Category Archives: Evolution

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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*Creative Commons License This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

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Filed under Ecology, Evolution, flatworms, worms

Should we save or should we get rid of parasites?

by Piter Kehoma Boll

Parasites are special types of organisms that live on or inside other lifeforms, slowly feeding on them but usually not killing them, just reducing their fitness to some degree. This is a much more discrete way to survive than killing or biting entire parts off, as predators (both carnivores and herbivores) do. However, different from these creatures, parasites are often regarded as unpleasant and disgusting. Yet parasitism is the most common way to get food in nature.

When I introduced the rhinoceros tick in a recent Friday Fellow, I mentioned the dilemma caused by it. Since the rhinoceros tick is a parasite of rhinoceroses, and rhinoceroses are threatened with extinction, a common practice to improve the reproductive fitness of rhinos is removing their ticks, but this may end up leading the rhinoceros tick to extinction.

This actually happened already with other parasites, such as the louse Coleocephalum californici, which was an exclusive parasite of the California condor Gymnogyps californianus. In order to save the condor, a common practice among veterinarians working with conservationists was to delouse the birds and, as a result, this louse is now extinct. The harm that the louse caused to the condor was so little, though, that its extinction was not at all necessary, being nothing more than a case of negligence and lack of empathy for a small and non-charismatic species.

The California condor louse Coleocephalum californici was extinct during a poorly managed campaign to save the California condor Gymnogyps californianus. Image extracted from https://www.hcn.org/blogs/goat/the-power-and-plight-of-the-parasite

The louse Rallicola (Aptericola) pilgrimi has also vanished forever during the conservation campaigns to save its host, the little spotted kiwi, Apteryx owenii, in another failed work.

The efforts to save the little spotted kiwi, Apteryx owenii, from extinction led to the extinction of its louse. Photo by Judi Lapsley Miller.*
The now extinct Rallicola (Aptericola) pilgrimi. Credits to the Museum of New Zealand.***

Another group of parasites that is facing extinction are fleas. The species Xenopsylla nesiotes was endemic to the Christmas Island together with its host, the Christmas Island rat, Rattus macleari. The introduction of the black rat, Rattus rattus, in the island led to a quick decline in the population of the Christmas Island rat, which went extinct at the beginning of the 20th century and, of course, the flea went extinct with it. The flea Acanthopsylla saphes has likely become extinct as well. It was a parasite of the eastern quoll, Dasyurus viverrinus, in mainland Australia. The eastern quoll today is only found in Tasmania, as the mainland Australia’s population went extinct in the mid-20th century. However, the flea was never found in the Tasmanian populations, so it is likely that it died away in mainland Australia together with the local population of its host.

The Manx shearwater flea Ceratophyllus (Emmareus) fionnus. Photo by Olha Schedrina, Natural History Museum.*

But things have been changing lately and fortunately the view on parasites is improving. A recent assessment was made on the population of another flea, the Manx shearwater flea, Ceratophyllus (Emmareus) fionnus. This flea is host-specific, being found only on the Manx shearwater Puffinus puffinus. Although the Manx shearwater is not at all a threatened species and has many colonies along the North Atlantic coast, the flea is endemic to the Isle of Rùm, a small island off the west coast of Scotland. Due to the small population of its host in this island, the flea has ben evaluated as vulnerable. If the Manx shearwater population in the Island were stable, things would be fine but, as you may have guessed already, things are not fine. Just like it happened in Christmas Island, the black rat was also introduced in the Isle of Rúm and has become a predator of the Manx shearwater, attacking its nests.

The Manx shearwater, Puffinus puffinus, is the sole host of the Manx shearwater flea. Photo by Martin Reith.**

Some ideas have been suggested to protect the flea from extinction. One of them is to eradicate the black rat from the Isle or at least manage its population near the Manx shearwater colonies. Another proposal is to translocate some fleas to another island to create additional populations in other Manx shearwater colonies.

