A Brief Overview of the Species
by Chip Hannum.
Just how many species of triops there are is a good question, and one that in spite of researching several papers on them I certainly don't feel confident to answer with any authority. There are a number of problems involved with a definitive list of who's who in the triops world. First, it's now been close to 50 years since anyone has attempted a serious review of the notostracans.
The last definitive paper on the
notostracan species was published in 1955 by Alan Longhurst. His paper was a
landmark in taxonomic classification of Triops and Lepidurus species and
is still the basis for defining most species. However, things have changed in
the intervening 45 years. A few new species have been discovered, such as Lepidurus
mongolicus. Some species that Longhurst chose to lump with other species or
define as a subspecies either have been explicitly redefined as separate species, notably
Lepidurus packardii, or in some cases other scientists have simply ignored Longhurst's
classification, as appears to be the case with Triops numidicus.
Other problems arise in that many of these species haven't been described from wild specimens since the 1900s or even 1800s - these species may be extinct or they may have never existed at all as separate species and merely missed proper classification by Longhurst because much of his classification was based upon published descriptions and their description was inaccurate.
Further, it is conceivable that separate but rare species "disappeared" into a similar species due to inaccurate or incomplete description. The problem is compounded even more in that with the exception of a few very well studied species, such as Triops cancriformsis and longicaudatus, most species have received very little attention from scientists. This results in there being next to nothing known about them and often little or no agreement about what is known. One scientist may still regard a certain species as existing in a geographic region while another considers the species description outmoded and invalid. It's all a very fuzzy and unfocused picture.
The second major problem with making any definitive statements on the notostracan species is that modern genetic analysis is calling even the agreed species classifications into question. The triops are remarkably similar in morphology. With the exception of a few distinct characters such as the supra anal plate on the telson of Lepidurus species, or the presence or absence of the second maxilla amongst various species, their gross anatomy is pretty much the same across the board.. You can use a general anatomical description to cover all the species without problem.
The obvious characters such as carapace shape, body ring number, and appendage number often vary significantly within defined species. You may have two triops of very similar appearance that are regarded as different species while two very different looking triops are considered the same species to an expert.
As a result of this conservation of morphology, species identification often comes down to rather obscure characters such as the absence or size of spines on a specific region of the carapace. Under current classification, two groups of triops may be classified as the same species due to certain physical characteristics even when geographic isolation and reproductive traits logically argue they're not. The most notable example of this situation is Triops longicaudatus. T. longicaudatus is found throughout the North American west and southwest, parts of Central and South America, the Galapogos Islands, Japan, New Caledonia, etc. There are at least two distinct morphological forms, long and short, characterised by distinct differences in body ring number range.
There are bisexual, hermaphroditic, and unisexual populations of every variation within T. longicaudatus. All are considered to be a single species (Longhurst proposed that the New Caledonian population was a subspecies but this appears to have been ignored by the scientific community at large). The argument has been that their eggs are so easily dispersed that speciation is unlikely with the triops and that reproductive differences are somehow inconsequential phenotypic variation. Genetic analysis is revealing the absurdity of this position. In just a few small studies, one in Japan and three in the U.S., at least five genetically different subspecies within T. longicaudatus have been found. Similar genetically different species have been found in L. couesii and T. newberryi. These are species that, although classified together, have been distinct lineages for millions of years and are reproductively isolated. The indication is that many triops species are actually be composed of multiple subspecies that currently remain undefined. Only with time and more research will a more complete picture of the the number and variety of triops species be accurately known.
At the most fundamental level, the division of the notostracans into the genera of Lepidurus and Triops is certainly valid. Not only is there the obvious morphological difference of the supra anal plate, life cycle, general body conformation, chromosome number and genetic analysis indicate these two genera have been separate for much of the triops' history on Earth. Among notable examples of their differences, Triops species require drying for the significant portion of their eggs to hatch whereas Lepidurus species eggs survive dessication but do not require it for hatching.
