Phylums are used commonly throughout biology as a principal taxonomic category that works to rank and group animals according to their behavior and characteristics.
One major factor that will class several animals as being in the same phylum is their habitat and where they are most commonly found to reside.
While new species and phylums are being discovered and altered each day, the majority of phylums are made up of creatures that are exclusively aquatic with there being 21 aquatic phylums and several others being quasi-aquatic creatures that can and will still live on land.
This is also the case with rotifers which are aquatic organisms that are extremely varied and make up the Phylum Rotifera.
The Phylum Rotifera is composed of thousands of aquatic organisms with fascinating characteristics, keep reading as we take a deep dive into the Phylum Rotifera and how rotifers act as an organism.
What Is A Rotifer?
Rotifers are the species that make up the Phylum Rotifera, and there are about 2200 variants of this creature.
All of them do possess very similar characteristics and biological make up however, they are all microscopic zooplankton who have an incredibly short life span between one to a few weeks at most.
Rotifers were first discovered in the late 17th century by English writer and scientist Rev. John Harris who studied and observed microorganisms which he had noticed in the rainwater while standing in a ‘Galley-Pot’ next to his window.
Later in 1703, Dutch businessman Antonie van Leeuwenhoek would go on to analyze the rotifer even more and would come to observe and discover most of its other forms and subclasses.
Rotifers have often been described as ‘Wheel animals’ which is the nickname given to them due to the ciliated corona located near the head of the organism.
This is also due to the rapid movement of rotifers and the cilia that makes them appear to whirl like a wheel.
They are all tiny free living planktonic pseudocoelomates, however they do have some unique characteristics that makes them largely stand out from other phylum groups.
All species of rotifers are known to reside in most freshwater ecosystems, which actually makes them an extremely important part of the aquatic ecosystem as they are a major food source for small predators who are either active carnivores or eat through filter feeding.
They have been found to mainly inhabit freshwater but also damp terrestrial habitats too, however the species that do inhabit this space will either live in damp soil or are commonly associated with moist lichens and mosses.
They therefore inhabit a range of ecosystems as long as they are unified by the presence of water, however it has been shown that even in temporarily dried or frozen areas, rotifers can still persist and live fairly normally as they are known for being survivors.
A few rotifer species are also accustomed to living in brackish or salt waters and while most variants are free living, some are truly planktonic meaning they will swim through the water while other kinds can travel by moving across a substrate.
In terms of food sources, the non-parasitic variants of rotifers will feed on decaying organic detritus, algae and dead bacteria, however some have even been seen to eat other rotifers.
They are filter feeders and obtain their food by directing it towards the mouth by using a current created by the movement of the corona.
These food particles travel to the mastax which is their jaw like system, then pass the digestive system and salivary glands and into the stomach before it is released through the anus.
They can also eat some larger organisms such as crustaceans and algae. Their muscular pharynx contains very small but hard jaws used for eating.
These are actually the hardest part of the rotifer and is what will appear on any fossilized organisms.
Appearance And Anatomy
Rotifers are an ancient animal however unfortunately, there have been a lack of fossils from them because of how soft and fragile their bodies are.
They are also incredibly small in stature, being on average between 0.004 and 0.02 inches.
Because of this, it means they can only ingest incredibly small particles of food that are usually up to 10 micrometers in size.
The body of most types of rotifers is made up of the head, trunk and foot.
The head contains the corona while the trunk contains all of the creature’s organs including the bi-lobed brain and small eyespots near the head for sensory awareness.
They are typically free swimming creatures, however the toes and extensions of the foot can secrete a sticky material that forms a holdfast that can help them stick to surfaces.
For most species, these parts of the body will work the same.
The head carries a corona of cilia which works to draw a vortex of water into the mouth which the rotifer will then sift for food.
The food is then grounded by the trophy or jaws which are located right behind the mouth and are found in almost all species within the Phylum Rotifera.
The brain is also known as the cerebral ganglion and is placed on the mastax with some other ganglia also being formed around the foot in species that have one.
Their sensory organs work in order to detect any changes in pressure or light with some of these possessing photoreceptors, mechanoreceptors and even chemoreceptors.
The stomach and reproductive organs are all held in the trunk and the foot contains a ‘toe’ which helps the rotifer attach itself to objects while drifting through the water.
