The Cave-Dwelling Salamander
If, by some whim, you decided to attach a live wire to another human, they probably wouldn’t thank you for it. In fact, it would likely be taken as an act of hostility, no matter the good faith in which you intended the gesture. Humans, it turns out, aren’t particularly well adapted to receive or sense electric currents. Yet one particular cave dwelling species has evolved this exact niche method for sensing its surroundings.
The entirely aquatic salamander, known as the olm (Proteus anguinus)1, is Europe’s most impressive cave-dwelling vertebrate. Or, more accurately, it is Europe’s only cave-dwelling vertebrate. In the Slovenian language, its name, človeška ribica, means “human fish” on account of its colour and fleshy skin. Part of the reason for this is that it has carved out a particular ecological niche for itself – an environment in which other amphibians would be hard pressed to live. This creature was first written about in 1689, where it was mistakenly (I am saddened to inform you) identified as a dragon.1
Classified as “vulnerable to extinction” and being the only species in its genus, the olm is considered an EDGE species.2 Yet due to its unusual characteristics and habitat, this is a fascinating species with many individual quirks. For example, owing to the dark depths in which it likes to reside, the olm is blind. For many animals, this would be quite the disadvantage. But this salamander has managed to power through as we’ll see below.
Sensory Perception & Electric Currents
The first, and perhaps least unique, method the olm has of sensing its environment is that of chemoreception.3 Now this may not be listed in the Child’s A-Z Guide of the Five Senses (quite a short book, you understand), but in the broadest sense, chemoreception is what we refer to as our sense of smell. The evidence for this chemical signalling was first given by the fact that sexually active males were found to defend their specific territory,4 and intruders actively avoided these sites. However, any nearby rivals did not avoid non-sexually active males. This was taken to mean that the sexually active males released pheromones (a kind of chemical signal) to warn off other members of the same species. Unsurprisingly, creatures so used to darkness quite rightly prefer to maintain their privacy! Their olfactory receptors – or ‘noses’ – are so sensitive that they can detect pieces of prey as small as 0.1g (that’s about one fifth the weight of a bean, in case you were wondering).5
Olfactory reception works because of the unique shape of individual molecules, which likely bind to specific proteins in the membrane of the animal. So even small amounts of a chemical can still bind to these proteins and cause a signal to be sent to the brain. These creatures can even detect the difference between living and dead prey – just based on the chemical signal!5
Now, an arguably more exciting method of sensing brings us back to electricity. Olms can use a method known as ‘electrolocation’ to navigate in cave systems. They have electrosensory ‘ampullary’ organs6 that can register weak electric fields. It is usually only aquatic organisms that have this organ, as water (being a polar molecule) can dissolve charged chemicals known as ions. Most commonly, we will see sodium and chloride ions in seawater, and we simply refer to this as salt! These ions can carry charge, and thus electric current, through water, making it the perfect environment in which to receive chemical signals. It has even been theorised that the olm can detect the earth’s magnetic field! To understand how, a little bit of physics understanding is required – but don’t worry, it’s not too tricky! Electric currents (containing charged particles) can create a very small and very local magnetic field. As the olm swim, they generate a small electric current which has its own magnetic field. The earth’s magnetic field can interfere with this local one, and causes a disturbance which the olm can detect via its electroreceptors – pretty neat!6
Perhaps more impressively, olm can likely even detect other members of its own species by this method – detecting what’s known as a bioelectric field. This is caused by ion-pumps in the gill membranes used to transport vital ions such as calcium (Ca2+) or potassium (K+).1
Despite all these clever methods of sensing, the olm still possesses something slightly akin to an eye. Although they have evolved for a life in darkness, they retain functional photoreceptors (light detecting sensors). These photoreceptors contain the protein ‘melanopsin’7, which is sensitive to short wavelength blue light.8 In other words, these creatures are unlikely to detect any light under water as blue light is reflected more easily by water (hence why the ocean appears blue). However, should these animals ever surface, a special mechanism will occur (and the same process happens in humans whenever we open our eyes!). Interestingly, the molecule called ‘retinal’ within melanopsin takes in UV light from the sun, which provides enough energy to break a double bond – a very strong chemical bond which cannot rotate. When this bond is broken, the chemical can then twist about the bond, and form a different form or ‘isomer’ of itself. This causes a signal to be sent to the brain and the olm ‘detects’ the light.
If they have no need to ‘see’, then why would this mechanism be useful? Well, when the olm does surface, and so detects light, the production of melanin is indirectly triggered.9 Melanin, as you may know, is a dark pigment often found in skin. So when an olm is placed in light for just a few hours, its normally pink skin will turn dark, and so protect it from the harmful UV rays! (These were the original self-tinting glasses, didn’t you know?).
Pollution Threats for the Olm
One of the reasons these creatures are listed as vulnerable is the often heavily polluted waters they live in. Due to such an abnormally low metabolism, they may be able to survive for up to ten years without food,10 but changes in the environment are much less predictable. For instance, known pollutants in olm habitats include chlorinated hydrocarbon pesticides, or PCBs (polychlorinated biphenyls).
