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By Giles Badger, Producer & Tom Parry, Researcher on Seven Worlds, One Planet

Over 1,800 species of cichlid fish live in Africa’s Rift Valleys lakes. In this crowded world, many cichlid species use ‘mouth-brooding’ to both incubate their eggs, and protect their young.

For several weeks after hatching, a mother’s mouth is a mobile crèche for her vulnerable fry. This remarkable strategy would be failsafe, were it not for the cunning of the cuckoo catfish, Synodontis multipunctatus. This small bottom dwelling fish lives on the bottom of lake Tanganyika, and here it employs the grisly tactic of brood parasitism.

Once they detect spawning cichlids on the bottom of the lake, shoals of catfish approach en masse, consuming the cichlid eggs whilst simultaneously laying their own.

The female cichlid unwittingly collects both her own eggs and those of the catfish before finding a protected nook to incubate her eggs in her mouth over a period of 2-3 weeks. However the cuckoo catfish eggs develop quicker, hatching first, whereupon the catfish fry either eat the cichlid fry in the mouth of their mother to be, or cause her to eject her fry prematurely.

The Seven Worlds, One Planet team wanted to film this behaviour for the Africa show. Across the series we were aiming to find new and surprising stories just like this.

Using drones and underwater photography, we captured the kaleidoscope of cichlid species, and the mouth brooding behaviour on camera. But to film the cuckoo catfish in action was an entirely different prospect.

The bottom of lake Tanganyika is murky with very little available light. Cuckoo catfish are extraordinarily shy, and their eggs are little bigger than a pinhead. On top of this the parasitism behaviour is highly unpredictable, so seeing, let alone filming it, is virtually impossible.

To capture this scene in detail, the Seven Worlds, One Planet team collaborated with scientists from the institute of vertebrate biology in Brno in the Czech Republic, who are studying the brood parasitism of this species to better understand how cichlids are able to detect this trickery. A month before filming, the team rigged bespoke lighting at a research aquarium, allowing the fish to get used to marginally increased light levels day by day. This, together with low light camera technology, enabled them to witness the behaviour first hand, and crucially do so without disturbing the fish.

It also meant the team could spend 14 hours every day for two weeks, waiting to film this rare behaviour - something that would not have been possible in the murky depths of Lake Tanganyika. Filming in tanks allowed the team to position cameras just centimetres from spawning cichlids and eventually capture this rare event in unprecedented detail.