Newly described 'parasol' sponges are graceful but deadly (to small crustaceans)

When most people think of sponges, they think of squishy, soap-filled kitchen sponges, or perhaps the graceful barrel sponges that grow around coral reefs. But in the dark depths of the ocean, some sponges have evolved into deadly predators, which trap and digest small, helpless prey. In a new paper in the journal Zootaxa, marine biologists describe three new species of predatory sponges that live in deep water off the coast of California and in the Gulf of California.

Most sponges are passive filter feeders, pumping seawater through their bodies and straining out microscopic single-celled organisms and bacteria. However, over the last 20 years, marine biologists have discovered that many deep-sea sponges are predators. These carnivorous sponges have adapted to food-poor habitats by ensnaring larger, more nutrient-dense prey.

Like tiny beach umbrellas sticking out of the deep-sea floor, the new species of sponges consist of thin vertical stalks capped by parasol-like disks with "numerous long filaments radiating in all directions from the parasol edge," according to the authors of the Zootaxa article.

Although the sponges' parasol-like structures are delicate and graceful to the human eye, they are death traps for tiny animals. Looking through a microscope at the filaments of collected sponges, the researchers observed dozens of miniscule, shrimp-like crustaceans embedded in small fluid-filled sacks called "cysts." Apparently, when one of these tiny crustaceans swims or drifts into a filament, it is trapped in a gauntlet of "chelae," which look like tiny, barbed fish hooks. The trapped prey are then enveloped within the body of the sponge and slowly digested.

Some of the new sponges also have small stalks atop their parasols, with conspicuous knobs or disks on the end. Researchers believe that, in mature sponges, these "apical antennae discs" produce packets of sperm that are released into the surrounding water.

Two of the newly named sponges were recorded on videotape by MBARI's remotely operated vehicles (ROVs) at depths of about 2,500 to 4,100 meters. All three of the new sponges are members of the genus Cladorhiza. These three new species join three previously known species of Cladorhiza sponges along the West Coast of North America.

One of the new sponges, Cladorhiza kensmithi, lives on the muddy seafloor off Central and Northern California. This sponge was named after MBARI deep-sea biologist Ken Smith, who has spent over 40 years studying deep-sea animals. A large number of C. kensmithi sponges were observed at "Station M," one of Smith's long-term research sites off the coast of Central California.

Cladorhiza kensmithi sponges are about 20 centimeters (8 inches) tall, and use root-like rhizoids to secure their stalks to muddy bottom.  When they were first observed during MBARI ROV dives, researchers gave them the nickname "Sputnik sponges" because their filaments and large, conspicuous antennae disks looked like antennae on a satellite.

Adult C. kensmithi sponges are widely distributed across the flat, muddy abyssal plain at Station M. However, smaller (and presumably younger) C. kensmithi sponges often grow in clumps on top of another type of sponge (a "plate sponge") that also lives in this area.

In contrast to the muddy habitat of C. kensmithi, the new sponge Cladorhiza mexicana was observed living on deep-sea lava flows at the southern end of the Gulf of California. This species can grow to at least 30 centimeters (one foot) tall.

A third new species, Cladorhiza hubbsi, was identified from a single specimen dredged up from the seafloor off Southern California in 1969 by researchers at the Scripps Institution of Oceanography. Only the top portion of this sponge was recovered, but that was enough to identify the sponge as a new species.

The researchers hope to follow up on this research by conducting more detailed studies of C. kensmithi sponges, which are relatively abundant at Station M. They suggest that using ROVs to study the feeding, growth, and reproduction of these sponges at Station M "would yield a wealth of novel information."