In this article we are going to go from describing fish to sea worms. In this case, we will talk about planariansThey are a group of flatworms (hence their name) whose class was formerly Turbellaria. For this reason, they are also called turbellarians. There are nearly 4500 known species of these worms, hence their importance. The vast majority of them are aquatic and live in benthic ecosystems. There are some other species adapted to humid environments.
If you want to know these sea flatworms in depth, in this post we will delve into their biology, classification and way of lifeWould you like to meet them?
Classification
The Turbellaria class It was considered a group of platyhelminthes that included all those that were not strictly parasites. However, with the passage of time and the development of taxonomy, This class has disappeared as a valid unit. Therefore, planarians are considered a paraphyletic group that includes free-living flatworms, mainly within Rhabditophora (with orders such as Tricladida and Polycladida). The acoelomorphs, which were traditionally grouped with the flatworms, are now recognized separately. These changes are due to a deeper study of the evolutionary line of these animals.
In a practical sense, when we talk about planarians we are referring to free-living flatworms that inhabit freshwater, marine waters and, to a lesser extent, humid soils. Two large groups stand out: tri-layered (abundant in fresh water) and the polyclades (mainly marine, with conspicuous colors). A common example of laboratory planaria is Schmidtea mediterranea (triclad), while in the marine environment genera such as Pseudoceros y Prostheceraeus (polyclades).
Key features
These are very small invertebrates and variable length. We can find specimens ranging from a millimeter to several centimeters in length. The largest planarians have a leaf-like shape or tape.
Most of these species are aquatic. Benthic species They are those that live in the depths, both marine and freshwater. Therefore, these worms are considered benthic organisms. Regarding their morphology, we can find a large number of epidermal cilia along your body.
Cilia are used to create microcurrents which promote movement and interaction with the substrate. In addition, its epidermis contains mucous glands which produce a lubricating mucus that facilitates movement and helps maintain moisture.
They present a dorsoventrally flattened body, bilateral symmetry and layered subepidermal musculature (circular, longitudinal and oblique) which, together with the cilia, enables precise gliding. Its digestive system is incomplete, with a single ventral buccopharyngeal opening and a eversible pharynx which is projected to capture and suck in food; the intestine is branched to distribute nutrients by diffusion.
As for excretion and water regulation, they lack kidneys but have protonephridia with flame cells that perform functions osmoregulatory and elimination of soluble waste.
Similar to flatworms
Planarians resemble flatworms morphologically because they have bilateral symmetry. This means that they have a longitudinal axis that separates two symmetrical body parts. They are triploblastic because we have three embryonic layers. The same thing happens with humans: we are triploblastic.
Unlike other organisms with bilateral symmetry, planarians and platyhelminthes They have no internal cavity true. They do not have a coelom, so they are classified as acoelomates.
Another remarkable feature is the absence of respiratory and circulatory systems. Without these systems, their ability to exchange oxygen and carbon dioxide with the environment depends on the diffusion through the integumentThis is the reason why their size is small or their body is very flat: this way they maximize the exchange surface.
So if they don't have these devices, how do they exchange oxygen and CO2? They do it through their body surface area. In addition, the digestive tract has ramifications so that nutrients can more easily reach all parts of the body. This gas exchange through the skin makes planarians very susceptible to dehydrationFor this reason they have to live in aquatic and humid environments.
El nervous system is concentrated in the head where several ganglia appear. From these ganglia two ventral nerve cords connected by transverse commissures depart, forming a ladder network. In the event that they are damaged, they may regenerate your body if they lose any part. They are even capable of regenerating their heads.
Special characteristics of planarians
As you can see, these animals are truly special and unique. Most of them have a free life, unlike parasitic flatworms. Living at the bottom, they have to feed on other small invertebrates or decomposing organic matter.
There are planarians that frequent the coastlines and reach their greatest diversity in coral reefs, where they can form large communities. Others have colonized freshwater habitats and some adapt to humid environments on land. The terrestrial ones choose dark and damp places, covered in leaf litter, with markedly nocturnal habits.
