Jack Dempsey

What it is

The Jack Dempsey is a medium-sized cichlid with a robust body and strong facial features. Its name references a famous boxer due to its aggressive look. Adults display dark purple-grey bodies with bright iridescent blue, green, and gold flecks. Two grey-black bars run across the top of the head between the eyes. Mature males have long pointed tips on their dorsal and anal fins, while females do not. The fish changes colour under stress or during breeding, often turning almost black. Aquarium variants include Gold, Electric Blue, and pink forms.

Where it lives

This species is native to freshwater habitats in southern Mexico and Honduras. It thrives in slow-moving, warm, and murky waters such as swamps, weedy canals, and drainage ditches. It also inhabits mud- and sand-bottomed rivers. Outside its native range, it has established populations in Australia, the United States, and Thailand. In Australia, it was found in a flooded quarry in New South Wales. It prefers temperatures between 22 and 30 degrees Celsius but can survive in cooler water down to 8 degrees.

How it lives

Jack Dempseys are carnivorous predators that eat worms, crustaceans, insects, and smaller fish. They are known for their aggressive behaviour and territorial nature. In the wild, they dominate space and compete fiercely for food. This aggression helps them survive in varied conditions but poses risks to native species where introduced. They are hardy and adaptable, capable of living in diverse water conditions. Their diet includes almost anything smaller than themselves, including frogs and invertebrates.

Breeding and life cycle

Females lay between 500 and 800 eggs on flat hard surfaces like rocks or logs. Both parents participate in incubating the eggs and guarding the fry after hatching. They show substantial parental care, often pre-chewing food for their young. However, stress or environmental disturbances can cause them to eat their own fry. During spawning, both genders become extremely aggressive towards other fish to protect their territory and offspring.

Fish and people

This fish is a popular choice for aquariums due to its striking appearance and personable mannerisms. It requires a tank with plenty of hiding places like caves. It is not suitable for community tanks because of its territorial aggression. In Australia, it is considered an invasive species likely released by aquarium owners. Eradication attempts using explosives in New South Wales failed to remove the population. Experts warn that imported ornamental fish pose significant biosecurity risks to local ecosystems.

Status and threats

The species is established in several non-native regions, causing concern for local biodiversity. In Australia, it is one of many aquarium fish that have impacted native aquatic ecosystems. Authorities attempted eradication in 2004 and 2005 but ceased further work. Non-native fish are implicated in the decline of many native Australian fish and frog species. The introduction of such species is considered a key threatening process, though import controls remain limited.

Relatives and look-alikes

It belongs to the cichlid family, known for complex behaviours and parental care. It is sometimes confused with other large aggressive cichlids. A natural genetic mutation has produced the Electric Blue variant, which is smaller and less aggressive than the standard form. It is distinct from the African Frontosa despite the nickname Mexican Blue Frontosa used in some regions.

Questions people ask about Jack Dempsey

Good reading and sources

• Jack Dempsey (fish) on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data