This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
More than 30 years after 50 critically endangered Hawaiian monk seals (Monachus schauinslandi) died of suspected algal toxic poisoning, the presence of ciguatoxins in living seals has finally been confirmed through a new, noninvasive test.
Ciguatoxins are produced by dinoflagellates, which live near coral and seaweed. The dinoflagellates are eaten by small fish, which are fed on by larger fish that are in turn consumed by predators such as seals and humans. Ingesting ciguatoxins produces an illness known as ciguatera, which produces gastrointestinal and neurological symptoms.
Unfortunately, ciguatoxins have not historically been easy to detect. Two of the 50 seals killed in 1978 tested positive for the neurotoxins using a method called mouse bioassay, where cultures were placed into live mice to see how they reacted. But mouse bioassay has more recently been criticized as not being effective in detecting algal toxins, and its practice has since been phased out in many places such as France, which started using chemical methods to detect the presence of toxins in oysters in 2010.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Looking for a better method, and seeking to understand the threats facing Hawaiian monk seals, researchers from the National Oceanic and Atmospheric Administration's Marine Biotoxins Program and Hawaiian Monk Seal Research Program turned to a different test developed in 2006 by the biotoxins program: the Neuro-2A cytotoxicity assay, which uses the mouse Neuro-2a neuroblastoma cell line rather than live mice and shows a greater sensitivity to toxins.
The results were conclusive: According to the abstract of a study published May 17 in the journal Environmental Science & Technology, "Tissue analysis from dead stranded animals revealed ciguatoxin activity in brain, liver and muscle, whereas analysis of blood samples from 55 free-ranging animals revealed detectable levels of ciguatoxin activity (0.43 to 5.49 picograms per milliliter P-CTX-1 equivalence) in 19 percent of the animals." The levels in the living seals were enough to poison rats; it was not proved, however, that the toxins were what killed the dead seals.
Seals that lived in pens near the shore were also tested and did not show any evidence of ciguatoxins.
The researchers say this is the first proof of transfer of ciguatoxins to marine mammals, and confirmation that the seals are exposed to and threatened by the toxins.
Hawaiian monk seals are critically endangered, numbering around 1,100 to 1,200 individuals. According to NOAA, Hawaiian monk seal populations are declining 4 percent annually. Other threats to the species include food shortages from overfishing, disease, low genetic diversity from being overhunted a century ago, and human interactions, such as getting tangled in fishing nets.
Photo courtesy U.S. Fish and Wildlife Service via Flickr