Largest squid have lowest anaerobic metabolic capacities

Hunting beneath the waves comes with its own unique set of challenges. As swimming predators grow, the drag holding them back increases when they speed up, and the chances of being detected by their victim also increase, so stalkers have to mount an even greater burst of speed to intercept prey. This requires that predatory fish increase their metabolic capacity as they grow. But how does the metabolism of squid that reside in the same environment alter as they increase in size: do their metabolic capacities increase as they grow? Intrigued, Lloyd Trueblood from La Sierra University, USA, and Brad Seibel from the University of Rhode Island, USA, went trawling and fishing for two species of squid – Dosidicus gigas ranging from 0.16 to 17,200 g and the more diminutive Doryteuthis pealeii (7–135 g) – to find out how their anaerobic capacity varied with size (p. 2710). The duo was amazed to discover that instead of increasing their anaerobic metabolic capacities, the largest animals from both species had the lowest capacities for their size.

Trueblood and Siebel suggest that the largest squid have reduced anaerobic metabolic capacities because they may not need to swim as fast as predatory fish when hunting; instead, they can reach for prey with their tentacles. They suspect that approaching more stealthily and striking from farther away also accounts for the squid's unusual metabolic characteristics. In addition, the duo explains that D. gigas largely dine on the same sized prey throughout their lives – unlike fish, which pursue larger and faster prey as they grow – so that their burst speed and power might decline because their prey don't speed up as the squid grow: ‘They can get away with being lazy’, says Trueblood.

They also explain that older D. gigas are protected from predation as fish tend to dine on prey that are 10–20% their own size, leaving larger D. gigas alone, and add that D. gigas migrate for part of each day down into the oxygen minimum zone, where they are protected from sharks and other top aquatic predators that cannot survive in the hypoxic conditions. The duo suggests that these factors could all have contributed to the reduction in the squids' anaerobic capacity as they grow.