"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

September 22, 2014

Kudoa islandica

Today's post features a newly described species of parasite, which is found in the muscles of some fish that are not exactly prized for their appearance. Regardless of how they look, these fish are commercially prized. But today's featured parasite has a queasy trick that ruins their host's value on the market - its tendency to liquefy fish fillet.

SEM photos of K. islandica spore (from the paper)
Kudoa islandica is a species of myxozoan parasite which infects a number of different marine fishes from the coasts of Iceland. The first of these are two species of wolffish - the Atlantic wolffish and the Spotted wolffish. Both have short bulldog-like faces and a formidable set of teeth to match. Wolffish is harvested for its flesh and it is commonly eaten in Iceland, but on top that, its skin can also be turned into a type of designer leather. The other host of K. islandica is the lumpfish, which is harvest for its flesh which are usually dried or smoked. Lumpfish eggs are also used as a caviar substitute.

Because of the many commercial uses for the wolffish, it was considered as a candidate for aquaculture and experimental farming of wolfish was initiated in the early 2000s. Samples of these farmed fish were also sent regularly to the Fish Disease Laboratory at the University of Iceland to examine them for any pathogens. It was during these routine examinations that K. islandica was discovered. While the parasite was not described at the time, its presence has been known informally for decades. Icelandic fishermen called soft-fleshed wolffish “hárasteinbítur”, which means “hairy wolffish” (the "hair" are the parasite's plasmodia stage).

Since it was initially found in farmed fishes, the scientists at the Fish Disease Laboratory decided to see if this parasite was also found in wild marine fish of Icelandic waters. They caught some wild wolffish and lumpfish from Bay Faxaflói off the west coast of Iceland and found that the wolffish had relatively light to moderate level of infected by K. islandica. In contrast, some of the lumpfish were more heavily infected. In fact, some of them so were so loaded with the parasite that large proportion of their flesh had been replaced by K. islandica plasmodia. This parasite proliferates in the fish's flesh, taking over much of the muscle fibres they invade. However, it does not seem to cause the fish much ill effect, and the lumpfish seems surprisingly fine with their muscle tissues being replaced by parasites, with no signs of inflammation or fibrosis.
Photo of infected lumpfish fillet (from the paper)
It is after the host has died that this parasite begins to unleash its mayhem. Heavily infected fish exhibit "soft flesh syndrome" which seems to be caused an enzyme that is activate by changes in pH which accompanies fish death. This cause the flesh to literally liquefy. In the wild, this process would liberate the infective stages of the parasite into the environment where they can be ingested by the next host in the life cycle, which are small invertebrates such as marine worms. This process cannot be halted by freezing and the melting fish fillets becomes unmarketable.

One of K. islandica's host - the lumpfish - is currently being trialled as a potential cleaner fish that can be used to combat sea lice in salmon farms. Considering that parasites from the Kudoa genus are generally are not picky about what fish it hops into, there is potential for K. islandica to jump host from lumpfish to salmon (which is already infected with its own Kudoa parasite - K. thyrsites), making it key priority to work out the ecology and life-cycle of this flesh-melting parasite.

Kristmundsson, Á., & Freeman, M. A. (2014). Negative effects of Kudoa islandica n. sp.(Myxosporea: Kudoidae) on aquaculture and wild fisheries in Iceland. International Journal for Parasitology: Parasites and Wildlife 3: 135-146.

September 8, 2014

Anelasma squalicola (revisited)

A few months ago I wrote a Dispatch for Current Biology about a newly published study on Anelasma squalicola - a parasitic barnacle that infects velvet belly lantern sharks. Unfortunately for most people, the Dispatch is behind a paywall, therefore I have decided to write a blog post about that study, which in turn is based on the Dispatch I originally wrote for Current Biology, so here it is.

Drawing of Anelasma squalicola and its host by Tommy Leung

The trouble with studying the evolution of parasites is that it is often hard to tell what evolutionary steps they took to get that way. Evolutionary selection pressures experienced by parasites can be quite different to those with a free-living life, thus parasites often bear very little resemblance to their non-parasitic relatives. For example, Enteroxenos oestergreni is a parasitic snail that lives inside a sea cucumber, but the adult stage of this snail is nothing more than a long, wormy string of gonads. To make things even more difficult, parasites are usually small and soft-bodied - which means they are not usually preserved as fossils and unlike say, birds or whales, there is not a good fossil record of various transitional form.

