The Lonely Life of the Cave Collembolan

For a few weeks last year, I had the job of sorting and identifying a collection of Collembola, springtails. Prior to doing this work, I had only the vaguest of understandings of springtail diversity: I knew that there were the round blobby ones, the long thin ones, and the ones that look a bit like sausages, but that was about as far as it went. Needless to say, there's a bit more to it than that.

Pseudosinella immaculata, copyright Andy Murray.

Pseudosinella is the largest genus of Collembola currently recognised, with over 280 described species. The greater number of those species are in Europe and North America, but various Pseudosinella have also been described from other regions of the world (there don't appear to be any from South America, but then I don't know how thoroughly anyone's looked). Pseudosinella species are mostly associated with subterranean habitats, from soil and litter to deep caves, with the highest diversity in the latter. According to a key at collembola.org, Pseudosinella are distinguished from related genera by having reduced eyes (with six or fewer ommatidia, as opposed to the eight ommatidia of other genera), and a bidentate mucro lacking a projecting lamella (the mucro is the claw-like structure at the end of the furcula, the posteroventral prong that forms a springtail's 'spring'). The key also distinguishes Pseudosinella from the similar genus Rambutsinella by it's not having the fourth antennal segment swollen as in the latter, but Bernard et al. (2015) described the species Pseudosinella hahoteana as also having the fourth antennal segment swollen so I'm not sure how reliable that feature is. Pseudosinella is very similar to another genus Lepidocyrtus, the main difference between the two being Pseudosinella's reduced eyes, and more than one author has raised the possibility that Pseudosinella may be a polyphyletic assemblage derived from Lepidocyrtus adapted for life underground.

As well as the reduced eyes, Pseudosinella tend to show a number of other features commonly associated with a subterranean lifestyle, such as a pale coloration and relatively elongate appendages. The claws of the feet also tend to become modified, with the larger of the two becoming longer and progressively narrower (Christiansen 1988). This latter feature is probably an adaptation to movement on the wet surfaces that predominate in caves. At a moderate length, the claws dig into the substrate surface more than those of surface-dwelling forms, allowing greater grip. At longer lengths, the claws are suited to allow the springtail to walk over the surface of the water itself (most springtails float on water surfaces due to their small size and low density, but not all can move with purpose in this position).

Pseudosinella hahoteana, from Bernard et al. (2015). Scale bar = 200 µm.

The aforementioned Pseudosinella hahoteana is worthy of extra attention, as it is one of a half-dozen springtail species endemic to caves on Rapa Nui, the landmass previously known as Easter Island. Many of you will be aware of the ecological catastrophe that beset Rapa Nui following human settlement, as its entire forest covering was cleared away. As a result of this clearing, the native fauna was also all but wiped out; no vertebrates survive, and of about 400 arthropods known from the island only about twenty are indigenous (Bernard et al. 2015). As such, the handful of minute animals clinging to survival in patches of ferns and moss at the entrance to caves represent a significant proportion of Rapa Nui's surviving native fauna.

REFERENCES

Bernard, E. C., F. N. Soto-Adames & J. J. Wynne. 2015. Collembola of Rapa Nui (Easter Island) with descriptions of five endemic cave-restricted species. Zootaxa 3949 (2): 239–267.

Christiansen, K. 1988. Pseudosinella revisited (Collembola, Entomobryinae). Int. J. Speleol. 17: 1–29.