The idea of a worm constructing a home inside a glass skeleton seven kilometers below the Pacific Ocean’s surface, in complete darkness and under pressure that would crush an unprotected human body in less than a second, seems almost ridiculous. However, when the Shinkai 6500 submersible surfaced last June with specimens that halted taxonomists in their tracks, researchers on board the JAMSTEC research vessel Yokosuka discovered precisely that. The hollow lattice of a hexactinellid sponge, which scientists sometimes refer to as a “glass sponge,” was home to two new polychaete worm species, now known as Dalhousiella yabukii and Leocratides watanabeae. Given the architecture involved, “glass castle” seems more accurate.
The Hades-named hadal zone is located between 6,000 and 11,000 meters below sea level. It’s a location that still defies intuitive comprehension. The cumulative weight of all that ocean is still sufficient to collapse the majority of known vertebrate biology, even though the pressure there does not double as it does close to the surface—rather, it compresses much more gradually as you descend from 6,000 to 11,000 meters. Fish biochemistry begins to deteriorate below a theoretical ceiling of 8,000 to 8,500 meters. Invertebrates, however, overcome it. Certain sea cucumbers, polynoid worms, and amphipods have been collected from depths greater than 10,000 meters. It turns out that what physics says should be impossible doesn’t really bother the worms.
The specificity of the adaptation is what gives the glass sponge discovery a truly peculiar feel. It would be amazing enough if these worms simply drifted to the seafloor and buried themselves in sediment. They developed to live inside a glass sponge, which is a structure composed of silica spicules and is essentially biological glass. They used the glass sponge as a shelter and, most likely, as a feeding station. A localized microhabitat with a marginally higher concentration of nutrients may be produced by the sponge’s filtering activity. Or maybe the stiff lattice protects the trench floors from amphipod scavengers. In all honesty, the scientists involved seem to find it exciting rather than frustrating that no one knows yet.
The expedition itself collected over 528 specimens from two unexplored areas off the coast of Japan: the Nankai Trough and the Shichiyo Seamount Chain. Following the dive, taxonomists from all over the world convened at JAMSTEC headquarters to categorize the findings. In the end, 38 new species were confirmed, and 28 more are still being investigated. That’s a startling figure from a single expedition, and it begs the silent, unsettling question: how much of the hadal zone is actually unknown if one submersible trip to two locations yields 38 new species? The majority of it, according to the mapping that is currently available.
The hadal zone makes up more than 40% of the ocean’s total depth range despite making up less than 25% of the world’s seafloor by area. Plate subduction zones, where one tectonic plate slides beneath another to form those long, narrow, V-shaped canyons, are where the majority of the deep trenches are located. However, a sizable chunk of the hadal seafloor is located in areas where exploration hasn’t really begun, mid-plate, and far from any tectonic boundary. In order to get to these locations, scientists working with NASA and WHOI have been creating tiny autonomous vehicles known as Orpheus AUVs. These vehicles are designed to withstand pressures that are more than 1,000 times higher than those at the surface and are able to maneuver through the narrow, rocky walls of trenches that no crewed vehicle could enter safely.
Observing this field’s growth gives me the impression that the hadal zone is subtly undermining presumptions, as uncharted territory always does. The deep trenches were thought to be biological deserts dotted with scavengers waiting for debris to fall from above, according to the old theory that life thins out with depth. Instead, complexity keeps appearing: amphipods’ gigantism, nematodes’ dwarfism, a high concentration of species that are unique to Earth, and now worms that seem to have decided that a glass sponge skeleton is a great place to raise a family. Researchers are still unsure if these worm species only use the sponge or if they will eventually be discovered in other substrates. However, the discovery itself serves as a reminder that adaptation can result in solutions that are difficult to predict at first glance when sufficient pressure and time are applied.
Just the carbon implications are worth considering. At the base of trenches, organic matter that sinks from the upper ocean builds up. This accumulation may have a significant impact on the global carbon cycle by storing carbon that might otherwise be in circulation. It takes more than just academic study to comprehend the biological workings of the hadal zone. It’s a component of a more comprehensive accounting that climate science is becoming more and more dependent on.
For the time being, the two glass-dwelling worms return to the literature with their Latin names and mystery, awaiting the next exploration that could shed light on the reasons behind their choice of architecture. Organisms that live inside glass, which is both strong enough to withstand the highest pressures on Earth and transparent enough to see through, seem fitting.