But why bother protecting parasites? Well, there are plenty of reasons. First, they comprise a huge part of the planet’s biodiversity and their loss would have a strong impact on any ecosystem. Second, they are an essential part of their host’s evolutionary history and are, therefore, promoters of diversity by natural selection. Removing the parasites from a host would eventually decrease its genetic variability and let it more vulnerable to other new parasites. Due to their coevolution with the host, parasites are also a valuable source of knowledge about the host’s ecology and evolutionary history, helping us know their population dynamics. We can even find ways to deal with our own parasites by studying the parasites of other species, and parasites are certainly something that humans managed to collect in large numbers while spreading across the globe.

Parasites may be annoying but they are necessary. They may seem to weaken their host at first but, in the long run, what doesn’t kill you makes you stronger.

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References:

Kirst ML (2012) The power and plight of the parasite. High Country News. Available at < https://www.hcn.org/blogs/goat/the-power-and-plight-of-the-parasite >. Access on 3 November 2019.

Kwak ML (2018) Australia’s vanishing fleas (Insecta: Siphonaptera): a case study in methods for the assessment and conservation of threatened flea species. Journal of Insect Conservation 22(3–4): 545–550. doi: 10.1007/s10841-018-0083-7

Kwak ML, Heath ACG, Palma RL (2019) Saving the Manx Shearwater Flea Ceratophyllus (Emmareus) fionnus (Insecta: Siphonaptera): The Road to Developing a Recovery Plan for a Threatened Ectoparasite. Acta Parasitologica. doi: 10.2478/s11686-019-00119-8

Rózsa L, Vas Z (2015) Co-extinct and critically co-endangered species of parasitic lice, and conservation-induced extinction: should lice be reintroduced to their hosts? Oryx 49(1): 107–110. doi: 10.1017/S0030605313000628

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*Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 International License.

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Filed under Conservation, Ecology, Evolution, Extinction, Parasites

Going deep with your guts full of microbes: a lesson from Chinese fish

by Piter Kehoma Boll

All around the world, many animal species have adapted to live in cave environments, places that are naturally devoid of light, either partially or entirely, and are, therefore, nutrient-poor habitats. The lack of light makes it impossible for plants and other photosynthetic organisms to survive and, as a result, little food is available for non-photosynthetic creatures. They rely almost entirely on food that enters the cave from the surface by water or animals that move between the surface and the depths.

Due to the lack of light in caves, animals adapted to this environment are usually eyeless, because seeing is not possible anyway, and white, because there is no need for pigmentation on the skin to protect from radiation or to inform anything visually. On the other hand, chemical senses such as smell and taste are often very well developed.

All these limitations make cave environments relatively species-poor when compared to surface environments. Or at least that is what it looks like at first. There are, of course, much less macroscopic species, such as multicellular animals, but those animals are themselves an environment and they may harbor a vast and unknown diversity of microrganisms inside them.

As you may know, most, if not all, animals have mutualistic relationships with microorganisms, especially bacteria, living in their guts. Those microorganisms are essential for many digestive processes and many nutrients that animals get from their food can only be obtained with the aid of those microscopic friends. The types of microorganisms in an animal’s gut are directly related to the animal’s diet. For example, herbivores usually have a high diversity of microorganisms that are able to break down carbohydrates, especially complex ones such as cellulose.

A recent study, conducted in China with fishes of the genus Sinocyclocheilus, compared the gut microbial diversity of different species, including some that live on the surface and some that are adapted to caves. All species of Sinocyclocheilus seem to be primarily omnivores but different species may have preferences for a particular type of food, being more carnivorous or more herbivorous.

The study found that cave species of Sinocyclocheilus have a much higher microbial diversity than surface species. But how can this be possible if there is a limited number of resources available in caves compared to the surface? Well, that seems to be exactly the reason.

Sinocyclocheilus microphthalmus, one of the cave-dwelling species used in this study. Photo extracted from the Cool Goby Blog.

