Cuckoo catfish egg traits as adaptations to brood parasitism

Brood parasites exhibit fascinating adaptations to encumber their hosts with all costs of parental care. So far, avian species have dominated the research on the behaviour, evolution, and ecology of brood parasites, and there are only a few studies outside birds. Cuckoo catfish (Synodontis multipunctatus) are the only obligate brood parasites among fish and have their eggs incubated by various mouthbrooding cichlid species in Lake Tanganyika, Africa. There are major differences in the development of brood parasitism between avian brood parasites and the cuckoo catfish. Birds provide parental care, and their eggs are fertilized internally. In contrast, egg fertilization in fish generally occurs externally, and Synodontis species are scatter-spawners that do not provide parental care. The eggs of cuckoo catfish are yellower, larger, and less adhesive compared to the eggs of other catfish in the same genus and, in many ways, resemble the eggs of their hosts. Since various adaptations to parasitic reproduction on the egg level are also well-known among avian brood parasites, making them classic examples of adaptation, this project aims to test the adaptive value and functionality of the characteristics of cuckoo catfish eggs. The coloration of the cuckoo catfish eggs might facilitate their acceptance by host females, and their reduced stickiness could be crucial for successful development in the mouth of cichlid females. Adaptations in one area are often linked to limitations in others. In this case, the development of conspicuous and well visible yellow eggs might exclude the fish from their original reproductive strategy, as the eggs would be easy prey for predators when scattered in the habitat. A field experiment will test the functionality of egg coloration. Whitish (achieved through special feeding) and naturally yellow catfish eggs will be presented to host females, and the acceptance rate between the different egg variants will be compared. To test if yellow eggs are under increased predation pressure, both yellow and whitish catfish eggs will be placed on standardized artificial structures in the habitat, and their survival rates will be monitored. The structures on the egg surface responsible for stickiness will be examined using electron microscopy. By treating these surface structures with enzymes, the adhesion of eggs from free-spawning species can be artificially reduced. These eggs can then be used in comparative experiments to investigate the effect of reduced stickiness on egg survival during incubation in the mouth of host females. The study presented here will demonstrate how adaptations (in the specific case egg characteristic adaptations) can affect traits at a similar level across taxonomic boundaries.