Category Archives: flatworms

Friday Fellow: Common Horse Tapeworm

by Piter Kehoma Boll

Flatworms are the fourth largest animal phylum after arthropods, mollusks and chordates and most species known to date belong to the clade Neodermata, which includes parasitic species such as flukes and tapeworms, some of which infect humans. Among tapeworms, the species that infect humans and belong to the genus Taenia are certainly the most popular, but it is expected that all vertebrates can have at least one tapeworm parasite, so that it is only a matter of time and opportunity for us to discover them all.

Among the species known to parasitize horses, the most widespread is Anoplocephala perfoliata, which I decided to call the common horse tapeworm. As tapeworms in general, the adult common horse tapeworm lives in the intestine of its definitive host, in this case a horse.

Different from species of Taenia, which can grow up to several meters in length, species of Anoplocephala are much smaller. The whole body of adult specimens measures about 5 to 8 cm in length and 1 to 2 cm in width and is divided into the same parts seen in other tapeworms. The anteriormost part of the body includes the scolex, which has 4 large suckers. Although the scolex of most tapeworms measures less than 1 mm, in the common horse tapeworm it reaches up to 3 mm.

A preserved specimen. Photo extracted from alchetron.com

After the scolex there is a small neck of undifferentiated tissue that grows constantly to build new proglottids, which form the rest of the body. Proglottids are connected to each other in a chain fashion and the posteriormost ones are continuouly lost and released into the environment. Each proglotid contains male and female sexual organs and is released when it contains mature eggs.

Mature proglottids are released in the environment through the horse’s feces and release their eggs on the ground and the vegetation. The eggs can survive outside a host for as long as 9 months. During this time, they hope to be accidentally ingested by oribatid mites that live in pastures. If this happens, the egg hatches inside the mite due to the mechanical action of the mite’s mouth parts and releases the first-stage larva called the oncosphere.

An egg of Anoplocephala perfoliata. The small 20-µm-diamter sphere is the oncosphere waiting to be released. Photo by Martin K. Nielsen, extracted from msdvetmanual.com

When the oncosphere reaches the mite’s gut, it is activated, probably via ions present in this environment, and uses a group of hooks to penetrate the mite’s tissues. After about 8 to 20 weeks, the oncosphere develops into a cysticercoid. This stage looks like an inverted miniaturized tapeworm inside a bladder-like vesicle, having already a protoscolex inside it.

While horses are grazing, they always ingest some invertebrates together with the grass. It they happen to ingest an infected mite, the cysticercoids are released during digestion, evert the protoscolex and attach to the intestinal walls of the horse. There, the tapeworm develops into an adult, restarting the cycle.

Attached to the intestine of their hosts, tapeworms do not feed on blood or other tissues as many parasites do. Instead of that, they collect nutrients directly from the host’s gut by absorbing them via the worm’s body surface.

For a long time its has been thought that the common horse tapeworm was a harmless parasite since most horses did not seem to have any symptom and the tapeworms were often only discovered during dissection after the horse’s death by other causes. The preferred area for the common horse tapeworm to attach is the caecum and the ileocaecal junction but in heavily infected animals some individuals can be found in suboptimal sites throughout the small and large intestines. In such heavily infected horses, the tapeworms can cause colics and even intestinal obstruction.

Large number of adult tapeworms in a heavily infected horse. Credits to Tomczuk et al. (2014).*

The common horse tapeworm can infect other equids as well, such as donkeys and zebras. Ironically, domesticated horses seem to be the most infected individuals exactly because horse owners treat them with anthelmintics. Most modern anthelmintics do not affect tapeworms and only remove other parasites, such as roundworms, which reduces competition and allows tapeworms to thrive.

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More parasitic flatworms:

Friday Fellow: Green-banded broodsac (on 09 March, 2018)

Friday Fellow: Salmon Fluke (on 11 January, 2019)

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References:

Gasser RB, Williamson RMC, Beveridge I (2005) Anoplocephala perfoliata of horses – significant scope for further research, improved diagnosis and control. Parasitology 131(1): 1–13. https://doi.org/10.1017/S0031182004007127

Tomczuk K, Kostro K, Szczepaniak KO, Grzybek M, Studzińska M, Demkowska-Kutrzepa M, Roczeń-Karczmarz M (2014) Comparison of the sensitivity of coprological methods in detecting Anoplocephala perfoliata invasions. Parasitology Research 113(6): 2401–2406. doi: 10.1007/s00436-014-3919-4

Wikipedia. Anoplocephala perfoliata. Available at < https://en.wikipedia.org/wiki/Anoplocephala_perfoliata >. Access on 11 June 2020.

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Friday Fellow: Lignano’s Macrostomum

by Piter Kehoma Boll

It’s time to come back to the fascinating flatworms and today I decided to talk about one of the most studied species in this group even though it was only formally described 15 years ago. Its name is Macrostomum lignano, or the Lignano’s macrostomum.

Macrostomum lignano.Photo by Lukas Schärer,**

Measuring 1 to 2 mm in length, the Lignano’s macrostomum belongs to the order Macrostomida, one of the basalmost flatworm groups. Its body is elongate and transparent, there are two small eyes close to the anterior end, which has a small rostrum (“snout”). The mouth is a little behind the rostrum. The posterior end is broad, forming a tail plate with many adhesive organs arranged in a U-shape.

