By Dr. Craig Smith, Chief Scientist
June 12, 2018
What is yellow, 40 centimeters (16 inches) long, has seven lips, 92 feet, and a spikey profile like a punk rocker? An abyssal sea cucumber of course (Figure 1)!
Human appreciation for nature’s habitats is often shaped by “charismatic megafauna”—those large animals with amazing shapes, colors, and behaviors uniquely adapted to a particular habitat. Classic examples are the elephants, rhinos, and giraffes of the Serengeti; polar bears and walrus of the Arctic; and penguins of the Southern Ocean. Does the abyssal Clarion-Clipperton Zone (CCZ) have any charismatic megafauna?
The enormous diversity of seafloor invertebrates in this region offers many candidates, but my favorites are the sea cucumbers, or holothurians. A particular evolutionary wonder is “Psychropotes longicauda,” which the ship’s crew has dubbed the “gummy squirrel” (Figure 2).
This animal is bright yellow with a red underbelly, and with its large tail, can be 80 centimeters (32 inches) long. The tail is held aloft like a squirrel’s but may actually be a sail—scientists have speculated that it is used to catch the weak deep-sea currents, allowing the cucumber to bounce along the seafloor in search of better feeding grounds. The gummy squirrel uses its 18 feeding palps, or “lips,” to ingest relatively food-rich particles of phytoplankton detritus that have sunk from the sunlit surface ocean five kilometers above. The animal’s bright yellow color might indicate the presence of a toxic chemical to deter consumption by rattail fish and other seafloor predators (nothing appears to eat Psychropotes even though it resembles a lime gummy bear).
The gummy squirrel was originally thought to be a “cosmopolitan” abyssal species, i.e., occurring below 3,000 meters depth in all ocean basins. However, a recent DNA study suggests that the gummy squirrel is comprised of at least two species, one found mostly in the Atlantic and another in the northeast Pacific. But prior to our DeepCCZ cruise, no gummy squirrels had been collected within 3,000 kilometers (~1,600 miles) of our study area in the western CCZ, where habitat conditions (for example, food availability, water depth) and evolutionary pressures may be different. Thus, the DeepCCZ gummy squirrels we have collected could well be a new species, or come from a distinct, previously unsampled, population.
The gummy squirrels, of course, are not alone in the abyss. So far during our DeepCCZ cruise, we have collected or observed at least 10 species of giant sea cucumbers across two Areas of Particular Environmental Interest (areas protected from mining), each species with distinct adaptions indicated by varying shapes and colors, numbers of feet and feeding appendages, presence/absence of “sails,” and behaviors ranging from plowing through the sediment to swimming up into the water column with the motions of a flamenco dancer.
Some of these sea cucumbers appear different from all known species and are likely to be new to science. We have also collected/observed many other animals, including anemones, sea lilies, sponges, cup corals, octocorals, worms, and crustaceans that are likely new to science. It is truly remarkable to be working in a region so poorly studied that many of its largest, most notable denizens, including giant sea cucumbers, have never been collected or described by scientists!