Hagfish lack jaws, and the teeth are carried on a cartilagenous dental plate associated with the large longitudinal and tubular retractor muscles. Previous studies had shown that the retractors had intracellular pH buffering capacities among the highest reported for vertebrate muscle, and as these animals indulge in bursts of feeding within the hypoxic body cavity of dead fish it was of interest to examine the role of anaerobic glycolysis in powering the dental plate retractor muscles. Both retractors of the New Zealand hagfish Eptatretus cirrhatus contain only large diameter white muscle fibres (XBAR = 200-mu-m), and show low capillary:fibre ratios relative to the red and pink fibres of the general body musculature. These white fibres are characterized by sparsely distributed small mitochondria with poorly developed inner membranes, low activities of enzymes unique to aerobic ATP production, and moderate activities of glycolytic enzymes. During bursts of continuous feeding for periods of from 1.5 to 6.45 min there was a linear increase in lactate concentration at rates of 19.6 and 17.4-mu-mol lactate min-1 g-1 wet wt muscle, respectively, for the longitudinal and tubular retractors, reaching a maximum of 132-mu-mol lactate g-1 wet wt muscle. While the glycolytic flux in the hagfish retractors during feeding may not reach levels encountered in white muscle of burst swimming tuna, the actual lactate concentrations achieved are among the highest ever reported for normal muscle work by unrestrained animals.
