For hardcore biologists this species will need little introduction, but I’m sure many will probably think I have just taken some pictures of sentient candle wax. The olm is, in fact, an aquatic, blind, neotenic cave salamander. It is endemic to the karstic limestone cave systems of the Dinaric Alps which run from the tip of NE Italy through Slovenia, Bosnia & Herzegovina, Croatia and to the north of Montenegro. Finding the olm was a major (though somewhat ambitious) target for me during my summer in Croatia. Without being a cave diver, your chances of seeing one are limited to when they are flushed out into rivers after heavy rain or a chance encounter in still or slow-moving water deep in the caves. I did end up seeing one in the latter of these scenarios and it was truly an emotional experience.
I thought my lucky charms had expired as my herpetology surveys were proving to be less than fruitful. This had gotten to the point where people were actively avoiding me and my surveys in order to increase their chances of seeing things. Worse still, that method was actually working… Towards the end of the trip, one student group came back bearing the news. They had seen an olm while on a cave excursion with Koen. Luckily, I was assisting our lead scientist, Dušan Jelić, with moving equipment for a cave dive he had planned later that afternoon in a nearby system. We agreed that I would help him and afterwards go and search for the olm with some others. We headed into the cave and retraced the steps of the other team. Approaching the subterranean pool, the excitement was high. This species was once recorded not moving for 7 years, surely it would still be there after three hours…
We arrive at the pool and there is no olm. Next thing I know, I’m taking off my boots, unzipping the bottom of my trousers and sliding through the cave mud into the 10oC water, desperately searching for the olm under some ledges not visible from the shore. Around the same time the freezing water had caused my genitals to retreat all the way up to my throat, I can hear Tom‘s voice start to tremble “I think our luck is up”. Defeated, I crawl onto the muddy bank and begin rambling – “There’s no way I can go back to camp now, I’ll never live this down. Nobody can be this unlucky. Just take my headtorch and leave me to be absorbed by the cave”.
Just as I was preparing to donate my body to the isopods, the olm swam out of a crack in the floor to come and see us. Magical!
Olms are weird as hell. Every single thing I learnt about this species only ever made them seem more improbable. Let’s take a look into what is known about the olm:
This species is the only member of the Proteus genus and diverged from other amphibians 110 million years ago. To put that in context, flowering plants only evolved 130 million years ago and humans split from chimps and bonobos a mere 6 million years ago. Earlier I mentioned that this species is neotenic (aka paedomorphic), that just means it remains in the juvenile form throughout its life. This is the same as what happens in an axolotl (Ambystoma mexicanum) which are common in pet shops. Juvenile features, like the external gills, are normally lost when amphibian larvae undergo metamorphosis (think tadpole into frog) but both the axolotl and the olm do away with that and instead undertake all the normal adult functions, like reproduction, in the juvenile form. Somewhat ironically, the olm is one of the longest living amphibians (max. age of around 100) despite never “properly” becoming an adult.
Amphibian metamorphosis is normally controlled by the thyroid gland. Axolotls can be experimentally induced to undertake metamorphosis with the hormone thyroxine which turns them into their adult, terrestrial form – the Mexican salamander – a process that never occurs naturally in the wild. Researchers have tried to do the same with olms, even using high concentrations of thyroxine, but nothing will make them metamorphose. Who knows what the adult form looks like..? Historically, these animals were thought to be the offspring of cave dragons, maybe they weren’t wrong after all…
The olm anatomy is weird too. On average they reach 20-30 cm and they have three toes on the front feet and two on the back. This is particularly good as the hind feet look like they have little oven mits on. Adorable. They also have tiny little vestigial eyes which actually are functional in young olms but undergo atrophy from around four months old. However, they do swim away from light which is sensed very poorly by the eyes and also, interestingly, by the skin. Chemoreceptors in the nose and mouth are also highly sensitive and allow far lower concentrations of chemicals to be detected than in other amphibians. Perhaps most important are the mechanoreceptors found in highly specialised inner ear cells enabling them to perceive sound from the water and the substrate. Experiments by Bulog & Schlegel (2000) showed that olm hearing sensitivity ranges between 10 – 15,000 Hz which is roughly around that of humans (20 – 20,000 Hz). There’s also a lateral line, like in fish, that can detect very small water distrubances.
All of these adaptations allow them to find food in the perma-dark. They eat detritus (general “stuff” on the bottom) and troglodytic (cave dwelling) isopods, molluscs and crustaceans. Food doesn’t always come easy though, so when they do find it they really pack it in and store all the excess energy as lipid and glycogen deposits in the liver. This helps when food is properly scarce allowing olms to survive up to 10 years (!) without eating by reducing their metabolic activity and going into a stasis. They reach sexual maturity at around 12 and females lay up to 70 eggs, each around 12mm diameter, which are placed in cracks/crevices and guarded. Females only reproduce once every decade or so which is why conservation of this species is so critical.
The olm is listed as “Vulnerable” by the IUCN with its biggest threat being water pollution. This species requires very clean, oxygen rich water to thrive and reproduce. Unfortunatley, many of the cave systems that the olms inhabit have big sink holes and shafts at the surface which are extremely convenient places for chucking your rubbish into (more so historically, though this is sometimes still an issue). As a result piles of crap end up in the caves and can leach harmful chemicals into the water. My friend Tiago works for Mossy Earth and is running a really cool project with Dušan to clean up the caves which will help to protect the olms and other endemic cave fauna from further harm. Find out more about that here (not sponsored, I just think they do good stuff!).
Despite being a secretive and relatively unknown species there has actually been some cool research on the olm. For example, one study used ultrasound to assess the health condition of olms and other physiological functions like the heart rate (average of 42 bpm). Check that out here, there’s even a video of the olm heart beating in the supplementary materials!
I hope this has given a small insight into the wonderful world of the olm and inspires you to find out more!
Quick facts:
Too many to choose from…
Further reading:
- http://www.edgeofexistence.org/species/olm/
- Bulog, B., & Schlegel, P. (2000). Functional morphology of the inner ear and underwater audiograms of Proteus anguinus (Amphibia, Urodela). Pflügers Archiv, 439(1), r165-r167.
- Balázs, G., Lewarne, B., & Herczeg, G. (2020). Extreme site fidelity of the olm (Proteus anguinus) revealed by a long‐term capture–mark–recapture study. Journal of Zoology, 311(2), 99-105.
- Holtze, S., Lukač, M., Cizelj, I., Mutschmann, F., Szentiks, C. A., Jelić, D., … & Hildebrandt, T. B. (2017). Monitoring health and reproductive status of olms (Proteus anguinus) by ultrasound. PloS one, 12(8), e0182209.
- Voituron, Y., de Fraipont, M., Issartel, J., Guillaume, O., & Clobert, J. (2011). Extreme lifespan of the human fish (Proteus anguinus): a challenge for ageing mechanisms. Biology Letters, 7(1), 105-107.
- https://www.youtube.com/watch?v=1Wg9_iPikks
Tales of Scales 