Exploring the Skagit River delta north of Seattle on the weekend. Found this beautiful spot where hayfields had just been cut. There is a drainage ditch on either side of the road (see image) that was full of hay clippings – looked like it had been there for several days.
Sampling the water revealed an amazing proliferation of life: at the least, one can count nematodes, rotifers, various types of ciliates, spirotrichs, dinoflagellates, and the many small green ovoid organisms (Euglena?):
Lacrymaria is truly a creature out of someone’s nightmares. Little known and underappreciated, this fast-moving, venomous predator extends its neck seven times its body length to engulf its victims, and has no hesitation in taking bites out of unlucky creatures too large to swallow. The lyrical name of Lacrymaria olor – “Tear of a Swan” – and its slender, willowy profile belie the ferocity of this creature:
Fortunately, this sinuous monster is unlikely to remove a toe the next time you step in the lake – at only 1/10 mm (100 microns) in length, it is one of the smaller ciliates – but it makes up in ferocity what it lacks in size. Lacrymaria is one of the litostomate ciliates, tiny creatures with unique, highly-specialized mouth structures.
Litostomatal ciliates are fascinating and beautiful, but their habits do n0t make them good neighbors:
“Litostomatean ciliates are predatory, with many extrusive toxicysts in oral area. They swallow algae, flagellates and ciliates and even rotifers. The cells are very mobile and some (e.g. Lacrymaria) show extreme rapid changes in shape.” (ref.: Rosati et al., J. Eukaryotic Microbiol. 50: 383, 2003). Courtesy W. van Raamsdonk, http://www.protozoa.nl.
This class is diverse, and includes many carnivorous protozoa, other, anaerobic ciliates that live commensally in the gut of many species, and the only human ciliated intestinal parasite, Balantidium coli.
The exact characteristics of this class have to date been poorly characterized; Gao notes, “…The class Litostomatea has been traditionally rather poorly defined as having an apically positioned cytostome, uniform somatic ciliation and a non-distinct oral apparatus…” Unlike many protozoa, where ingestion can occur at any site on the membrane, this group of organisms sports an apical cytostome or “mouth”. The latter consists of an indentation specialized for phagosytosis, with an underlying support structure composed of two sets of microtubules. The cytosome is usually, but not always, associated with a cytopharnx:
“…a long, tube-like structure that forms the invagination associated with the cytostome…it is typically directed towards the posterior of the cell, often hooking around a central nucleus. The length of the cytopharynx varies during the cell cycle, however the average length is 8 µm. Much like the cytostome, a set of microtubules form an association with the cytopharynx. Two sets of microtubules follow the path of the cytopharynx in cells. These sets of microtubules form a gutter-like structure that surrounds the cytopharynx…”
“…macromolecules…pass into the lumen of the cytopharynx and are transported to the posterior end of the cell where they are put into budding vesicles that are transported to other parts of the cell. The cytopharynx in this way acts much like a straw that sucks macromolecules to the posterior end of the cell…” (From Wikipedia, “Cytostome.”
Genetic analysis has upset and reorganized traditional morphological classification of single-celled organisms. Gao et al., 2016 have recently constructed a classification of the ciliates based on both morphology and several different genetic markers:
On the basis of modern morphological and genetic evidence, the Litostomatea (on the left in the above diagram) have three subclasses:
Haptoria: comprising Lacrymariida, Haptorida, Didiniida, Pleurostomatida and Spathidiida
Trichostomatia: Symbiotic ciliates in the gut of vertebrates. Also includes Balantidium coli, the only known ciliated intestinal parasite of humans.
Rhynchostomatia: The most recently-created subclass, includes Tracheliida and Dileptida. The distinguishing characteristic of this subclass is the location of the mouth at the base of a long proboscis, the latter covered with cilia and possessing toxicysts used to stun prey. The Dileptids (e.g., Dileptus) are aggressive predators, “…equipped with a long, mobile proboscis lined with toxic extrusomes, with which they stun smaller organisms before consuming them…” (Wikipedia, Dileptus). Dileptus‘ hunting habits have been described as “rapacious.” The Tracheliida are similar to the Dileptida but vary in the mouth structure and arrangement of cilia.
