According to marine biologists from the Monterey Bay Aquarium Research Institute (MBARI), a ‘vampire squid’ called Vampyroteuthis infernalis uses two thread-like filaments to capture bits of organic debris that sink down from the ocean surface into the deep sea, unlike its relatives the octopuses and squids, which eat live prey.
About 100 years ago, scientists hauled the first vampire squid up from the depths of the sea. Since that time, perhaps a dozen scientific papers have been published on this mysterious animal, but no one has been able to figure out exactly what it eats.
It’s easy to imagine the vampire squid as a nightmarish predator. It lurks in the eternal midnight of the deep sea, has a dark red body, huge blue eyes, and a cloak-like web that stretches between its eight arms. When threatened, it turns inside out, exposing rows of wicked-looking ‘cirri.’ In reality, the vampire squid is a soft-bodied, passive creature, about the size, shape, and color of a football. A ‘living fossil,’ it inhabits the deep waters of all the world’s ocean basins at depths where there is almost no oxygen, but also relatively few predators.
A new study, published in the Proceedings of the Royal Society B: Biological Sciences, shows that vampire squids eat mostly ‘marine snow’ – a mixture of dead bodies, poop, and snot. The dead bodies are the remains of microscopic algae and animals that live in the waters farther up in the ocean, but sink down into the depths after they die. The poop consists of fecal pellets from small, shrimp-like animals such as copepods or krill. The snot is mostly debris from gelatinous animals called larvaceans, which filter and consume marine snow using mucus nets.
In addition to looking at the stomach-contents of vampire squids from museum collections, the marine biologists used remotely operated vehicles (ROVs) to collect live vampire squids and study their feeding habits in the laboratory. They also examined high-definition videos of vampire squids taken by ROVs. Finally, they examined vampire squid arms and feeding filaments under optical and scanning electron microscopes.
The videos showed that vampire squids often drift motionless in the water, extending one of their thin filaments – up to eight times as long as the animal’s body – like a fishing line.
Under the microscope, the researchers observed that the vampire squid’s suckers were covered with cells that produce mucus, which the animal apparently uses to collect and glue together individual particles of marine snow. Their filaments are covered with tiny hairs and a dense net of sensory nerves, which makes them extremely sensitive to touch.
When looking at vampire squids’ stomach contents, the researchers did not see bones or pieces of individual animals that would indicate the vampire squids had captured live prey. Instead, they saw mostly amorphous bits of broken-up organic debris. The only prey they saw that might have been eaten alive were the remains of tiny crustaceans that sometimes ‘hitchhike’ on sinking mucus nets or clumps of the marine snow.
“The vampire squid’s filament is likely a multifunctional organ that is deployed to detect and capture detrital matter but at the same time may detect the presence of predators and perhaps small living prey,” said co-authors Dr Hendrik Hoving and Dr Bruce Robison.
The organic detritus that forms the bulk of the vampire squid’s diet would not seem to be particularly nutritious. However vampire squids complement their frugal diet with an extremely energy-efficient lifestyle and unique adaptations. Their bodies are neutrally buoyant, so they don’t have to expend energy to stay at a particular depth. Even better, they don’t have to swim to find food, but simply extend their filaments to collect food that drifts past them.
Finally, vampire squids don’t have to expend much energy avoiding predators, because they live at depths where there is so little oxygen that few other animals can survive. Conveniently, these deep, low-oxygen zones are often found where there is an abundance of life near the sea surface, which in turn creates lots of marine snow for vampire squids to eat.
“Because of its unique adaptations, the vampire squid is able to permanently and successfully inhabit the center of the oxygen minimum zone, an otherwise hostile environment where the vampire squid’s predators are few, and its food is abundant,” Dr Hoving explained.
Bibliographic information: Hendrik J. T. Hoving and Bruce H. Robison. Vampire squid: detritivores in the oxygen minimum zone. Proc. R. Soc. B, published ahead of print September 26, 2012; doi: 10.1098/rspb.2012.1357