On Saturday, 30 January 2016, one of Postojna Cave’s tour guides noticed that there was an olm egg attached to the glass of the aquarium, which is located in the Concert Hall of Postojna Cave. Next to the egg, there was a pregnant female olm watching it carefully.
They noticed that the pregnant female olm had only one egg, which she was defending fiercely. Soon after the biologists had arrived, one of the other olms approached the “mother” and the egg. The pregnant female responded rather impetuously and bit the intruders’ side, thus scaring it away. Judging by what the two biologists had just witnessed, it was clear that keeping everything the way it was would not give the pregnant female olm any peace and quiet and that the egg would be under threat. Therefore, they decided to remove the other olms from the aquarium and immediately started doing everything necessary to do so. They captured the other six olms, while at the same time making sure not to upset the pregnant female olm more than absolutely necessary.
Given the experience of two years ago, a threat to the egg could also be posed by the freshwater amphipods Niphargus stygius, to which the egg may look like a delicious morsel. However, during the past month, the olms were not fed, therefore only a few amphipods are currently present in the aquarium, usually hidden among the gravel.
Now all there is left to do is wait and see if the pregnant female lays any more eggs, as the egg laying usually takes place for several days. We will see what happens. The female olm may lay more eggs, but it may also happen that she simply reabsorbs the remaining eggs that have not been laid yet, thus saving the energy for the next round of breeding.
It seems that the conditions inside the aquarium are very favourable for the olms, as our 2013 “miracle” has happened again. In the following six days the olm has laid two more eggs. The mother is about a week into a 20-day period where, hopefully, she will lay as many as 30 to 60 eggs. Several weeks later, they are expected to hatch.
At present, all of the eggs are under a rock, where the olm took to immediately after the other olm specimens had been moved from the aquarium.
An infrared camera is our latest addition, which provides a revealing insight into the life of Slovenia’s most famous animal family. “What is he/she up to when the lights are turned off” is not just what many parents wonder about, but the scientists who are keeping an eye of the olms as well.
Our two biologists still visit the aquarium at least three times daily; as for visitors’ access to the aquarium, it has been somewhat restricted to make sure the camera flash of the curious visitors, who do not abide by the rules, is not too stressful for the olms or her eggs. The infrared camera recordings reveal the secrets of the olm’s reproduction even when we are not anywhere near. The recording shows a visible bulge on the abdominal part of the olm’s body. The olm gets attached to the surface where she is about to lay an egg, and after 20 minutes the egg is laid. Some eggs show signs of breaking open, which means that the olm’s young are very likely to hatch from them.
Cave visitors can watch the olm live on a special screen inside the cave. While our two biologists are keeping an olm diary, a website for live coverage of the olm family is also being set up, which will soon allow all of you to watch the “darkest” family show.