Striped Killifish

Geographical Origin and Habitat: Fundulus majalis is native to the coasts of the Western Atlantic Ocean, distributed from the state of Massachusetts to northern Florida in the United States. It inhabits shallow coastal waters, lagoons, brackish estuaries, salt marshes, and tidal pools. It is a euryhaline species, capable of tolerating wide and rapid fluctuations in salinity, often moving in large schools over sandy or muddy bottoms.

Taxonomy and Morphology: It belongs to the family Fundulidae, genus Fundulus. It presents an elongated, robust, and tapered body with a dorsally flattened head and an upward-directed mouth, ideal for feeding at the surface. It reaches a maximum adult length of about 15 centimeters, although average sizes in the aquarium are smaller. The scales are large and cycloid.

Social Behavior: It is a gregarious fish and a very active swimmer that forms cohesive schools in nature. In the aquarium, it must be kept in groups of at least 6 specimens to avoid stress and display natural behaviors. It swims mainly in the upper and middle levels of the water. It is a peaceful but very energetic species that can intimidate excessively slow fish.

Coloration and Sexual Dimorphism: The background coloration is silvery or olive-grayish with metallic reflections. The sexual dimorphism is among the most evident in the family: adult males present 15 to 20 dark vertical stripes along the flanks and a black spot on the dorsal fin during the breeding period, while adult females present 1 to 3 black horizontal longitudinal stripes.

Aquarium Setup: It requires a spacious aquarium of at least 150-200 liters to house a group. The tank must offer wide open spaces for free swimming and must be set up with a bottom of fine sand, as these fish love to rummage in the substrate. It is advisable to insert plants resistant to brackish water (such as Vallisneria or macroalgae) and rocks. A secure cover is essential.

Diet and Feeding: It is a voracious and opportunistic omnivore. In nature, it feeds on small crustaceans, insect larvae, worms, plant debris, and algae. In the aquarium, it readily accepts any type of dry food (flakes and granules), but benefits greatly from frozen or live foods such as mysis, brine shrimp, bloodworms, and chopped krill. Supplement with spirulina-based foods.

Water Quality: Very tolerant, but prefers brackish or marine water with specific gravity between 1.005 and 1.020. Temperature: 15-25°C (tolerates lower temperatures, being a temperate species), pH: 7.5-8.2, GH: 12-25. It is essential to ensure efficient filtration and regular water changes to keep nitrogenous compound levels low.

Compatibility and Cohabitation: It can coexist with robust fish of similar size in brackish or temperate marine aquariums. Excellent tankmates are other members of the genus Fundulus, Cyprinodon, or peaceful marine fish like medium-sized gobies. Avoid cohabitation with very small species that could be mistaken for food or with slow fish with long fins.

Aquarium Breeding: Breeding in the aquarium is possible and is stimulated by variations in salinity and temperature. The eggs, adhesive and equipped with filaments, are laid among the vegetation or in the sand. They do not provide parental care and may prey on the eggs; to collect the offspring, it is preferable to use a spawning mop or remove the parents after mating.

Risks and Diseases: The risk of jumping out of the tank is very high, especially during the early stages of acclimation. It is a robust species that rarely falls ill if water quality is maintained properly; however, it can contract bacterial skin infections or fin rot if water values degrade or if kept in water that is too fresh or thermally unstable for a long time.