- Last updated on 1999.09.27 by Frans Janssens
Checklist of the Collembola: Progress in Cave Collembolan Taxonomy and Biogeography

Kenneth A. Christiansen, Department of Biology, Grinnell College, PO Box V3, Grinnell, IA 50112-0806, USA

Fig.1. Troglomorphic features
A symposium Taxonomy and Systematics of cave organisms in the 21st Century a look ahead.


This talk was presented at a Symposium on "Taxonomy and Systematics of Cave Organisms in the 21st Century", held at the Convention of the National Speleological Society in Sewanee Tennessee August 4th 1998.

Cave Biogeography

has a great need for more knowledge concerning the phylogeny of cave Collembola (Fig.1) and the relationship between this and the microgeographic distribution of species. The widespread distribution of genera along with extremely limited dispersal of troglobitic or highly troglomorphic species combined with ability to readily distinguish the cave dependent (i.e. clearly adaptive) features from the cave independent features gives a very unusual opportunity to distinguish between lineage members and levels of evolutionary adaptation. We already have a very large database for the form genus Pseudosinella in our BUGS DELTA system which could probably be used for such an analysis.

Tab.1. Quantifying troglomorphy
In 1961 I developed a system for quantifying troglomorphy in cave Entomobryidae (Tab.1). Much later Culver and I applied this system to all described cave Entomobryidae found in North and Central America (Tab.2). When this was done many interesting patterns were found. A most characteristic one is seen here in the Sinella barri lineage, where the most troglomorphic species occupy a small segment and as we radiate out from there a series of spatially larger and less troglomorphic species ranges occur (Fig.2). There is a major need for the application of systems such of evolutionary and geographic analysis to the well studied European and Japanese faunas.

Information, now totally lacking, is needed concerning the relationship between genetic differentiation and morphological similarity brought about by troglomorphic convergent and parallel evolution. In
Fig.2. Levels of troglomorphy in cave populations of the S. barri lineage
particular we need information concerning the relationships between specific nucleic acid sequences and the adaptive features seen in troglomorphy. The functional Collembolist network we now have should greatly facilitate such studies

Another fascinating problem concerns the mystery of how to explain the occurrence of widely disjunct highly troglomorphic, troglobitic species, having very similar cave dependent and cave independent features but lacking any related less troglomorphic cave or surface species. An excellent example of this is Pseudosinella espana and espanita. An explanation encompassing a whole range of now extinct surface and cave species stretching from the caves Southern Missouri where espana lives to Central Kentucky and North central Tennessee (Fig.3) where espanita lives, seems highly improbable. There are a number of cases of similar disjunct forms in both North America and Europe.

Fig.3. Levels of troglomorphy and locations of the four populations of the P. espana lineage
In areas outside of western Europe the inadequate nature of the collections available for study remains a major problem for students of cave Collembola. A few regions have been very well sampled and these include most of the central and eastern parts of the U.S. but even here very thorough collections shows many surprises. Thus when about 5 years ago David Hubbard starting sending me Virginia cave Collembola I thought that I already had a pretty good knowledge of what was there. I was astonished to find in the materials he eventually sent 8 new species of the genus Arrhopalites, which along with the three previously known species represented the largest number of species of the genus found in any state of the Union or any other region of equivalent size. His samples also included 4 new species of Pseudosinella and a scattering of species of other genera, including the first cave species of the genera Oncopodura and Schaefferia found east of the Mississippi. Not all regions prove so rich on close examination. Thus several hundred samples from The Missouri cave project uncovered only a single undescribed subspecies. Nevertheless sampling in some areas is very weak. This is particularly true of the Rocky Mountain States which still represent a large lacuna in our knowledge of U.S. cave Collembola faunas. Many other parts of the world are even less well known and China and S. E. Asia have barely been scratched.

