More Than 70 New Species of Water Mites Discovered via DNA and Color Markings
By Samuel Bolton, Ph.D.
Picture a bubbling brook, gurgling over a hard bed of rocks and gravel. As a child, you were drawn to explore that strange and stony world, replete as it was with all manner of unfamiliar life forms. Freshly imbued with an unbridled curiosity for all things new, you were a natural naturalist. You noticed how small animals would contend with tumultuous waters, either by clinging to pebbles or swimming nimbly against the rushing current. Through the rippled surface of the water, you glimpsed some of the many different arthropod inhabitants, including crawfish, hellgrammites, and stonefly nymphs. And perhaps, with an inspection container in hand, you observed among the grit even smaller arthropods, including midge larvae and numerous water mites.
There ended one of your earliest zoological escapades, ill-resourced as you were for further investigation. If you had been equipped with a good magnifying glass, you would have noticed that many of those water mites were ornamented with an assortment of different and often striking color markings. Most of those mites belong to a single genus, Torrenticola, which is the dominant genus of water mite in most rocky or sandy streams. Water mite taxonomists often ignore color markings because they can vary dramatically within a species. But, as a young, inexperienced, and untrained naturalist, you probably would have suspected that the different markings can be used to distinguish different taxa—and, in many cases, you would have been right!
As I write this, a monumental tome on the taxonomy of Torrenticola sits on my desk. It is the recently completed doctoral thesis of Ray Fisher, Ph.D., who is based at the University of Arkansas, and its content spans a mammoth 687 pages. I cannot claim to have read it all, but I have marveled at the many beautiful images. This remarkable thesis, which has just been published as a single monograph in ZooKeys, has raised the total number of species of Torrenticola in the United States and Canada from 18 to 90.
Fisher approaches the taxonomy of his water mites in a highly meticulous, integrative, and original way. Species are described and delimited via the careful synthesis of multiple types of data, including geographic distribution, morphology, and DNA. And in cases where only a single specimen of a species was available, Fisher made certain he had DNA as well as other data.
What’s most interesting and innovative about Fisher’s monograph is the importance of color to help describe and characterize species. Indeed, the first thing I notice as I flick through the pages of his thesis is the sheer volume of color photographs. This is not the usual way that new species of mites are presented. Traditionally, mite descriptions feature drawings, which can only capture shape. Color markings, if available, are often ignored or given little attention. As I mentioned above, part of the reason for this is that color markings can vary greatly within a species. But another important reason is that most mite taxonomists use clearing agents to make their specimens transparent enough for viewing under a compound microscope. Consequently, when a mite is first viewed up close, the markings (if they were ever present) are either greatly faded or completely lost.
But Fisher took care to image his specimens before placing them in clearing agents. Therefore, he could add descriptive characters based on the color markings he observed on the main body of the mites. Because Fisher also used DNA to help delimit species, he could determine which color-based characters were taxonomically useful and which ones were not. An integrative approach enabled him to tease out the taxonomic signal from color-based characters that other mite biologists (acarologists) had long abandoned. Moreover, in some cases color provides the only visible way of telling species apart.
Fisher, who has an infectious passion for Torrenticola, has regaled me with numerous examples of his discoveries on color. One of my favorites involves a species from the Ouachita Mountains of Arkansas that has an unusual blood-red color-marking on its body. Because of the way this marking is smeared, it resembles a sunset, hence the name of the species epithet, solisorta. This species is almost identical to one that is broadly distributed in the east of the United States. The only way that they can be differentiated is by the blood-red smear, which is absent in the eastern species. It is comparatively rare that the only feature that can be used to distinguish two species of mites can be noticed under a low-magnification microscope from a mere glance. And yet had Fisher opted to skip the imaging step before clearing his mites, he would have almost certainly missed this feature.
Fisher, who grew up in Kentucky, has an encyclopedic knowledge of arthropods, which must be largely the product of his interminable curiosity. Unlike most of us, Fisher never left that brook. But he is certainly not the sort of biologist who is so greatly invested in other forms of life that he shuns his own species. He has a great passion for teaching and sharing his expertise, and I know few scientists, if any, who are as much admired and appreciated by their own students. It helps, no doubt, that his great body of knowledge is matched by his patience and compassion.
Part of the reason I write this article is that I feel vehemently that we need innovative biologists like Fisher to address one of the most important impediments to describing the world’s species. While taxonomists laboriously describe new species, the many species that have not yet been discovered are becoming threatened with a range of ever-intensifying anthropogenic influences. Taxonomists are clearly up against the clock, and there is an urgent need to increase the pace and detail of descriptions. Detail must increase because, without the discovery and documentation of new descriptive characters, we would remain oblivious to the subtle differences that distinguish a new species from the dozens of already described ones that it closely resembles. And with each new suite of available characters comes the potential to describe many more species.
Fisher’s monograph is in stark contrast to the works of most early mite taxonomists. Whereas Fisher’s descriptions are rich in detail and an assortment of beautiful and highly informative images, early descriptions of mites often contain a dearth of information. In many cases early taxonomists would use only a short paragraph to describe an entire species; figures were often either omitted or inadequate by today’s standards. But the need for more and more detail is, of course, an impediment to the also-urgent need to raise the pace of taxonomic description. However, Fisher’s new emphasis on color-based characters has alleviated rather than increased the workload of description. Color markings are easy to capture with a camera, but they obviously cannot be drawn with a stylus. So Fisher had to photograph his mites, saving himself much work and time in the process. Therefore, Fisher’s discoveries on the importance of color will help to overcome decades of acarological inertia for some or perhaps all colorful groups, such as Torrenticola; there is no longer any case to be made for drawing when imaging does the job far more effectively. And, because color markings also add many new and important characters to the inventory of available characters, Fisher’s discoveries will aid future taxonomic efforts by providing useful, new detail, as well as pace, to descriptions.
Samuel Bolton, Ph.D., is Curator of Acari at the Florida State Collection of Arthropods, in the Florida Department of Agriculture and Consumer Services’ Division of Plant Industry. Email: email@example.com