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Supergiant amphipods from the deep sea

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These supergiant amphipods are almost 10 times larger than normal amphipods and seem to exhibit several rare characteristics. (Photo: University of Aberdeen)

A joint U.K. and New Zealand expedition to the Kermadec Trench, north of New Zealand, turned up specimens and sightings of supergiant amphipods. Amphipods are a type of crustacean normally around 2-3 cm long. But the creatures discovered in the Kermadec Trench were more than ten times this size. The findings represent both the biggest amphipod specimen ever caught and the deepest they have ever been found.

Seven amphipods were caught and nine captured on film by the team from the University of Aberdeen, in Scotland, and the National Institute of Water and Atmospheric Research (Niwa), in New Zealand. The team used a specially designed ultra-deep submergence technology created by the University of Aberdeen's Oceanlab to deploy a camera system and trap to depths of up to almost ten km. The largest amphipod brought back up to the ship was 28 cm long, and the largest recorded on film was 34 cm long.

Large amphipods, termed "supergiants," had been captured by deep-sea cameras before in the 1970s, and specimens were captured in the 1980s off the coast of Hawaii. They have also been seen in the Antarctic, where they grow up to 10 cm, which may qualify them as giants but not supergiants. The individuals caught off New Zealand appear to be the same species as these other supergiants, but a definitive positive identification requires more study of the animals' morphology.

Ashley Rowden of Niwa was a member of the expedition. He says he felt both surprise and excitement upon seeing the supergiant amphipods hauled up on the boat: surprise because they were not expecting to see such large individuals and excitement because everyone immediately knew this meant something of interest. "It was one of those moments when you are returned to the sheer wonder that is at the core of deep-sea study," Rowden said. "After so many voyages and samples, and the concerns about whether this particular expedition will be successful, there is a tendency to sometimes lose touch with basic excitement of seeing animals from so deep in the ocean."

Very little is known about supergiant amphipod behavior and biology. Rowden says that most of what we know comes from a 1986 paper published after the discovery of supergiants off the coast of Hawaii. Supergiant amphipods are primarily scavengers, feeding on the dead carcasses of larger animals that fall to the ocean bottom. They are found on the seafloor. And supergiants don't appear to be common where they are found; while smaller amphipods can be found in groups of hundreds or thousands, supergiants have thus far been found only in low numbers.

The team's next step is to compare the genetic structure of these amphipods to the Hawaiian supergiants to determine whether they truly are the same species. Rowden says the answer will indicate something about the evolution of deep sea species and the connectivity between deep sea habitats. Rowden and the rest of the team are also trying to figure out why this particular species grows to such large sizes. So far, scientists are speculating about the role of environmental conditions at the locations where they have been found (such as low temperatures, low oxygen levels, and infrequent food sources), and even considering the hypothesis that they are very long-lived animals, perhaps a "relic fauna" from earlier conditions when food was in greater supply.

The supergiant amphipods captured in Kermadec Trench are currently residing in Wellington, New Zealand awaiting further study. Meanwhile, Rowden and the rest of the team are gearing up for the team's next expedition this month. Based on the results of their first trip, there's no telling what they'll find.

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These supergiant amphipods are almost 10 times larger than normal amphipods and seem to exhibit several rare characteristics. (Photo: University of Aberdeen)
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Mary Bates, Ph.D.

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