Deformities in Aquarium Fish: Genetic, Nutritional and Environmental Causes

Why fish develop curved spines, missing gill covers and other skeletal deformities: inherited factors, vitamin C deficiency, temperature during development, injury and toxicants.

Overview

Skeletal deformities in fish include sideways curvature of the spine (scoliosis), downward or upward curvature (lordosis and kyphosis), fused vertebrae, shortened bodies, jaw malformations and missing or shortened gill covers (opercula). A fish pathology nomenclature published in the Journal of Toxicologic Pathology (INHAND, chapter 12) groups these under vertebral and bone deformation and notes that vertebral deformation is a commonly encountered lesion, well described in salmonids, Senegalese sole and zebrafish. The same deformity can have several unrelated causes, so the appearance of a fish alone rarely identifies the cause.

The causes reported in the literature fall into four broad groups: inherited (genetic) factors, nutrition, conditions during early development such as temperature, and environmental or physical insults including infection, toxicants and injury.

Genetic causes

Some spinal curvature is heritable. The curveback guppy is a laboratory lineage of Poecilia reticulata founded from a single curved male crossed with a normal female. Studies of this line found that inbreeding showed a strong genetic influence on curvature, that curve onset, progression and final magnitude varied between individuals, and that some curves resolved before maturity. Computed tomography showed no vertebral breakage or fusion in these fish. A later mapping study (BMC Genetics, 2011) located a major locus acting in a recessive manner that explained 82.6% of the phenotypic variance in curve susceptibility.

In farmed gilthead seabream (Sparus aurata), designed matings showed that sires with lordosis, lack of operculum or vertebral fusion produced more offspring with the same deformity, with heritability estimates of 0.34 to 0.46; the authors concluded that these deformities have a genetic origin. Reduced genetic variation also matters: in Atlantic salmon (Salmo salar), fully homozygous experimental lines developed a high level of complete vertebral fusions. Inbreeding raises the chance that harmful recessive alleles are carried in two copies and expressed; this is covered in more detail in the separate guides on line breeding and inbreeding depression and on selective breeding.

Nutritional causes

Vitamin C (ascorbic acid) is the best-documented nutritional cause. Many fish lack the enzyme L-gulonolactone oxidase and cannot make vitamin C, so it must come from the diet. Vitamin C is required to form collagen, and therefore bone matrix and connective tissue. According to the MSD Veterinary Manual and WikiVet, deficiency causes spinal lordosis and scoliosis, stress fractures and deformation of the gill covers and gill lamellae; farmers and hobbyists sometimes call the condition broken back disease.

A controlled study in channel catfish (Ictalurus punctatus) fingerlings fed purified diets without vitamin C found deformed spinal columns, haemorrhages, fin erosion, darkened skin and reduced bone collagen after 8 to 12 weeks. Fish given 30 mg of vitamin C per kg of diet showed none of these problems over 22 weeks, although 60 mg/kg was needed to prevent distortion of gill-filament cartilage. The INHAND chapter also lists phosphorus imbalance alongside vitamin C as a dietary factor associated with vertebral deformity.

Temperature and development

Conditions during embryonic and larval development influence skeletal formation. In golden pompano (Trachinotus ovatus) larvae reared at 26, 29 and 33 °C, jaw deformity was highest at 33 °C and lowest at 26 °C, even though growth was faster at the higher temperatures. In Atlantic salmon, embryo incubation temperature (4 °C versus 8 °C) altered the number of some skeletal elements, such as supraneural bones and fin rays, though not the vertebral count. INHAND also lists abnormal temperatures and low pH among the conditions associated with vertebral deformity.

Environmental, infectious and physical causes

Infection: parasitic, bacterial and viral infections have been associated with vertebral abnormalities (INHAND).
Toxicants: heavy metals and several pesticides and industrial chemicals, including polychlorinated biphenyls, are associated with vertebral deformity; exposure to oxytetracycline antimicrobials has been reported to cause bone deformity and fracture.
Physical injury: trauma and electrocution can deform or fracture bone, and WikiVet notes that a curved spine seen on radiographs may come from trauma or inheritance rather than diet.
Ageing: INHAND lists age-related changes among causes of vertebral deformation, and zebrafish show reduced vertebral bone mass with age.

Missing and shortened gill covers

Gill-cover defects have more than one origin. Lack of operculum showed measurable heritability in gilthead seabream, while vitamin C deficiency deforms the opercula and gill cartilage. In farmed Atlantic salmon and rainbow trout, fish health staff interviewed in a 2026 study named aggression from underfeeding, with fish nipping each other's gill covers, as the main cause of shortened opercula. In gilthead seabream with a defect on one side, 61% had regained external integrity after 16 months; fish with defects on both sides recovered less well.

Prevention

Feed a complete diet within its storage life, since vitamin C is lost in old or badly stored feed.
Feed fry adequately and avoid overcrowding to reduce fin and gill-cover nipping.
Keep incubation and rearing temperatures within the range documented for the species.
Avoid repeatedly breeding from fish that show the same deformity, and avoid close inbreeding in small lines.
Keep toxicants such as heavy metals and pesticides out of the water, and treat infections promptly.