Hydaticus transversalis, a vulnerable new species to mainland Finland

“Do you find good species in some urban ponds than other urban ponds?” My opponent, Prof Nigel Willby, asked me last week when I was defending my PhD thesis “Effects of landscape connectivity and predator-prey interactions on diving beetle (Dytiscidae) diversity in Finnish urban ponds”.

“If the definition of ‘good species’ is rare species that beetlers hunt for, then I didn’t get any in my urban ponds because what I found were all common species,” I answered.

Yet, this Monday, my first working day of postdoc life told me I gave an incorrect answer to my opponent: I was checking the diving beetle samples I collected this summer. Suddenly, a medium-sized dytiscid specimen caught my eyes (Figure 1)! It looks similar to Hydaticus seminiger, a common species in Finland, but it has different patterns on its elytra from what Hydaticus seminiger has (Figure 2)!

Figure 1 Hydaticus transversalis (female) occurred in Munkinpuisto, Helsinki. ©Wenfei Liao

Figure 2 Hydaticus seminiger (left), a common species in Finland; Hydaticus transversalis (right), a vulnerable species in Finland. Picture from Nilsson & Holmen 1995.

Immediately, I opened the bible of the Nordic diving beetle species (Nilsson & Holmen 1995). The interesting specimen turned out to be a female of Hydaticus transversalis, a vulnerable species that did not exist in mainland Finland before 2015 (Figure 3; Rassi et al 2015)! It may have spread northwards with recent climate change, just as Cybister lateralimarginalis has.

• Read Cybister, an Immigrant in Finland 

Figure 3 My record of Hydaticus transversalis in Munkinpuisto, Helsinki, is the first record of this species in mainland Finland! This species was only recorded in the Åland of Finland before my record.

According to the Nordic dytiscid bible (Nilsson & Holmen 1995), Hydaticus transversalis use permanent ponds with dense vegetation in open surroundings as their main habitat, which was exactly the type of habitat where I recorded my observation of this species. Hydaticus transversalis are also recorded in small water bodies and water bodies with forested surroundings (Frelik & Pakulnicka 2015). This species and many other dytiscid species overwinter out of water, which means they also need suitable terrestrial habitats to complete their life cycle. When conserving species, it is crucial to understand what habitats they use and how they use habitats, so that we can make conservation plans both at the local scale and at the landscape scale.

Figure 4 The vegetated spot in Munkinpuisto, Helsinki, where I made the first record of Hydaticus transversalis in mainland Finland. The specimen was found in early August 2021. This photo was taken in April 2020 before the species was recorded. ©Wenfei Liao

Read in Chinese: 《横纹丽龙虱——芬兰大陆的新物种》

References

Frelik A & Pakulnicka J (2015). Relations between the structure of benthic macro-invertebrates and the composition of adult water beetle diets from the Dytiscidae family. Environmental entomology, 44(5), 1348-1357. https://doi.org/10.1093/ee/nvv113

Liao W (2021) Observation of Hydaticus transversalis in Munkinpuisto, Helsinki. Click

Nilsson AN & Holmen M (1995). The Aquatic Adephaga (Coleoptera) of the Fennoscandia and Denmark. Ii. Dytiscidae: II-Dytiscidea. Brill.

Rassi, P., Karjalainen, S., Clayhills, T., Helve, E., Hyvärinen, E., Laurinharju, E., … & Silfverberg, H. (2015). Kovakuoriaisten maakuntaluettelo 2015.