Rotifers actually take up a unique position as an organism because its unique structure could even class it as an animal.
Rotifers have specialized organ systems alongside a complete digestive tract that includes a mouth and an anus.
These features are common animal characteristics and is why rotifers can sometimes be called animals despite being microscopic.
Their body is telescopic being incredibly small and recognisable by its extendible and transparent cuticle covering which suggests that rotifers may be close relatives of roundworms.
The body can take a few forms including taking a spherical shape, being flattened or even resembling a worm.
Their body wall is made of a thin cuticle with tufts of cilia near the anterior to make up the corona portion at the top of the body used for feeding.
Life Cycle And Reproduction
All types of rotifers live extremely short lives, usually being alive for no longer than a week and usually no more than a few days.
In terms of reproduction, this largely depends on the specific species.
Majority of them are truly direcious, having either male or female genitalia, and reproduce sexually.
There are however also a few rotifer species that produce asexually through a process called parthenogenesis which is a method of gender determination which promotes a fast population growth and extensive colonisation.
Parthenogenesis happens by apomixis and it involves the replication of an egg through the process of mitosis which results in diploid cells which are direct clones of the parent.
This process therefore does not require fertilization by a sperm.
This method of parthenogenesis has meant that some rotifer species, such as the bdelloid, have been able to survive centuries reproducing without having sex.
Within this process and for some species, no males will be produced causing the amitotic female rotifers to only produce diploid via mitosis.
There is a chance male forms of the species can be produced allowing sexual reproduction, though this would mean a female would need to produce micdic females capable of sexual reproduction which can still be rare.
The other primary form of reproduction in rotifers is amphoteric reproduction which was first discovered through the monogononta organisms, however it has been seen to also be the case in a few other variants of the Phylum Rotifera.
Females in these species are amphoteric and are therefore capable of producing both male and female organisms.
They will use haploid eggs for the male, and diploid eggs for females.
Female rotifers are much better suited in the reproduction cycle for rotifera organisms, the males are often much more short lived and have no digestive system and a single testis.
Classes Of Phylum Rotifera
Rotifera organisms are separated into three primary classes being the Monogononta, Bdelloidea and Seisonidea.
While there is common ground in terms of general appearance and habitat, with so many species of rotifera there are some with unique characteristics that can differ from those in other groups.
The monogonata class is the largest group comprising over 1600 species.
They are all assumed to be free swimming, sessile and dioecious with only one gonad, while females have one ovary with a vitellarium.
The males within this class are most often structurally reduced and are generally ephemeral, with them usually being present in the plankton usually for just a few days up to a full week.
Monogononta rotifers are either raptorial or microphagous, however very few of them are classed as parasitic.
The appearance of rotifers in this class also differs slightly, the corner near the head takes more of a broad to narrow disk shape or is even vase shaped possessing long setae used for the capturing of prey.
There are a few key advantages the monogononta rotifers have over the other classes: For one, the generation times in mangonadas are much shorter than the other groups, as a result their population level trait responses to selection regimes will be much faster in mangonadas.
They also will regularly and easily engage in sexual reproduction under ideal conditions and sexually produced disposing eggs have been observed to hatch very quickly.
This essentially allows for the recruitment of new genotypes within an incredibly short time span, and although sexual recombination can initially slow down evolution in populations due to ‘genetic slippage’, it has still been shown to strongly increase genotypic diversity in experimental populations which therefore broadens the genetic basis.
Another noticeable feature of monogononta rotifers is that they are the only known metazoans that can be kept under steady state conditions in chemostats.
At a steady state, the size and demographic of the population remain constant because mortality and dilution losses are compensated by food limited population growth.
Bdelloidea rotifers are an incredibly unique creature because they are classed as the world’s most radiation resistant animal due to their incredible ability to absorb and retain massive amounts of radiation without feeling any side effects.
Radiation rays that are absorbed are measured by Grays and while tens of these would be more than enough to kill a human, observations have shown that bdelloid rotifers were found to absorb as many as 1,000 Grays and live a completely normal life.
While this would normally cause major alterations or deficiencies in other animals, the only drawback and limitation faced by the bdelloids is a 10% reduction to their egg laying capacity, however the fact this massive amount of radiation does not make them sterile is fascinating in itself.
