Frozen sperm helps revive endangered fish species

Jim Crocker
22nd November, 2025

Frozen sperm helps revive endangered fish species

Figure 1 from study demonstrates the optimization of cryopreservation conditions for Tokyo bitterling testes, showing histological analysis of testes at different maturation stages, cell viability post-thaw, and confirmation of germ cell preservation using immunostaining.

Image adapted from: Yamakawa et al. / CC BY (Source)

Key Findings

  • Researchers developed a method to preserve the genetic material of an endangered fish, the Tokyo bitterling, via germ cell cryopreservation and transplantation
  • Frozen Tokyo bitterling germ cells successfully revived the species’ genetic lineage when transplanted into a related fish, the oily bitterling, after sterilizing the recipient
  • This technique offers a potential solution for conserving endangered fish species and may even allow revival of species if germ cells were previously stored
Freshwater ecosystems are experiencing a global biodiversity crisis, with many species facing extinction due to habitat loss, pollution, and overexploitation. Fish are particularly vulnerable, and conservation efforts are often hampered by the difficulty of maintaining genetic diversity in small, isolated populations. Traditional conservation methods, like breeding programs, can struggle to preserve the full genetic spectrum of a species, potentially reducing its ability to adapt to future environmental changes. This is especially true for species with unusual reproductive strategies. Researchers at Tokyo University of Marine Science and Technology[1] have developed a novel approach to preserving the genetic material of an endangered fish, the Tokyo bitterling (Tanichthys kuhiei). This fish is unique because it lays its eggs inside the gill chambers of freshwater mussels. Both the bitterling and the mussels are declining due to riverbed degradation – specifically, the covering of gravel beds with concrete and silt. The Tokyo bitterling is now listed as endangered and faces a high risk of extinction. While conservation efforts are underway, their success is limited. The study focused on preserving the fish’s “germ cells” – the reproductive cells (sperm and eggs) that carry the genetic information. Unlike trying to freeze and revive mature eggs or embryos, which is difficult in fish due to their size and yolk content, the researchers targeted undifferentiated germ cells: primordial germ cells, spermatogonia, and oogonia. These are early-stage cells that can develop into either sperm or eggs. The team successfully froze these cells in liquid nitrogen, a process called cryopreservation. The key innovation was then transplanting these frozen, preserved germ cells into another, related species – the oily bitterling (Tanichthys albonubes). However, the oily bitterlings needed to be prepared to accept the donor cells. The researchers used a technique called “dnd knockdown” to eliminate the oily bitterling’s own germ cells, effectively sterilizing them. This created space for the transplanted Tokyo bitterling germ cells to migrate to the oily bitterling’s developing gonads (reproductive organs). Remarkably, the transplanted cells matured and began producing eggs and sperm. When these recipient fish were bred, they produced offspring that were genetically identical to the original Tokyo bitterling – possessing the same genetic makeup and physical characteristics. This demonstrated that the frozen germ cells had successfully revived the genetic lineage of the endangered fish. This research builds upon earlier work highlighting the importance of genetic diversity in preventing extinction[2]. That study found that threatened species often have significantly lower levels of genetic diversity compared to their non-threatened relatives, which compromises their ability to adapt and survive. The Tokyo bitterling study offers a potential solution to this problem by providing a way to safeguard genetic resources even when populations are critically low. Furthermore, the success of this method contrasts with concerns that genetic factors may not play a significant role in extinctions, with some arguing that species are often driven to extinction before genetic issues become critical[2]. This study demonstrates that genetic factors can be crucial, and preserving genetic diversity is a vital component of conservation. The method developed by the Tokyo University of Marine Science and Technology team isn’t limited to just these two bitterling species. They found the technique applicable to a wide range of fish species, suggesting it could be a valuable tool for conserving other endangered fish populations. The researchers suggest that this approach could even potentially revive species that are already extinct, provided a germ cell bank was established before their disappearance. The study also indirectly addresses the challenges of conserving entire freshwater ecosystems[3]. While broader conservation efforts often focus on habitat restoration and pollution control, this technique provides a targeted approach to preserving the genetic foundation of vulnerable species within those ecosystems. Although conserving habitats is paramount, this method offers a safety net for genetic diversity, particularly important given that prioritizing conservation based on terrestrial species may not adequately represent the needs of freshwater fauna[3].

Animal ScienceMarine BiologyEvolution

References

Main Study

1) Production of offspring via the transplantation of frozen germ cells from Tokyo bitterling, a fish on the brink of extinction

Published 19th November, 2025

https://doi.org/10.1038/s41598-025-24449-y

Related Studies

2) Most species are not driven to extinction before genetic factors impact them.

Journal: Proceedings of the National Academy of Sciences of the United States of America, Issue: Vol 101, Issue 42, Oct 2004

3) One-quarter of freshwater fauna threatened with extinction.

https://doi.org/10.1038/s41586-024-08375-z


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