December 10 - Parvatrema margaritense

Trematodes are famous for the complexity of their life cycles, but some species demonstrate something beyond all expectations. A good example is Parvatrema margaritense which belongs to Gymnophallidae, a relatively small family of trematodes circulating in coastal ecosystems. As studied in the Bartents and White Seas in Russia, sexual reproduction of this parasite occurs in the digestive tract of the common eider duck (Somateria mollissima). Eggs containing miracidia disperse into the water and the bivalves Turtonia minuta get infected. The cercaria develop in sporocysts, are shed and then seek out and penetrate into the second intermediate host, the gastropod Margarites helicinus. The cercariae occupy the extrapallial cavity of their hosts, lose their tails and form metacercariae. For the majority of trematodes, the latter would eventually be infective for the definitive hosts. However, P. margaritense’s metacercariae are parthenogenetic; they produce the next generation of metacercariae, also parthenogenetic. These subsequently yield mature metacercariae that can continue the cycle inside the eiders. (In addition to these findings, other gymnophallid metacercariae from Sakhalin and the Kuril Islands in Russia were shown to produce cercariae which are capable of re-infecting the host species they originate from, so creating a peculiar sub-cycle).

One cercaria established in M. helicinus can thus give rise to as many as 1.5 – 2 thousand invasive metacercariae (clones). This is about 100 times as many as the number of cercariae shed by a single T. minuta a day, providing a significant multiplication of a parasite. Another curious observation is that metacercariae in the extrapallial cavity of M. helicinus don’t cause any of the common parasite-related troubles and might better be call commensals. The broader exciting discussion on these topics as well as on their evolutionary implications can be found in the referred papers.

References

Galaktionov K. V. An experimental study of the unusual life cycle of Parvatrema sp. (Trematoda: Gymnophallidae). Parazitologiya (1996), 30, 487–494 (in Russian).

Galaktionov K.V. Phenomenon of parthenogenetic metacercariae in gymnophallids and aspects of trematode evolution. Proc. Zool. Inst. Russ. Acad. Sci, 310. 2006: 51-58.

Galaktionov K.V., Irwin S.W.B., Saville D.H. One of the most complex life-cycles among trematodes: a description of Parvatrema margaritense (Ching, 1982) n. comb. (Gymnophallidae) possessing parthenogenetic metacercariae. Parasitology (2006), 132: 733-746.

Galaktionov K. V. A description of the parthenogenetic metacercaria and cercaria of Cercaria falsicingulae I larva nov. (Digenea: Gymnophallidae) from the snails Falsicingula spp. (Gastropoda), with speculation on an unusual life-cycle. Systematic Parasitology (2007), 68 (2), 137-146.

Contributed by Anya Gonchar.

December 9 - Cancellaria cooperi

On this blog, we've had all kinds of blood-suckers - leeches, bats, ticks, and lice. But when it comes to vampirism, a snail doesn't usually come to mind, but that's exactly what today's parasite is - a blood-sucking snail. Cancellaria cooperi is a snail that appears to have specialised to feed on the blood of the Californian Torpedo Ray, Torpedo californica. These snails spend most of their time inactive, buried in the sand and waiting for the next potential victim. But when a torpedo ray comes along, this mollusc springs into action. Equipped with an extremely keen sense of smell, C. cooperi is capable of detecting the slightest trace of ray mucus, and observations of trails left by these blood-thirsty snails indicate that they can sniff out a ray from as much as 24 metres (about 80 feet) away. Upon making contact, the snail begin touching and exploring the dorsal surface of the ray with extended tentacles, before extending its proboscis and making a small incision with its scalpel-like radular teeth. It then insert its proboscis into the wound and begin its blood-sucking session, which can last for up to 40 minutes. This snail appears to be a specialised parasite of the California torpedo ray, and has no interest in approaching other benthic fishes which are common in its local area, though they have been observed to feed on the Angel Shark (Squantina californica) in laboratory settings. Surprisingly, the torpedo ray seems unperturbed by the experience of being felt up by a snail before getting cut and probed and having its blood-sucked by the vampiric mollusc. But then, torpedo rays seems to be generally unresponsive to most forms of prodding and mechanical stimuli.

Source: O'Sullivan, J.B., McConnaughey, R.R. and Huber, M.E. (1987) A blood-sucking snail: the Cooper's Nutmeg, Cancellaria cooperi Gabb, parasitizes the California Electric Ray, Torpedo californica Ayre. Biological Bulletin 172: 362-366.

Post by Tommy Leung and photo by Lovell & Libby Langstroth.

December 8 - Sarcotaces arcticus

Scarcely resembling a crustacean so much as some bloated, otherworldly maggot, Sarcotaces arcticus is virtually invisible when it swims up the backdoor of its preferred host, the rockfish. Attaching like a tick to the delicate membranes of the rectum, it begins to gorge on blood and releases an enzyme that causes the host's own body to grow a protective bag or "gall" around the intruder. A bag made of anus. Now impossible to dislodge, the parasite freely grows to around the size of a golf ball, which would be bad enough if the host were our size, but we're talking about a somewhat smaller fish here. Not only does the parasite make a sleeping bag out of rectum skin, it grows too large to ever fit back out either way. All the while, the parasite reproduces and releases its microscopic young, making it not only rude for the host to break wind at social gatherings, but positively terrifying.