At present the biggest problem in Collembolan taxonomy is the shortage of taxonomists outside Europe. The dominance of European specialists has historical roots. Entomology in Europe has always been less closely attached to applied Entomology than elsewhere. Thus people in research positions were freer to follow their interests rather than the interests of the citizenry. Since Collembola have no parasitic species and very few agricultural pests this led to a small number of students of the group outside Europe. The present day shortage in the U.S. is not as a result of lack of interest but of lack of funding. Every year I get inquiries from somebody wanting to start work on Collembola in the U.S. or Canada. This poses a problem because I know full well if they continue work in Entomology they will almost certainly end up studying Aphids or Corn borers or some other group with direct clear human impact. In addition for reasons somewhat less clear U.S. Soil science has tended to ignore the soil arthropod meso and microfauna and concentrate on chemical, microbial and macro fauna. This also makes it harder to get funding for Collembola research. The situation is even worse if anyone wants to study Collembolan taxonomy. Present funding is such that people interested in alpha taxonomy are severely discouraged from pursuing this. Even within Europe, with an ample supply of taxonomists, some are unable to get employment or funds for research. The huge success of molecular biology, leading to the commercial horn of plenty of molecular biological biotechnology, has so strongly colored the funding picture that research not using molecular techniques is virtually impossible to fund. Collembola pose a very serious problem in this area since so many species are extremely rare and difficult to collect. Earlier, the small size of Collembola and the difficulty of rearing cave forms posed a serious problem but newer techniques have greatly reduced this. Very few investigators have started to work with molecular techniques in Collembola and none of these has an interest in cave forms, probably partly because they prefer to work with species whose collection is a bit less strenuous but probably mostly because they are associated with organizations emphasizing soil science. The great majority of Collembolan specialist work primarily with soil forms.

Eventually the possibility exists of using computers and biochemical analyses to overcome the shortage of working taxonomists but this is unlikely to be available within the next 30 years for Collembola studies. In caves a major problem which must be settled is the nature of the cavernicole species. Is every discrete gene pool to be considered a separate species, or only those which show some evolutionary adaptive unity?

Even if at some time in the future specimens captured in the field may be quickly or even immediately analyzed using molecular techniques, and then referred to a computer base system to see what species or population they most closely resemble in DNA or protein structure, this would only be useful if we could show the relationship between such an analysis and the evolutionarily significant morphology, physiology and behavior - i.e. troglomorphy. At present we are still a long way from this.

Molecular studies have been extremely valuable in determining large scale relationships and at the other extreme relationships between populations of a species of Collembola. They have been much less useful in mid level taxonomy. Molecular techniques have also been particularly valuable with larger organisms having limited distributions. Some such groups tend to draw attention and thus be well collected (fish, amphibia, large crustaceans, beetles and crickets are examples). They have been much less used with smaller cave organisms, particularly arthropods such as Collembola. They have never dealt with the questions of the genetic basis of troglomorphic features and the genetic relationships among widely dispersed troglophile and troglobite species. Advances in technology should make these problems more amenable to analysis, but the question of available funding, and thus research interest, remain unresolved.

What will biospeleological Collembola taxonomy be like in the next 30 years? Various scenarios come to mind and I shall present three ranging from what I consider to be the most dismal to the most bright:

  1. Funding for taxonomy continues to focus more and more on genetic studies. Eventually this leads to a relatively static or diminishing knowledge of alpha and beta taxonomy of many groups such as Collembola. At the same time there is a much greater facility for genetic analyses and funding for such studies. The inability to match this information with troglomorphic features leads to a characterization of cave populations solely on the basis of genetic structure and the move towards abandoning of binomial nomenclature in favor of characterization of populations and eventually species as clusters of genetic formulae, stored and analyzed in computers. The result is a diminished interest in cave microarthropods such as Collembola as model systems for evolutionary and ecological study.
  2. Funding for taxonomy increases as The biological community realizes that this is the essential first step for any ecological or evolutionary analysis; however, this is largely on a contract or piecework basis. This combined with relatively cheap and easily used molecular techniques, as well as increased facility in using computer scanning techniques for taxonomic analysis, results in a melding of genetic, morphological and behavioral information to develop a new sound and widely accepted picture of cave species as groups of evolutionarily and genetically united taxa for the few researchers still studying evolution of cave forms.
  3. An increasing number of researchers recognize that cave organisms represent a unique possibility for recognizing the genetic basis of adaptive features and distinguishing these genes from those representing non or marginally adaptive ones. This combined with the new and easier techniques outlined in scenario 2 results in cave organisms being widely used to study evolution. The result is a very good picture of the mechanisms and results of evolution in cave organisms and these become widely used in general evolutionary and ecological studies.
Fondly do I hope that scenario 3 comes to pass and fervently do I pray that scenario 1 not become reality.

Tab.2. Quantifying troglomorphy in Entomobryidae of North and central America

Note: While this was aimed at Biospelological systematics most would apply to Collembola taxonomy in general.