Biomarkers of homeostasis, allostasis, and allostatic overload in decapod crustaceans of distinct habitats and osmoregulatory strategies: an empirical approach

What makes a competent aquatic invader? Considering saline niches of invertebrates and ray-finned fishes

Aquatic invasive species are of growing concern globally, especially in fresh water. The problem is intensified by climate change, which often causes salinization of coastal fresh waters. Animals deal with salinity through the function of osmoregulation, and osmoregulatory ability can be informative when considering invasive potential. A species is said to be 'euryhaline' if it can tolerate a wide range of salinities, either through osmoregulation (tightly controlling its extracellular fluid osmolality) or osmoconformation (matching the osmotic concentration of its internal fluids with that of the environment). Euryhaline animals display a large fundamental saline niche (FSN); i.e. a wide physiological tolerance of salinity change. However, the range of salinities of the habitats where a species actually occurs define its realized saline niche (RSN). Importantly, aquatic species living in stable habitats (i.e. those with little variation in salinity) will have a small RSN, but may have large FSNs, depending on their evolutionary history. Species with large FSNs are more likely to be successful invaders of new habitats with different salinities. Here, I propose the term 'osmotic comfort' as a concept that is associated with the FSN. The core of the FSN corresponds to ∼100% osmotic comfort, or 'optimum salinity', putatively meaning minimum stress. Physiological markers of osmotic comfort can provide raw data for mechanistic niche modelling in aquatic habitats. A species with a larger FSN is more likely to remain 'osmotically comfortable' in a different saline habitat, and is less likely to suffer local extinction in fresh waters, for example, that undergo salinization.

Effects of hypo-osmotic shock on osmoregulatory responses and expression levels of selected ion transport-related genes in the sesarmid crab Episesarma mederi (H. Milne Edwards, 1853)

This study examined the osmoregulatory responses to hypo-osmotic shock in the commercially and ecologically important crab Episesarma mederi (H. Milne Edwards, 1853). After the acclimation for one week at a salinity of 25 PSU, Adult males E. mederi were immediately exposed to salinities of 5 PSU and 25 PSU (the control group). The time course of changes in haemolymph osmolality, gill Na+/K+ ATPase (NKA) activity, oxygen uptake rates, and mRNA expression levels of ion-transport related genes, including the NKA-α subunit, V-type H+ATPase (VT) and Na+/K+/2Cl-(NKCC), were determined. The results showed that E. mederi was a strong hyperosmoregulator after exposure to 5 PSU, achieved by modulations of NKA activity in their posterior gills rather than the anterior gills. The crabs acclimated to 5 PSU increased oxygen uptake, especially during the initial exposure, reflecting increased energetic costs for osmotic stress responses. In the posterior gills, the NKA activities of the crabs acclimated to 5 PSU at 3, 72 and 168 h were significantly higher than those in the control group. Elevated NKA-α subunit expression levels were detected at 6 h and 12 h. Increased expression levels of VT and NKCC were identified at 6 h and 12 h, respectively. Our results indicate that elevated gill NKA activity at 3 h could result from enzyme activity and kinetic alterations. On the other hand, the gill NKA activity at 72 and 168 h was sustained by elevated NKA-α subunit expression. Hence, these adaptive responses in osmoregulation enable the crabs to withstand hypo-osmotic challenges and thrive in areas of fluctuating salinity in mangroves and estuaries.

A systematic evaluation on the relationship between hypo-osmoregulation and hyper-osmoregulation in decapods of different habitats

