Whales give clues

 
By Sumana Narayanan
Last Updated: Saturday 04 July 2015

TSUNEMI KUBODERASEAFARERS’ tales abound with giant octopuses that emerge from the deep seas to rip apart ships. They may be fantasy stories but have a grain of truth in them. Giant octopuses and squids do exist at the bottom of the sea—but rarely do they show up, and certainly don’t go about attacking ships. Marine scientists know of the existence of different species of cephalopods (mainly octopuses and squids) in the ocean deep from rare photographs and specimens caught by whaling ships, not much else. Some of the mystery surrounding them unravelled when a group of researchers decided to analyze the stomach contents of three sperm whales found beached near the Bay of Biscay, France. These whales are known to prey on cephalopods which have beaks unique to each species. The beaks, used to hold and bite the prey, are made of chitin, a hard-to-digest complex carbohydrate. The beaks of 19 different cephalopods of varying ages were found in the whales’ abdomen. A chemical analysis of the beaks helped scientists learn about the food habits and habitats of these elusive creatures. The beaks were examined to determine the ratio between two carbon isotopes (carbon 13 and 15) and two nitrogen isotopes (nitrogen14 and 15). The nitrogen isotopes indicated the position of the cephalopods in the food chain. The carbon isotopes indicated whether they prefer to spend time close to the ocean floor, near the coast or in the open ocean. “This way we can investigate the food chain hierarchy of hundreds of specimens of virtually inaccessible species and gather vital infor - mation about them,” said Vlad Laptikhovsky from UK administered Falkland Island Fisheries Department. The carbon and nitrogen levels of the cephalopods were also compared with that of other marine animals like sharks to understand their inter-relationship. The researchers found the carbon levels did not vary much indicating cephalopods and some other sea animals live in overlapping habitats. The nitrogen levels varied a little more, but not much, indicating the two groups were closely bunched in the food chain. The results changed accepted notions about some cephalopods. It showed the vampire squid (Vampyro - teuthis infernalis) does not live in the nether region as the name indicates. This was inferred from the low carbon levels in its beak. “Higher carbon is seen in near-bottom prey species (they live off carbon rich food available near the sea floor) and if found in a predator, it indicates it was feeding closer to the bottom,” said Laptikhovsky. The beak of the giant squid Architeuthis dux had high carbon levels revealing it is a bottom sea dweller. The giant squids, Taningia danae and Lepidoteuthis grimaldi, live at a depth of 200 to 2,000

The giant squid T danae,
thought to be sluggish,
is as aggressive as a
sperm whale

metres and the flying squid, prefers to live closer to the coast, the researchers concluded. The research threw up a few surpri - ses. Some open sea species of squids had high carbon levels, not low carbon as one would expect. This meant they spent the initial phase of their life near the sea floor closer to the coast where carbon rich food is plenty and changed habitat later in life. Different species of giant squids were found to occupy different positions in the food chain. Going by their size they should have stayed close by like other heavy marine predators. It also turned out the giant squid T danae, thought to be sluggish, occupies the same place in the food chain as a sperm whale meaning it is equally aggressive and predatory. The findings were published online on March 18 in Biology Letters. The paper is important in understanding the role of cephalopods, a major marine group, said Marek Lipinski, researcher with marine and coastal management department, South Africa. “Ocean decides our survival on the planet and monitoring its health indicators through marine life is vital. For this we must first know how the marine ecosystem works,” Lipinski said.

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