Drinking and water permeability in the Pacific hagfish, Eptatretus stoutii

Hypoxia modifies calcium handling in the Pacific hagfish, Eptatretus stoutii

Hagfishes may encounter low dissolved oxygen in their natural habitats, a consequence of association with hypoxic sediments and their feeding behaviour. In teleost fish, hypoxia exposure decreases ion uptake, speculated to be a mechanism for energy conservation. Although hagfishes osmoconform, they do regulate extracellular fluid concentrations of divalent cations such as calcium. The current study hypothesised that exposure of hagfish to hypoxia (0.4 kPA, 24 h) would reduce calcium uptake (determined via in vitro isolated skin and gut epithelial transport assays) and calcium accumulation (determined by in vivo whole animal exposures, using radiolabelled calcium (⁴⁵Ca) to assess newly acquired calcium). A decrease in in vitro epidermal uptake was observed at sub-environmental calcium levels (10 μM), but not at environmental calcium levels (10 mM). No changes in gut calcium uptake were determined. Conversely, hypoxia led to a more rapid in vivo accumulation of calcium in tissues (skin, muscle, liver, heart, plasma, brain), mediated mostly by a significant increase in accumulation at the gill. These differences were only apparent after 1-h of exposure to the radiolabel (i.e., the last hour of the 24-h hypoxia exposure) and were not observed after 3 and 24 h periods of radiolabel exposure. This outcome was the opposite of the hypothesised effect. The reasons for a more rapid accumulation of calcium in hypoxic hagfish are unknown but may relate to roles for calcium in enhancing hypoxia tolerance in hagfishes or could be a consequence of changes in ventilatory frequency.

Reductionist approaches to the study of ionoregulation in fishes

The mechanisms underlying ionoregulation in fishes have been studied for nearly a century, and reductionist methods have been applied at all levels of biological organization in this field of research. The complex nature of ionoregulatory systems in fishes makes them ideally suited to reductionist methods and our collective understanding has been dramatically shaped by their use. This review provides an overview of the broad suite of techniques used to elucidate ionoregulatory mechanisms in fishes, from the whole-animal level down to the gene, discussing some of the advantages and disadvantages of these methods. We provide a roadmap for understanding and appreciating the work that has formed the current models of organismal, endocrine, cellular, molecular, and genetic regulation of ion balance in fishes and highlight the contribution that reductionist techniques have made to some of the fundamental leaps forward in the field throughout its history.

The good, the bad and the slimy: experimental studies of hagfish digestive and nutritional physiology

The hagfishes provide valuable insight into the physiology of feeding, digestion and nutrient absorption by virtue of unusual and unique features of their biology. For example, members of this group undergo long periods of fasting, and are the only vertebrates known to absorb organic nutrients across their epidermal surface. Such properties engender significant attention from researchers interested in feeding and feeding-related processes; however, the practical realities of employing the hagfish as an experimental organism can be challenging. Many of the key tools of the experimental biologist are compromised by a species that does not readily feed in captivity, is difficult to instrument and which produces copious quantities of slime. This Commentary provides critical insight into the key aspects of hagfish feeding and digestive processes, and highlights the pitfalls of this group as experimental organisms. We also suggest key research gaps that, if filled, will lead to better understanding of hagfishes, and we consider how this group may advance our knowledge of feeding, digestion and nutrient absorption processes.

The effects of salinity and hypoxia exposure on oxygen consumption, ventilation, diffusive water exchange and ionoregulation in the Pacific hagfish (Eptatretus stoutii)

Hagfishes (Class: Myxini) are marine jawless craniate fishes that are widely considered to be osmoconformers whose plasma [Na⁺], [Cl^-] and osmolality closely resemble that of sea water, although they have the ability to regulate plasma [Ca²⁺] and [Mg²⁺] below seawater levels. We investigated the responses of Pacific hagfish to changes in respiratory and ionoregulatory demands imposed by a 48-h exposure to altered salinity (25 ppt, 30 ppt (control) and 35 ppt) and by an acute hypoxia exposure (30 Torr; 4 kPa). When hagfish were exposed to 25 ppt, oxygen consumption rate (MO₂), ammonia excretion rate (J_amm) and unidirectional diffusive water flux rate (J_H2O, measured with ³H₂O) were all reduced, pointing to an interaction between ionoregulation and gas exchange. At 35 ppt, J_H2O was reduced, though MO₂ and J_amm did not change. As salinity increased, so did the difference between plasma and external water [Ca²⁺] and [Mg²⁺]. Notably, the same pattern was seen for plasma Cl^-, which was kept below seawater [Cl^-] at all salinities, while plasma [Na⁺] was regulated well above seawater [Na⁺], but plasma osmolality matched seawater values. MO₂ was reduced by 49% and J_H2O by 36% during hypoxia, despite a small elevation in overall ventilation. Our results depart from the "classical" osmorespiratory compromise but are in accord with responses in other hypoxia-tolerant fish; instead of an exacerbation of gill fluxes when gas transfer is upregulated, the opposite happens.