Basic morphology of the Lignano’s macrostomum. Credits to Lengerer et al. (2014).*

The Lignano’s macrostomum was first collected in marine samples in the city of Lignano on the Adriatic Sea coast in northern Italy in 1995 and soon revealed to be very suitable for laboratory cultures. The natural environment of this species includes the sand and other sediments near the shore. It avoids light and, when at rest, remains attached to the substrate by its tail plate. It feeds on small organism, especially diatoms, which it ingests using its cylindrical pharynx, similarly to how most flatworms eat.

Also like most flatworms, the Lignano’s macrostomum and other macrostomids have special stem cells called neoblasts that fill their body. All differentiated cells in the body come from neoblasts and are continuously replaced by them, since its differentiated cells cannot continue reproducing. Neoblasts also give the Lignano’s macrostomum an impressive regenerative ability like that of many other flatworms such as planarians.

Even before its formal description in 2005, the Lignano’s macrostomum had already been identified as a potentially new model organism. It can be easily cultured in laboratory in Petri dishes and fed with diatoms. Its body has about 25,000 cells, which is a number small enough to facilitate studies on development, regeneration, ageing and gene expression and that is exactly what has been done in the past decades.

The Lignano’s macrostomum is hermaphrodite. The body contains two testes and two ovaries. The male copulatory apparatus contains a stylet, a hardened penis-like copulatory organ. When two macrostomums mate, they touch their ventral surfaces in a yin yang fashion (just like the guys from last week) and exchange sperm. This behavior is easily observed in laboratory and led the Lignano’s macrostomum to become a model organism for the study of sexual selection as well. But wait! Sexual selection in a hermaphrodite organism? Yes! I discussed this topic some time ago here.

Macrostomum lignano, reciprocal mating behaviour
Two Lignano’s macrostomums mating in the yin yang position. Photo by Lukas Schärer.***

Sometimes, when two macrostomums meet, they don’t find their partner that attractive, so having their eggs fertilized by that guy is not of their interest from the female side. However, their male side is still as male as any other and they want to fertilize as many eggs as possible. As a result, if the partner is not good enough, they still want it as a female but not as a male. The other guys is not interesting in being a female only though, so copulation only occurs if both partners accept to receive each other sperm. “I let you fertilize my eggs if you let me fertilize yours.” So that’s what they do.

A pair of flatworms, Macrostomum lignano, mating. See how the white one, in the end, bends over itself and sucks the other guy’s sperm out of the female pore in order to get rid of it. Notice, however, in the last drawing, that the sperm cells are still attached to the female pore. It did not work. Image extracted from Schärer et al. (2004).

However, after they delivered the sperm into each other’s body, they separate and may never see each other again. So the female side evolved a strategy to select better sperm. When the “bad partner” moves away, a macrostomum that received low-quality sperm bends over itself, connects its pharynx to its female genital pore, and sucks the other guy’s sperm out before it has the chance to fertilize its eggs. A clever strategy, right? But remember: just as this guy is getting rid of the other guy’s sperm, the other guy may be doing the same with this guy’s sperm. So a strategy must evolve to prevent the female personality to discard the sperm. And that is exactly what happened! The sperm cells of the Lignano’s macrostomum have hard bristles pointing backward that, when the cells is pulled back, enter the tissues in the female copulatory apparatus and remain stuck. Trying to pull them out is just like trying to pull porcupine quills out of the skin.

Watch the behavior in video.

Now the male side recovered the advantage that the female side would have if the bristles were not there. But this is evolution, and its effect on hermaphrodites is like having two different personalities fighting each other in the same body.

Life is not easy anywhere.

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References and further reading:

Egger B, Ladurner P, Nimeth K, Gschwentner R, Rieger R (2006) The regeneration capacity of the flatworm Macrostomum lignano—on repeated regeneration, rejuvenation, and the minimal size needed for regeneration. Development Genes and Evolution 216:565–577. doi: 10.1007/s00427-006-0069-4

Ladurner P, Schärer L, Salvenmoser W, Rieger RM (2005) A new model organism among the lower Bilateria and the use of digital microscopy in taxonomy of meiobenthic Platyhelminthes: Macrostomum lignano, n. sp. (Rhabditophora, Macrostomorpha). Journal of Zoological Systematics and Evolutionary Research 43(2):114–126. doi: 10.1111/j.1439-0469.2005.00299.x

Lengerer B, Pjeta R, Wunderer J et al. (2014) Biological adhesion of the flatworm Macrostomum lignano relies on a duo-gland system and is mediated by a cell type-specific intermediate filament protein. Frontiers in Zoology 11:12. doi: 10.1186/1742-9994-11-12

Mouton S, Willems M, Braeckman BP, Egger B, Ladurner P, Schärer L, Borgonie G (2009) The free-living flatworm Macrostomum lignano: A new model organism for ageing research. Experimental Gerontology 44(4):243–249. doi: 10.1016/j.exger.2008.11.007

Pfister D, De Mulder K, Hartenstein V et al. (2008) Flatworm stem cells and the germ line: Developmental and evolutionary implications of macvasa expression in Macrostomum lignano. Developmental Biology 319(1):146–159. doi: 10.1016/j.ydbio.2008.02.045

Pfister D, De Mulder K, Philipp I et al. (2007) The exceptional stem cell system of Macrostomum lignano: Screening for gene expression and studying cell proliferation by hydroxyurea treatment and irradiation. Frontiers in Zoology 4:9. doi: 10.1186/1742-9994-4-9

Schärer L, Joss G, Sandner P (2004). Mating behaviour of the marine turbellarian Macrostomum sp.: these worms suck, Marine Biology 145 (2) doi: 10.1007/s00227-004-1314-x

Wasik K, Gurtowski J, Zhou X et al. (2015) Genome and transcriptome of the regeneration-competent flatworm, Macrostomum lignano. PNAS 112(40):12462–12467. doi: 10.1073/pnas.1516718112

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A balanced diet may kill you sooner… if you are a land planarian

by Piter Kehoma Boll

There’s one thing that I should do more often here, and that is presenting my own research for the readers of the blog, so today I am going to do exactly that.