They have no cuticle and the body surface is a single layer of cells with ciliaIn some larger species the cilia are reduced. Beneath the skin there is a layer of muscles and glands connected to the surface by pores, which secrete mucus and other substances to stay moist and make moving easier.
To get around they have several options. The smaller aquatic ones use the cilia to propel themselves and move. On the other hand, the larger ones that do not have marked cilia resort to muscle contractions to crawl or swim. The terrestrial ones are capable of throw strings of snot to climb rocks and branches.
In marine polyclades, some planarians present calcareous or siliceous structures microscopic, reminiscent of spicules, providing rigidity and a annular appearance to the body. In addition, they usually exhibit aposematic colorations (conspicuous) to warn of defensive substances and prevent predation.
It is frequently confused with nudibranchs (sea slugs). Although they share a flattened shape and bright colors, nudibranchs are molluscs with distinct features (cerata, external gills), while planarians are flatworms with eversible pharynx and ciliated epidermis without shell.
Often confused with nudibranchs, planarians are only related to them by the sometimes flattened shape of their bodies, being otherwise completely different. Planarians or flatworms belong to the phylum of the platyhelminthes (F. Platyhelminthes) is composed of four classes; three of these classes are made up of organisms that are not free-living, being parasites. These are the classes Monogenea, Trematoda, and Cestoda, which include well-known animals such as tapeworms and liver flukes. Only one class, Turbellaria (turbellarians), includes free-living organisms, whether freshwater or marine, and this article will address them.
Reproduction
Planarians are capable of reproducing both sexual , the asexuallySome reproduce by means of fragmentation of their bodies, cloning themselves by transverse fission. They can also do so asexually by gemmation in certain groups.
However, the most well-known and common form of reproduction is sexual. To do this, planarians perform internal fertilization by means of copulation with another individual. They are hermaphrodites simultaneous, so each specimen has functional ovaries and testes.
In many planarians, in addition to potential self-fertilization, it is common to cross-fertilizationSome species exhibit traumatic copulation (injection of the penis as a sting through the integument) for sperm exchange. After copulation, the fertilized eggs and nutrient cells are encapsulated in ovoid cocoons attached to the substrate; juveniles similar to adults emerge from them.
In freshwater tricladides the predominant species is direct development (without larva), while in many marine polycladids the eggs can give planktonic larvae that mature in the water column before settling.
They lack a coelom, but present specialized gonads. The testes connect via ducts to muscular penises; the oviducts converge toward a vagina or genital atrium. The exact architecture of the reproductive system is key to taxonomic diagnosis of polyclad marine species.
Institutional: https://www.youtube.com/watch?v=0IDO9E6KgE8
Sensory anatomy and nervous system
The cabeza Many planarians have two eye spots or ocelli light-sensitive, acting as photoreceptors; they may be accompanied by atria (lateral folds) with a high density of chemoreceptors. These structures help them search food and avoid bright light.
At the neural level, the brain is a pair of bilobed ganglia connected to two ventral longitudinal cords with commissures, constituting a network in ladder. This system exhibits cephalization, with multipolar neurons, abundant interneurons, and local circuits. Although simple, it meets the functional criteria of a primitive brain and coordinates the animal's behavior.
The molecular similarities of neuronal proteins with vertebrates and the bilobed organization have made planarians key organisms to study the evolution of the nervous system, neuronal plasticity and sensory integration in simple systems.
Regeneration and stem cells (neoblasts)
Planarians have a extraordinary regenerative capacity. Tiny fragments can regenerate an entire animal in days or weeks. The cellular basis is the neoblasts, pluripotent stem cells that represent a significant fraction of total cells and are virtually the only ones that divide in adults.
Following injury, neoblasts migrate to the damaged site, proliferate, and form a blastema that differentiates in the lost tissues. Classical experiments have shown that ablation of neoblasts prevents regeneration and that clonogenic neoblast can restore it, demonstrating its pluripotency.
Regeneration combines growth and morphallaxis (reorganization of existing tissues) to restore proportions and symmetry. Positional cues, such as the pathway Wnt/β-catenin, determine anteroposterior axes; their manipulation can generate heteromorphosis (for example, two heads or two tails). muscle cells They appear to store positional information and reprogram their gene expression after amputations.
