FISH FACTS: Unwelcome surprises on the filleting table

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IN my job as an aquatic animal health specialist (aka. “Fish Doctor”), I’m asked to examine abnormal marine life on a regular basis. Some of the more frequent inquiries from recreational fishers are those which relate to infections of the fillet. Virtually all wild fish are infected by various parasites and disease agents. However, without specialist training of what to look for, the vast majority of infections are virtually invisible to the casual fisher. But not so for infections of the fillet. Infections of the skeletal muscle of fish are those unsuspected, often unwelcome, and sometimes shocking surprises you occasionally see on the filleting table. Sure, these fillet infections often lead to disappointment as the anticipation of a tasty meal quickly evaporates. But looking at the problem with a “glass half full” attitude finds that these memorable encounters with parasites are simply reminders that fish are important parts of a much bigger aquatic food chain. And when you start to learn more about the life cycles of the parasites themselves, the initial disappointment can be replaced by fascination as we are afforded a brief glimpse of the many unseen wonders of the aquatic world.

One of the more memorable fillet invaders are the parasitic copepods called Sarcotaces. Parasitic copepods are small crustaceans that usually infect the outside surfaces of the skin and gills of fish (i.e. they are ecto-parasitic). Sarcotaces, on the other hand, are endo-parasitic in that the juvenile female copepods latch onto the skin of the host fish, then burrow into the muscle head first. Once the fish is infected the Sarcotaces stays fixed in the same place and is eventually covered by the host skin, except for the very last body segments which remain exposed to the water. Once in place the female copepod feeds on fish muscle and blood and grows quickly to resemble a pear shaped brown blob, several centimeters long. Male copepods are attracted to females by pheromones and join them inside their cyst cavity. Well protected in their food filled cyst inside the fillet, the female copepods produce many tens of thousands of eggs which are fertilised by the male before they are released into the surrounding water to infect other fish directly.

Such is the growth of these parasites, the cysts become swollen and can be visible externally to the naked eye. After a lifespan of many months (possibly even years, as Sarcotaces and the similar related Ichthyotaces infections seem to be more common in older fish), the copepods die, leaving behind the cyst in the fillet. The cyst now contains copious amounts of black fluid, resulting from breakdown of the copepod and products from the blood that the parasite has engorged. It is this nasty fluid that is released by the filleting knife, contaminating the flesh when infected fish are taken for the table. Fortunately, endo-parasitic copepod infections are reasonably rare, being most common in larger, long lived tropical groupers like Maori cod and coral trout. However, they are also found in long lived temperate deepwater species like ling and rockfish.

Nothing spoils the anticipation of a good feed of fish like an infection of the fillet by the Sarcotaces copepod. This one is from a brown Maori cod. Photo by Blake Boscacci.

Besides the spectacular Sarcotaces, there are plenty of other, much smaller parasites that can also infect fish muscle. Many are members of the Myxozoa, a group of parasites that are related to jellyfish. Myxozoans are microscopic parasites that have complicated lifecycles, with most species living inside various tissues of fish half the time, spending the other half of their lifecycle in various invertebrates including annelids, polychaete worms or bryozoans. One well known myxosporean is Myxobolus cerebralis, which infects the brain of rainbow trout causing “whirling disease”. It spends the other half its life inside tubifex worms in the mud. However, when filleting fish you are much more likely to encounter something like Kudoa, a group of myxozoans which infect the skin of fish and enter the bloodstream, eventually lodging in various organs including the muscle of the fillet. Here they multiply and form small white cyst-like inclusions filled with spores in the muscle fibres. The spores are highly resistant and are released into the water upon the death of the host. Once released into the water, the spores are eaten or filtered out by the invertebrate alternate host, inside which further development takes place. The alternate host then releases the actinosporean stages that infect new fish hosts. Heavy infection by myxosporeans can cause a range of issues for infected fish, including spinal deformities, organ dysfunction and even death. However, unlike Sarcotaces, when infecting the fillet muscle the problems caused by Kudoa and other similar myxosporeans are not only aesthetic. They affect the taste and heavy infections can also cause enzymatic liquification of the fillet after the host dies, turning it to mush (as in kingfish in SE QLD infected by the related Unicapsula). These enzymes can also cause human health issues including vomiting and diarrhea.

These white worm-like inclusions in the flesh of this mackerel tuna are actually cysts caused by infection with the parasitic cnidarian Kudoa spp.

Heavy Kudoa spp. infection in the fillet of a yellowfin tuna. Sushi anyone?

A range of parasitic worms can also infect the fillet of fish. These include tapeworm larvae which can heavily infect the flesh of certain fish species, such as barracouta and mulloway. In tapeworm infections, the parasite firstly infects crustaceans like prawns or copepods, which are then eaten by the fish. Individual fish which prey upon large numbers of crustaceans can become massively infected with tapeworm larvae, giving a spaghetti-like appearance to the fillet. The lifecycle is then completed when the infected fish is eaten by a shark or a large predatory fish. There the adult tapeworm lives out its life in lower intestine, passing millions of eggs into the water which then hatch into larval forms that re-infect the crustaceans.

A heavy infection of the fillet by tapeworm larvae. The adult worms usually live in sharks or rays or large predatory fish.

Of course, there are many other permutations of this theme where different types of parasites spend part of their lives inside fish as they span between 2, 3 or sometimes even 4 levels of the food chain. Whether your fillet has one or hundreds of tiny black cysts, a few larger yellow spots, a red sore, or a green tinge, these all represent infection by various different types of parasite which have come along for a ride. But marine parasitology being what it is, in most cases we have only really scratched the surface of what is going on. Most of the parasite lifecycles are unknown and remain to be teased out. Sometimes really nasty infections, like the completely black fillets of a tropical grouper in the photo below, are probably new to science and remain to be properly described. As anglers, you are at the front line. So keep your eyes peeled and stay alert around that fillet table!

This nasty infection of the skeletal muscle of a tropical grouper is extremely unusual. What was initially thought to be a routine Sarcotaces problem in this case is most likely caused by an undescribed protozoan parasite. More samples are needed to confirm the identity of the parasite involved.

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