Rotifers Revisited - Including Sessile and Colony Forming Rotifers
by Dr. Robert Berdan
September 4, 2020
Stephanoceros fimbriatus is a sessile rotifer a member of the clade Collothecaceae (a clade is a group of organisms believed to have evolved from a common ancestor). 100X Darkfield microscopy. This rotifer bears tentacles that function in the capture of small prey.
Introduction
Rotifers are small metazoans, multicellular organisms having about a 1000 cells found in pond water, streams, lakes, soil, moss, lichen and marine environments. They form an important part of the food chain and are used in research to study aging, nutrition and are raised in aquaculture to feed small fish fry. These animals were discovered shortly after the first light microscopes were developed in 1590's. There are currently 2149 species and most live in fresh water. They have the ability to form cysts (xerosomes - Wallace et. al., 2010) that can survive for decades if their pond dries up. Rotifers range in size from about 0.04 to 2 mm or 40 micrometres to 2000 micrometres so a stereo and light microscope are essential to study them. They come in a wide variety of shapes and many appear alien-like. They do not cause any disease or ailments that affect human beings yet they are found almost everywhere around the planet including the Antarctic. The main purpose of this article is to share some new information and images of these magnificent organisms.
Platyius quadricornis rotifer - dorsal view with brightfield microscopy 200X.
Rotifers are believed to disperse by hitchhiking on birds feet and their cysts can be blown by the wind. When these tiny creatures are viewed under a microscope they can fascinate almost anyone for hours. I have been photographing rotifers for almost 50 years and they still intrigue me. For those new to Rotifers you might want want to look at two of my other articles: Photographing Rotifers or Photography of Rotifers II. I use a variety of microscope techniques to photograph them including: Brightfield, Darkfield, Rheinberg, Phase contrast, Fluorescence, Polarizing and Differential Interference Contrast (DIC) microscopy to capture the beauty of these organisms. At the end of this article I include some reference articles for those that might want to learn more about these common but fascinating metazoans.
Interesting fact: Scientists recently discovered a substance made by rotifers (Rotaria rotaria - a Bdelloid rotifer) that can paralyze worms that cause schistosomiasis, a dangerous infection that affects 200 million people world wide (P.A. Newark 2019 Howard Hughes Medical Institute Blog - science paper - PDF). Schistosomiasis is also known as snail fever and bilharzia, a disease caused by parasitic flatworms called schistosomes. Symptoms include abdominal pain, diarrhea, bloody stool, or blood in the urine.
Squatinella mutica rotifer as viewed in bright field microscopy 200X. You can see a DIC image of another species of this rotifer on this web page.
Keratella quadrata darkfield microscopy 200X
Lecane quadridentata - Rheinberg lighting 200X
Bdelloid rotifer stained with Acridine Orange 200X fluorescence microscopy.
Syncheata pectinata
There are 37 species of Synchaeta (Wilke et. al., 2019). About half the species are found in marine or brackish waters. I first observed them swimming in pond water that I placed into a Petri dish. These rotifers rotate while swimming forward and upward toward the surface. On the anterior end of the rotifer there are a number of sensory setae or tufts of cilia. In order to photograph and film them I compressed them slightly under a coverslip to slow them down.
Synchaeta sp rotifers feed mainly on bacteria, algae and other microscopic living organisms. Most rotifers have a transparent cuticle so their internal organs can easily be observed. DIC microscopy 400X.
Synchaeta sp 200X DIC microscopy.
Synchaeta sp 400X by phase contrast microscopy
Synchaeta sp 400X by darkfield microscopy
A comparison of Synchaeta sp A) bright field microscopy B) darkfield microscopy C) phase contrast microscopy D) DIC microscopy
Diagram from Wilke et al. (2019) - PDF
Diagram from Wilke et al. (2019) - PDF
Synchaeta sp 400X by Rheinberg microscopy which is a simple technique that can be added to any microscope for the cost of about $30 - filters sold on Ebay.
