Giants from the ocean floor

THE kraken of Norse legends and Scylla and Charybdis of ancient Greece were not just mythical beasts, writes Richard Collins

Giants from the ocean floor

One of their tribe was caught by fishermen 190km off the Kerry coast last month; a spectacular beast, it had tentacles 6m long.

The giant squid, a relative of slugs and snails, is one of the world’s largest invertebrates. A related creature, the ‘colossal’ squid, is even bigger; females can weigh a quarter of a tonne, males being a third lighter. The eight arms have suckers with sharp tooth-like rings. Fish and other squid are their main prey. Victims are grasped, drawn towards the beak, and shredded by the creature’s abrasive tongue.

Only six giant squid have been caught off Ireland in 350 years. These creatures live so far down in the sea that their behaviour is impossible to study. The first photographs of a live one were only taken in 2004 and moving footage wasn’t obtained until 10 years later. Being soft-bodied, squid leave few fossil traces. Only their hard parts, the beaks and the chitin surrounding their suckers, are deposited on shore or turn up in fishing nets.

These beasts exhibit deep-sea gigantism, the tendency of creatures living deep in the ocean to become enormous. The trait, shared by sharks, Japanese spider crabs and sun fish, has puzzled scientists. Size matters, but why? Light doesn’t penetrate the cold sterile depths. Photo-synthesis isn’t possible, so getting enough food to sustain such huge bodies must be challenging. Nor are squid long-lived animals which grow slowly over many decades; they don’t appear to live beyond the age of five.

Japanese spider crab.

Japanese spider crab.

Is gigantism just an instance of Bergmann’s rule? In 1847, the German biologist Carl Bergmann observed that creatures living in cold places tend to be larger than their cousins in warmer ones. Large bodies, with greater bulk compared to surface area, conserve heat better than small ones. Temperatures are lower deep in the oceans than at the surface, so big beasts might fare better there. However, the drop in temperature isn’t great enough to account for such huge increases in size. Bergmann’s rule can’t be the whole story.

The giant squid’s great enemy, the sperm whale, dives to great depths to hunt it. Whales exhibit gigantism, though not of the deep-ocean variety; being mammals, they must have access to the surface to breath. The blue whale, which migrates along our west coast, is not only the largest animal alive today but the largest one ever to have existed.

Evolutionary biologist Graham Slater and colleagues discuss whale gigantism in a paper just published in the Proceedings of the Royal Society B. His team examined the fossil records of extinct whales, early descendants of hoofed animals which ventured out to sea around 40m years ago. They found that whales remained relatively small until comparatively recently. Gigantism, defined as a length exceeding 10 metres, only emerged among the baleen whales around 4m years ago. For 90% of their history, whales were comparatively small.

The blue whale is the largest animal to have ever existed

The blue whale is the largest animal to have ever existed

Gigantism, which appeared independently among several whale families, was not a response to predation by sharks, nor was it an adaptation to lower ocean temperatures. The research team’s computer models suggest that a superabundance of food was the driving force behind the sudden increase in size. The onset of Ice Age conditions, when the climate changed, gave rise to colder well-oxygenated waters, teeming with life. The bigger a creature, the more food it can take in and the less energy it needs to expend harvesting it. Whales, the scientists argue, became big to avail of a food bonanza.

G Slater et al. Independent evolution of baleen whale gigantism linked to Plio-Pleistocene ocean dynamics. Proceedings of the Royal Society B. 2017.

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