Rotifers: Exquisite Metazoans1 | Integrative and Comparative Biology | Oxford Academic
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. In some recent treatments, rotifers are placed with acanthocephalans in a 5 Predators; 6 Genome size; 7 Gallery; 8 References; 9 External links The exact relationship to other members of the phylum has not yet been. Phylogenetic relationships of Acanthocephala based on analysis of 18S The evolutionary relationships of rotifers and acanthocephalans. Acanthocephalans are most easily distinguished by the eversible proboscis at the I does not solve the relationships between Rotifera and Acanthocephala.
Most species of rotifers are about to micrometers long. However a few species, such as Rotaria neptunia may be longer than a millimeter Orstan Rotifers are thus multicellular creatures who make make their living at the scale of unicellular protists. The name "rotifer" is derived from the Latin word meaning "wheel-bearer"; this makes reference to the crown of cilia around the mouth of the rotifer.
The rapid movement of the cilia in some species makes them appear to whirl like a wheel. At left, you can see a photomicrograph identifying basic anatomical features of Epiphanes brachionus. The general body plan of a rotifer consists of four basic regions: In most species, the head carries a corona crown of cilia that draws a vortex of water into the mouth, which the rotifer sifts for food.
The food itself is ground by the trophi jawslocated 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. It is the cuticle that suggests rotifers are close relatives of roundworms and arthropods. Within the body are the stomach and reproductive organs. The final region of the rotifer body is the foot; this foot ends in a "toe" containing a cement gland with which the rotifer may attach itself to objects in the water and sift food at its leisure.
As rotifers are microscopic animals, their diet must consist of matter small enough to fit through their tiny mouths during filter feeding.
There was a problem providing the content you requested
Rotifers are primarily omnivorous, but some species have been known to be cannibalistic. The diet of rotifers most commonly consists of dead or decomposing organic materials, as well as unicellular algae and other phytoplankton that are primary producers in aquatic communities.
- Phylum Acanthocephala: Characters, Classification and Affinities
Such feeding habits make some rotifers primary consumers. Rotifers are in turn prey to carnivorous secondary consumers, including shrimp and crabs. As well as their morphology and feeding habits, reproduction in rotifers is rather unusual. Several types of reproduction have been observed in rotifers. Some species consist only of females that producetheir daughters from unfertilized eggs, a type of reproduction called parthenogenesis.
In other words, these parthenogenic species can develop from an unfertilized egg, asexually. Other species produce two kinds of eggs that develop by parthenogenesis: These individuals copulate resulting in a fertilized egg developing within the rotifer. The males survive long enough to produce sperm that fertilize eggs, which then form resistant zygotes that can survive if the local water supply should dry up.
Key to Australian Freshwater and Terrestrial Invertebrates
The eggs are released and hatch in the water. If the egg develops in the summer, the egg may remain attached to the posterior end of the rotifer until hatching.
A particular class of rotifers called bdelloids can be found living in almost all freshwater environments, and occasionally in brackish and marine waters. See also the section Phylogenetic quandaries, below.
When major portions of the ICL are thickened the bodywall is somewhat less flexible; such species are called loricate.
Species lacking such thickening are called illoricate. Although the ICL has little taxonomic significance, it is synapomorphic with phylum Acanthocephala, indicating a close phylogenetic relationship. Challenges—Rotifers appear to be very different from acanthocephalans. What is the phylogenetic relationship between these taxa? See also Phylogenetic quandaries. Class Seisonidea is dioecious with gametogenesis taking place by ordinary meiosis. Class Bdelloidea diploid reproduces exclusively by ameiotic parthenogenesis.
Class Monogononta haplodiploid exhibits a cyclical parthenogenesis with the asexual cycle amictic females dominating Fig. Sexual reproduction in monogononts mixis occurs only after being triggered by specific environmental signals e. Mixis results in the production of mictic females which produce haploid eggs or, if fertilized, diploid embryos resting eggs or cysts.
Resting eggs undergo obligatory diapause, eventually hatching as amictic females. Thus, the ecological and reproductive roles of amictic and mictic females differ, and it is of interest to understand the strategies that allot resources to each Aparici et al. Challenges— 1 What is the optimal sex allocation of amicitic vs. Has this character been lost in these species, are local environmental signals unable to trigger mixis, or have sexual periods been missed due to infrequent sampling?
Other challenges—Three other interesting features of the monogonont life cycle await additional study Fig. What is the mechanism of this switch in nature? Pseudosexual eggs have been reported in a planktonic species of Keratella, but this may simply be due to the fact that inadequate sampling has missed the brief period of sexuality Nogrady et al.
The significance of this phenomenon appears to be a risk-spreading strategy under conditions of low food. Does amictic diapause occur in other species? Selection for male dwarfism may be a strategy that maximizes male production with a minimum reproductive cost. Is there a pattern to distribution of male dwarfism in rotifers and is it reflected in their habitats?
