"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

February 23, 2016

Anisakis pegreffii

Raw fish are eaten all over the world. However, when preparing fish fillet for a meal, one might come across some parasitic worms, much to some people's shock and revulsion. Most of these parasitic worms are anisakid nematodes, and these parasites had other plans for that fish before they ended up on someone's chopping board.
Photo of hagfish (left) by Linda Snook, photo of anisakid worms in hagfish (right) from Fig. 1 of the paper
Anisakids is a family of parasitic nematodes which really gets around. It is found in oceans all around the world and they have complex lifecycles that carry them across most part of the oceanic food web, infecting all kinds of marine animals from krill to fish to whales. The larval stages of the parasite are found in crustaceans, squid, and fish, while sexually-mature adult worms live in the gut marine mammals such as whales and seals where they mate, and produce prodigious number of eggs which are released into the ocean to start the lifecycle again. Sometime humans can interrupt this cycle, resulting in a disease call anisakiasis.

While the larval worms are usually found in the gut of their fish host, after their host dies, the worm tend to migrate into the muscle, where they are sometimes found by people preparing fish fillets for a meal. Since its lifecycle takes place in the open sea, anisakids have incorporated many marine animals into acting as their hosts at various stages of their development.

One seemingly unlikely animal that they have incorporated is the hagfish. Hagfish themselves are often mistakenly consider as a parasite when they are in fact mostly scavengers. This misconception has probably arisen from their habit of burrowing into the body cavity of dead and dying fish while feeding, however unlike the other living lineage of jawless fish - the lamprey - hagfish do not exhibit any parasitic habits.

Hagfish are also well-known as living slime machines, able to produce buckets worth of slime that act as a deterrent to many would-be predators. But despite their slime defence, this does not stop them from being eaten by a variety of large marine animals as well - some of which happens to be the final host for anisakid nematodes. Of course hagfish have also been incorporated into culinary dishes from around the world, which means they can also be a potential source of anisakiasis.

In the study being featured today, scientists examined 265 hagfish purchased from a fishing port in northeastern Taiwan, composing of four different species. From that sample they were able to find eight species of anisakids. By far the most common species was one call Anisakis pegreffii, which made up about 80% of the worms that were found. And not all the hagfish were equally parasitised - the host with the most was Eptatretus burgeri, otherwise known as the Pacific hagfish. Anisakis pegreffii has also been found in many other marine fish including anchovies, cod, and mackerel, and their final hosts are dolphins. So what's the advantage of using hagfish as a part of their lifecycle?

Because hagfish often scavenge on dead fish, they might actually be a slimy saviour for many larval anisakid worms found in those fish. Since the final host of these worms (marine mammals), don't usually go around picking up dead rotting fish from the seafloor, anisakid nematodes in dead and dying host would have usually been consigned to the same fate as their fish host.

However, the intervention of the hagfish can keep these worms in circulation, giving them another chance of reaching their final host where they can reproduce. Thus, the scavenging hagfish act as a slimy saviour for these parasites on their life's journey.

Luo, H. Y., Chen, H. Y., Chen, H. G., & Shih, H. H. (2016). Scavenging hagfish as a transport host of Anisakid nematodes. Veterinary Parasitology 218: 15-21

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