Giant larvaceans make their houses from mucus

Technology

THE deep sea is full of fantastical creatures. Gelatinous pink sea pigs shovel food with arms like tiny sea anemones. Delicate tripod fish stand on chopstick-like stilts. Barreleye fish have transparent heads that reveal all their internal workings. Giant larvaceans hold a well-deserved spot on this list of curiosities. Shaped like oversized sperm, with a head and wide tail, the faintly blue tunicates are just ten centimetres long. But their “houses”, ephemeral structures which the creatures build from a film-like mucus, can be up to a metre across.

Larvaceans are a familiar sight to deep-ocean biologists. But, as Bruce Robison at the Monterey Bay Aquarium Research Institute (MBARI) told a conference in California earlier this month, it is only in the past few years that scientists have been able to study the animals in their native environment. Doing so has helped to answer the question of just why there is so much life on the floor of the deep ocean, where food was thought to be scarce.

A larvacean’s house comes in two parts. The inner house is made up of two symmetrical lobes and looks a bit like a translucent brain that hovers just above the animal. Around those lobes is a larger and less well-defined external house. Both act as filters, channelling nutrient-rich water towards the larvacean inside. Once they become clogged, roughly every 24 hours, the larvacean whisks itself out with a flick of its tail and builds a fresh dwelling. The abandoned houses collapse in upon themselves and sink to the sea floor.

As they sink, these discarded dwellings take with them all the particles trapped in their filtration systems, much of which is dead organic matter. By shadowing the sinking structures with remotely operated submersibles called ROVs, Dr Robison measured their rate of descent at around 800 metres per day. By contrast smaller, free-floating organic particles, known as “marine snow”, sink at a rate of just centimetres a day. Marine snow is the main food source for deep-ocean ecosystems. But because it sinks comparatively slowly, microbes in the water are able to digest much of the food along the way, consuming it before it reaches the ocean floor. Since the sinkers move much faster, microbes have less time to nibble away at their cargo before they reach the bottom.

That finding has helped solve a long-standing mystery in marine biology. Ken Smith, one of Dr Robison’s colleagues, studies the ecosystems at the bottom of Monterey Bay, more than 4,000 metres below the surface. The organisms living there seemed to be expending significantly more energy than the marine snow was providing. The difference, Dr Robison believes, can be accounted for by discarded larvacean houses. He reckons they could account for a third of the energy available on the ocean floor. And what is true of Monterey Bay is probably true elsewhere, too. Larvaceans have been observed all around the Pacific and Atlantic oceans.

Larvacean houses may also help to explain why microscopic particles of plastic have been discovered in the ocean’s deepest depths. Plastic fragments should float, but if they become trapped inside the snot-like globs of discarded larvacean houses, they can be dragged down into the abyss.

Kakani Katija, another of Dr Robison’s colleagues, has designed a laser scanner for the aquarium’s ROV. She has injected microplastics into the water around giant larvaceans and watched as they were sucked into the filters. The plastic ended up in the animal’s faeces and houses, both of which ferried them into the deep. At the conference this month, she described collecting sinkers from the deep ocean that were full of microplastics. What effect those plastics are having on the rest of the deep-sea world is, for now, unknown.