Rotifers need to eat too!
by AJ Leiden
BIOL/WATR 361, Spring 2015
Bdelloidea and Monogononta
The invertebrate phylum Rotifera is an ubiquitous presence in freshwater environments around the world. Especially in Europe and the Americas where there is freshwater, even in small temporary puddles, there are almost certainly rotifers. One factor that contributes to their pervasiveness is the diversity of feeding strategies exhibited in rotifers. Some rotifers function as sessile filter feeders, while others are motile predators that consume a variety of other aquatic microorganisms. These different approaches to feeding and the abundance of rotifers in freshwater ecosystems results in rotifers holding an important and varied role within freshwater food webs everywhere. This scope of this report is to cover the methods which rotifers use to feed and reflect upon how that affects their role in food webs.
The phylum Rotifera contains three classes: Bdelloidea, Monogononta, and Seisonidea. The vast majority of the bdelloid and monogonont rotifers live in freshwater. Those in Seisonidea are commensal on marine crustaceans only and will not be discussed within the scope of this paper. A unique aspect of rotifer anatomy is at the anterior end of a rotifer there is a corona: a ciliated structure rotifers use for movement and to create water currents towards their mouths. Just beyond the corona lies the mastax, a muscular structure containing trophi, which resemble a pair of antlers designed to grind up any food the rotifer may collect. The assemblage of the trophi can be represented by eight different models. Six of these models are applicable to the Monogononta, and of the other two, one of each pertains to the Bdelloidea and Seisonidea (Melone et al., 1998). The variation of the trophi within Monogonanta is indicative of the variety of feeding patterns exhibited by those rotifers. The homogeneity of the bdelloid and Seisonidea rotifers is representative of the overall similarity of feeding methods within each class.
Most bdelloid rotifers feed on particles, whether through filter feeding, or scraping the sediment of a river/lake. Bdelloids typically feed on a variety of materials but selectively choose what they feed upon based on size of the particle (Melone et al., 1998). Analysis of bdelloid trophi with a scanning electron microscope confirmed a general homogeneity of the design of bdelloid trophi with slight differences among the orders of bdelloid rotifers (Melone et al., 1998). This pattern of selective filter feeding is found within monogononta as well. A study was conducted on different species of the genus Branchionus, in the class Monogononta. Cultures of the rotifers were fed algae of several approximate sizes and beads of varying μm-scale diameters. They found that members of this genus ingested food very selectively on particle size, with no other apparent criteria controlling the selectivity of feeding (Rothhaupt, 1990). The study also found that body size of varying individuals of the same species did not affect the selected particle size for feeding (e.g. larger individuals did not select larger particles). The author suggested this may be due to the eutelic body plan exhibited by rotifers. Many rotifers that live in benthic environments survive by scraping food off of sediment or browsing by collecting particles floating through the water (Ricci & Balsamo, 2000).
While it is a common practice among rotifer species, filter feeding is not the only method that rotifers use to feed. Some rotifers have been reported to feed by piercing algae and taking in the cytoplasm from the algal cells (Ricci & Balsamo, 2000). Many rotifer species also function as predators on other members of the freshwater microbial community. As a result of the general lack of discrimination towards food sources beyond size, many filter feeding species can and do ingest other microorganisms. Herbivorous rotifers have been known to feed on small ciliates, bacteria, protozoans, and other microbes. The opposite is also true, predatory rotifers will feed on larger algae particles indiscriminately (Ricci & Balsamo, 2000). Many rotifers are direct predators on other members of the microbial community. Members of the genus Asplancha will even consume other rotifers, both of different species or smaller organisms of the same species (Nandini et al., 2003). Rotifers are known to predate on bacteria quite frequently. Some rotifers are often found in wastewater where they feed on the abundant bacteria that consume the waste organic matter and have since found use within the wastewater treatment process (Arndt, 1993). Rotifers have also long been known as predators on ciliates. Some rotifers have been reported to consume up to 50 ciliates per day (Gilbert & Jack, 1993). Add in that rotifers readily consume protozoans as well, and it becomes clear that rotifers have a large role in the structure of an aquatic microbial food web. Predatory rotifers also exhibit a large amount of selectivity in what they eat. For example the genus Asplancha mentioned before will preferably consume rotifers of the genus Branchionus because they are of an appropriate size, reproduce rapidly and are generally abundant, over smaller colonial rotifer species or ciliates (Nandini et al., 2003).
A study on the feeding of bdelloid rotifer species found within biofilms of a river also demonstrated a high degree of selectivity in food consumption. The epilithic biofilm of a river is the microbial community attached to the rocks/sediment at the bottom of the river. It is a very ecologically active component of a river ecosystem. The study found that rotifers consumed primarily cyanobacteria and diatoms, heavily favoring the cyanobacteria (Mialet et al., 2013). Studying relationships such as these can be important to understanding how freshwater microbial communities function. Research such as this is of particular interest when one considers the water quality issues associated with some species of toxic cyanobacteria.
Rotifers are found in such a great diversity of freshwater environments that there is a large capacity to research the roles of rotifers in food webs of many different freshwater systems. While the microbial community on the benthic material of a river and the planktonic community of a hypertrophic lake can be two very different communities structurally, both contain rotifers acting as predators upon other organisms. A study on the feeding habits of two planktonic rotifer species, Anuraeopsis fissa and Branchionus angularis in hypertrophic lake environments provides similar insight. Hypertrophic lake environments are predominated by phytoplankton, many of which are incapable of being consumed. The study demonstrated that these two species readily consumed bacteria within this environment, functioning as a means of transferring bacterial biomass through the food web (Miracle et al., 2014).
Despite the evidence of the abundance of rotifers globally, there is not a large quantity of information on what factors affect the distribution of rotifers. Primarily speaking, rotifers inhabit either benthic substrate, biofilms, or are a part of the planktonic community of a freshwater body (Ricci & Balsamo, 2000). However, the environmental settings that affect which species of rotifer appear where have not been examined very much. Some research has indicated that the largest factors influencing rotifer populations include pH, total nitrogen, food availability, predation and even some competition from other microorganisms (Devetter, 1998). That study occurred in an artificial reservoir and is generalized, so the information may not be wholly indicative of common themes that influence rotifers everywhere. Other research has also suggested that rotifer distribution follows food sources such as organic material and microbial biofilms (Ricci & Balsamo, 2000). A quick consideration of these suggestions will find that it is quite logical for rotifer distribution to be significantly influenced by food availability. This goes hand in hand with the discussion of the varying feeding methods that rotifers utilize. If a river or lake were to have suitable sediment environments at the bottom, benthic rotifers that browse, scrape and filter feed would certainly proliferate. If an environment has a large microbial community with various ciliates, bacteria, or protozoans, then predatory rotifers are likely to prosper.
Rotifers are a phylum with several distinguishing traits that allow them to thrive in freshwater environments all over. They have a wide range of feeding patterns that result in species of rotifers being found in a considerable number of environments within freshwater ecosystems. While there is plenty of knowledge of the general behavior and ecology of rotifers there still remains a great capacity to explore the conditions that influence rotifer distribution and the role that rotifers play in food webs.
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