A balanced population of rotifers is a key indicator of a healthy aquatic ecosystem. Thus, rotifers play an important ecological role.
Every member of the ecosystem is essential. Rotifers and other zooplankton function as bottom consumers and food sources for bigger animals. Rotifers are dioecious organisms having either male or female types. The females are always larger than the males sexual dimorphism. In some species, the female can be up to ten times bigger than the male. In many species, males are short-lived and with no digestive system males are degenerate due to the loss of half part of DNA.
These males only have a single testis as the internal organ. Two types of reproduction have been observed in rotifers. Some species consist only of females that produce their daughters from unfertilized eggs, a type of reproduction called Parthenogenesis. In other words, these parthenogenic species can develop from an unfertilized egg asexually. The white arrows point to the visible ciliated parts of a developing embryo. Red eyespots are also visible. Other species reproduce by Sexual reproduction.
Gender determination of these rotifers is unusual. A fertilized egg develops into a female and an unfertilized egg develops into a male. In this case, the male rotifers have only a half copy of genome DNA called haplodiploidy but still can produce sperm. Photo source: Developmental biology interactive. Mating between females and males results in fertilized eggs developing within the female rotifer. The eggs are released and hatch in the water. These resting eggs are pretty resistant in a dormant form that can survive when the local water supply dries up.
The thick shell around the eggs provides great protection. Many species can switch between Asexual Parthenogenesis and Sexual Reproduction according to the condition of habitation. This forms a Life Cycle of rotifers. Usually, sexual reproduction happens when the environment becomes harsh dry, cold, or lack of food.
The rotifer species can be preserved as the resting eggs in a resistant dormancy until the condition becomes habitable. In a nutrient-rich environment, rotifers reproduce by Asexual Parthenogenesis with only females. Upon encountering an environmental cue becoming harsh; for example, during the winter , rotifers adapt to Sexual Reproduction. To do so, asexual females produce sexual daughters via meiosis.
If unfertilized, these haploid eggs develop into males who fertilize sexual females to produce diploid dormant eggs. These resting eggs can survive until the condition is turning favorable for example, spring and then develop into asexual females. Rotifers are unique in that they are born with all their cells. Ideally, rotifers thrive in humid conditions and mild to warm temperatures.
During the harsh condition like winter or dry seasons, rotifers can survive through a process known as Cryptobiosis another famous example is Tardigrades , a. Rotifers enter cryptobiotic life by stopping all metabolic processes and shrinking into a dormant state. When environmental conditions return to being hospitable, the rotifers can quickly return to their normal state. Rotifer eggs can also withstand drying by staying dormant for many years.
Rotifers are also superior to survive radiation damages due to their DNA repair capability. In many aspects, rotifers are truly extremophiles meaning organisms with the ability to thrive in extreme environments.
Observing rotifers is relatively easy. Rotifers are almost everywhere. Finding rotifers is possible in specimens collected from all kinds of humid environments. You can try collecting water samples with bottom sediments from scums and ponds. Rotifers also like to grow on decayed leaves and aquatic plants.
Mosses and lichens are great places to find rotifers, too. You can place a piece of plant samples in a petri dish with some water for several days and observe rotifers migrate out. For example, I successfully saw rotifers and water bears recovered from a piece of frozen moss collected during the winter. You may even see rotifers growing from hatching eggs. Photo source: The Canadian Nature Photographer.
Here is a great resource to do so. Rotifers are important model organisms to study genetics. Scientists are fascinated by how rotifers developed so many different species and morphologic diversity through asexual reproduction. Recently, scientists found that bdelloid rotifers can incorporate foreign DNA from fungi, plants, and bacteria into their genome and creating a mosaic of DNA.
This kind of horizontal gene transfer is rare in multi-cellular eukaryotes, but it has been found that bdelloid rotifers contain a high proportion of horizontally transferred genes.
Scientists believe the contribution of foreign DNA resulted in the diversity of the rotifer family. Some of the foreign genes were defective, whereas others were intact and active. Massive horizontal gene transfer may represent an important force in bdelloid evolution. Photo source: Science.
Rotifers are also a great model organism to study aging. Rotifers have many advantages including, ease of culture, a short life span of 2 weeks — 2 months, and simplified organ systems analogous to humans. For example, scientists studied calorie restriction reducing average daily caloric intake can extend the life span in rotifers.
Rotifers are typically free-swimming and truly planktonic organisms, but the toes or extensions of the foot can secrete a sticky material forming a holdfast to help them adhere to surfaces. The head contains sensory organs in the form of a bi-lobed brain and small eyespots near the corona. Figure 1. Shown are examples from two of the three classes of rotifer. EPA; scale-bar data from Cory Zanker. The rotifers are filter feeders that will eat dead material, algae, and other microscopic living organisms, and are therefore very important components of aquatic food webs.
Rotifers obtain food that is directed toward the mouth by the current created from the movement of the corona. The food particles enter the mouth and travel to the mastax pharynx with jaw-like structures.
Food then passes by digestive and salivary glands, and into the stomach, then onto the intestines. Digestive and excretory wastes are collected in a cloacal bladder before being released out the anus. Rotifers are pseudocoelomates commonly found in fresh water and some salt water environments throughout the world. Figure 2 shows the anatomy of a rotifer belonging to class Bdelloidea. The food itself is ground by the trophi jaws , located just behind the mouth in the pharynx throat.
Trophi are found in almost all rotifers, and are characteristic organs of the phylum Rotifera. The body of the rotifer is externally but not internally segmented. The body is telescopic, with a semi-flexible, extendible, transparent cuticle covering.
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