Collecting Rotifers
Collecting rotifers requires a pair of rubber boots and a few plastic jars. I also sometimes use a plankton net and a golf ball retriever to extend and move the collection bottles through the water (see The Scoop bottle by T. Pattinson 2017). You will also find rotifers clinging to water plants so be sure to include some in your samples. The larger rotifers can be seen by eye if you hold the jar up to a light source. I like to place some of the collected water and plant material into a shallow plastic Petri dish and then examine the water with a stereomicroscope at 10-40X. This way I can select individuals to put on a microscope slide for examination with my compound light microscope. With practice you can begin to identify some of the species with the stereomicroscope on the basis of how they swim or move. Bdelloid rotifers swim or crawl like an inch worm. Other rotifers like Testudinella patina and Synchaeta pectinata often twirl through the water as they swim. It's fun to watch the organisms swim in three dimensions whereas on a microscope slide they are limited to swimming in two dimensions. The rotifers shown in this article were collected near Calgary in ponds next to the Bow river, from the Bearspaw area in Calgary Northwest and Winchell lake about 30 minutes from where I live in Calgary. All these rotifers were collected during the spring and summer of 2020. Rotifers can be collected in winter under the ice (T. Virro et al. 2009 - PDF) and I hope to trycollecting some in winter.
The Turtle rotifer Testudinella patina brightfield microscopy 200X
The Turtle rotifer Testudinella patina darkfield microscopy 200X
The Turtle rotifer Testudinella patina Rheinberg lighting 200X
The Turtle rotifer Testudinella patina viewed with a polarizing microscope 200X. Polarized light shows banding in the two large muscles. The bands are called A (dark) and I bands (light). Studies of skeletal muscle with an electron microscope reveal that the I bands are composed primarily of thin actin filaments and the A bands contain thicker myosin filaments - to learn more about muscle structure see electron micrographs of a sarcomere on wikipedia.
The Turtle rotifer Testudinella patina viewed with phase contrast microscopy 200X
The Turtle rotifer Testudinella patina by Differential Interference Microscopy (DIC) 200X, note the two red eyes and the bands in the two large muscles which are used to pull the head into the lorica.
The Turtle rotifer Testudinella patina pseudo colored with Adobe Photoshop 200X
Testudinella rotifers were first described by Bory de Saint-Vincent in 1826. They were named because of their shell-like circular lorica. They can retract their ciliated head and foot. They have two red eyes and their musculature is highly visible. These rotifers are found world wide in fresh water and they rotate when swimming. Forty-six species have been described with eleven of them being found in salt water. The are even found living in the film of water in some arctic mosses. Most are female that reproduce by parthenogenesis (B. Talon - Zooplankton of the Great Lakes - Central Michigan University).
Colonial Rotifers
Two families of rotifers are colonial: Flosculariidae and Conchilidae. Colonial rotifers can include from a few to over 200 rotifers attached to each other, though some colonies can exceed 1000 rotifers (Wallace, 1987). The reason some rotifers from colonies is that it is believed that it offers an energetic advantage or predatory defense mechanism. Colony formation appears to deter tactile feeders. No predatory rotifers form colonies and all are microphagous i.e. eat bacteria and small phytoplankton.
Conochilus hippocrepis
I collected these rotifers from Winchell lake near Water Valley, Alberta in August. This small lake is frequented by those that enjoy fishing and bird lovers who like to capture photos of loons and osprey. I used a small plankton net and when I placed a water sample into a Petri dish I noticed a small colony resting on the bottom. I placed the colony on a slide for observation with my light microscopes. Colonies of C. hippocrepis commonly contain 2 to 160 animals. This genus is distinguished by large paired lateral antennae and they are made up of round-vase shaped organisms with a long retractile foot with no toes. The band of cilia on the apical end has a horse-shoe like corona and double band of cilia. When the colony becomes large it may split into daughter colonies. One organism that preys on this rotifer is a Calanoid copepod Parabroteas sarsi. I also captured several calanoid copepods in this lake (see photo below).
Conochilus hippocrepis brightfield microscopy 100X this colony is made up of 18 individuals
Conochilus hippocrepis darkfield microscopy 100X
Conochilus hippocrepis Rheinberg lighting 100X
Conochilus hippocrepis polarization microscopy 100X Note the red eyes.
Conochilus hippocrepis polarization microscopy 200X.
Conochilus hippocrepis phase contrast microscopy 400X The round structures near the base appear to be eggs.
Conochilus hippocrepis DIC microscopy 100X.
Conochilus hippocrepis DIC microscopy 400X.
Calanoid copepod from Winchell Lake 100X Darkfield microscopy - these copepods feed on colonial rotifers like Conochilus.
