Medaka Research

A Thorough Analysis of Spinal Curvature in Medaka

Genetics and Environment Secrets to Raising Healthy Medaka This article is based on translations from several Japanese blog articles. Table of Contents What is “spinal curvature” in medaka? Specific symptoms of spinal curvature Effects on lifespan and health Main causes of spinal curvature in medaka Genetic factors Nutritional deficiency / nutritional imbalance Water quality deterioration and stress Physical damage Burden on females due to excessive egg carrying How to respond and improve the environment for prevention Conclusion Many keepers feel anxious when they notice that the body shape of medaka they have carefully raised at home has become curved without them realizing it. This condition, known as “spinal curvature,” not only affects appearance but is also deeply related to the health and quality of life of medaka. Spinal curvature does not occur due to a single cause; rather, it develops through a complex interaction between genetic factors and the breeding environment. In this article, we scientifically clarify why spinal curvature occurs in medaka, explain its main causes, and provide detailed guidance on how to handle medaka that already show symptoms, as well as concrete preventive measures for raising healthy medaka. What Is “Spinal Curvature” in Medaka? Spinal curvature in medaka refers to a condition in which the spine (backbone) is abnormally bent or deformed, rather than maintaining a normal streamlined shape. Specific Symptoms of Spinal Curvature Although symptoms vary, the following conditions are commonly observed: Curved body shape: The body appears loosely bent, twisted, or curved in an S-shape or wave-like form. Head tilt or raised back: The head may appear sharply bent downward from the back, or part of the back may appear noticeably raised or bulging. Timing of occurrence: Many cases are congenital (present at birth), but some females begin to show spinal curvature after they start spawning. Effects of Spinal Curvature on Lifespan and Health Because spinal curvature is a skeletal abnormality, it affects the quality of life of medaka. Impact on daily life: Individuals with skeletal abnormalities may have difficulty swimming and, in some cases, difficulty feeding compared to normal medaka. Impact on reproduction: Individuals with poor skeletal structure are not suitable as broodstock for maintaining a line. Spinal curvature carries a high risk of passing undesirable genes on to the next generation. Main Causes of Spinal Curvature in Medaka The causes of spinal curvature in medaka can be broadly divided into genetic factors and environmental factors. Genetic Factors Among the causes, genetic factors require particular attention. Wavy gene (Wy gene): There exists a gene related to spinal curvature in medaka known as the “Wavy gene.” When this gene is expressed, it causes wave-like or curved deformation of the spine. Expression of recessive genes: Even if parent fish appear straight-bodied and well-formed, they may carry harmful recessive genes that cause spinal abnormalities. Over successive generations, these genes may appear as visible traits. Repeated inbreeding increases genetic concentration, leading to inbreeding depression or weakness, making hidden harmful recessive genes more likely to surface. High heritability: Congenital spinal curvature is said to be particularly heritable. In some documented cases, breeding broodstock with spinal curvature resulted in nearly 100% of offspring inheriting the same condition. Characteristics of Hikari body types: Hikari-type medaka have symmetrical dorsal and anal fins, which creates upward and downward pulling forces on the body. Because of this structure, they are said to be more prone to developing spinal curvature compared to other body types. Nutritional Deficiency / Nutritional Imbalance (The source material does not provide detailed discussion regarding specific vitamin or mineral deficiencies as direct causes of spinal curvature.) Water Quality Deterioration and Stress Sudden environmental changes and stress during breeding can also contribute to deformities. Rapid temperature changes: Especially during the egg stage, when cells are forming the body, sudden water temperature fluctuations may influence development and result in deformities such as spinal curvature. Excessive aeration: Strong aeration during the fry stage, when bones are not yet fully developed, is also cited as a factor that may increase the likelihood of spinal curvature. Physical Damage Stress on immature bones: Handling fry whose bones are not yet formed—such as scooping them with a net—can place stress on the skeletal structure and is considered an unfavorable factor. Burden on Females Due to Excessive Egg Carrying Some individuals are known to develop spinal curvature after spawning begins. However, spinal curvature that appears after spawning is said to be less likely—or unlikely—to be inherited compared to congenital cases. How to Respond to Medaka with Spinal Curvature and Improve the Environment for Prevention Once spinal curvature occurs, it is difficult to restore the body to its original shape. However, with appropriate care and preventive measures, the risk of future occurrences can be greatly reduced. How to Handle Medaka That Already Show Spinal Curvature Exclusion from breeding (selection/culling): Because of the risk of inheritance, such individuals should not be used for breeding purposes. Proper selection is essential to prevent undesirable genes from being passed on. Ethical breeding practices: Selection or culling does not mean abandoning the fish. It is important not to euthanize them unnecessarily and to care for them responsibly until the end of their lives. Ornamental keeping: When viewed from above, medaka with spinal curvature often look similar to normal individuals. Allowing them to live calmly in a biotope or outdoor setting can be a suitable option. Genetic Considerations for Prevention To maintain healthy medaka bloodlines, selection is the most important and indispensable practice. Careful selection of broodstock: Choosing broodstock that does not exhibit spinal curvature is the first and most important step. Strict body shape selection: Selection should not focus solely on color or fin length, but also on body shape, physique, and skeletal structure. Even slight irregularities should be grounds for exclusion. Medaka bred by breeders who enforce strict selection are said to produce remarkably high-quality offspring. Moderate introduction of new bloodlines: Rather than continuing the same line indefinitely, crossing with different bloodlines can reduce the risk of recessive genetic defects becoming expressed. However, if selection after introducing new bloodlines is insufficient, inbreeding weakness may quickly reappear. Nutritional Considerations for Prevention (The source material does not provide specific nutritional recommendations for preventing spinal curvature.) Maintaining Water Quality and Environment for Prevention Environmental control during developmental stages directly impacts deformity prevention. Egg incubation environment: When collecting eggs, eliminating temperature differences between the parent tank and the incubation container is an important preventive measure. Avoiding sudden temperature changes during development is critical. Fry management: During the fry stage, when bones are still immature, gentle aeration should be used. Rough handling with nets should also be avoided. Conclusion Spinal curvature in medaka is often a deformity caused by a combination of genetic factors—such as the Wavy gene—and environmental stressors such as sudden temperature changes. Rather than excessively fearing inbreeding, consistently practicing strict selection during daily breeding can greatly reduce the risk of undesirable genes becoming fixed within a line. This is similar to agriculture, where only healthy, well-shaped seeds are selected and planted year after year. By removing poor seeds and continuing to select good ones, it is possible to maintain strong and desirable bloodlines over many generations. Back to top