This biology has driven the use of planarians in stem cell research, aging (maintenance of telomeres in asexual lines) and tissue regeneration, with potential applications in regenerative medicine and bioengineering.
Ecology, diet and environmental role
In the natural environment, planarians are opportunistic predators and scavengers of marine invertebrates and freshwater. They feed on ciliates, rotifers, small crustaceans, worms, sponges, bryozoans, tunicates and mollusks. They detect prey by chemoreception and envelop food with mucus; their eversible pharynx injects enzymes for external predigestion and suction.
They play a key role in the population control of invertebrates and in the recycling of organic matterIn freshwater ecosystems, their presence and diversity are often associated with good water quality, since they are sensitive to contaminants and chemical alterations.
In coastal and reef marine environments, polyclades contribute to the dynamics of benthic communities, preying on sponges and bryozoans and participating in complex food webs.
Planarians in aquariums: presence, pests and control
In home aquariums, especially sweet water, planarians frequently appear as accompanying faunaThey tend to remain discreet, acting as detritivores and recycling aids. However, in conditions of excess food and lack of predators, they can proliferate.
To prevent and control its population, it is recommended: manage the organic load (regular siphoning, removing remains, moderating portions), promoting biological balance (introduce, if appropriate, species that prey on them, such as some compatible fish or inverts) and avoid aggressive chemical treatments that also harm them mollusks and other invertebrates.
Some fans apply temporary temperature increases or helminthicides in extreme cases, but these measures carry risks and must be carried out judiciously, always prioritizing the aquarium well-beingIn most cases, proper management of the substrate and feeding reduces their numbers without complications.
Species and research models
Several species are references in laboratory and teaching. Among freshwater tricladids, the most notable are Schmidtea mediterranea (model for genetics and regeneration, with sexual and asexual strains), Dugesia japonica y Girardia tigrina (widely used in education). These species are easy to maintain and show robust regeneration.
In the sea, polycladids such as Prostheceraeus y Pseudoceros presentan bright colors, tentacles formed by folds of the margin and a powerful protractile pharynx; they live under stones, among algae, and on benthic colonies such as sponges and bryozoans.
In addition to regeneration, planarians are used in environmental toxicology due to its sensitivity to chemicals and its simple anatomy, with assays that measure epithelial integrity, behavior and regeneration for assess toxicity of substances.
Learning, memory and classical controversies
Planarians are capable of learning by conditioningModern studies show that they can retain information even after regenerate the head, suggesting distributed memory storage or reprogramming of the new brain from systemic signals.
Old research that proposed memory transfer by ingestion of trained planarians do not have current support due to methodological problems and biases. Even so, these works prompted the exploration of the bases bioelectrical and molecular of memory and its persistence during regeneration.
Often confused with nudibranchs, planarians are only related to them by their sometimes flattened body shape, and are otherwise completely different. Planarians, or flatworms, belong to the phylum Platyhelminthes (F. Platyhelminthes), which is composed of four classes. Three of these classes are made up of organisms that are not free-living, being parasites. These are the classes Monogenea, Trematoda, and Cestoda, which include well-known animals such as tapeworms and liver flukes. Only one class, Turbellaria (turbellarians), includes free-living organisms, whether freshwater or marine, and this article will discuss them.
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How is it?
It is a planarian that reaches 3 cm in length. It has a dorsiventrally flattened, oval, and leaf-shaped body. On the anterior part, it has two tentacles formed by the folding of the margin of its body itself; in fact, the name of the genus Prostheceraeus means 'antennae pointing forward'. The rest of the margin also has small undulations, but never as pronounced as the tentacles. It is creamy white, similar to ivory, with black stripes running the length of its body. This is where the species name comes from, vittatus, which means 'adorned with bands'. Its entire body is covered by a layer of cilia, a typical feature of turbellarians, which allow them to move through the rocky substrate and, in some cases, through the sediment. It is worth noting the accumulation of simple eyes (ocelli) in the anterior area of the body, just below the tentacles, as well as their dispersion throughout the margin of its body.
Where do you live?