As I mentioned, species of Sinocyclocheilus are omnivores. On the surface, they have plenty of food available and can have the luxury of choosing a preferred food type. As a result, their gut microbiome is composed mainly by species that aid in the digestions of that specific type of food. In caves, on the other hand, food is so scarce that one cannot chose and must eat whatever is available. This includes feeding on small amounts of many different food types, including other animals that live in the cave and many different types of animal and plant debris that reach the cave through the water. Thus, a much more diverse community of gut microorganisms is necessary for digestion to be efficient.

Look how the number of different genera of bacteria is much larger in the cave group (right) than in two groups of surface species (left and center). Image extracted from Chen et al. (2019).

More than only an increased diversity by itself, the gut community of cave fish also showed a larger number of bacteria that are able to neutralize toxic compounds of several types. The reason for this is not clear yet but there are two possible explanations that are not necessarily mutually exclusive. The first states that water in caves is renewed in a much lower rate than surface waters, which promotes the accumulation of all sort of substances, including metabolic residues of the cave species themselves that can be toxic. The second hypothesis is of greater concern and suggests that this increased number of bacteria that are able to degrade harmful substances is a recent phenomenon caused by an increase in water pollutants coming from human activities, which is promoting a selective pressure on cave organisms.

The diverse gut microbiome of cave fish is, therefore, a desperate but clever strategy to survive in such a harsh environment. Nature always finds a way.

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More on cave species:

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

Don’t let the web bugs bite

Friday Fellow: Hitler’s beetle

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Reference:

Chen H, Li C, Liu T, Chen S, Xiao H (2019) A Metagenomic Study of Intestinal Microbial Diversity in Relation to Feeding Habits of Surface and Cave-Dwelling Sinocyclocheilus Species. Microbial Ecology. doi: 10.1007/s00248-019-01409-4

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Filed under Bacteria, Ecology, Evolution, Fish

Looking like lichens: leaves disguised as tree trunks to avoid being eaten

by Piter Kehoma Boll

We are all familiar with animals of many species that developed interesting mechanisms to avoid being eaten. This includes, for example, animals that look like plant parts:

The famous giant leaf insect, Phyllium giganteum. Photo by Bernard Dupont.**

and animals that merge with the background:

An East African jackal, Canis mesomelas, in the Savanna. Can you spot it? Photo by Nevit Dilmen.***

There are also animals that look like other, unpalatable or dangerous, animals, in order to push predators away:

The edible viceroy butterfly Limenitis archippus (top) mimicks the poisonous monarch butterfly Danaus plexippus (bottom). Credits to Wikimedia user DRosenbach. Photos by D. Gordon E. Robertson and Derek Ramsey.***

But we rarely think that plants also use this sort of mechanisms to avoid being eaten. There are, however, some recorded cases of similar behaviors in plants. One case is that of the plant Corydalis benecincta, whose leaves commonly have the brownish color of the surrounding rocks:

The leaves of Corydalis benecincta look like the rocks found in its natural habitat. Photo extracted from http://www.svenlandrein.com/yunnancollections/10CS2204.html

Recently, a study on plants of the genus Amorphophallus found another interesting case of mimicry. This genus, which includes the famous titan arum, usually develops a single large leaf that in some species can attain the size of a small tree or shrub. Such a gigantic leaf seems to be a perfect meal for some herbivores but, to avoid them, many species of this genus developed a series of marks along the petiole of their leaf that look like lichens or cyanobacteria.

Cyanobacteria-like marks on the petiole of Amorphophallus gigas (A); Cyanobacteria-like plus lichen-like marks also on A. gigas (B); And lichen-like marks on A. hewittii (C) and A. dactylifer (D). Extracted from Claudel et al. (2019).

With this mimicry, the petioles, which are quite tender, end up looking like a hard and old trunk that does not look that interesting as a meal for most herbivores. The lichen marks are so well represented that they can even be associated with real lichen genera. For example, the marks seen on the figures B and C above look like lichens of the genus Cryptothecia.

