Four new species of rare carnivorous sponge have been found off the coast of southern California.

Thriving in the lightless depths of the deep sea, some sponges were discovered to be carnivorous around two decades ago. Since then, only seven species have been discovered in the northeastern Pacific ocean.

Yet a new study led by the Monterey Bay Aquarium Research Institute has found four new types of "killer" sponge, living on the seabed between the Pacific Northwest and Baja California.

The animals, which resemble small shrubs, are covered in tiny hairs which consist of microscopic hooks, used to ensnare small animals such as amphipods. Once the prey is trapped, it is engulfed and digested by the sponge cells. The process takes just a few days until an empty shell is left.

Killer sponge Asbestopluma monticola
A large group of Asbestopluma monticola sponges grow on top of a dead sponge on Davidson Seamount, off the Central California coastMBARI

MBARI researchers videotaped the new sponges on the seafloor, then collected a few samples for taxonomic work and species-reference collections.

Lonny Lundsten, Henry Reiswig, and William Austin, who discovered the sponges, stated they had found "numerous crustacean prey in various states of decomposition".

Sponges feed on bacteris and single-celled organisms which are filtered from the surrounding water. They contain specialised cells called choancytes, whose tails move to create a flow of water which brings food to the sponge.

"To keep beating the whip-like tails of the choancytes takes a lot of energy," Lundsten explained. "And food is hard to come by in the deep sea. So these sponges trap larger, more nutrient-dense organisms, like crustaceans, using beautiful and intricate microscopic hooks."

Two of the sponges have the genus Asbestopluma. Asbestopluma monticola, one of the sponges, was first collected from the top of Davidson Seamount, an extinct underwater volcano off the Central California coast.

A second new species, Asbestopluma rickettsi, was named after marine biologist Ed Ricketts, who was immortalised in John Steinbeck's Cannery Row.

The third and fourth new species of carnivorous sponges were observed near deep-sea hydrothermal vents, where plumes of hot, mineral-rich water flow out of the seafloor. One new species, Cladorhiza caillieti, was found on recent lava flows along the Juan de Fuca Ridge, a volcanic ridge offshore of Vancouver Island.

The fourth sponge, Cladorhiza evae, was found far to the south, in a newly discovered hydrothermal vent field along the Alarcon Rise, off the tip of Baja California.