Radiation will usually heavily damage the DNA of rotifers by completely shredding it apart, as it does with other animals.
The sheer fact that bdelloids can not only survive huge amounts of radiation, but also that their offspring are fertile as well means they must possess the ability to repair these break ups.
It may seem baffling as to why bdelois rotifers have the most resistant body to radiation on the planet when areas where they reside are usually entirely radiation free.
The reason seems to be in evolution, it has been studied and observed in recent years that drought and radiation pose a similar sort of challenge in terms of how they affect and alter DNA, so the fact bdelloids have adapted their bodies to drier environments in turn seems to have also made them very resistant to radiation.
Just like other rotifer species, bdelloid rotifers are firstly freshwater invertebrates residing wherever there is a good surrounding presence of water.
They also reproduce asexually and have been doing so for about 80 million years.
They possess the unique ability to completely dry out at any point during their life cycle and reside in a dormant state before becoming rehydrated.
This method has actually allowed the species to colonize a huge number of habitats which may seem a bit more unreliable since the water can evaporate easily.
This is what makes them resistant to radiation, but what also sets them apart from the other rotifer classes as bdelloids have evolved to a point where they can not only survive drier environments, they can actually survive in them for extensive periods of time.
The final major class of rotifers is the seisonidea who are quite unique especially in terms of the reproduction cycle and structure of the sexes.
The male and female species within this class are usually of equal size and both possess paired gonads.
They are considered to have diverged from traditional rotifers quite early on.
These species are the only rotifers whose reproduction is strictly bisexual, however they also possess the strange feature of the males of the species being well developed.
Seasoned rotifers can often be referred to as the more primitive group in the Phylum Rotifera due to features such as their paired retrocerebral organ, nervous system and trophi that are less developed than other species.
They also possess peculiar features like a unique mastax, obligatory amphimixis, endolecithal eggs and the complete absence of copulatory organs in males and of vitellaria in females.
They are also noticeable for their large and elongated bodies and their reduced corona.
There are on the other hand some features that they do share with other rotifer species including a lateral antennae, cellular stomach wall and paired gonads.
The head of these species takes a flattened shape with the mastax being visible with its strong muscles.
No eyes are evident and in males, a long spoon shaped groove can be seen but is lacking in females, demonstrating the strange feature in this class of males having well structured bodies.
The neck can also be withdrawn into the ventral area of the trunk and it contains ducts of cephalic glands and of the esophagus and muscles.
The trunk takes an oval-like shape and due to its lack of a hump, gives the trunk a more slender appearance.
How To Culture Rotifers
There are two primary ways to culture rotifers that will work the best and produce your intended results.
Continuous culture involves using a larger container and introducing rotifers into the container at about 10 to 20 per milliliter, along with adding some plankton into the container.
The rotifers will multiply, however as they do, a portion of the population is removed daily and this avoids overpopulation.
This process can also be carried out with the use of excess food where it is fermented in a bucket for approximately 2 weeks and is then used to produce algae which is fed to rotifers.
Batch culture on the other hand is done through a slightly different process and involves inoculating algae with freshwater rotifers at between 20 and 30 degrees celsius.
You can then add phytoplankton or another suitable food source for the rotifers to feed on.
How Best To Observe Rotifers
Luckily, observing and analyzing rotifers is not a very complicated process as long as you have the right equipment, thus while rotifers are certainly invisible to the naked eye, they can easily be observed and watched in their exported habitats through the help of a microscope.
When you are extracting rotifers from a chosen sample, it can be best to use a pipette and draw water from the area surrounding clumps of soil or plant matter in the sample.
The sample should then be quickly transferred to a slide so that the rotifers do not adhere to the sides of the pipette.
It is also advised to avoid using cover-slips under a light microscope because rotifers are known to easily be disturbed and can form into an indiscernible ball, making them far harder to analyze clearly.
You can also decide to add food to the slide to observe the rotifers swimming as long as they do not end up becoming stuck to the slide, and is a perfect way to analyze their general characteristics.
The Phylum Rotifera is made up of rotifers and while there are a huge number of these aquatic animals split into several groups, certain species can possess characteristics and qualities that can marginally set them apart from the others.
This is not as much the case with appearances and general habitat, however in terms of reproduction, anatomy and preferred environment, these can differ quite a lot.
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