High Swimming and Metabolic Activity in the Deep‐Sea EelSynaphobranchus kaupiiRevealed by Integrated In Situ and In Vitro Measurements

Several complementary studies were undertaken on a single species of deep‐sea fish (the eelSynaphobranchus kaupii) within a small temporal and spatial range. In situ experiments on swimming and foraging behaviour, muscle performance, and metabolic rate were performed in the Porcupine Seabight, northeast Atlantic, alongside measurements of temperature and current regime. Deep‐water trawling was used to collect eels for studies of animal distribution and for anatomical and biochemical analyses, including white muscle citrate synthase (CS), lactate dehydrogenase (LDH), malate dehydrogenase (MDH), and pyruvate kinase (PK) activities.Synaphobranchus kaupiidemonstrated whole‐animal swimming speeds similar to those of other active deep‐sea fish such asAntimora rostrata. Metabolic rates were an order of magnitude higher (31.6 mL kg−1h−1) than those recorded in other deep‐sea scavenging fish. Activities of CS, LDH, MDH, and PK were higher than expected, and all scaled negatively with body mass, indicating a general decrease in muscle energy supply with fish growth. Despite this apparent constraint, observed in situ burst or routine swimming performances scaled in a similar fashion to other studied species. The higher‐than‐expected metabolic rates and activity levels, and the unusual scaling relationships of both aerobic and anaerobic metabolism enzymes in white muscle, probably reflect the changes in habitat and feeding ecology experienced during ontogeny in this bathyal species.