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  • Introduction

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  • Secondary sexual characteristics of fish of the genus Oncorhynchus and SDGs

    • SDGs2 SDGs14 SDGs top

      In Japan, we have developed a hatching and release business that takes advantage of the salmon's tendency to return to their mother rivers and obtains them as highly productive fishery resources. We aim to upgrade this business to a more robust and sustainable food production system that can adapt to ecosystem considerations and climate change, and through our research we hope to contribute to the SDGs (Zero Hunger, Enriching the Oceans).

      The United Nations has designated the decade beginning in 2021 as “the Decade of Marine Science” to contribute to the SDGs. Marine science as defined by the UN includes the field of fisheries.

  • Secondary sexual characteristics of anadromous salmonids the genus Oncorhynchus

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      (left figure)

      河川: river

      産卵・受精: spawning and fertilization →ふ化: hatching →浮上: surfacing →稚魚: juvenile降海回遊: descending migration

      サケ，カラフトマス: chum salmon and pink salmon その他の遡河性サケ属魚類: other anadromous salmonsパー: parr →銀化変態: smoltification →スモルト: smolt

      海洋: ocean

      索餌回遊: foraging migration

      成長: growth →性成熟開始: start of sexual maturity →母川回帰: return to mother river

       

      (right figure)

      成熟に伴う外部形態の変化: changes in external form with maturity

      未成熟魚（外洋回遊中）: immature fish (migrating on the open sea) 河川に回帰した成熟魚: mature fish returning to rivers


    • Among fish that migrate between the sea and rivers, those that go upstream to spawn are called anadromous migratory fish. Salmonids (Genus Oncorhynchus: hereafter referred to as “salmons”), which are representative of this group, are single spawners with a life history in which most species, with the exception of a few species, die after spawning in the river. During their foraging migration in the ocean, both sexes look similar, but when they return to rivers and mature, they show completely different external morphologies.  These are called secondary sexual characteristics, and many of them are thought to be used for fighting between males and appealing to females. Previous studies have also shown that 11-ketotestosterone (11-KT), one of the major male sex hormones (androgens) in bony fishes, is involved in the development of secondary sexual characteristics.


  • Dorsal ridge of fish of the genus Oncorhynchus

    • カラフトマス背隆起成熟に伴う変化

               Structure of pink salmon dorsal ridge (bottom) and changes with maturity (left)

                  カラフトマス背隆起の構造
      　　　　皮膚: skin 棘上筋: supraspinatus muscle 疎性結合組織: sparse connective tissue
      　　　　不完全神経間棘: incomplete intraneuronal spine 筋肉: muscle 神経棘: neural spine 末端領域: endpoint region
      　　　　三日月形領域: crescentic region 内側中隔: medial septum

      A: immature male B: maturing male C: mature male


    • In male salmons, pink salmon and sockeye salmon, dorsal ridges develop markedly with sexual maturity. Although past studies have published that the dorsal ridge is packed with a tissue mass of “cartilage” known as fibrocartilage based on gross observations, and the “cartilage theory” has been supported by domestic and international salmon experts. However, our research group has analyzed the dorsal ridge of pink salmon from a microanatomical perspective.

      The results revealed that the bone at the center of the dorsal ridge is a hard bone called the “incomplete intraneuronal spine”, which becomes longer, thicker, and at a greater angle to the body axis with maturation. In addition, cartilage tissue previously reported to be present around the bone was not observed and was actually “sparse connective tissue”.

      It has also been found that the main components of this tissue are mostly type I collagen, which is the same type of collagen contained in skin, and water. Furthermore, mature male individuals had higher levels of 11-KT in their blood than immatures or females, suggesting the involvement of male sex hormones in the formation of the dorsal ridge.


  • Bent snout of fish of the genus Oncorhynchus

    • カラフトマス鼻曲がり発達の機序

                         Mechanisms of pink salmon snout bend development (left) and changes associated with maturation (bottom)

                      カラフトマス鼻曲がり発達 成熟に伴う変化


      (left figure)

      上顎: upper jaw 軟骨組織: cartilage tissue 軟骨細胞: cartilage cells 雄性ホルモン受容体: male hormone receptor 雄性ホルモン（11-KT）: male hormone (11-KT) 活性化軟骨細胞: activated cartilage cells

      下顎: lower jaw 結合組織: connective tissue 海綿状骨組織: trabecular bone tissue 骨芽細胞: osteoblast 雄性ホルモン: male hormone 未成熟骨芽細胞: Immature osteoblasts 雄性ホルモン受容体: male hormone receptor 下顎骨: mandible

       

      (right figure)

      外洋を回遊中の未成熟魚: immature fish migrating in the open sea 河川への遡上を前に沿岸を回遊中の成熟途上魚: maturing fish migrating along the coast before returning to the river 産卵のために河川に回帰した成熟魚: mature fish returning to river to spawn

       


    • In male salmons, the snout grows into a hooked snout with sexual maturity. Our research group has elucidated the developmental mechanism of bent snout formation and analyzed whether male sex hormones are involved.

      The results showed that chondrocytes and cartilage matrix increased in the maxilla, while trabecular bone tissue added to the tip of the dentary bone developed and elongated in the mandible. Furthermore, genetic analysis by PCR and histological analysis by immunostaining revealed the localization of male hormone receptors in both jaws, suggesting the involvement of male hormones. Blood analysis also showed that the blood 11-KT concentration increased with maturity, with higher 11-KT concentrations in mature males compared to mature females, suggesting that the bent snout structure develops via male hormones.


  • Axillary process of fish of the genus Oncorhynchus

    •      サケ属魚類の腋突起         サケ属魚類の腋突起

    • (right figure)

      サケ成熟個体腋突起幅: axillary process width of mature salmon

      腋突起長: length of axillary process ハイピューラル長: hypural length 雄: male 雌: female


    • Salmons have a lobe-like structure called an axillary process near the ventral fin. Although their existence has been known for some time, the function of axillary processes remains unclear. Previous studies have reported that the axillary process elongates during smoltification (adaptation to seawater) associated with oceanic descent and that males are slightly wider than females in mature individuals, suggesting that it may be involved in swimming in the ocean or be a secondary sexual characteristic. Therefore, our research group is analyzing the effects of excision of axillary processes on swimming using a device called a gyration tank, and at the same time, we are investigating their involvement in swimming and secondary sexual characteristics by detecting male hormone receptors through tissue staining and genetic analysis.

      As described above, various things have been learned through detailed examination of the “shapes” of the various secondary sexual characteristics. It is hoped that the biological significance of each structure will be further clarified.


  • References

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    Salmon and Trout SDGs2 SDGs14