Field | Crayfish alert at Žumberak Mountains

Hi everyone, and welcome back to my blog. I took a month+ long break, during which I focused on my health and final exams at my University. At the same time, BIUS – Biology Students Association was preparing for their annual field trip, that I really wanted to be a part of!
BIUS is an association that gathers many Biology students from our department and focuses mainly on field trips, excursions, and expert lectures, all in order to complement and expand our Biology-related knowledge about certain topics. BIUS is also a publisher behind In Vivo Magazine, for which I serve as editor-in-chief.


Firstly, however, I would like to write a bit about my love for scientific (and a little less scientific) field trips. My primary love is the lab, after all. However, I grew up in a tiny village, surrounded by a living world – woods, animals, endless fields of tall grass… I actually started to think about studying something science based, perhaps Biology even, way back in the primary school, after wanting to identify all the bugs and spiders I would find in my front yard.
During my first two years of Bachelor’s degree, the thought of going out to the field didn’t really cross my mind, but it all changed in the middle of my third year, when I realized that something was lacking in my life, and that something turned out to be raw nature.

Photo by Đina Nola

This year’s big field excursion lasted for eight days, but I was only able to attend for the last four. Usually, BIUS organizes this kind of excursions twice a year, in May and September, but due to the pandemic, it was completely moved to the end of September, when situation in Croatia improved. Every year, a new terrain is explored, usually switching between continental and marine area. For this year, the leadership chose the Žumberak Mountains which are located on a border with Slovenia, and are approximately one hour drive from Zagreb. Žumberak is a mountain range divided into two parts, the Samobor Hills and the Žumberak Hills, both comprising the protected nature park Žumberak – Samobor Hills. It is home to many plant, fungal, and animal species, some of which are endangered or sensitive.


At first, shortly after arriving I was planning to spend every day with a different group, but in the end, I spent all the days driving around with the Crustacean group. I wasn’t sure how much fun is that going to be, since I knew very little about freshwater crayfish, apart from researching crayfish plague for a little while as an undergrad during an elective lab course.
I was already familiar with two members, Lena and Ljudevit Luka, since we are the same generation and took multiple classes together, and I also knew Anita and Karla a little bit; the whole group was very determined to carry out their research but with the sprinkle of carefreeness. I didn’t feel excluded for one bit and they were extremely patient with me taking photographs and filming videos.

So, what did Crustacean group actually do? Anita kindly explained their goals:

  • monitoring of the species Austropotamobius torrentium, also known as stone crayfish (how many specimens, in which streams are they located, what gender…)
  • taking swabs of crayfish cuticles in order to check for crayfish plague pathogen; this is later investigated by using the PCR method
  • taking water samples using special filters in order to check for crayfish presence; this is later investigated by analyzing the eDNA (environmental DNA)

How does that actually look like out in the field?
The first thing we did every morning, was to check the map and the roads; sometimes, we drove for more than one hour to reach a destination. Then we walked up to a stream, which sometimes proved to be rather tricky, since some seemed to dry up overnight. The most important thing we did before and after walking in every stream, creek or puddle was to disinfect our rubber boots, in order not to accidentally transfer pathogens to different habitats.

The group was very active even before my arrival, so we checked some permanent streams where they already set up special crayfish traps, that were actually made of old plastic bottles, with some tasty hot dog sausages in them. (Don’t worry, those traps are reusable! They just have to be washed thoroughly.) After taking out crayfish, one by one, they are measured and gently rubbed with a toothbrush, in a special buffer, to collect possible crayfish plague pathogen. Every tube containing that buffer is then labeled and safely stored. Crayfish are carefully released back into the stream, at the same place where they were found.

However, sometimes we went to streams for the first time, which meant no traps. So how do you catch a crayfish then? With hands. Usually they were hiding under rocks, but what most people probably wouldn’t expect, is that they are freakishly fast. Still, even during night-time catching & release, every member of the group was highly skilled in catching them. They could also easily discern female from male specimens, and Ljudevit Luka readily explained how, and also sent additional images (the ones below). In short, the main difference is that male crayfish have gonopods, while females don’t. (Gonopods are modified legs that are substantial during mating.)

A female stone crayfish

In four days that I spent with this wonderful group, I learned a lot and had a really amazing time. I wanted this post to focus mostly on crayfish, but I’m planning to post another one, where I will write a little bit more about travelling, our camping site, and wonderful nature I was able to document. I also took many videos, which I’m currently editing in one coherent, presentable, work, which I initially planned to release at the same time as this article, but life got a little bit in the way.
I sincerely hope you liked this write-up, and will read my next one as well!


Here you can find social media of some of the members of the Crustacean group, as well as the KarioAstacidae website, a student project led by Ljudevit Luka and Lena, which focuses on Astacidae populations in Zagreb.

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