Tiny, wimpy, scorpions
Scorpions are badass, we all know that. With two mean pincers at the front to hold you in place with, before then inflicting you with their venomous sting; these small creatures are the stuff of some sensitive types’ nightmares, and would terrify us all if they were the size of their ancient ancestors which at up to 2.5 meters long were likely the largest arthropods that ever lived.
Closely related to the true scorpions are the whip scorpions; one of the three scorpion sister-groups, which lack a venomous stinger but possess instead a whip-like tail, from which some can spray acid, alongside extra-large pincers for crushing their prey.
Finally, there are the micro-scorpions. Tiny relatives of the aforementioned scorpion-groups, they grow no larger than 3mm long, and look a lot like dinky whip scorpions. These miniature predators live in a multitude of different habitats; mostly preferring to hunt within damp soils but some being found in unusual habitats such as within sandy beaches. They all have one thing in common though: they all hate sunlight.
And so some species of micro-scorpions have during their many million years of existence, found their way into caves. One species (Eukoenenia spelaean) has been found only in a low number of Slovakian caves. Tiny and elusive, they are very rarely studied due to the sheer difficulty of getting hold of the blasted things to work on in the first place, and so investigation on them has been restricted to basic research on their anatomy. The species has comparatively large, scissor-shaped mouthparts and so it had been assumed that they simply fed on other tiny cave-dwelling invertebrates, as most other self-respecting arachnids would be busy doing.
A recent study in PLos one by a team of Slovak and Czech scientists however, has shown that the truth can be more surprising than would be expected. Examining the (above) mouthparts (chelicera) under an electron microscope showed they are more complex than their previous ‘big and shaped like scissors’ description had let on. The chelicera have many small ‘teeth’ fringing them, giving them a comb-like structure.
Through analysis using histology and confocal microscopy, it was shown that the studied wild-caught scorpions had in fact been feeding primarily on cyanobacteria (bacteria which are broadly thought to be precursors to plants). Histology typically involves cutting very thin slices of the animal, staining them, and then looking at the slices under a microscope. Confocal microscopy (a favourite technique in my research on pyrosomes) involves staining sections of the animal, and then making the stained cells glow (fluoresce) by firing laser beams at the sample, whilst observing the fluorescence through a specialised microscope.
These analyses showed the guts contained cyanobacteria, which are known to have existed for at least 3.5 billion years-not bad considering earth is estimated to be around 4.2 billion years old, alongside what are thought to be specialised molecules to aid the digestion of these photosynthetic cells.
The study thus suggests that these mini creatures do not feed on flesh as most other arachnids do, but instead use their tiny chelicera to scrape the cyanobacteria off of the rock in the caves they live in.
This vegetarian lifestyle makes this species very unusual: they are members of the arachnids, the group that contains spiders, mites, harvestmen, and the scorpions. Virtually all of this group feed either on their prey or the blood of the animals they parasitise. The ancestors of these micro-scorpions will have been predators themselves, but likely took to this form of grazing as a response to finding themselves in caves with little prey.
Even when in the most inhospitable habitats, with little food, life finds a way; even if it means once-voracious predators having to instead slum it scraping green cells off of a cave wall rather than go hungry. Evolutions a determined bugger like that.