Can isopods help in biological control by removing dead eggs in fish hatcheries?

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By Milthon Lujan

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Schematic figure of the foraging features of A. aquaticus during fish embryo incubation. Source: Kucska et al., (2024); Scientific Reports, 14(1), 1-11.

Fish hatcheries face a constant battle: ensuring high survival rates of delicate fish eggs. While factors such as genetics and water quality influence this, dead eggs pose a significant threat. These eggs become breeding grounds for bacteria and fungi like Saprolegnia, damaging both the remaining eggs and newly hatched larvae.

Current solutions include chemical treatments and manual removal of dead eggs. However, chemicals can be harmful, and manual extraction is often impractical, especially for smaller eggs.

A new study by scientists from the Hungarian University of Agriculture and Life Sciences experimented to gain a better understanding of Asellus aquaticus‘ foraging activity during fish egg incubation.

The Problem: Dead Eggs and Diseases

The success of fish egg hatching depends on various internal and external factors. While genetics and egg quality are crucial, the environment plays a significant role. Ideal temperature, salinity, and minimal presence of predators and pollutants are essential. However, bacterial and fungal infections pose a significant threat during incubation, especially in hatcheries.

Unfortunately, dead eggs are like time bombs. They attract harmful pathogens like bacteria and fungi, endangering the health of remaining eggs and hatched larvae. Pathogens like Saprolegnia and Flavobacterium naturally exist in water systems. The sticky outer layer of fish eggs provides a perfect breeding ground.

The study highlights a critical link between dead eggs and diseases. Dead eggs not only attract harmful pathogens but can also contribute to their spread. This can have the following effects:

  • Pathogen breeding ground: Dead eggs provide a feast for bacteria and fungi, allowing them to multiply rapidly.
  • Domino effect: These pathogens then spread to healthy eggs and larvae, causing diseases and death.
  • Oxygen consumption: A large number of bacteria compete with beneficial bacteria for oxygen, further stressing the environment.
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Current Solutions and Their Limitations

Hatcheries often use techniques like egg disinfection to combat pathogens. While chemical treatments exist, they have varying degrees of success and can harm the environment.

The study also mentions the manual removal of dead eggs as a possible solution. However, this method is only practical for larger eggs with good visibility and is not feasible for smaller species or those incubated in turbulent environments.

Biological Control: Isopods to the Rescue

Asellus aquaticus could be the answer hatcheries have been looking for. These small crustaceans are nature’s cleaning crew and feed on decaying organic matter, including fungi.

Why Isopods?

Recent research suggests that Asellus aquaticus, a freshwater isopod, can act as a biological control agent and offers several benefits:

  • Targeted cleaning: Studies show that Asellus aquaticus specifically consumes dead eggs, leaving healthy eggs and larvae intact. This reduces the breeding ground for harmful pathogens.
  • Disease prevention: By removing dead eggs, Asellus aquaticus can help prevent the spread of diseases that can pass from dead to live eggs. This can improve the overall health of the hatchery.
  • Reduced oxygen demand: Dead and decaying eggs create a feast for bacteria, which in turn consume large amounts of oxygen. Asellus aquaticus helps by removing this food source, potentially improving oxygen levels for developing fish.
  • Ecological solution: Unlike chemical treatments, Asellus aquaticus is a natural and sustainable method for keeping hatchery environments clean.

Researchers studied the feeding behavior of Asellus aquaticus during zebrafish egg incubation. These millimeter-sized creatures, also known as water slaters, are detritivores, meaning they chew on decaying organic matter. The study placed Asellus aquaticus with dead zebrafish eggs, healthy eggs, and even hatched larvae.

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Fascinating Results

Asellus aquaticus specifically targeted dead eggs, leaving healthy eggs and larvae intact. The number of isopods and the duration of their exposure significantly affected the amount of dead eggs consumed. This suggests that Asellus aquaticus could act as natural cleaners in aquatic environments, removing potential infection sources for developing fish.

Benefits for Fish Farming

These findings have intriguing implications for fish hatcheries. Asellus aquaticus could potentially be used as a biological control mechanism. Their presence could:

  • Reduce fungal and bacterial growth: By consuming dead eggs, Asellus aquaticus can limit the food source for harmful microbes, creating a healthier environment for developing fish.
  • Improve egg survival rates: By removing dead eggs, A. aquaticus could help prevent pathogen spread to healthy eggs, potentially leading to higher survival rates.


The study also suggests a broader role for A. aquaticus. By consuming dead organic matter, they can help maintain cleaner water quality in natural ecosystems. Additionally, their ability to detect dead eggs could be explored to manage fish populations in specific environments.

More research is needed to fully understand the potential of Asellus aquaticus in aquaculture and environmental management. However, this study opens doors to exciting possibilities for harnessing the power of tiny nature’s cleaners.

The open access was funded by the Hungarian University of Agriculture and Life Sciences, while the study was financed by the National Research Development and Innovation Office of Hungary and the New National Excellence Program of the Ministry for Culture and Innovation of the National Research, Development and Innovation Fund.

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Bence Ivánovics
Hungarian University of Agriculture and Life Sciences, Páter Károly U. 1., Gödöllő, 2100, Hungary
Email: ivanovics.bence@uni-mate.hu

Reference (open access)
Kucska, B., Ngọc, Q. N., Ivánovics, B., Staszny, Á., Havasi, M., Vranovics, K., Griffitts, J. D., Varga, Á., Urbányi, B., & Müller, T. (2024). Removal of dead fish eggs by Asellus aquaticus as a potential biological control in aquaculture. Scientific Reports, 14(1), 1-11. https://doi.org/10.1038/s41598-024-57903-4