Post by Jonathan Wojcik (of BogLeech fame - check out his cool parasite gear) and photo by Jonathan Martin (and if you're brave, check out the video that he took as well!).

December 7 - Trypanosoma cruzi

Years after returning from the long voyage of the Beagle, Charles Darwin started to feel not-so-well - dizziness, muscle spasms, fatigue and other symptoms. Some speculate that the naturalist may have acquired Chagas Disease while he was traveling in South America. This malady is caused by a single-celled parasite known as Trypanosoma cruzi. These parasites are transmitted by triatomine bugs such as Rhodnius prolixus, commonly called "assassin bugs" or "kissing bugs", the latter because of their tendency to take blood meals around the mouth. The parasites are passed out in the bug's feces and enter the bite wound where they travel through the blood and can take up residence in numerous organs. The acute form may go unnoticed and the chronic form can take decades to set in. Infected people often die of heart failure, due to the damage done to this organ. Animals such as opossums, bats, and livestock serve as reservoirs for the parasites. In 2009, a paper documented transfer of genes from the trypanosome to humans followed by inheritance in the children of these Chagas Disease patients!

December 6 - Scaphanocephalus expansus

One way to look at parasites is as generalists or specialists. Generalists are non-host specific and while some infect a group of related hosts, others may infect completely unrelated hosts. Specialists infect only one species of host. This strange-looking trematode with wing-like expansions on the anterior end is an intestinal parasite of the osprey, Pandion haliaetus. The osprey is so highly evolved that it is the only species in its own family, Pandionidae. Perhaps because of its evolutionary isolation, the osprey has an unusual number of helminth specialists, and because its diet is 99% fish, almost all of them probably use fish as an intermediate host.

Contributed by Mike Kinsella.

December 5 - Haemophilus influenzae

Did you get a flu shot this year? Good. But, it's not going to protect you against this pathogen, Haemophilus influenzae. This gamma proteobacterium, a member of the Pasteurellaceae, was mistakenly thought to be the agent responsible for influenza until the 1930's when the actual culprit, viruses, were found. That said, these bacteria can still cause a whole slew of illnesses such as lower respiratory tract infections, pneumonia, ear infections, and meningitis. H. influenzae also holds another important distinction - it was the first bacterium to have its entire genome sequenced - this was published in 1995.

December 4 - Megarhyssa macrurus

Megarhyssa macrurus is a species of parasitoid wasp. These insects seek out their hosts - larval horntail wasps - by tapping trees with their antennae until they sense their vibrations and scent. The females then bore into the wood with enormous ovipositors (now thought to be tipped with metals such as zinc!) and then she injects an egg into the larvae. Her offspring will hatch out and consume the body of its host and then use similarly metal-laden mandibles to emerge from the wood. Male M. macrurus are wandering around trees listening as well - they seek out newly emerging females to mate with.

December 3 - Schistosoma mansoni

Schistosoma mansoni is an unusual parasitic trematode of humans. Unusual, because unlike most other trematodes, schistosomes are dioecious, i.e. have separate sexes and also unusual in that they are longer and more worm-like than most other flukes. Eggs are released in the host's feces and hatch into miracidia, which go on to infect freshwater snails and reproduce to form cercaria. The cercaria then seek out human hosts and penetrate their skin and make their way to the blood stream. The worms seek out a member of the opposite sex and if one is found, the pair will settle down in mesenteric blood vessels and begin to start a family. The couple is monogamous - the female actually lives within a groove in the male's body called the gynaecophoric canal (as shown in photo) and there she will churn out more than 300 eggs a day. S. mansoni is a major public health threat in parts of Africa, Asia, and South America where is causes a chronic disease known as either schistosomiasis or bilharzia.

December 2 - Myleusnema bicornis

Myleusnema bicornis is a species of parasitic nematode belonging to the family Kathlaniidae which is found in the intestine of a small herbivorous freshwater fish call Pacoucine (Myleus ternetzi). This nematode has a few morphology features which are rather unusual. Firstly, while most nematodes have a relatively straightforward-looking anterior end, M. bicornis has a separate, narrow cephalic region that can be extended or retracted (see photo), superficially resembling the proboscis of acanthocephalans such as Profilicollis altmani and Echinorhynchus salmonis. Additionally, male M. bicornis worms have a pair of postcloacal horns located at the posterior end, a feature that is absent in all other species of nematode in the kathlaniid family.

Source: Moravec, F. and Thatcher, V.E. 1996. Myleusnema bicornis gen. et sp. n. (Nematoda: Kathlaniidae), an intestinal parasite of a freshwater serrasalmid fish, Myleus ternetzi, from French Guiana. Folia Parasitologica 43:53-59.

Contributed by Tommy Leung.

December 1 - Crossobothrium antonioi

Crossobothrium antonioi is a species of tapeworm that was just described last year after finding it in the broadnose sevengill shark (Notorynchus cepedianus), off the coast of Argentina. This tapeworm may win the worldwide contest for having the most "cojones" as the colloquial saying goes - A mature worm can have over 60 segments or proglottids, and in every single one of them, C. antonioi has over 700 testes! That sounds rather formidable - until you realize that the entire tapeworm is only about 50 mm long.