Abstract
Few studies have addressed how reduced water salinity affects cardiovascular and metabolic function in marine euryhaline fishes, despite its relevance for predicting impacts of natural salinity variations and ongoing climate change on marine fish populations. Here, shorthorn sculpin (Myoxocephalus scorpius) were subjected to different durations of reduced water salinity from 33 to 15 ppt. Routine metabolic rate decreased after short-term acclimation (4–9 days) to 15 ppt, which corresponded with similar reductions in cardiac output. Likewise, standard metabolic rate decreased after acute transition (3 h) from 33 to 15 ppt, suggesting a reduced energetic cost of osmoregulation at 15 ppt. Interestingly, gut blood flow remained unchanged across salinities, which contrasts with previous findings in freshwater euryhaline teleosts (e.g., rainbow trout) exposed to different salinities. Although plasma osmolality, [Na+], [Cl−] and [Ca2+] decreased in 15 ppt, there were no signs of cellular osmotic stress as plasma [K+], [hemoglobin] and hematocrit remained unchanged. Taken together, our data suggest that shorthorn sculpin are relatively weak plasma osmoregulators that apply a strategy whereby epithelial ion transport mechanisms are partially maintained across salinities, while plasma composition is allowed to fluctuate within certain ranges. This may have energetic benefits in environments where salinity naturally fluctuates, and could provide shorthorn sculpin with competitive advantages if salinity fluctuations intensify with climate change in the future.
Highlights
Many coastal marine environments are characterized by large fluctuations in water salinity, which challenges the osmotic homeostasis of animals inhabiting these environments
standard metabolic rate (SMR) significantly decreased throughout the 4-day exposure period (F3,13 = 3.535, P = 0.045), but no evidence was found of an interaction effect between time and treatment (F3,13 = 0.176, P = 0.911), suggesting that time spent in the respirometers affected both treatment groups and was, unaffected by the water salinity
The SMR of uninstrumented shorthorn sculpin in fullstrength seawater (33 ppt) in the present study is consistent with previously reported values for this species under similar conditions (Sandblom et al 2014)
Summary
Shorthorn sculpin (Myoxocephalus scorpius, Linnaeus, 1758) were caught in baited traps by a local fisherman near Grundsund (Gullmarsfjorden) on the west coast of Sweden (Fig. 1). Sculpins were transported in aerated insulated cooler bins to the University of Gothenburg and held in a 1500 L tank (salinity: 33 ppt; temperature: 10 °C; 12:12 h light:dark photoperiod) for a minimum of 4 weeks prior to experiments to ensure acclimation to the new environment. The bottom of the tank was covered with gravel and half clay pots to supply suitable bottom substrates and dark hiding places for the fish. Fish were fed to satiation once a week with whole juvenile herring (Clupea harengus) and fasted for 6 days prior to experiments. All experimental protocols were approved by the ethical committee in Gothenburg (permit #165-2015)
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