seawater (SWcontrol) 34aaa, and hypersaline water (HSW) 60aaa] for 14 days was used to investigate the osmoregulatory abilities of Plotosus lineatus through measurements of blood chemistry, muscle water content (MWC), Na⁺/K⁺-ATPase (NKA) specific activity and ion transporter expression in gills, DO, kidney and intestine. Ion transporter expression was determined using immunoblotting, immunohistochemistry (IHC) and quantitative polymerase chain reaction (qPCR). HSW elevated mortality, plasma osmolality and ions, and hematocrit, and decreased MWC indicating an osmoregulatory challenge. NKA specific activity and protein levels were significantly higher in DO compared to gill, kidney and intestine at all salinities. NKA specific activity increased in kidney and posterior intestine with HSW but only kidney showed correspondingly higher NKA α-subunit protein levels. Since DO mass was greater in HSW, the total amount of DO NKA activity expressed per gram fish was greater indicating higher overall capacity. Gill NKA and V-ATPase protein levels were greater with HSW acclimation but this was not reflected in NKA activity, mRNA or ionocyte abundance. BW acclimation resulted in lower NKA activity in gill, kidney and DO. Cl^- levels were better regulated and the resulting strong ion ratio in BW suggests a metabolic acidosis. Elevated DO heat shock protein 70 levels in HSW fish indicate a cellular stress. Strong NKA and NKCC1 (Na⁺:K⁺:2Cl^- cotransporter1) co-localization was observed in DO parenchymal cells, which was rare in gill ionocytes. NKCC1 immunoblot expression was only detected in DO, which was highest at HSW. Cystic fibrosis transmembrane regulator Cl^- channel (CFTR) localize apically to DO NKA immunoreactive cells. Taken together, the demonstration of high NKA activity in DO coexpressed with NKCC1 and CFTR indicates the presence of the conserved secondary active Cl^- secretion mechanism found in other ion transporting epithelia suggesting a convergent evolution with other vertebrate salt secreting organs. However, the significant osmoregulatory challenge of HSW indicates that the DO may be of limited use under more extreme salinity conditions in contrast to the gill based ionoregulatory strategy of marine teleosts.

Pass the salt: physiological consequences of ecologically relevant hyposmotic exposure in juvenile gummy sharks (Mustelus antarcticus) and school sharks (Galeorhinus galeus)

Estuarine habitats are frequently used as nurseries by elasmobranch species for their protection and abundant resources; however, global climate change is increasing the frequency and severity of environmental challenges in these estuaries that may negatively affect elasmobranch physiology. Hyposmotic events are particularly challenging for marine sharks that osmoconform, and species-specific tolerances are not well known. Therefore, we sought to determine the effects of an acute (48 h) ecologically relevant hyposmotic event (25.8 ppt) on the physiology of two juvenile shark species, namely the school shark (Galeorhinus galeus), listed by the Australian Environmental Protection and Biodiversity Conservation Act as 'conservation dependent', and the gummy shark (Mustelus antarcticus), from the Pittwater Estuary (Australia). In both species, we observed a decrease in plasma osmolality brought about by selective losses of NaCl, urea and trimethylamine N-oxide, as well as decreases in haemoglobin, haematocrit and routine oxygen consumption. Heat-shock protein levels varied between species during the exposure, but we found no evidence of protein damage in any of the tissues tested. Although both species seemed to be able to cope with this level of osmotic challenge, overall the school sharks exhibited higher gill Na⁺/K⁺-ATPase activity and ubiquitin concentrations in routine and experimental conditions, a larger heat-shock protein response and a smaller decrease in routine oxygen consumption during the hyposmotic exposure, suggesting that there are species-specific responses that could potentially affect their ability to withstand longer or more severe changes in salinity. Emerging evidence from acoustic monitoring of sharks has indicated variability in the species found in the Pittwater Estuary during hyposmotic events, and together, our data may help to predict species abundance and distribution in the face of future global climate change.

Physiological responses to hypersalinity correspond to nursery ground usage in two inshore shark species (Mustelus antarcticus & Galeorhinus galeus)

Shark nurseries are susceptible to environmental fluctuations in salinity because of their shallow, coastal nature; however, the physiological impacts on resident elasmobranchs are largely unknown. Gummy (Mustelus antarcticus) and school sharks (Galeorhinus galeus) use the same Tasmanian estuary as a nursery ground; however, each species has distinct distribution patterns that are coincident with changes in local environmental conditions, such as increases in salinity. We hypothesized that these differences were directly related to differential physiological tolerances to high salinity. To test this hypothesis, we exposed wild, juvenile school and gummy sharks to an environmentally-relevant hypersaline (120% SW) event for 48 h. Metabolic rate decreased 20-35% in both species, and gill Na+/K+ ATPase activity was maintained in gummy sharks but decreased 37% in school sharks. We measured plasma ions (Na+, K+, Cl−), and osmolytes (urea and trimethylamine oxide (TMAO)), and observed a 33% increase in plasma Na+ in gummy sharks with hyperosmotic exposure, while school sharks displayed a typical ureosmotic increase in plasma urea (∼20%). With elevated salinity, gill TMAO concentration increased by 42% in school sharks and by 30% in gummy sharks. Indicators of cellular stress (heat shock proteins HSP70, 90, 110, and ubiquitin) significantly increased in gill, and white muscle in both a species- and tissue- specific manner. Overall, gummy sharks exhibited greater osmotic perturbation and ionic dysregulation and a larger cellular stress response compared to school sharks. Our findings provide physiological correlates to the observed distribution, and movement of these shark species in their critical nursery grounds.

Effects of short-term hyper- and hypo-osmotic exposure on the osmoregulatory strategy of unfed North Pacific spiny dogfish (Squalus suckleyi)

The North Pacific spiny dogfish (Squalus suckleyi) is a partially euryhaline species of elasmobranch that often enter estuaries where they experience relatively large fluctuations in environmental salinity that can affect plasma osmolality. Previous studies have investigated the effects of altered salinity on elasmobranchs over the long term, but fewer studies have conducted time courses to investigate how rapidly they can adapt to such changes. In this study, we exposed unfed (no exogenous source of nitrogen or TMAO) spiny dogfish to hyper- and hypo-osmotic conditions and measured plasma and tissue osmolytes, nitrogen excretion, and changes in enzyme activity and mRNA levels in the rectal gland over 24h. It was shown that plasma osmolality changes to approximately match the ambient seawater within 18-24h. In the hypersaline environment, significant increases in urea, sodium, and chloride were observed, whereas in the hyposaline environment, only significant decreases in TMAO and sodium were observed. Both urea and ammonia excretion increased at low salinities suggesting a reduction in urea retention and possibly urea production. qPCR and enzyme activity data for Na(+)/K(+)-ATPase did not support the idea of rectal gland activation following exposure to increased salinities. Therefore, we suggest that the rectal gland may not be a quantitatively important aspect of the dogfish osmoregulatory strategy during changes in environmental salinity, or it may be active only in the very early stages (i.e., less than 6h) of responses to altered salinity.