For years, EEB Ph.D. student Yu Kai Tan, who specializes in the division of Mollusks, heard rumors of a deep-sea carrier snail that could hover above the seafloor. Found off the coast of Curaçao, this creature was said to leap into the water column and briefly levitate before settling back onto the sea floor. To Yu Kai, this unique behavior sounded almost unbelievable. With a permit and lab bench access sponsored by Caribbean Research and Management of Biodiversity (CARMABI), a non-profit foundation on the island of Curaçao in the Dutch Caribbean, Yu Kai was given the chance to find out for himself whether this phenomenon was fact or fiction.

To see these Caribbean carrier snails (Onustus caribaeus) active in their environment is no easy feat. Living at depths of roughly 150 to 300 meters below sea level, the only way to reach them requires a submersible. For Yu Kai, this was his first time aboard the compact vessel. Inside, the occupant lies face down to observe the seafloor through a domed viewing window while a pilot navigates and a technician operates the collection equipment.

As Yu Kai descended in the submersible, the underwater landscape shifted dramatically. Shallow coral reefs gradually gave way to non-photosynthetic corals, then to glass sponges. Below 200 meters, the reefs disappeared entirely, replaced by an expansive plain of sand. Without the submersible's lights, the view is in near-total darkness as very little sunlight can reach this depth.

Yu Kai was surprised by how much smaller the animals appeared than he expected when he spotted them through the curved windows of the submersible. “It was challenging to spot a carrier snail as it looked like a tiny fleck on the seafloor,” he said. “Then I began to recognize the snails’ distinctive forms, and I realized they were everywhere!”

Unlike the shallow-water species, Xenophora conchyliophora, which are somewhat rarer, the deep-sea species were more common. Even more intriguing was their spatial distribution. Carrier snails dominated certain stretches of the seafloor, followed by areas occupied almost exclusively by sea urchins (Paleopneustes cristatus) or hermit crabs. This pattern suggested an unexpected partitioning of space among deep-sea animals.

A submersible compact vessel (Photo credit: Yu Kai Tan).

 

To collect specimens at these depths, the team used a basket mounted to the front of the submersible to scoop up snails along with surrounding shells, rocks, and debris. Water jets were used to remove fine sand before samples were secured. Across multiple deep dives, the team collected more than 20 carrier snails, a significant amount compared to the five specimens obtained during seven weeks spent shallow-water collecting in Guadeloupe the year prior.

Despite close observation, the snails did not exhibit the rumored hovering behavior. “Whether that behavior occurs under different conditions, or whether the story has been exaggerated over time, remains an open question,” says Yu Kai.

While the carrier snails were Yu Kai’s primary focus, the dives yielded several unexpected discoveries. The team found worms living inside the umbilicus, the hollow underside of the snail shells. Although similar associations are known from a deeper Atlantic species, Onustus longleyi, they had not previously been documented in this shallower species. Even more surprisingly, the worms belong to an entirely different family, raising new questions about symbiont–host specificity and preference.

Perhaps the most significant discovery was the presence of small, inconspicuous hoof snails (family Hipponicidae) cemented to the shells of the carrier snails. These snails are protandric hermaphrodites that form stacked colonies: individuals closest to the substrate typically settle as males and later transition to females as additional snails attach. Females are substantially larger than males.

 

Spirobranchus giganteus in Piscadera Bay Curacao (Photo credit: Yu Kai Tan).

 

Although similar hoof snails had been reported from the underside of O. caribaeus in expedition notes dating to 1983, genetic sequencing and soft body anatomical analysis revealed that this population does not belong to the family that scientists once believed. It appears shell characters may have misled researchers for decades. Instead, these snails are almost certainly a species new to science.

Researchers also documented a coral species living exclusively on the underside of the carrier snails, one that specialists at CARMABI were unable to identify. While DNA sequencing remains challenging, preserved specimens and photographs of living colonies now provide essential material for future study.

In addition to surveying potential attachment surfaces in the snails’ environment and comparing them with the snails’ actual attachments to demonstrate selective choice, Yu Kai collected 126 specimens across Mollusca, Annelida, Cnidaria, Porifera, Arthropoda, and Echinodermata to add to the museum’s collections.

At a time when coral reefs worldwide are declining, the expedition offered rare glimpses of a deep-sea ecosystem that remains active, structured, and surprisingly rich in life and, perhaps most interestingly, species and associations still unknown to science.

“It was incredibly exciting to realize we weren’t just observing a known species, but something entirely new,” said Yu Kai. “Finding both a previously unrecognized snail and an unidentified coral living on these snails highlights just how much biodiversity in deep-sea ecosystems is still waiting to be discovered.”

Coral on Base of Onustus caribaeus Curacao (Photo credit: Yu Kai Tan).
Onustus caribaeus from Curacao (Photo credit: Yu Kai Tan).
Onustus caribaeus from Curacao (Photo credit: Yu Kai Tan).