The Challenger Deep in the Pacific is a region where only the most extreme survive, with the darkness, intense pressure, and cold temperature acting as ecological gatekeepers to this underwater world. Only recently is this hidden frontier coming to light.
Few creatures live at its depths, but there have been some stout beasties discovered, including sea cucumbers, single-celled organisms called xenophyophores, and shrimp-like amphipods. The latter, located at the southern end of the deepest region of the Mariana Trench, is not like any other known.
Typically, the shell of an amphipod dissolves in such a severe environment 10,900 meters (35,760 feet) below the ocean's surface. However, one species, Hirondellea gigas, has evolved a survival strategy to outwit even the Challenger Deep – the little creature makes its own armor out of aluminum. This is even more remarkable considering aluminum is not a readily accessible resource in the ocean.
"H. gigas is the first organism to use aluminum in the world," lead author Hideki Kobayashi told IFLScience. So how does this pint-sized powerhouse do it?
Publishing their results in the journal PLOS One, the team removed the exoskeletons of H. gigas with tweezers and scissors, then examined them using scanning electron microscopy and X-ray spectrometry. Their results revealed a peak of aluminum, as well as calcium, magnesium, potassium, and sodium, in various parts of the exoskeleton. Calcium was the most abundant, but aluminum was widely distributed throughout the entire body too, particularly in the tail and along the edges of the feet.
"From XRD analysis, we found calcium was originated from the crystal calcite," said Kobayashi. "Their habitat is around 10,000 meters deep, and much deeper than Carbonate Compensation Depth (5,000-6,000 meters). Calcite in the exoskeleton is very important for crustaceans and is similar to human bone."
To understand where all this aluminum comes from, the team took sediment samples from the Challenger Deep and exposed it to the chemicals in the creature's stomach. They found that the metal-rich sediment reacts with a byproduct of H. gigas's diet. When the amphipod eats plant debris buried in the sediment, it usually consumes some sediment as well. The reaction frees up aluminum ions that, when exposed to the alkaline seawater, become a protective aluminum gel.
"As soon as aluminum ions released from the body touch seawater, they become aluminum hydroxide gels," said Kobayashi. "This is a unique chemical property of aluminum ions with respect to pH."
Follow-up tests suggest that the armor acts as protection from stress caused by high pressure. So far, no aluminum has been found in the exoskeletons of other crustaceans. The authors, therefore, believe this may be unique to amphipods in the hadal zone – the deepest marine habitat on Earth.