Since the 1970s, a parasitic worm that infects fish, squid, whales, dolphins and sometimes even us has increased globally by 283-fold, according to a new meta analysis.
That's no small amount, and yet because this creature is so tiny and the oceans so vast, it's somehow evaded our notice until now. Not even the researchers themselves can figure out why this parasite is "growing like gangbusters", or what it could possibly mean in the long run.
Known as the "herring worm" or Anisakis simplex, this particular parasitic nematode can hide in raw seafood; if mistakenly eaten by humans, it can cause symptoms similar to a bad case of food poisoning.
Other mammals aren't so lucky. While the fishing industry might be inconvenienced by a rising tide of Anisakis, this parasite poses a much bigger threat to cetaceans, such as whales and dolphins.
Unlike humans, these marine mammals can get stuck with Anisakis for years, and they don't have seafood processors and sushi chefs around to carefully clean out their catch.
Aquatic and fishery scientist Chelsea Wood says consumers don't need to be too worried; she herself continues to eat sushi regularly. Given that the fishing industry hasn't even noticed this increase in the parasitic worm, any risk of us ingesting it probably remains quite low. But the same can't be said for marine mammals.
"It's not often considered that parasites might be the reason that some marine mammal populations are failing to bounce back," says Wood.
"I hope this study encourages people to look at intestinal parasites as a potential cap on the population growth of endangered and threatened marine mammals."
The impact of this rising parasite on marine mammals is currently unknown, but if the numbers are right, cetaceans are facing a much greater risk of contracting Anisakis than half a century ago.
Analysing a total of 123 papers, the researchers reveal an astonishing increase in Anisakis abundance over a 53 year period from 1962 to 2015. On average, they explain, this means we have gone from finding less than one worm in every 100 hosts to more than one worm in every single host caught.
The global scale of this analysis was too great to pin down any one factor driving this growth, but Wood has a strong suspicion she knows what's going on.
"My gut is that this is about the improvements we've made in marine mammal conservation," Wood told ScienceAlert.
"The time frame of our study directly overlaps with when a bunch of really important marine mammal legislation went into effect like the Marine Mammal Protection Act in 1972 and the international whaling commission moratorium on commercial whaling which came in the 1980s."
Strangely enough, however, not all marine mammal parasites are increasing. The authors found another similar parasite, called Pseudoterranova, which infects fish, sea lions and other seals, remained relatively stable throughout the same time period.
Wood says they were expecting it to be the other way around, given how much seals and sea lions have thrived in recent years and how much whales have struggled. So maybe Anisakis is increasing because its life cycle has to pass through fewer hosts. Or perhaps it has something to do with the fitness of cetaceans versus seals.
The problem is, we have no baseline for what 'natural' looks like. The rising number of ocean worms could be a sign of the ecosystem thriving, or they could represent a growing threat to already endangered and vulnerable creatures like Hector's dolphin (Cephalorhynchus hectori).
The only studies available for analysis come from near history, and by this time, humans had already changed the oceans in drastic ways.
This raises the question: is the abundance of Anisakis increasing in response to human impacts, like fishing, pollution, or climate change, or is it recovering alongside an exploited marine mammal host?
Right now, we just can't say for sure. Wood thinks the most plausible explanation is that some marine mammals are doing well, leading to an increase in the parasite at the expense of more vulnerable creatures who now face an increased risk of infection. We simply need more research.
"This is the story of only two parasite species among millions that are extant, and we encourage others to use historical ecology approaches to track change across a diversity of marine parasite species," the authors write.
"Only then will we have the data to indicate whether contemporary oceans are facing a 'rising tide' of marine disease."
In the Arctic, where Anisakis flourishes, we often lack long-term data, even for the best known parasites and their diseases. And if we don't know where they're going or where they've been, we can't predict how diseases will change with the times.
In this case, we completely missed out on the rise of Anisakis, and while it might not matter to humans this time, some day it very well could.
"There are way bigger infectious disease threats that people have to worry about, particularly for now," says Wood. The effects of Anisakis are pretty mild, she says, and while it's certainly not fun to barf, we've got bigger fish to fry.
The study was published in Global Change Biology.