Facts Of Life On Planet Parasite

We've come to the end of yet another year and all that it entails in the field of parasitology. As with last year, we have continued to the feature guest posts by student from the University of New England ZOOL329/529 class of 2014, who wrote about fungus that kills water bears, midges that suck blood from mosquitoes, and wasps that zombifies cockroaches and many more. In addition to student guest posts, there were some conference coverage (Part 1, Part 2) mixed in as well.

As for some of the parasites that were featured this year, we looked under the sea - and found that it was filled with shark-suckers, face-huggers, brood-blockers, and egg-mimics. While they sound like the monsters of science fiction horror, but they are non-fiction of the real world, and they are not monsters, but simply living things trying to get on with their life - admittedly in ways that somewhat terrifies us.

This year, we learned about parasite that can take a reproductive toll on their host, such as a lovecraftian parasitic copepods that infect flamboyant sea slugs, a peculiar barnacle which sticks itself in the flesh of a shark and can castrate its host, a tiny crab that brood-blocks its limpet host, and a copepod that masquerading as a lobster egg so they can feast on the brood of its host.

When they're not killing their hosts' broods one way or the other, they outright disintegrate them. We learn about the parasite that kills a species of "killer shrimp" by dissolving them into shrimp paste, but not before causing the crustacean to bring themselves out into the open to the waiting maw of its cannibalistic cousins. Other parasites like myxozoans do not kill their host outright, but when their fish hosts do die, it cause their flesh melt into mush, much to the dismay of fishermen.

But it's not just aquatic critters that are the target of parasites - they rumble in the jungle too, and are found in larger terrestrial animals like rhinos and monkeys, as well as smaller ones like crickets. In the case of the cricket, some parasites actually bring their terrestrial host into the aquatic realm by manipulating the host's behaviour. Other parasites mess with their host's sense of smell. And some parasites don't alter behaviour directly but just gets in the way - the worm that gets in the eyes of prairie chickens (and other birds), and fish are not faring any better, with a parasite that literally get all up in their face.

And there is no escape from parasitism - parasites are found everywhere, even in deep sea hydrothermal vents. And they do more than just gross us out or cause their host to suffer - they can also cause changes in their hosts that sends a ripple effect into the surrounding ecosystem too. Parasites are ubiquitous, diverse, and a major components of this planet's biological diversity. Parasitism is as much a fact of life as feeding, fighting, and f…reproducing - that is unless a parasite gets in the way of your ability to do that last thing…

We will back next year to bring you more posts on parasite research which you might not have read about elsewhere - so here's to another year of more parasitology science! Bring on 2015!

P.S. If you can't wait until next year for your parasite fix, you can check out some of my other parasite-related writing on The Conversation on the important ecosystem roles played by some parasites here and on parasites that blind their hosts here. As well as writing this blog, I have also been doing a regular radio segment call "Creepy but Curious" where I talk about parasitic (and non-parasitic organisms) like hairworms, emerald jewel wasps, killer sponges, vampire snails, colossal squids, second-hand vampires, and melting seastars. You can find links to all these and more on this page here.

Gnathia maxillaris

Today's blog post features a study in which an infestation at an aquarium allowed a group of scientists to work out the life cycle of a common parasite. Now, we are not talking about your lounge room fish tank, but the biggest exhibition tank at Aquarium of Barcelona. The exhibition aquarium, call Oceanarium, measures 37000 cubic metres and is home to over 3000 fish of 80 different species. But amidst those 80 different species, they have a parasite which has made its way into the mix.

Adult female with larval brood (left) and newly-hatched zuphea (right)
Photos from Fig. 1 of the paper

The parasite in question - Gnathia maxillaris - belongs to a family of little blood-sucking crustaceans call Gnathiidae (we have previously featured gnathiids on this blog here). You can think of them as being like ticks of the sea - not only are they blood suckers, but they also alternates between a blood-feeding and a free-living stage during their development (like a tick). The parasitic stage of a gnathiid is called a Zuphea - it needs to attach and feed on a host for a while before it drops off to moult into its next stage call a Pranzia. The pranzia is free-living stage, but it doesn't stay that way for long, as the next step of its development is to grow into a slightly larger zuphea which jumps right back onboard a fish for a blood meal. A gnathiid needs to go through this parasitic-then-not-parasitic-then-parasitic-again development cycle three consecutive times (each successive stages are called Z1, P1, Z2, P2, Z3, P3) before it can become an adult (and you thought going through puberty was bad!)

There are over 190 known species of gnathiids from all across the world, but the full life-cycle has only been described for four of those species, and now G. maxillaris join that very short list. Even though G. maxillaris is relatively well-studied and fairly widespread across the Atlantic Ocean as well as the Baltic and Mediterranean seas, the complete life-cycle of G. maxillaris was unknown until now because much of this parasite's development takes place out of sight on the open sea.

But the infestation at Aquarium of Barcelona provided scientists with a great opportunity to study this life-cycle. They harvested G. maxillaris larvae by exploiting their natural attraction to light; at night, they turned on a set of light installed at the bottom of the aquarium, then pump the sea water through a fine-meshed plankton net that have also been placed there to trap the parasite larvae.

Clockwise from upper left:
Adult female, adult male, female carrying eggs
From Fig. 2 of the paper

With the harvested parasites, they exposed them to different species of potential fish hosts to observe their behaviour. They noticed that newly-hatched zuphea (Z1) cannot feed on blood because their mouthpart is so small the fish blood cells cannot fit through them. Instead, they feed on lymph and have to subsequently grow into the larger zuphea stages before they can incorporate blood into their diet.

They also discovered that G. maxillaris has different preference for specific parts of the fish's body, and this has consequences for the parasite's growth. While they can attach pretty much anywhere on the fish's body, they have a taste for the base of the fins, near the gill covers, or around the eyes - basically areas of high blood flow and where it would be harder for the fish to rub them off. They also noticed zuphea that attach themselves to the fish's fin feed for longer and takes more time to develop into a pranzia, most likely because there is less blood flow there than other parts of the body, so the parasite needs to stick around for longer to get a full meal.

In all, G. maxillaris' entire life-cycle takes about three months to complete, but that is if the water temperature is at 17.5 °C; if the surround temperature is 20 °C, then the parasite would take only two months to complete this cycle. At higher temperature, the female parasites also grew larger and produced more eggs. This is particularly pertinent to the current situation because one of the (many) consequences of increasing ocean temperature might mean in the future, the seas will be filled with more gnathiids that grow faster than ever before, which is bad news for fish. Not only are they blood-suckers, like ticks on land, gnathiids can also act as vectors for various other parasites.

While an infestation of tiny "ticks of the sea" might not be the best news for a national aquarium, when life hands you an infestation - you might as well do some science with it!

Reference:
Hispano, C., Bulto, P., & Blanch, A. R. (2014). Life cycle of the fish parasite Gnathia maxillaris (Crustacea: Isopoda: Gnathiidae). Folia Parasitologica 61: 277-284.