Lichen of the species Cryptothecia striata, which seems to be mimicked by the marks in Amorphophallus gigas and A. hewittii. Photo by Jason Hollinger.*

How and why this marks evolved across Amorphophallus species is still not well understood. Despite the hypothesis that they help the plant mimic a tree trunk, some species with small leaves also have those marks, while some with large leaves do not have any marks or have them in simpler patterns. The titan arum Amorphophallus titanum is a good example of the latter:

Amorphophallus titanum is the largest species of Amorphophallus but displays a considerably simple lichen-like pattern. Photo by flickr user Björn S.**

For a long time, plants were regarded as less dynamic organisms than animals, but in recent years our knowledge about them is increasing and showing that they are actually very versatile creatures that developed similar creative and complex strategies.

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Reference:

Claudel C, Lev-Yadun S, Hetterscheid W, & Schultz M 2019. Mimicry of lichens and cyanobacteria on tree-sized Amorphophallus petioles results in their masquerade as inedible tree trunks. Bot J Linn Soc 190: 192–214.

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Filed under Botany, Evolution

One more species joins the Husband-Killers Club

by Piter Kehoma Boll

Leia em português

Sexual cannibalism is the act of eating a sexual partner right before, during or right after copulation. Despite being a considerably rare behavior, its occurrence is very popular among the general public.

When sexual cannibalism occurs, it usually consists of the female eating the male. Two popular cases are those of mantises and of spiders, especially the black widow. This phenomenon, at least among black widows, is much rarer than most people think.

Female mantis eating a yummy male. Photo by Oliver Koemmerling.*

Although sometimes sexual cannibalism occurs because one of the partners mistakes the other for food, in many species it is a evolutionary selected strategy that assures that the female will eat enough for the offspring to develop properly. It may look horrible from our human point of view, especially if we think from the perspective of the male, but we must remember that passing your genes to the next generation is the main purpose of most organisms and, if the male succeeded in fertilizing the female’s eggs, his life has served his purpose and he can die happily.

Sexual cannibalism is, of course, almost exclusively observed among predators, which is kind of obvious. And, as I said above, is commonly performed by the female. One group that is famous for its female-empowered species is the insect order Hymenoptera, which includes bees, ants, wasps, sawflies, among others. Since many hymenopterans have some degree of sociality, in which societies are composed almost exclusively of females, and males are generated only for the purpose of reproduction, it is curious that sexual cannibalism has never been recorded in this group… until now.

A recently published study examined the mating behavior of a small parasitoid wasp, Gonatopus chilensis. This species belongs to the family Dryinidae, of which all species lay their eggs on insects of the suborder Auchenorrhyncha, which includes cicadas, leaf hoppers, plant hoppers, among others. The larvae, after hatching from the egg, feed on the hosts. Adult females of dryinid wasps are also voracious predators and feed on the same species on which they fed as larvae.

Male of Gonatopus chilensis (left) inseminating a female (a), and female eating a male (b and c). Extracted from Virla & Espinosa (2019).*

After copulation, females of G. chilensis were often observed trying to capture the males in the same way they capture their prey. However, in only one occasion the female was successful in capturing the male and ate its gaster (the large round portion that forms most of the abdomen in wasps). Since only one instance of cannibalism was observed, it may be a rare phenomenon in this species, but since several attempts to capture the male were seen, it seems that eating the male is an interesting idea for the females.

This is the first known case of sexual cannibalism in hymenopterans and, therefore, an important record that increased the number of groups in which this behavior is known to occur.

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You may also like:

Badass females are unpopular among praying mantids

Male dragonflies are not as violent as thought

Having few females turns male tortoises into rapists

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Reference:

Virla EG, Espinosa MS (2019) Observations on the mating behavior of a dryinid and first record of sexual cannibalism in the hymenoptera. Acta Ethologica. doi: 10.1007/s10211-019-00315-9

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Filed under Behavior, Entomology, Evolution

Caught in the act: Insect sex preserved in amber

by Piter Kehoma Boll

A recently published paper describes a new species of insect of the order Zoraptera from two specimens found in mid-cretaceous amber from northern Myanmar.

