A colossal slickhead, photographed in profile.

The most massive ‘massive sardine’ discovered in the deep waters of Japan

Even with the intense research focus of the last twenty years, the deep sea is still almost entirely unexplored. New species are par for the course every time a fresh sample is recovered from the abyssal plain. The vast biodiversity of the deep seafloor is offset by a biomass deficit; the denizens of the deep sea, with a few notable exceptions, are few and far between, their size often limited by the paucity of food available to them. While giants like the Japanese King Crab or the Giant Deep-sea Isopod do occur, the vast majority of deep-sea species are relatively small. 

The discovery of new species in the deep ocean is common, but the discovery of new giants in the deep sea is extremely rare. 

Last month, a research team from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) led by Dr. Yoshihiro Fujiwara and the Deep-sea Biodiversity Research Group announced the discovery of a new species of slickhead from the deep waters around Suruga Bay. Weighing in at 25 kilograms and measuring 140 centimeters, the Colossal Slickhead, described from four specimens recovered from depths greater than 2000 meters, is the largest species of slickhead (a group of deep-sea fishes found in waters deeper than 1,000 meters) yet described

In Japan, slickheads are commonly called sekitori iwashi–’massive sardines’. In recognition of its immense size, the researchers gave this most massive of massive sardines the common name yokozuna iwashi, after the title given to champion sumo wrestlers. 

The colossal slickhead is unusual for two other characteristics beyond its size. First, it was discovered in a heavily fished area, where substantial scientific research as well as commercial fishing occurs, making the discovery of such a large species surprising. And it has a very high trophic position (a measure of its place in a food web and an metric for assessing what a species eats): it may be a top predator that is not just scavenging on opportunistic food falls, but actively hunting for prey that more than likely includes other active predators. The colossal slickhead’s trophic position is higher even than most known species of deep-sea shark and is the highest on record for any species found in Suruga Bay.

Dr. David Shiffman is a shark researcher who specializes in trophic ecology. “Stable isotope values provide a long-term average measure of diet, with higher nitrogen values associated with a higher position on the food chain,” says Shiffman. “the colossal slickhead has higher nitrogen values than some large sharks in the region!”

Does this mean that the colossal slickhead is the ultimate deep ocean predator? “This result doesn’t necessarily mean they’re the deep sea’s biggest baddest predator.” says Shiffman. “Animals generally  have nitrogen values one trophic level higher than what they eat, which can’t tell us if they’re actively hunting or simply scavenging, and a fish scavenging on already-dead predators is perhaps a little less intimidating than one who hunts and kills those predators.” 

With only four individuals to study, Shiffman feels that any conclusions about the trophic ecology of the Colossal Slickhead may be premature. “I’d like to see a higher sample size to confirm that these aren’t outliers; there are some statistics we run on stable isotope food web data that require a minimum sample size of about 20 individuals.”

It is unlikely we will see large sample sizes for the colossal slickhead anytime soon. Though massive, these fish remain incredibly rare. Whether they are sparse in distribution due to naturally or artificially low population sizes, or if, as active swimmers, they are simply able to avoid generalist sampling equipment remains unclear. The increased presence of passive observation platforms in the deep sea–baited and unbaited camera traps and environmental DNA samplers–which can register biodiversity without disturbing animals are already dramatically increasing researchers capacity to observe more elusive species.

What does the discovery of the colossal slickhead mean for deep-sea mining? As with many deep ocean discoveries, bottom time is the greatest limitation to scientific exploration. The vast majority of deep-sea hydrothermal vents, for example, have received the equivalent of a long weekend of direct observation, if they have been observed at all. As deep-sea mining operations establish a long-term presence on the seafloor, especially the deep abyssal plain, the potential for discovery increases exponentially.

With robotic assets constantly observing the deep seafloor, and a continued commitment to partnerships with independent researchers and institutions, mining companies can be at the forefront of not only next-generation resource extraction, but biological discovery. Where and how those discoveries come into conflict with continued extraction will be a defining feature of the relationship between commercial enterprise, scientific advancement, and environmental conservation in the coming decades.

Featured image: Colossal Slickhead, from Fujiwara et al. 2021

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