Anyone who wants to understand this fascinating class of ciliates should not miss reading Vdacny and Foissner’s “Monograph of the Dileptids (Protista, Ciliophora, Rhynchostomatia)”(http://www.zobodat.at/pdf/DENISIA_0031_0001-0529.pdf). Although focused on the Dileptids, this exhaustive (529 pages), yet very readable treatise is inclusive, well-organized and thoughtfully written. It proves much information, as well as many drawings and images, of ciliate ultrastructure, that generalize to the structure of all protozoa.
If, as I have, you feel discouraged and ignorant because you struggle to classify tiny creatures neatly into phylum, class, order, family, genus and species, take heart. Genetic analysis has made thousands of hours of academic study and morphological classification completely irrelevant. Careful classifications based on overall anatomy, ciliary structure, or the gonadal morphology of worms have been turned on their heads. See Wikipedia’s article on taxonomy to appreciate the evolution of this science, and the degree of present-day flux in our thinking about the slots into which we put the living world around us.
We have a much improved understanding of the structure of the Tree of Life. This does not, however, mean that this new information has simplified the life of the microscopist. All but the most general classification systems are morphing continually as branches die off and new limbs grow. Those still suffering from an inferiority complex should consult the papers by Gao et al or Zhang et al’s “Insights into the phylogeny of systematically controversial haptorian ciliates based on multigene analyses” and admire the beautiful, multicolored taxonomic trees and RNA maps. These would look lovely on a wall or embroidered into seat covers, but tend to induce stomach pain in anyone short of a graduate-level genetic taxonomist. I often tell students, “If you are confused, and think that nothing about this subject makes sense, it may be that you understand the topic.” If you are confused, it may be that the subject is confusing rather than that you are stupid. This is true of most of protozoan taxonomy today. If you don’t understand the scheme, wait until next Wednesday and it will probably be different.
You are better to learn the 36 presently accepted phyla, get a general idea of the different classes of water creatures, learn the common species, then enjoy the pretty animals. Often, just separating an acorn worm (Hemichordata) from an annelid (Annelida) or a thorny-headed worm (Acanthocephala), or distinguishing a Stentor from a Vorticella (and maybe a couple of species of each) is as far as most of us have time to go.
The reader is referred to David Goldstein’s Micscape article, “The Classification of Living Things” for further thoughts on this topic. Those who want to feel much better about their humble understanding should peruse Walter Dioni’s well-researched article, “An Annotated Key for Species of the Family Stentoridae and Two Related Families: An Excursion Through the Taxonomic Maze” and muse on the torturous progression of the taxonomy of the simple Stentor.
If you are STILL feeling badly, carefully read Vd’acny’s recent (2013) paper “The Chaos Prevails: Molecular Phylogeny of the Haptoria (Ciliophora, Litostomatea).” and make sure that you take CAREFUL note of Figure 5. Then consume a moderate quantity of hydroxylated ethane, go to bed, and pull the covers over your head for an hour. When you emerge, you will feel better with the knowledge that much smarter people than you struggle with taxonomy and are busy messing up everything that you THINK you know, so why bother?
Ehrenberg, C.G. Die Infusionsthierchen als vollkommene Organismen, Vols I & II. Leipzig, 1838.
Gao, F. et al. “The All-Data-Based Evolutionary Hypothesis of Ciliated Protists with a Revised Classification of the Phylum Ciliophora (Eukaryota, Alveolata)”. Scientific Reports No. 24874 (2016). http://www.nature.com/articles/srep24874
Vd’acny, P. et al. “Phylogeny and classification of the Litostomatea (Protista, Ciliophora), with emphasis on free-living taxa and the 18S rRNA gene.” Mol Phylogenet Evol. 59:510-22 (2011). http://www.ncbi.nlm.nih.gov/pubmed/21333743