As a result of this, some Lepidurus species have adapted to life in shallow lakes which are permanent or very nearly so, either experiencing short, seasonal dryings or freezing every year, but otherwise remaining stable. These Lepidurus populations hatch out in the spring and survive until creeks or floods allow larger predators to temporarily establish themselves in the summer - it would be difficult to impossible for Triops to survive in such an environment. Similarly, Triops tend to be found in hotter climates where the pools are guaranteed to dry predictably versus the cooler climates many Lepidurus are found in. Due to the lack of required dessication, many Lepidurus species are adapted to a life cycle with multiple generations present while most Triops live in single generation populations. Genetically, all Triops species have a haploid number of n = 4 ( with the exception of T. australiensis, which has a haploid number of n = 5, the Australians do it differently as usual). The Lepidurus species haploid number is n = 6.
Further, the superspecies classifications will remain stable for the most part. The genetic analyses, while indicating separate genetic lines, aren't indicating that these cryptic species haven't shared an ancestor with their current named species in 100 million years but rather spans like 15 million years, which is a fairly minor thing when you're as stable of an order as the notostracans. A single species like Triops longicaudatus will find itself split into multiple subspecies but most will remain some form of Triops longicaudatus insertyournamehere. In the end, it's going to come down to getting a far more complete phylogenetic picture based on actual genetic analysis and working from there.
Inevitably, politics will play a role as well in the reclassification of species. If a triops goes from having a range across eastern Europe to being located exclusively on a small mountain range at an altitude of 1000 - 1500 meters in France there may be ramifications involving international endangered and threatened species laws and agreements. Just such a controversey occurred in the U.S. with Lepidurus packardii. Longhurst had claimed that it wasn't a separate species but rather a minor variant of L. apus., not even a proper subspecies. The difference is significant, as L. packardii, it is a species limited to a very small geographic area in California which is under frequent development, as L. apus, it's merely a representative population of one of the most common and widespread species in the world.
It took hearings with testimony from multiple experts before the U.S. government for it to be officially recognized as a separate species and placed on the endangered species list. As vindication, genetic analysis has verified that it is indeed a separate and distinct species. All very well and fine for L. packardii, but the problem scenario is what will happen when it appears that "everyone is special". If it turns out that populations separated by only a few hundred miles actually represent separate subspecies or, worse, separate superspecies, then these little tadpole shrimp could find themselves the equivalent of the spotted owl in the court rooms. The show is far from over and it may be decades before any sort of modern consensus is reached.
The result of this situation is that the species information presented here is only so accurate. There may be a species listed here that in a 1963 paper that I haven't seen was declared definitively to actually be an aberrant specimen of some other species. I am also not going to make any great effort to sort out the fine gradations of already recognized subspecies and will group them with their superspecies. This is partly because I am not certain at all times which of these subspecies names are still currently considered valid and, moreso, because many scientists publishing on triops aren't certain themselves. There may be one researcher who lumps all of the triops he's looked at under T. cancriformis while another makes the distinction between populations of T. cancriformis cancriformis, T. cancriformis simplex and T. cancriformis mauretanicus.
For those not familiar with the three name nomenclature system, the third name is used to distinguish between subspecies. T. cancriformis cancriformis is the "standard" T. cancriformis and the others are subspecies. Something is declared a subspecies when it is different enough to be distinctly identified from the major species but still not so different as to represent a clade of its own. A good example would be the difference between the grey wolf Canis lupus and an American Husky Canis lupus familiaris
The below table lists all known species that I have found reference to and their subspecies. Clicking on the names will take you to another page with more complete information where possible.
|Lepidurus batesoni||Triops granarius|
|Lepidurus couesii||Triops newberryi|
|Lepidurus lemmoni||Triops numidicus|
This section, more so than any other on the site, is unfinished and in need of contributions from others. If you have reference to a Lepidurus or Triops species not listed, or a reference on why a certain species or subspecies shouldn't be listed, please email Chris at CodeMonkey@vt.edu Donations of species photographs would be greatly appreciated as well.