The preserved couple. They did not leave descendants but were eternized in science. Credits to Chen & Su (2019).*

But the most impressive thing about this new pre-historic species, named Zorotypus pusillus, is the fact that the fossil contains a male and a female that apparently died while they were mating. This is concluded because the two individuals are very close to each other and the male has an elongate structure coming out of his abdomen, which is probably the aedeagus or intromittent organ, a penis-like organ found in most zorapterans and used to deliver sperm into the female.

A detail of the posterior end of the male showing the aedeagus or intromittent organ. An anatomical reconstruction is shown to the right. Credits to Chen & Su (2019).*

The order Zoraptera contains a very small number of species, currently 44 extant ones and 14 fossils. They are very small, live in groups and look like tiny termites, although they are not closely related to them. Most extant species mate with the male introducing its aedeagus into the female to deliver sperm, but at least one species, Zorotypus impolitus, does not copulate. In this species, the male deposits microscopic spermatophores on the abdomen of the female.

The discovery of the preserved mating behavior in this species from the cretaceous period indicates that the mating behavior seen in most extant species was already used by species living 99 million years ago. The origin of zorapterans is not well known yet, but this and other fossil species indicate that they exist at least since the beginning of the cretaceous.

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Reference:

Chen X, Su G (2019) A new species of Zorotypus (Insecta, Zoraptera, Zorotypidae) and the earliest known suspicious mating behavior of Zorapterans from the mid-cretaceous amber of northern Myanmar. Journal of Zoological Systematics and Evolutionary Research. doi: 10.1111/jzs.12283

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Hagfish: Another Phylogenetic Headache

by Piter Kehoma Boll

Years ago, I wrote a post about the problematic Acoelomorpha and their controversial position among bilaterian animals. Now I am going to talk about another headache: hagfish.

Hagfish are primitive chordates that make up the class Myxini. They are marine animals that live at the bottom of the sea and feed mainly on polychaete worms that they pull out of the substrate. However, they are also scavengers and have a peculiar behavior in which they perforate the body of dead fish and enter it, eating the dead animal from inside out.

Specimen of the Pacific hagfish Eptatretus stoutii. Photo by Jeanette Bham.*

Morphologically, hagfish are characterized by the presence of a cartilaginous skull, like vertebrates, but lack a vertebral column, keeping the notochord, the dorsal cartilage-like structure of chordates, during their whole lives. Due to this lack of vertebrae, the hagfish were classified outside of the vertebrates, but united to them due to the presence of the skull. Thus, Myxini was seen as the sister-group of Vertebrata and both together formed the clade Craniata.

Among the vertebrates, most extant groups have a jaw that evolved from modified gill arches, making up the clade Gnathostomata. The only animals with a vertebral column that lack jaws are the lampreys (Petromyzontiformes) and, although this lack of jaws is shared with hagfish, it is not usually seen as a synapomorphy uniting these groups. In hagfish, the jawless mouth have lateral keratin plates with tooth-like structures that act somewhat like the true jaws of Gnathostomata, but working from the sides and not from above and below. In lampreys, on the other hand, the mouth is circular and have keratin tooth-like structures arranged circularly.

General organization of the head of hagfish, lampreys and jawed vertebrates, with special attention to the mouths. Extracted from Oisi et al. (2012).

There are a lot of morphological features that unite lampreys to vertebrates and separate them from hagfish, the main one being the already mentioned vertebrate column. Likewise, lampreys and jawed vertebrates have dorsal fins while hagfish lack them. Lampreys also have lensed eyes in common with jawed vertebrates, while hagfish have simple eyesposts without lenses or even associated muscles.