As you may know, the group of organisms with which I work is the family Geoplanidae, commonly known as land planarians. Here in Brazil, the most speciose genus is Obama, of which I have talked in previous posts. This genus became considerably famous after one of its species, Obama nungara, became invasive in Europe, which called attention of the public especially because of the curious name of this genus, even though it has nothing to do with the former president of the United States.

Anyway, during my Master’s study, it became clear that species in the genus Obama feed on soft-bodied invertebrates, mainly slugs and snails, although some species also feed on earthworms or even other land planarians. Obama nungara, for example, feeds on all three groups, although it seems to have some preference for earthworms.

A specimen of Obama anthropophila with its testicle freckles. Photo by myself, Piter K. Boll.*

One common species of Obama in urbans areas of southern Brazil is Obama anthropophila, whose name, meaning “lover of humans” is a reference to this habit precisely. This species has a uniformly dark brown dorsal color, sometimes mottled by the mature testicles appearing as darker spots on the first half of the body. The diet of this species includes snails, slugs, nemerteans and other land planarians, especially of the genus Luteostriata, and more especially of the species Luteostriata abundans, which occurs very often in urbans areas too.

Watch Obama anthropophila capture different prey species.

So I wondered… if O. anthropophila feeds on different types of invertebrates, does it mean that each type provides different nutritients, so that a mixed diet is necessary or more beneficial than one composed of a single prey type? To assess that, I divided adult specimens of O. anthropophila into three groups, each receving a different diet:

Group Dela: fed only with the common marsh slug, Deroceras laeve
Group Luab: fed only with the abundant yellow striped planarian, Luteostriata abundans
Group Mixed: fed with both prey species in an alternating way

The results were not what I expected. The Mixed group showed a lower survival rate than the groups receiving a single diet. Another interesting feature was that the Mixed group showed a tendency to skip the slug meal and eat only the planarian after some days receiving the alternating prey types.

Based on the hypothesis that a mixed diet is more nutritious, I was expecting the Mixed group to have the best performance, or at least being similar to the single-diet groups if there was no increase in nutritional value with an additional prey type. However, the results indicate that a mixed diet may be bad for the planarian, at least if the animal has to eat a different food on every meal.

We don’t know what causes this, but my idea is that maybe different prey types demand different metabolic processes, such as the production of different enzymes and stuff, and having to constantly reset your metabolism is too costly. As a result, the fitness of specimens receiving such a diet decreases and the animals start to avoid one of the food types, because eating less is less dangerous than mixing food.

A “pregnant” Obama anthropophila about to ley an egg capsule. Photo by myself, Piter K. Boll.*

Another interesting aspect is that planarians receiving a mixed diet, even though they died earlier, laid heavier egg capsules than the single-diet groups. Heavier egg capsules generally mean that they have more embryos or are more nutrient for the embryos, increasing the reproductive success. But how can a dying animal be better at reproducing than a healthy one?

Well, this may be related to the terminal investment hypothesis. It is thought, and proven in some groups, that an organism may increase its investment on reproduction when future reproductive events are not expected, i.e., when the organism “realizes” it is about to die, it puts all its effort to reproduce in order to garantee that its genes will pass successfully to future generations.

We cannot be sure about anything yet. More studies are necessary to better understand the relationship of land planarians and their food. What we can assure is that, just like Obama nungara, O. anthropophila may end up in Europe or anywhere else soon because its relatively broad diet and its proximity to humans make it a potential new species to be spread accidentally around the world.

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Reference:

Boll PK, Marques D, & Leal-Zanchet AM (2020) Mind the food: Survival, growth and fecundity of a Neotropical land planarian (Platyhelminthes, Geoplanidae) under different diets. Zoology 138: 125722.

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The land planarian community of FLONA-SFP and how it gets along

by Piter Kehoma Boll

(First of all, I wish it were Bolsonaro, that piece of diarrhea-shaped cancer, who were dying by fire instead of the Amazon forest.)

(Now let’s go to the post itself:)

The São Francisco de Paula National Forest (FLONA-SFP) is a protected area for sustainable use in southern Brazil. Its was originally covered by Araucaria forest but currently is composed of a mosaic of the native forest and plantations of Araucaria, Pinus and Eucalyptus trees. This protection area is one of the main study areas of Unisinos’ Planarian Research Institute, where I conducted my undergradate, Master’s and PhD studies.