It is usually found under stones or among algae, in rocky and shallow areas, up to 10 m deepThey have rarely been seen at greater depths. Throughout the Mediterranean, in parts of the eastern Atlantic, the English Channel, and the North Sea.
How does it feed?
Like the rest of the planarians, they are carnivorous and feed on ciliates, rotifers, sponges, small crustaceans, and worms, among others. Despite this, their preferred food is the ascidians of the genus CarnationThey have the ability to detect food at a certain distance by chemoreceptors. These organisms when they are going to eat project the pharynx They have folded teeth in the middle of their bodies and suck in food. Since they don't have an anus, all indigestible food is expelled through the mouth, and digestible food is distributed throughout the complex digestive system, which, being so branched, allows nutrients to reach all corners of the body, always by diffusion.
How does it reproduce?
Like most planarians Prostheceraeus vittatus is hermaphrodite, that is, each animal has both sexes, male and female. It is usual for the cross-fertilization with another individual. These organisms can inject their penis through their partner's skin to exchange sperm. After copulation, the fertilized egg or eggs, along with several nutrient cells, remain to form a small cocoon fixed to stones or vegetation.
Can it be confused?
Due to their particular coloration, they are unmistakable. The error often arises when classifying them, confusing them with nudibranchsIf we take into account that its body is dorsoventrally flattened and that its tentacles are folds of the margin, it is understood that it is not a mollusc but a planarian.
Curiosities
· They have a great capacity of regeneration: A new individual can emerge from each fragment. They can wrap their prey with mucus and pre-digest them with enzymes. To avoid being eaten, they secrete toxic substances and their coloration warns predators. · In some species, behavior has been observed during fertilization in which they attempt avoid being fertilized while trying to fertilize their partner to save energy costs.
Taxonomy
Division: platyhelminthes, Class: Turbellaria, Infrafilo: Rhabditophora, Order: Polycladida, Suborder: Cotylea, Family: Euryleptidae, Gender: Prostheceraeus
How is it?
It is a planarian that can reach 4 centimeters in length. Like the rest of the planarians, its body is dorsoventrally flattened, oval and leaf-shaped. On the anterior part, it has two tentacles formed by the folding of the body's own margin. The rest of the margin has small undulations. The median dorsal surface has a darker bulge than the rest of the body, which is thinner and more transparent. It is brown, with white spots scattered and the margin lighter. It has an accumulation of ocelli in the anterior area, under the tentacles, as well as dispersion of these along the margin. Its body is covered with cilia, a typical feature of turbellarians, which allows them to move through the rocky substrate and in some cases through the sediment. In addition, they can also swim.
Where do you live?
It normally lives sheltered in rocky substrates and on benthic colonies such as sponges or bryozoans. They can be found throughout the Mediterranean and in parts of the eastern Atlantic.
How does it feed?
They are carnivorous and feed on sponges, bryozoans, tunicates, and small crustaceans. They detect food from a distance thanks to chemoreceptorsThey project the pharynx and suck the food, distributing the nutrients through their branched intestineThe indigestible is expelled through the mouth.
How does it reproduce?
Its hermaphrodites and cross-fertilization is common. After copulation, the eggs remain in a small cocoon attached to the substrate.
Can it be confused?
Color variation may exist, but their morphology makes them relatively easy to identify. The most common confusion is with nudibranchs, but planarians lack the characteristics of molluscs and present ciliated epidermis and evertible pharynx.
Curiosities
· Highlights its regeneration capacity from fragments, with limits depending on the size of the segment. · They can immobilize prey with mucus and enzymes to then absorb nutrients. · They secrete repellent substances to avoid predation. · In some Australian planarians, behavior of “penile sword” to maximize fertilization and minimize being fertilized.
Taxonomy
Division: platyhelminthes, Class: Turbellaria, Infrafilo: Rhabditophora, Order: Polycladida, Suborder: Cotylea, Family: Pseudocerotidae, Gender: Pseudoceros
Planarians, far from being simple organisms, constitute a group key in biology for their regeneration, their ecological role and their usefulness as experimental models to understand stem cells, aging, neurobiology, and response to pollutants. Understanding their diversity, morphology, and behavior allows us to appreciate them as valuable components of ecosystems and first-rate research tools.