PCBs were used by electrical manufacturing companies due to their inability to conduct electricity, and high thermal stability.11 As late as 1983, these chemicals were dumped into the Krupa river in Slovenia where this species can be found. Recently, PCBs were found to be as concentrated as 1500 micrograms per gram – which is almost thirty times higher than normal levels in the Krupa river!
These chemicals were found to be highly concentrated in the fat and liver tissues of the olm. The most abundant form of PCB in water was a version known as a di-ortho substituted PCB, whereas in sediment the chemical was mono-substituted. Di-ortho substituted PCB has more chlorine atoms on the molecule, and hence it will be more polar (soluble in water), although the weight of the molecule may also contribute to distribution. This has consequences when digested by the animals. For instance, PCBs can bind to lipid (fat) molecules and be incorporated to tissue – the strength of this binding is dependent on the number of chlorine atoms on the molecule. As such, it was found that the olm could only partially get rid of the low chlorinated PCBs – whereas the di-ortho substituted PCB molecules accumulated to what would normally be considered ‘dangerous levels’ and took longer to degrade.11
Another type of dangerous pollution is that of metals. For example, cadmium and mercury are often in high levels near olm habitats – and this can be worrying for both their welfare and survival. They do, fortunately, have some adaption to this. Their livers can produce a protein called metallothioneins which helps to detoxify these harmful metals by chelating (binding) to them.12 The proteins stop the toxic Cd2+ ions from the metal from entering further into the body’s system.
Although it is fortunate that these creatures have some adaption, it is likely that in the past, such large-scale pollution is what led to these animals becoming vulnerable in the first place. As such, it is vital that we try and stop pollution from reaching these unique animals in whatever ways we can, lest we drive them to extinction…
Not So Cool, Now: Climate Change Dangers
We may all be micro-focussed on the impacts climate change will have on visible biodiversity. But it is so easy to forget about the hidden yet mighty olm. ‘Out of sight, out of mind’ as the saying goes, and indeed they often are.
The embryos of the olm normally develop at temperatures between 9-11°C, up to a lethal temperature of 20°C.12 However, embryos reared at 17°C became more vulnerable to external threats, showing how vital it is to have a reliable temperature in olm habitats. This is only as controllable as climate change – meaning this should be a real focus for the protection of these creatures, as well as the consideration of pollution.
Now you can imagine that deep underground waters would not become too hot, particularly not in the Northern hemisphere. Yet even today, high temperatures of up to 14°C are noted. With global warming, local climates could become unpredictable and these wonderful creatures could become extinct.
Do They Really Deserve This Much Fuss?
If you’ve managed to read about a creature that can sense by electricity, self-tint, and feel the earth’s magnetic field – and this question still needs answering, I may have failed my task. Simply put, these animals are an EDGE species because of how unique and vulnerable they are. We can continue to focus on the easy animals – the ones we can visibly see suffering – but we must not forget about the creatures that are out of sight. The ones hidden by millennia of evolution in deep, underwater caves.
The olm is a marvellous species, one that will last for many more millennia, but only if we act now to protect this species.
Here at WAWA Conservation, we value the natural world for what it is and we believe that the variety that lies in all earth’s biodiversity makes the world a more wondrous and better place to be. If you too want to help save weird and wonderful animals from extinction, then please consider supporting WAWA and together we can help keep the wild weird.
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References
RSC Referencing (https://edu.rsc.org/download?ac=14556)
1 The Olm: https://en.wikipedia.org/wiki/Olm (accessed: 23/1/2021)
2 IUNC Redlist: https://www.iucnredlist.org/ (accessed: 24/1/2021)
3 J.P. Durand, J. Parzefall, B. Richard, Behavioural Processes, 1982, 123-134
4 O. Guillaume, Journal of Zoology, 2011, 78, 167-173
5 P. Dumas, B. Chris, Behavioural Processes, 1998, 43, 107-113
6 Bulog B., Schlegel P. et al. (2002) Non-visual orientation and light-sensitivity in the blind cave salamander, Proteus anguinus (Amphibia, Caudata). In: Latella L., Mezzanotte E., Tarocco M. (eds.). 16th international symposium of biospeleology; 2002 Sep 8–15; Verona: Societé Internationale de Biospéologie, pp. 31–32.
8 Melanopsin: https://en.wikipedia.org/wiki/Melanopsin#Structure (accessed: 24/1/2021)
9 Chromatophore: https://en.wikipedia.org/wiki/Chromatophore#Melanophores) (accessed: 24/1/2021)
10 160. Olm: http://www.edgeofexistence.org/species/olm/ (accessed: 24/1/2021)
11 M. Pezdirc, E. Heath et al., Chemosphere, 2011, 84, 987-993
12 J.P. Durand, B. Delay, Journal of Thermal Biology, 1981, 6, 53-57