After studying the land planarian community of FLONA-SFP for many years, we conclude that it includes a fairly large number of species. Take a look at some of them and how cool they are:

Obama ladislavii, the Ladislau’s leaf-like flatworm. Photo by Piter Kehoma Boll.*
Obama anthropophila, the brown urban leaf-like flatworm. Photo by Piter Keehoma Boll.*
Obama josefi, the Josef’s leaf-like flatworm. Photo by Piter Kehoma Boll.*
Obama ficki, the Fick’s leaf-like flatworm. Photo by Piter Kehoma Boll.*
Obama maculipunctata, the spotted-and-dotted leaf-like flatworm. Photo by Piter Kehoma Boll.*
Cratera ochra. The ochre crater flatworm. Photo by Piter Kehoma Boll.*
Luteostriata arturi, the Artur’s yellow striped flatworm. Credits to Instituto de Pesquisas de Planárias, Unisinos.**
Luteostriata ceciliae, the Cecilia’s yellow striped flatworm. Photo by Piter Kehoma Boll.*
Luteostriata pseudoceciliae. The false Cecilia’s yellow striped flatworm. Credits to Instituto de Pesquisas de Planárias, Unisinos.**
Luteostriata ernesti, the Ernst’s yellow striped flatworm. Photo by Piter Kehoma Boll.*
Luteostriata graffi, the Graff’s yellow striped flatworm. Photo by Piter Kehoma Boll.*
Supramontana irritata, the irritated yellowish flatworm. Photo by Piter Kehoma Boll.*
Pasipha backesi, the Backes’ shiny flatworm. Photo by Piter Kehoma Boll.*
Pasipha brevilineata, the short-lined shiny flatworm. Photo by Piter Kehoma Boll.*
Matuxia tymbyra, the buried Tupi flatworm. Photo by Piter Kehoma Boll.*
Choeradoplana iheringi, the Ihering’s swollen-throated flatworm. Photo by Piter Kehoma Boll.*
Choeradoplana benyiai, the Benya’s swollen-throated flatworm. Photo by Piter Kehoma Boll.*
Choeradoplana minima, the lesser swollen-throated flatworm. Photo by Piter Kehoma Boll.*
Cephaloflexa araucariana, the Araucaria’s bent-headed flatworm. Photo by Piter Kehoma Boll.*
Paraba franciscana, the Franscican colored flatworm. Photo by Piter Kehoma Boll.*
Paraba rubidolineata, the red-lined colored flatworm. Credits to Instituto de Pesquisas de Planárias, Unisinos.**
Imbira guaiana, the Kaingang bark-strip flatworm. Photo by Piter Kehoma Boll.*

Land planarians live in the leaf litter of forest soils and prey on other invertebrates. The 22 species shown above are the ones found in FLONA-SFP that are formally described but there are still some awaiting description. We could say that there are at least 30 different species coexisting in this protected area.

How can they all persist together? Isn’t there any sort of competition for food? Thinking of that, I conducted my master’s research investigating the diet of those and other land planarians. My results suggest that, although some species share many food items, most of them have a preferred food or an exclusive food item that could be considered what Reynoldson and Pierce (1979) called a “food refuge”.

Here is what we know about the FLONA-SFP’s species until now:

  • Obama ficki feeds on slugs and snails and seems to prefer large slugs;
  • Obama ladislavii feeds on slugs and snails and seems to prefer snails;
  • Obama maculipunctata feeds on slugs and snails with unknown preference;
  • Obama anthropophila feeds on slugs, snails and other land planarians, especially of the genus Luteostriata, and prefers the latter;
  • Obama josefi apparently feeds on other land planarians only;
  • All species of Luteostriata feed exclusively on woodlice;
  • Species of Choeradoplana apparently feed on woodlice and harvestmen;
  • Cephaloflexa araucariana apparently feeds on harvestmen;
Obama ladislavii capturing a slug. Photo by Piter Kehoma Boll.*

The diet of the remaining species is still completely unknown but, based on other species of the same genera, it is likely that species of Pasipha feed on millipedes, species of Paraba feed on slugs and planarians, and Imbira guaiana feeds on earthworms.

Luteostriata ernesti near some juicy woodlice. Photo by Piter Kehoma Boll.*

There are plenty of different invertebrate groups that share the leaf litter with land planarians. Despite the apparently simple anatomy of these flatworms, they were able to adapt to feed on different types of prey and have muscular and pharyngeal adaptations for that. And attempt to relate anatomical adaptations to the diet of land planarians was part of my PhD research. As soon as it is published, I’ll make a post about it. There are some nice results!

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More on land planarians:

Friday Fellow: Abundant Yellow Striped Flatworm

Friday Fellow: Ladislau’s Flatworm

Darwin’s Planaria elegans: Hidden, extinct or misidentified?

How are little flatworms colored? A Geoplana vaginuloides analysis

Obama invades Europe: “Yes, we can!

The fabulous taxonomic adventure of the genus Geoplana

The hammerhead Flatworms: Once a mess, now even messier

The New Guinea flatworm visits France: a menace

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References:

Boll PK & Leal-Zanchet AM 2015. Predation on invasive land gastropods by a Neotropical land planarian. J. Nat. Hist. 49: 983–994.

Boll PK & Leal-Zanchet AM 2016. Preference for different prey allows the coexistence of several land planarians in areas of the Atlantic Forest. Zoology 119: 162–168.

Leal-Zanchet AM & Carbayo F 2000. Fauna de Planárias Terrestres (Platyhelminthes, Tricladida, Terricola) da Floresta Nacional de São Francisco de Paula, RS, Brasil: uma análise preliminar. Acta Biologica Leopoldensia 22: 19–25.

Oliveira SM, Boll PK, Baptista V dos A, & Leal-Zanchet AM 2014. Effects of pine invasion on land planarian communities in an area covered by Araucaria moist forest. Zool. Stud. 53: 19.

Reynoldson TB & Piearce B 1979. Predation on snails by three species of triclad and its bearing on the distribution of Planaria torva in Britain. Journal of Zoology 189: 459–484.