Stephanoceros fimbriatus
Stephanoceros fimbriatus belongs to the clade Colletheaceae and lacks a typical rotiferan corona. This rotifer produces a short-lived larva which attaches to a substrate and undergoes metamorphosis within 1.5-4 hours after hatching. The larva functions to disperse the species. The larval corona is replaced with the adult infundibulum (a funnel-shaped cavity or structure) and a cup shaped head develops from the anterior foregut. The infundibulum has 5 tentacles that function in prey capture. Collothecid rotifers are sessile on submerged vegetation and are often found associated with Utricularia macrorihiza also called bladderworts a genus of carnivorous plants found in fresh water (A. Hochberg and R. Hochberg, 2017).
The tentacles of Stephanoceros bear a series of short and elongate cilia called setae and their arrangement differs between species. Adults are permanently sessile and are one of the most beautiful rotifers to watch. I have only found three specimens over the summer of 2020.
Stephanocereos fimbriatus 200X DIC microscopy - this is a sessile rotifer enclosed within a mucous sheath.
Stephanocereos fimbriatus 200X DIC microscopy.
Miscellaneous Fresh Water Rotifers
Below are a variety of other rotifers I have encountered by collecting in ponds around Calgary. In some jars they are abundant and in other collected samples they may be absent all together.
Brachionus quadridentatus DIC microscopy 200X.
Brachionus quadridentatus DIC microscopy 200X with foot extended.
Notholca acuminata dorsal head extended, DIC microscopy 200X.
Notholca acuminata lorica dorsal surface DIC microscopy 200X.
Platyias quadracornis - 200X DIC microscopy.
Platyias quadracornis - 200X DIC microscopy - focus stack.
Bdelloid rotifer retracted, stained with Acridine orange 200X - fluorescence microscopy.
Notommata copeus rotifer 200X darkfield microscopy.
Lophocharis sp side view Rheinberg lighting 200X. C.F. Rousselet studied rotifers in the 1800's and invented the Rousellet compressorium to hold rotifers from moving during observation with the microscope.
Keratella quadrata 200X DIC microscopy focus stack.
Notommata sp rotifer species, Rheinberg lighting 200X.
Euchlanis sp DIC microscopy 200X.
Notommata copeus rotifer dorsal view brightfield microscopy 100X.
Notommata copeus rotifer dorsal view darkfield microscopy 100X.
Notommata copeus rotifer dorsal view DIC microscopy 100X.
Bdelloid rotifer? stained with Acridine orange fluorescence microscopy 200X.
Lecane bulla stained with Acridine orange fluorescence microscopy 200X.
Asplanchnopus multiceps rotifer side view darkfield & Rheinberg microscopy 100X.
Mytilina sp side view Rheinberg lighting 200X.
Monogononta rotifer Notommata copeus dorsal view 200X DIC microscopy.
Monogononta rotifer Notommata copeus dorsal view 200X DIC microscopy.
Above is a single celled ciliate - Stentor coeruleus. These one-celled organisms can be 1-2 mm in length. What is unusual about this photo is that it shows an ingested rotifer which is a more complex multicellular microorganism, but in the micro-world size matters. Bright field microscopy 50X.
Above is a phase contrast image showing the rotifer inside a digestive vacuole within the single celled Stentor.
While there may be only a few thouscand species or rotifer described so far, their latin names make it challenging to remember them, but their shapes and behaviours seem easier to imprint in my memory. In spite of looking for them for decades I still have only encountered a few hundred species, but I look forward to discovering new ones in each catch.
Summary
Many rotifers have distinct sizes and shapes and are relatively easier to identify compared to ciliates of which there may be as many as 30,000 species. The size of rotifers is ideal for observation with both a stereo and light microscope and the use of a variety of microscope lighting techniques can reveal different structures. Confocal laser scanning microscopes are currently being used in research in conjunction with fluorescent dyes and antibody-labels to study the development of the rotifer nervous and musculature systems. The discovery that one rotifer offers a substance that can paralyze worms that cause schistosomiasis will hopefully stimulate more research into these interesting creatures. For those that own a microscope you can start looking for rotifers in your backyard by investigating water in a bird bath, soil, moss, tree lichen and even water from your eves-troughs. No organisms are insignificant and any species may reveal undiscovered benefits. For those looking for an interesting hobby that is not too expensive and doesn't require much travel, a microscope can reveal on undiscovered world, and it will also serve as a magnifying lens for your mind. RB
Acknowledgements I would like to thank Dr. R. Shiel from Australia for pointing out some of my errors in identification of certain species of rotifers which have now been corrected and updated. See the reference and taxonomic key to Rotifers of Australia below by Dr. R. Shiel.
