Like the cuckoo bird it shares a name with, the cuckoo catfish shirks its parental duties. The East African lake swimmer abandons its eggs to the care of other species. Now, researchers have learned how it pulls off this trick, and how its offspring get in on the action.
More than 90 species of birds, including cuckoos, are “brood parasites.” They don’t build their own nests; instead, they rely on other birds to care for their young. But the cuckoo catfish (Synodontis multipunctatus) seems to be the only other vertebrate with this strategy. In East Africa’s Lake Tanganyika, the catfish has a very particular target: thick-lipped cichlid fish that use their mouths as nurseries to raise their young. When the cichlid lays her own eggs, which she then scoops into her mouth, riotous catfish couples sneak under her to lay and fertilize their own eggs at the same spot. In the chaos created, the cichlid scoops up both her eggs and theirs.
Last year, researchers showed that some cichlid moms are smart enough to avoid picking up the catfish eggs. Now, those same researchers—along with a second team—have shown how the catfish are fighting back. In their laboratory at the University of Colorado in Boulder, evolutionary ecologist Marcus Cohen and his colleagues compared the development of the catfish and the cichlid eggs. Catfish eggs developed faster, hatched sooner, and were bigger than cichlid eggs laid at the same time, which puts the catfish young at an evolutionary advantage, Cohen’s team will report next month in the Philosophical Transactions of the Royal Society B.
Those differences are bad news for the young cichlids, who start to hatch just about the time the catfish embryos need to start to feed. The catfish intruders use their wide jaws and extra teeth to devour the new hatchings headfirst. If the catfish run out of cichlid hatchlings, they start to chow down on each other.
Meanwhile, another team has discovered that bigger eggs and faster development give the young catfish a second advantage. Martin Reichard, a biologist at the Czech Academy of Sciences Institute of Vertebrate Biology in Brno, and his colleagues found that catfish eggs that were rejected or missed by the cichlid parent end up hatching and developing just fine outside the cichlid’s mouth. But when the cichlid mom lets her brood out while she feeds, the young catfish can jump inside when she scoops her fry back up, Reichard’s team reports in the same issue of the journal.
This two-stage ability for the fish to invade sets them apart from parasitic birds, says Sheena Cotter, an evolutionary ecologist at the University of Lincoln in the United Kingdom who was not involved in the work. “That definitely doesn’t happen in birds.”
This ability of catfish young to survive on their own also suggests the catfish—unlike the cuckoo—has not yet fully evolved a dependence on the cichlid to raise its young, Reichard and his colleagues write. The tendency for the cichlids to take in catfish young may have even encouraged the evolution of this dependency in the first place, they add.
These papers should encourage researchers to look for more examples of brood parasitism, Cotter says. “It’s entirely possible you can get this happening in other systems.”