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Friday Fellow: Salmon Fluke

by Piter Kehoma Boll

Leia em Português

Everybody knows salmons, especially the Atlantic salmon, Salmo salar, and many of us love to eat this fish species as well. However, I’m not here to talk about the Atlantic salmon itself, but to talk about one of its closes companions and antagonists, the salmon fluke.

Scientifically known as Gyrodactylus salaris, the salmon fluke is a flatworm of the clade Monogenea, a group of ectoparasites that infect mainly fish. As its name suggests, the salmon fluke infects salmons, such as the Atlantic salmon, and closely related species, such as the rainbow trout Onchorhynchus mykiss.

Several salmon flukes on a host. Photo by Tora Bardal. Extracted from https://www.drivaregionen.no/no/Gyrodactylus-salaris/

The salmon fluke was first discovered in 1952 in salmons from a Baltic population that were kept in a Swedish laboratory. Measuring about 0.5 mm in length, the salmon fluke attaches to the skin of the fish and is too small to be seen with the naked eye. The attachment happens using a specialized organ full of tiny hooks, called haptor, located at the posterior end of the body. When feeding, the salmon fluke attaches its mouth to the surface of the fish using special glands in its head and everts its pharynx through the mouth, releasing digestive enzymes on the fish, dissolving its skin, which is then ingested. The wounds caused by the parasite’s feeding activity can lead to secondary infections that can seriously affect the salmon’s health.

Artificially colored SEM micrograph of five specimens of Gyrodactylus salaris. Credits to Jannicke Wiik Nielsen. Extracted from https://www.vetinst.no/nyheter/kan-gyrodactylus-salaris-utryddes-i-drammensregionen

Different from most parasitic flatworms, monogeneans such as the salmon fluke have a single host. During reproduction, the hermanophrodite adults release a ciliated larva called oncomiracidium that infects new fish. A single fluke can originate an entire population because it is able to self fertilize.

During the 1970’s, a massive infection by the salmon fluke occurred in Norway following the introduction of infected salmon strains. This led to a catastrophic decrease in the salmon populations in the country, affecting many rivers. Due to this evident threat to such a commercially important species, several techniques have been developed to control and kill the parasite. The first developed methods included the use of pesticides in the rivers, but those ended up having a negative effect on many species, including the salmons themselves. Currently, newer and less aggressive methods have been used.

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References:

Jansen, P. A., & Bakke, T. A. (1991). Temperature-dependent reproduction and survival of Gyrodactylus salaris Malmberg, 1957 (Platyhelminthes: Monogenea) on Atlantic salmon (Salmo salar L.). Parasitology, 102(01), 105. doi:10.1017/s0031182000060406

Johnsen, B. O., & Jenser, A. J. (1991). The Gyrodactylus story in Norway. Aquaculture, 98(1-3), 289–302. doi:10.1016/0044-8486(91)90393-l

Meinilä, M., Kuusela, J., Ziętara, M. S., & Lumme, J. (2004). Initial steps of speciation by geographic isolation and host switch in salmonid pathogen Gyrodactylus salaris (Monogenea: Gyrodactylidae). International Journal for Parasitology, 34(4), 515–526. doi:10.1016/j.ijpara.2003.12.002

Wikipedia. Gyrodactylus salaris. Available at < https://en.wikipedia.org/wiki/Gyrodactylus_salaris >. Access on December 26, 2018.

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Think of the worms, not only of the whales, or: how a planarian saved an ecosystem

by Piter Kehoma Boll

Leia em português

Due to the massive interference of human practices on natural habitats during the past decades, ecosystem restoration has become a trend in order to try to save what is still savable. Unfortunately, the effort of ecologists and other experts alone is not enough to achieve that, and a larger section of the society needs to be engaged in helping reach the goals. In order to do so, it is common to appeal to the beauty and cuteness of endangered species, which usually include mammals and birds, since they are more likely to caught the public’s attention. However, most of the endangered species are invertebrates or other less charismatic beings, and they are often ignored even by biologists.

Hopefully, things are able to change on this matter. Recently the first ecosystem restoration directed to save an invertebrate was successful, and I am here to tell you about it.

The invertebrate in question is a freshwater planarian named Dendrocoelum italicum. It was discovered in 1936 in a cave in northern Italy named Bus del Budrio. Inside the cave, there was a small freshwater pool, about 5 × 5 m or little more, caused by a waterfall from a small stream coming through a narrow elevated corridor. The species is apparently found only in this pool and nowhere else.

There are no available photos of Dendrocoelum italicum, but it should look similar to the widespread Dendrocoelum lacteum seen here, but D. italicum lacks the eyes. Photo by Eduard Solà.*

During the 1980’s, a pipe was installed to divert the water from the stream to a nearby farm. The waterfall ceased to exist and the pool dried up permanently. The planarian survived in a very narrow rivulet that formed inside the cave and some small isolated ponds resulting from water drips. This critical condition of the population was discovered in 2016 by a research group from the University of Milan. They informed the administrators of the cave about the situation and, together, the team started to raise awareness about the situation of the cave among the citizens that benefitted from the reservoir formed by the diverted water, which made the farmer responsible for diverting the water agree to remove the artificial structure.

Image of the inside of the cave. Photo by Livio Mola. Extracted from https://www.naturamediterraneo.com/forum/topic.asp?TOPIC_ID=57050

The removal happened on December 3, 2016 after all the planarians occurring in the rivulet were collected and stored in plastic tanks inside the cave. When the waterfall was restored, it quickly started to fill the old pool again and, one day later, the planarians were released into the pool.