Some of the traits shared between hagfish and lampreys, just as the lack of jaws, are usually seen as a primitive state that changed in jawed vertebrates, or have clearly evolved independently. For example, both hagfish and lampreys have only a single nostril, while jawed vertebrates have two, but this is likely a primitive character. Adult hagfish and lampreys have also a single gonad, but this appears in hagfish by an atrophy of the left gonad, so that only the right one develops, while in lampreys the left and right gonads fuse into a single organ.

Specimens of the least brook lamprey Lampetra aepyptera. Photo by Jerry Reynolds.*

Therefore, morphologically, it seems logical to consider hagfish as a sister group of vertebrates, which include lampreys and jawed vertebrates. It is also important to mention that there are more groups of jawless vertebrates that are currently extinct, such as the class Osteostraci, one of several fossil groups traditionally called ostracoderms. Although lacking a jaw as well, these vertebrates had paired fins just like jawed vertebrates. Thus, the phylogenetic organization of these major groups based on morphology would be as shown in the figure below:

The craniate hypothesis, where hagfish are a sister-group to vertebrates.

However, in the last decades, the use of molecular phylogenetics has challenged this view by grouping hagfish and lampreys into a monophyletic clade that is sister-group of jawed vertebrates. But how could this be possible? Such a relationship would imply that the primitive state of hagfish is the result of secondary loss.

The cyclostome hypothesis. Hagfish are a sister-group to lampreys.

Evidence from fossils could help clarify this issue, but most fossils that have been associated with hagfish have not good enough morphological characters preserved to assess their correct phylogenetic position. Recently, however, a well preserved hagfish fossil from the Cretaceous helped to elucidate part of the hagfish phylogeny. The divergence between lampreys and hagfish, considering previous knowledge, was usually put around the early Cambrian period, just after the beginning of the divergence of most animal phyla, but with data of the new fossil, it is pushed to a more recent point in time, around the Early Silurian, more than 130 million years after. This new fossil, named Tethymyxine tapirostrum, clearly lacks a skeleton or dorsal fins as seen in lampreys and jawed vertebrates, but has several characters shared with extant hagfish.

Fossil of Tethymyxine tapirostrum found in Lebanon. Extracted from Miyashita et al. (2019).

At least two synapomorphies can be found uniting hagfish and lampreys and separating them from jawed vertebrates. The first one are the teeth, which in these two groups are composed of keratin plates. The second one is the organization of the myomeres, the series of muscles arranged along the body of chordates in a somewhat segmented fashion, that in both hagfish and lampreys begin right around the eyes.

Considering the evidence from molecular data, the new fossil that makes it likely that hagfish and lampreys diverged more recently if they form a monophyletic group, and the likely true synapomorphies uniting these two jawless vertebrate groups, it seems that hagfish and lampreys are indeed sister-groups, forming a clade called Cyclostomata and sister-group of the jawed vertebrates Gnathostomata. If this is really the case, then the apparently more primitive features of hagfish are in fact the result of secondary losses and its ancestor likely had a more vertebrate look, with a vertebral column, dorsal fins and lensed eyes.

But let’s keep watching. Things may change again in the future as new data become available.

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References:

Miyashita T, Coates MI, Farrar R, Larson P, Manning PL, Wogelius RA, Edwards NP, Anné J, Bergmann U, Palmer AR, Currie PJ (2019) Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. PNAS 116(6): 2146–2151. doi: 10.1073/pnas.1814794116

Oisi Y, Ota KG, Kuraku S, Fujimoto S, Kuratani S (2012) Craniofacial development of hagfishes and the evolution of vertebrates. Nature 493: 175–180. doi: 10.1038/nature11794

Wikipedia. Cyclostomata. Available at <
https://en.wikipedia.org/wiki/Cyclostomata >. Access on March 25, 2019;

Wikipedia. Hagfish. Available at <
https://en.wikipedia.org/wiki/Hagfish >. Access on March 25, 2019.

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Filed under Evolution, Paleontology, Systematics, Zoology