Decapods occupy all aquatic, and terrestrial and semi-terrestrial environments. According to their osmoregulatory capacity, they can be osmoconformers or osmoregulators (hypo or hyperegulators). The goal of this study is to gather data available in the literature for aquatic decapods and verify if the rare hyporegulatory capacity of decapods is associated with hyper-regulatory capacity. The metric used to quantify osmoregulation was the osmotic capacity (OC), the gradient between external and internal (hemolymph) osmolalities. We employ phylogenetic comparative methods using 83 species of decapods to test the correlation between hyper OC and hypo OC, beyond the ancestral state for osmolality habitat, which was used to reconstruct the colonization route. Our analysis showed a phylogenetic signal for habitat osmolality, hyper OC and hypo OC, suggesting that hyper-hyporegulators decapods occupy similar habitats and show similar hyper and hyporegulatory capacities. Our findings reveal that all hyper-hyporegulators decapods (mainly shrimps and crabs) originated in estuarine waters. Hyper OC and hypo OC are correlated in decapods, suggesting correlated evolution. The analysis showed that species which inhabit environments with intense salinity variation such as estuaries, supratidal and mangrove habitats, all undergo selective pressure to acquire efficient hyper-hyporegulatory mechanisms, aided by low permeabilities. Therefore, hyporegulation can be observed in any colonization route that passes through environments with extreme variations in salinity, such as estuaries or brackish water.

The application of allostasis and allostatic load in animal species: A scoping review

Principles of allostasis and allostatic load have been widely applied in human research to assess the impacts of chronic stress on physiological dysregulation. Over the last few decades, researchers have also applied these concepts to non-human animals. However, there is a lack of uniformity in how the concept of allostasis is described and assessed in animals. The objectives of this review were to: 1) describe the extent to which the concepts of allostasis and allostatic load are applied theoretically to animals, with a focus on which taxa and species are represented; 2) identify when direct assessments of allostasis or allostatic load are made, which species and contexts are represented, what biomarkers are used, and if an allostatic load index was constructed; and 3) detect gaps in the literature and identify areas for future research. A search was conducted using CABI, PubMed, Agricola, and BIOSIS databases, in addition to a complementary hand-search of 14 peer-reviewed journals. Search results were screened, and articles that included non-human animals, as well as the terms "allostasis" or "allostatic" in the full text, were included. A total of 572 articles met the inclusion criteria (108 reviews and 464 peer-reviewed original research). Species were represented across all taxa. A subset of 63 publications made direct assessments of allostatic load. Glucocorticoids were the most commonly used biomarker, and were the only biomarker measured in 25 publications. Only six of 63 publications (9.5%) constructed an allostatic load index, which is the preferred methodology in human research. Although concepts of allostasis and allostatic load are being applied broadly across animal species, most publications use single biomarkers that are more likely indicative of short-term rather than chronic stress. Researchers are encouraged to adopt methodologies used in human research, including the construction of species-specific allostatic load indexes.

Osmoregulatory power influences tissue ionic composition after salinity acclimation in aquatic decapods

Decapod crustaceans show variable degrees of euryhalinity and osmoregulatory capacity, by responding to salinity changes through anisosmotic extracellular regulation and/or cell volume regulation. Cell volume regulatory mechanisms involve exchange of inorganic ions between extra- and intra-cellular (tissue) compartments. Here, this interplay of inorganic ions between both compartments has been evaluated in four decapod species with distinct habitats and osmoregulatory strategies. The marine/estuarine species Litopenaeus vannamei (Lv) and Callinectes danae (Cd) were submitted to reduced salinity (15‰), after acclimation to 25 and 30‰, respectively. The freshwater Macrobrachium acanthurus (Ma) and Aegla schmitti (As) were submitted to increased salinity (25‰). The four species were salinity-challenged for both 5 and 10 days. Hemolymph osmolality, sodium, chloride, potassium, and magnesium were assayed. The same inorganic ions were quantified in muscle samples. Muscle hydration (MH) and ninhydrin-positive substances (NPS) were also determined. Lv showed slight hemolymph dilution, increased MH and no osmotically-relevant decreases in muscle osmolytes; Cd displayed hemolymph dilution, decreased muscular NaCl and stable MH; Ma showed hypo-regulation and steady MH, with no change in muscle ions; As conformed hemolymph sodium but hypo-regulated chloride, had stable MH and increased muscle NPS and ion levels. Hemolymph and muscle ions (especially chloride) of As were highly correlated (Pearson, +0.83). Significant exchanges between hemolymph and muscle ionic pools were more evident in the two species with comparatively less AER regulatory power, C. danae and A. schmitti. Our findings endorse that the interplay between extracellular and tissue ionic pools is especially detectable in euryhaline species with relatively lower osmoregulatory strength.