The ecosystem was monitored during the following two years until January 2018. The number of planarians varied greatly during the survey, but was not significantly larger after the restoration from what it was before. However, there was a significant increase in the population of a bivalve species, Pisidium personatum, and a small increase in the population of a crustaceon of the genus Niphargus. Additionally, annelids of the family Haplotaxidae, that were absent in the cave, appeared after restoration. Thus, it is clear that the ecosystem benefited from the reappearance of the pool.

Thanks to the efforts of those researchers, Dendrocoelum italicum now has a better chance to avoid extinction. However, this is not an isolated case. There are many cave-dwelling planarian species all around the world living under similar conditions, usually restricted to a single small pool inside a single cave. Many of those occur, or occurred, as D. italicum, in Italy, but the help came to late for some of them. For example, a closely related species, Dendrocoelum beauchampi, was discovered in 1950 in a cave in northwestern Italy named Grotta di Cavassola, but a recent survey found no planarians inside the cave, which seems to have suffered great alteration due to human activities. Similarly, the species Dendrocoelum benazzi was discovered in 1971 in central Italy in a cave named Grotta di Stiffe, but nowadays, with the cave open to turists and its water polluted, the planarians disappeared. It is very likely that both D. beauchampi and D. benazzi are now extinct. The situation is the same for other Italian species.

Out of Italy, a recently described species living a similar small environment is the Brazilian cave planarian Girardia multidiverticulata, which is known to occur in a small pool about 10 m² inside a cave named Buraco do Bicho in the Cerrado Biome.

Girardia multidiverticulata is a planarian species restricted a small 10 m² pool inside a cave in Brazilian cerrado. Credits to Souza et al. (2015)**

The case of Dendrocoelum italicum shows us it is possible to save small endemic populations of threatened habitats, but we need the help of the public. Let’s hope other ecosystems have a similar happy ending.

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References:

Manenti R, Barzaghi B, Lana E, Stocchino GA, Manconi R, & Lunghi E 2018. The stenoendemic cave-dwelling planarians (Platyhelminthes, Tricladida) of the Italian Alps and Apennines: conservation issues. Journal for Nature Conservation.

Manenti R, Barzaghi B, Tonni G, Ficetola GF, & Melotto A 2018. Even worms matter: cave habitat restoration for a planarian species increased environmental suitability but not abundance. Oryx: 1–6.

Souza ST, Morais ALN, Cordeiro LM, & Leal-Zanchet AM 2015. The first troglobitic species of freshwater flatworm of the suborder Continenticola (Platyhelminthes) from South America. Zookeys 470: 1–16.

Vialli PM 1937. Una nuova specie di Dendrocoelum delle Grotte Bresciane. Bollettino di zoologia 8: 179–187.

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Planarian: a living vessel built by unicellular organisms?

by Piter Kehoma Boll

Not long ago I talked about the peculiar genome of the freshwater planarian Schmidtea mediterranea and how it challenges our view of many cellular processes. Now what if I told you that planarians also challenge our view of what a multicellular organism is?

We all know that organisms may be either unicellular or multicellular, but sometimes it is hard to tell them apart, especially in what are called colonial organisms, in which clones of unicellular individuals may live together.

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Green algae of the genus Volvox are somehow at the boundary between unicellular and multicellular organisms. Although usually considered a colony of unicellular organisms, they behave somewhat like a multicellular organism. Photo by Frank Fox.*

So where lies the boundary between gathered unicellular organisms and true multicellular organisms? One of the ideas is that for a group of cells to be considered a single unit (an organism) they must have high levels of cooperation and low levels of conflict between each other and, perhaps more important than that, they have to be dependent on the association in order to survive.

As most recent evolutionary theories predict, cooperation increases with genetic similarity. As a result, multicellular organisms are (almost) always composed of cells having the exact same genetic material, i.e., they are all clones. We know, however, that during DNA replication mutations may occur, so that eventually at least some cells of an adult and many-celled organism may have become genetically distinct. This leads to a need to find a way to fix this problem by reincreasing genetic similarity, and the way most organisms found to do that is by allowing only one lineage of their cells, the germ cells (which produce the gametes) to generate the next generation. Thus, each transition from one generation to the next passes through a “zygotic bottleneck”, i.e., a new organism is always generated from a single original cell, the zygote, which assures that the genetic similarity is always brought back.

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A developing new egg, with an embryo that is created from a single original cell, the zygot, assuring a higher genetic similarity of the cells in the whole organism. Photo by Stéphanie Bret.**

The zygotic bottleneck is not a rule for a lot of species though. Many animals and plants are able to reproduce asexually by budding, fission or many other ways. In such cases, the new organism usually is built from several different lineages of the parent organism. For example, some succulent plants may generate a new organism from a dettached leaf and several cells of the original leaf start to reproduce and together they build the new plant and, as each lineage may have suffered different mutations, the offspring is not necessarily composed of genetically identical cells. Nevertheless, even such organisms, which are able to reproduce asexually, still retain the ability to generate zygotes through sexual reproduction, which eventually “cleans that mess”.