In Europe, where the situation has always been better, taxonomic training appears to be healthy, while in North America the number of taxonomists has fallen to a critically low level. In all, what we know about rotifer taxonomy, biogeography, and records of invasions are limited to those places were rotifer workers live or where they have had the opportunity to sample. In practical terms, much of the world, including the U. Challenge—How can we increase taxonomic training and study?
ECOLOGY Rotifers are important components in freshwater ecosystems Until the s contributions of rotifers to the trophic dynamics of lakes were frequently overlooked. However, although individual biomass is minute, large population size, coupled with high turn-over rates make rotifers an important component of food webs Herzig, ; Starkweather, ; Walz, Adding to their importance is the fact that rotifers are eaten by invertebrate predators and are also the first food of fish fry, thereby making their energy available to higher trophic levels.
While recent work indicates that rotifers are significant components of microplankton community structure, the magnitude of that importance has not been completely explored Arndt, ; Berninger et al. Challenges— 1 How important are rotifers to microplankton food webs in a variety of habitats? Population biology Examination of annual population cycles of rotifers from a variety of habitats indicates that species abundance can differ markedly from year to year Herzig, Unfortunately, even short-term sampling schedules weekly can miss the details of population peaks Berner-Fankhauser, Thus variability, coupled with difficulties involved in sustaining intensive sampling schedules, makes studying population dynamics difficult.
Use of chemostats along with particle-counting technology, however, permits systematic manipulation of experimental conditions, thereby allowing tests of population growth models Walz, For example, Boraas et al.
Challenges— 1 How can sampling techniques be improved to assess spatial and temporal variation within populations? Rotifers have been used in pollution monitoring either as bioindicator species or as part of the saprobic assessment system for some time. In addition, because rotifers are relatively easy to culture and sensitive to pollutants they have become important tools in ecotoxicological testing ET.
Bdelloidea - Wikipedia
Other work in this field has focused on cellular and enzymatic biomarkers as indicators of sublethal effects Snell and Janssen, Recent examination of UV radiation as a factor influencing survival and reproduction of aquatic organisms has provided valuable information on the effects of UV on rotifers e. This work indicates that there is a wide range in tolerance, with Keratella spp. Challenges— 1 How robust are these testing protocols when only a few species have been used and when LC50s vary widely even among members of the same genus?
In addition, many species possess a retrocerebral organ, comprising paired subcerebral glands, an unpaired retrocerebral sac, and ducts leading to the coronal surface. Challenges— 1 What is the function of the retrocerebral organ? A few neuro-pharmacological investigations have shown that cholinergic systems function in at least 12 species representing six families and that adrenergic and catecholaminergic systems have been observed in other species, including both bdelloids and monogononts e.
Challenge—Are the details of rotifer neuroanatomy and neuophysiology uniform across a wide array of taxa? Several researchers have studied the effects of environmental stimuli on rotifer behavior including movement Charoy et al. However, efforts by Joanidopoulos and Marwan have yielded some extraordinary insights to the relationship between neuroreceptors and behavior. While holding male Asplanchna sieboldi on the tips of microcapillary tubes, these workers stimulated their coronal sensory receptors using the chemical and mechanical stimuli associated with male-female encounters.
These stimulations triggered specific male mating responses. Challenge—Will we employ these and other procedures such as microelectrode technology, dye injection, cell labeling, photometry, and video imaging to better characterize rotifer neurobiology?
Aging studies Use of rotifers as models for aging makes sense as they are eutelic, have short lifespans, and are inexpensive to culture under conditions suitable for life history analysis. However, although the earliest aging work dates from the s, there has never been a consistent effort to examine a variety of species or to systematically test all aging theories using rotifers.
Some of the studies that have been done have examined effects of maternal age, pattern of reproduction, photoperiod, and various antioxidants on aging and life history. However, of the dozen or so theories of aging, only four have been examined using rotifers Enesco, According to Enesco, the rate of living theory is not substantiated, but there is support for the free radical damage and calcium theories cf. Based on the fact that chemical signals are known to evoke a variety of life history events in rotifers, Enesco also proposed programmed aging as a plausible model for rotifer aging.
In this model, the withdrawal of a reproductive hormone serves as the programmed signal for termination of life. Challenges— 1 Can other theories of aging be examined using rotifers? Coloniality in rotifers is not understood Unlike some colonial invertebrates, rotifers do not share energy resources among colony members. Therefore, the fact that colony formation should be found at all is remarkable; intuitively one would expect that their only interactions would be as mates, competitors, or in predator-prey relationships Wallace and Snell, Challenge—How is coloniality linked to the sessile life-style?
Regardless of size, colonies form by one of three different methods termed allorecruitive, autorecruitive, and geminative Fig. Coloniality may have three important implications Wallace and Snell, Challenge—The methodologies appear to be available to test these hypotheses, but is it possible to do so?