Parasitism has evolved in many different groups of animals, including crustaceans. Various lineage of crustaceans have independently evolved to be parasitic, some of them are so well-adapted that most people would not recognise them as crustaceans if they were to encounter one. Some barnacles have also jumped on the parasitism bandwagon, of which the most well-known is Sacculina which infects and castrate crabs.  The body plan of Sacculina and other rhizocephalans bear little resemblance to the filter-feeding species often found attached to rocks or the hull of ships. Superficially, it resembles some kind of exotic plant (perhaps Audrey II from the Little Shop of Horrors)- there is the bulbous reproductive organ call the Externa which protrudes from the host's abdomen, but the rest of the parasite is actually found inside the body of the crab in the form of an extensive network of roots called the Interna.

Aside from the rhizocephalans, there are only two known genera of parasitic barnacles - one of which is the star of this post. Anelasma squalicola is one of those rare parasites that has retain some remnants of its non-parasitic past. Its host is the velvet belly lantern shark - a deep water fish also known as the shark that warn off predators by wielding a pair of "light sabers". But such armament offers no protection against A. squalicola. This barnacle attaches to the shark's body and burrow into its flesh. Anelasma squalicola digs into the shark using its peduncle - for non-parasitic stalked barnacle, that is the structure they use to stick themselves onto a fixed surface. In A. squalicola, the peduncle embeds itself into the shark's muscles, then sprouts numerous branching filaments that sucks the life blood out of the host. As a shark can sometimes be infected with multiple A. squalicola, this can really take a toll and this parasite has been known to cause host castration.

There are of course, other barnacles that attached to marine animals like whales and turtles, but they are not truly parasitic as they still feed strictly by filtering food from the water instead of feeding off the host like A. squalicola. One group - the Coronuloidea - are specialists at this particular life-style. In fact, some of them do not merely stick to their host, they are partially buried in the host tissue and have special structures to anchor them firmly in place. So it seems likely that the coronuloids might be the predecessor to a full-blown parasite like A. squalicola, right? Even though they have kept up their filter-feeding life-style, they are already embedded in the host's body, so one can imagine that it is only one step away from feeding directly from the host itself.

But as plausible as that story may sound, according to the new study by Rees and colleagues, their analysis shows that the closest living relative of A. squalicola is not the coronuloids but is actually...[drumrolls]...a filter-feeding goose barnacle! The ancestor of A. squalicola seems to have taken up the parasitic life-style about 120 million years ago in the early Cretaceous, when the sea was filled with marine reptiles. It was also during this period that more "modern" sharks underwent a dramatic increase in their diversity. Given the lack of any other known stalked barnacles with similar life-styles and its relatively ancient origin, could A. squalicola be the remnant species from a group that was once far more diverse, rather like the coelacanth or the tuatara?

But what about the Coronuloidea? Why did they not go "full parasite"? Considering the radical changes the ancestor of A. squalicola underwent from a life of filter-feeding to one parasitising a shark, why have none of the coronuloids done the same? Especially seeing how they seem to be in such a prime position to do so.

The affinity of A. squalicola to modern rock-clinging barnacles should remind us that evolution does not always go the way we imagine it to be. You can come up a plausible hypothesis (like A. squalicola evolving from the coronuloid barnacles) that seem rather believable, but ultimately it has to face the data. The evolution history of any organism is a convoluted tale, and sometimes it can challenge our expectations.

Leung, T. L. (2014). Evolution: How a Barnacle Came to Parasitise a Shark. Current Biology 24: R564-R566.

Rees, D. J., Noever, C., Høeg, J. T., Ommundsen, A., & Glenner, H. (2014). On the Origin of a Novel Parasitic-Feeding Mode within Suspension-Feeding Barnacles. Current Biology 24: 1429-1434

For another take on this story, I also recommend Ed Yong's post about the paper here.