But in planarians things get really strange. First of all, let’s explain some basic things about planarians. They have, as you know, a remarkable regeneration ability. This happens due to the presence of stem cells called neoblasts that fill their bodies. Those neoblasts are able to generate all cell types that make up the planarian’s body. In fact, all cells in a planarian must come from neoblasts, because, as weird as it may be, all differentiated cells in a planarian body ARE UNABLE TO UNDERGO MITOSIS! Once a neoblast differentiates into any kind of cell, it is condemned to die in a few days without ever letting descendants. All cells in a planarian body are therefore constantly replaced by new ones coming from neoblasts. The only lineage of differentiated cells that is still able to reproduce is that of the germ cells, which, as in other multicellular organisms, assure that the next generation will consist of organisms with genetically homogeneous cells.

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Girardia tigrina, a freshwater planarian. Photo by Wikimedia user Slimguy.**

Several freshwater planarians, however, have lost their ability to generate sexual organs and, as a result, germ cells. In order to reproduce, they must rely on a form of asexual reproduction, which in this case happens by transversal fission of the body and posterior regeneration of the missing parts. In these populations, the zygotic bottleneck disappeared completely and, as a result, any non-lethal mutation in the neoblasts is retained in the organism, leading to a population of genetically distinct neoblasts inside a planarian.

Therefore, considering the fact that asexual planarians are not genetically homogeneous, having several different neoblast lineages in the same body, and that the neoblasts are the only cells able to reproduce and continue the species, a recent publication by Fields and Levin (see references) suggests that asexual planarians are nothing more than a very complex environment built by neoblasts in order to survive. Considering that each neoblast is an independent cell, which only needs the environment (the planarian) to survive, but does not need other neoblasts, planarians, at least the asexual ones, do not seem to have reached completely the requirements of high internal cooperation and low internal competition to be considered multicellular organisms.

We could interpret the neoblasts as unicellular organisms that live together, cooperating to build a complex environment, the planarian body, with their own sterile descendants (the differentiated cells), as if they were a group of queen ants living among sterile castes. Kind of mind blowing, huh? But it actually makes sense.

If you want to read more about it and understand every detail in this theory, read the references below.

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Maybe you’d like:

Endosperm: the pivot of the sexual conflict in flowering plants

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References:

Fields, C; Levin, M. (2018) Are Planaria Individuals? What Regenerative Biology is Telling Us About the Nature of Multicellularity. Evolutionary Biology: 1–11.

West, S. A.; Fisher, R. M.; Gardner, A.; Kiers, E. T. (2015) Major evolutionary transitions in individuality. PNAS 112 (33): 10112-10119. https://doi.org/10.1073/pnas.1421402112

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Friday Fellow: Green-Banded Broodsac

by Piter Kehoma Boll

Parasites are very speciose, and I often feel that I’m not giving enough space for them here, especially when I bring you a flatworm, which is likely the group with the largest number of parasite species. So let’s talk about one today at last.

The first parasitic flatworm I am introducing to you is Leucochloridium paradoxum, the green-banded broodsac. It is a member of the flatworm group Trematoda, commonly known as flukes and, as all flukes, it has a complex life cycle.

Adults of the green-banded broodsac live in the intestine of various passerine birds of North America and Europe. The eggs they lay reach the environment through the bird’s feces and are eventually ingested by land snails of the genus Succinea.

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Adult individual of Leucochloridium paradoxum (left), an infected intermediate hose, a sail of the genus Succinea (center) and the sporocysts along the snail’s internal organs (right). Images not to scale. Extracted from http://medbiol.ru/medbiol/dog/0011a975.htm

Adults of Leucochloridium paradoxum are very similar to the adults of other species of the genus Leucochloridium, the main differences being seen in the larval stages. Inside the body of the snail, the eggs hatch into the first larval stage, the miracidium, which inside the snail’s digestive system develops into the next stage, the sporocyst.

The sporocyst has the form of a long and swollen sac (the broodsac, hence the common name) that is filled with many cercariae, which are the next larval stage. The sporocyst than migrates towards the snail’s eye tentacles, invading them and turning them into a swollen, colorful and pulsating structure that resembles a caterpillar. In this stage of infection, the poor snail is most likely blind and cannot avoid light as it normally does. As a result, it becomes exposed to birds that mistake it for a juicy caterpillar, eating it eagerly.

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A poor snail of the species Succinea putris with a broodsac in its left eye stalk. There is only one terrible fate for this creature. Photo by Thomas Hahmann.*

When the snail is eaten, the sporocyst burst and the several cercariae are released. In the bird’s intestine, they develop into adults and restart the nightmarish cycle.

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References:

Rząd, I.; Hofsoe, R.; Panicz, R.; Nowakowski, J. K. (2014) Morphological and molecular characterization of adult worms of Leucochloridium paradoxum Carus, 1835 and L. perturbatum Pojmańska, 1969 (Digenea: Leucochloridiidae) from the great tit, Parus major L., 1758 and similarity with the sporocyst stages. Journal of Helminthology 88(4): 506-510. DOI: 10.1017/S0022149X13000291

Wikipedia. Leucochloridium paradoxum. Available at < https://en.wikipedia.org/wiki/Leucochloridium_paradoxum >. Access on March 8, 2018.

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You know nothing, humans! A planarian genome challenges our understanding of how life works

by Piter Kehoma Boll

We finally have a rather complete sequencing of a planarian’s genome, more precisely, of the planarian Schmidtea mediterranea, which is an important model organism for the study of stem cells and regeneration.

