Impact of the thick-shelled river mussel (Unio crassus) on the habitat preference and predator-prey interactions between European bullhead (Cottus gobio) and the freshwater shrimp Gammarus pulex

Abstract

Freshwater mussels play a crucial role as ecosystem engineers, contributing to various ecological functions, including nutrient recycling, biofiltration, and habitat modification. Both live mussels and their discarded shells also play integral roles in providing and enhancing habitats for other organisms by furnishing physical structure, stabilising sediments, and promoting bioturbation. This study employs microcosms to investigate the impact of the thick-shelled river mussel, Unio crassus, a highly imperilled temporary parasite on fish, on one of its host fishes European bullhead, Cottus gobio, (from now on bullhead) the macroinvertebrate Gammarus pulex, and the ensuing dynamics of predator-prey interactions.

In this study, gammarids were allowed to choose habitat with or without mussels in two substrate types: large or small stones, and additional exposure to bullhead chemical cues was manipulated. Bullheads were offered the same habitat choice, with infestation with mussel glochidia being altered rather than predator cues. Survival trials for gammarids were finally conducted while varying the following factors: infestation status in the fish predators, substrate size, and mussel presence.

The study revealed that, in the absence of predator cues, gammarids preferred habitats containing mussels. No result was shown in the habitat preference of bullhead, even though the bullheads showed a non-significant preference tendency for mussels in the larger substrates. Furthermore, gammarids demonstrated significantly higher survival rates in mussel habitats with small but not with large substrates. These findings may contribute to an enhanced understanding of the multifaceted ecosystem functions of imperilled mussels.

Popular Abstract

Do freshwater mussels alter the behaviour of its host fish and the host fish prey?

In the ecosystem of a river, mussel beds can create localised hotspots for nutrient recycling and storage that can lead to higher aquatic insect densities. The physical structure provided by mussels also serves as habitat for many organisms, contributing to increased habitat complexity. Given the global interest in preserving ecosystem functions, and the services provided by freshwater mussels, research on these functions is highly warranted. This is particularly true in the case of the endangered and ecologically significant thick-shelled river mussel, Unio crassus. Organisms, specifically fish and aquatic insects, which live in the same habitat as mussels, engage in interactions and may experience potential effects due to the presence of mussels in that habitat.

Some freshwater mussels undergo a parasitic life cycle: the mussel larvae attach to the gills or fins of suitable host fish. Depending on mussel species, the length of the parasitic stage varies from days to months, and upon successful completion, the juvenile mussel lets go of the host and starts to live as an independent mussel. This parasitic interaction has various effects on host fish behaviour, including their general activity levels, habitat use and feeding.


The research question of this study was: Do the presence of freshwater mussel U. crassus alter the behaviour of its host fish and the host fish prey, freshwater shrimp? To address this question the host fish and the freshwater shrimp
were exposed to a combination of habitats with and without mussels and on different bottom material.

This study used laboratory tanks to investigate the impact of the thick-shelled river mussel, a highly endangered temporary parasite on fish, on one of its host fishes: the European bullhead and also the impact of mussels on a freshwater shrimp. Lastly, the study investigated how the presence of mussels impacts the interactions between bullheads and their prey, freshwater shrimp.

In the first experiment freshwater shrimp were placed into tanks with big stones or into tanks with small stones and allowed to choose their respective habitat; i) with or ii) without mussels. Additionally, bullhead chemical cues were added to half of the tanks.

In the second experiment bullheads were placed into tanks with big stones or into tanks with small stones and allowed to choose their preferred habitat; i) with or ii) without mussels. Additionally, the bullheads were either infested with mussel larvae or they were not.

Finally, in the third experiment, survival trials of freshwater shrimp were conducted, while varying the following factors: infestation status in the bullheads, the size of stones, and mussel presence.

Result & Conclusion
A significantly higher proportion of freshwater shrimp was found in the mussel habitat in the absence of predator cues compared with presence of predator cues. In the second experiment, bullhead habitat preference, there were no differences between treatments. Freshwater shrimps survived at a significantly higher rate in the mussel habitats, especially within the smaller stones.

This study proposes that mussels exert an influence on gammarids and on predator-prey interactions under certain substrate choices. The findings suggest that mussel beds in combination with small stones serve as refuges, enabling prey organisms to avoid predation and therefore significantly contributing to the broader ecological dynamics in aquatic ecosystems.


Master’s degree project in Aquatic Ecology, 30 credits, 2024
Department of Biology, Lund University

Advisor: Anders Nilsson
Department of Biology, Lund University

• author: Elmlund, Anna Malin
• supervisor: Anders Nilsson; Sebastian Rock
• organization: Degree Projects in Biology
• course: BIOM02 20231
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9146732
• date added to LUP: 2024-01-24 14:51:05
• date last changed: 2024-01-24 14:51:05