References & Links
T. Wilke, W. H. Ahlrichs, O.R.P. Bininda-Emonds (2019) A weighted taxonomic matrix key for species of the rotifer genus Synchaeta (Rotifera, Monogonnta, Synchaetidae) ZooKeys 871: 1-40 - PDF
J. Gao et al. (2019) A rotifer-derived paralytic compound prevents transmission of schistosomiasis to a mammalian host. PLOS Biology https://doi.org/10.1371/journal.pbio.3000485 - PDF
T. Pattinson (2017) The Freshwater Microscopist Part 3. Available on Blurb. I own 4 of his 5 books, the books are easy to read with lots of great ideas and information.
A. Hochberg and R. Hochberg (2017) Musculature of the sessile rotifer Stephanoceros fimbriatus (Rotifera:Gnesiotrocha: Collothecaceae) with details on larval metamorphosis and development of the infundibulum. Zoologisher Anzeiger 268:84-95 - PDF
A. Örstan and M. Plewka. (2017) An Introduction to Bdelloid Rotifers and their Study - www.quekett.org/starting/microscopic-life/bdelloid-rotifers
A. Labuce and S. Strake (2017) An overview of Synchaeta Ehrenberg 1832 (Rotifera: Monogononta: Synchaetidae) species in the Eastern Gotland Basin, Baltic Sea with complementary characteristics for the trophi of S. fennica Rousselet, 1902 and S. monpus Plate from 1889. Aquatic Ecology 66:287-294 - PDF
D. Fontaneto and W.H. De Smet (2015) - Chapter 4 Rotifera - In book: Handbook of Zoology, Gastrotricha, Cycloneuralia and Gnathifera. Volume 3, Publisher: de Gruyter Editors: Andreas Schmidt-Rhaesa pp 217- 300 - excellent review - download PDF
N.S. Iakovenko et al. (2015) Antarctic bdelloid rotifers: diversity, endemism and evolution. Hydrobiologia 761, 5-43 - Springer Verlag
P. Meksuwan, P. Pholunthin and H. Segers (2013) The Collothecidae (Rotifera, Collothecacea) of Thailand, with the description of a new species and an illustrated key to the Southeast Asian fauna. ZooKeys 315:1-16 - PDF
R.L. Wallace, T. W. Snell and H.A. Smith (2010). Phylum Rotifer Chapt 13 in Thorp and Covich's Freshwater Invertebrates. Academic Press, NW. 3rd edition. pp 225-271.
T. Virro, J. Haberman, M. Haldna, K.Blank (2009) Diversity and structure of the winter rotifer assemblage in a shallow eutrophic northern temperate Lake Võrtsjärv (Central Estonia).
Aquatic Ecol 43: 755-764 - PDF
M.V. Sorensen (2005) The Musculature of Testudinella patina (Rotifera, Flosculariacea), Revealed with CLSM. Hydrobiologia 546: 231-238. 10.1007/s10750-005-4202-6 - PDF
R. J. Shiel (1995) A guide to identification of Rotifers, Cladocerans and Copepods from Australian inland Waters. Research Guide - PDF
W.T. Edmondson (1959) (ed) Fresh Water Biology 2nd edition, John Wiley. NY pg 420-507 - read online version free.
Rotifers as Art. All my photographs can be purchased as digital images for printing and personal use starting at only $20 - see my pricing.
Note: Educators and students may use my images freely for reports and teaching, all other uses or for commercial use please contact me. If you use my images I appreciate attribution and a link back to this web site. Note magnifications are approximate - 100X means taken with 10X objective, 200X with a 20X objective, 400X with a 40X objective.
Authors Biography & Contact Information
Bio: Robert Berdan is a professional nature photographer living in Calgary, AB specializing in nature, wildlife and science photography. Robert retired from Cell\Neurobiology research to pursue photography full time many years ago. Robert offers photo guiding and private instruction in all aspects of nature photography, Adobe Photoshop training, photomicrography and macro-photography. Portrait of Robert by Dr. Sharif Galal showing some examples of Robert's science research in the background.
Email at: rberdan@scienceandart.org
Web sites: www.canadiannaturephotographer.com
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Phone: MST 9 am -7 pm (403) 247-2457.