In case you don’t know, planarians have a remarkable ability of regeneration, so that even tiny pieces are able to regenerate into a whole organism. They are like a real-life Wolverine, but somewhat cooler! This amazing ability is possible due to the presence of a group of stem cells called neoblasts that can differentiate into any cell type found in the planarian’s body. In fact, all differentiated cell types in planarians are unable to undergo mitosis, so that neoblasts are responsible for constantly replacing cells in every tissue. But we are not here to explain the details of planarian regeneration. We are here to talk about Schmidtea mediterranea‘s genome!

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Look at its little cock eyes saying “I will destroy everything you think you know, humans!” Photo by Alejandro Sánchez Alvarado.*

A rather complete genome of S. mediterranea has been recently published and its analysis reveal some astonishing features.

First of all, 61.7% of S. mediterranea‘s genome is formed by repeated elements. Repeated elements are basically DNA strands that occur in multiple copies throught the genome of an organism. They are thought to come from the DNA of virus that was incorporated to the host’s DNA. In humans, about 46% of the genome is formed by repeated elements. Most repeated elements of S. mediterranea belong to unidentified families of retroelements, thus suggesting that they are new undescribed families. Those repeats are very long, having more than 30 thousand base pairs, which are not known to exist in other animals. The only other group of repeated elements with a similar size is found in plants and known as OGRE (Origin G-Rich Repeated Elements). The long repeat found in Schmidtea was therefore called Burro (Big, unknown repeat rivaling Ogre).

But certainly the most surprising thing about S. mediterranea‘s genome is the lack of many highly conserved genes that are found in most eukaryotes and that were thought to be essential for cells to function properly.

Schmidtea mediterranea lacks genes responsible for repairing double-stranded breaks (DSBs) in DNA, which would make them very likely to suffer a lot of mutations and sensitive to anything that induces DSBs. However, planarians are known to have an extraordinary resistance to gamma radiation that induces DSBs. Do they have other repairing mechanisms or is our current understanding about this process flawed?

gene_loss

Several “essential” genes and their presence (in green) or absence (in red) in several animals. Schmidtea mediterranea lacks them all. Image extracted from Grohme et al. (2018).**

Another important gene that was not found in S. mediterranea is the Fatty Acid Synthase (FASN) gene, which is essential for an organism to synthesize new fatty acids. Planarians therefore would have to rely on the lipids acquired from the diet. This gene is also absent in parasitic flatworms and was at first thought to be an adaptation to parasitism but since it is absent in free-living species as well, it does not seem to be the case. Could it be a synapomorphy of flatworms, i.e., a character that identifies this clade of animals?

That is not enough for little Schmidtea, though. More than that, this lovely planarian seems to lack the MAD1 and MAD2 genes, which are found in virtually all eukaryotes. Those genes are responsible for the Spindle Assembly Checkpoint (SAC), an important step during cell division that prevents the two copies of a chromosome to separate from each other before they are all connected to the spindle apparatus. This assures that the chromosomes will be evenly distributed in both daughter cells. Errors in this process lead to aneuploidy (the wrong number of chromosomes in each daughter cell), which is the cause of some genetic disorders such as the Down syndrome in humans. Planarians do not have any trouble in distributing their chromosomes properly, so what is going on? Have they developed a new way to prevent cellular chaos or, again, is our current understanding about this process flawed?

Let’s wait for the next chapters.

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Reference:

Grohme, M. A.; Schloissnig, S.; Rozanski, A.; Pippel, M.; Young, G. R.; Winkler, S.; Brandl, H.; Henry, I.; Dahl, A.; Powell, S.; Hiller, M.; Myers, E.; Rink, J. C. (2018). “The genome of Schmidtea mediterranea and the evolution of core cellular mechanisms”. Nature. doi:10.1038/nature25473

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Friday Fellow: Duckweed Chain Flatworm

by Piter Kehoma Boll

Today we have one more flatworm in our team. It is part of the most bizarre group of flatworms, the so-called Catenulida. Our fellow is called Catenula lemnae, which I adapted as the “duckweed chain flatworm”.

The duckweed chain flatworm is a very small animal, measuring about 0.1 mm in width and about two or three times this size in length. It is found worldwide in freshwater lakes and ponds and is likely a complex of species, but more detailed studies are needed to make it clear. As other catenulids, it lives close to the substract, being considered a benthic animal, and feeds on other smaller organisms, such as small invertebrates and algae. It is usually a dominant species in the community of benthic microanimals, such as microturbellarians, in places where it is found.

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A chain of several connected individuals (zooids) of Catenula lemnae. Photo by Christopher Laumer.*

The word catenula, meaning “little chain” in Latin, was given to these animals because of their peculiar way of vegetative reproduction. The organism frequently divides transversally close to the posterior end, giving rise to new organisms that are genetically identical to the original one. However, the new animals often remain connected to each other for a long time before splitting, and as this asexual reproduction continues, it eventually turn them into a chain of connected individuals (called zooids). This chain swims elegantly using its cilia as if it were a single individual.

Most recent studies mentioning the duckeed chain flatworm are simply surveys of the species composition of a certain area or broad phylogenetic studies on the catenulids or flatworms in general. Little is known about the ecology, behavior and population structuring of this species, unfortunately.

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References:

Braccini, J. A. L.; Leal-Zanchet, A. M. (2013)  Turbellarian assemblages in freshwater lagoons in southern Brazil. Invertebrate Biology132(4): 305–314. https://dx.doi.org/10.1111/ivb.12032

Marcus, E. (1945) Sôbre Catenulida brasileiros. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, série Zoologia, 10: 3–113.

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