Acute metabolic responses of two marine brachyuran crabs to dilute seawater: The aerobic cost of hyper regulation

Hepatus pudibundus (flecked box crab) is a stenohaline osmoconfomer, and restricted to marine habitats. Callinectes danae (swimming crab Dana) lives in coastal/estuarine waters and is a weak hyper regulator. There is no consensus on which strategy is more expensive metabolically face salinity challenges: conformation with higher dependence on cell volume regulation, or hyper regulation, alleviating the need for intense cell volume regulation. Crabs were probed for their acute response to dilute seawater through exposures to salinities 35 parts per thousand, 30 parts per thousand, 25 parts per thousand, and 20 parts per thousand for 2, 4, and 6 h. Hemolymph osmolality, lactate, and ions (chloride, sodium, magnesium, potassium) were assayed, as well as muscle water content. Water dissolved oxygen, ammonia, and pH levels were also measured. H. pudibundus conformed for osmolality and displayed increase in muscle hydration along the decrease in salinity down to 25 parts per thousand, while C. danae efficiently maintained hemolymph osmo ionic stability, consumed more oxygen, acidified more the water, and released more ammonia. In 25 parts per thousand, both species spent energy: H. pudibundus putatively controlling cell volume, and C. danae regulating hemolymph concentrations. In 20 parts per thousand, H. pudibundus closed itself, avoiding the contact of the interface epithelia with the external environment and producing much lactate, whereas C. danae spent more energy (aerobic) in extracellular osmo ionic stability. Under these conditions, anisosmotic extracellular regulation (together with additional cell volume regulation) is more oxygen consuming than osmoconformation with a putatively more intense challenge to cell volume. The exposure to hyposalinity limits the occupation of estuarine environments by H. pudibundus in short and middle term.

Automated summary

Hepatus pudibundus is a stenohaline osmoconfomer, while Callinectes danae is a weak hyper regulator. The metabolic cost of dealing with salinity challenges is not clear, as it is uncertain whether conformation or hyper regulation is more expensive.