Is the Parental-Care Behaviour of Aequidens Paraguayensis (Cichlidae) Optimal?

Abstract

AbstractAequidens paraguayensis from South America is a biparental larvophilic mouthbrooder. Adhesive eggs are laid on a substrate and fanned until hatching (substratebrooding). The hatching larvae are taken into the mouth by the female. Subsequently the parents relieve each other in mouthbrooding their offspring (larvophilic mouthbrooding). In comparison with pure substratebrooders-which are thought to represent the ancestral state-the time lag between spawning and hatching is shortened. This is interpreted as due to a general selective advantage of the mouthbrooding technique over the substratebrooding technique. Why then didn't evolution favour the transition from mouthbrooding of larvae to mouthbrooding of eggs (ovophilic mouthbrooding) thus allowing the extension of mouthbrooding up to spawning like in other cichlid species? An hypothetical answer is that this is not due to the lack of the adequate selection pressure but may relate to the nonavailability of appropriate ovophilic variants. This would imply that the parental care behaviour in this species is not optimal. To support this hypothesis experimental evidence on how broodcaring is regulated is given: 1. The uptake of larvae at hatching is induced by stimuli from the hatching wrigglers. Mouthbrooding can be induced at any time of the substratebrooding phase by presenting larvae to the adults. 2. The readiness to take up larvae into the mouth is maintained by the simultaneous presence of larvae within the mouth and of egg-like structures outside of it. 3. Females with induced uptaking readiness are not able to retrieve adhesive eggs from the substrate. 4. Females without uptaking readiness fan adhesive eggs, but eat loose eggs. 5. Females with induced uptaking readiness take up loose eggs in their mouths and mouthbrood them successfully at least until hatching. They behave like ovophilic mouthbrooders. Experimentally created ovophilic variants can be used to test the non-optimality hypothesis in nature or under semi-natural conditions. It is concluded that at least two independent traits must change simultaneously within the same generation to produce an ovophilic variant out of a larvophilic ancestor: on the one hand induction of an egg eating inhibition together with mouthbrooding readiness, on the other hand loss of adhesiveness of eggs. In the neodarwinian view of evolutionary change this is only possible if there is pleiotropy of the traits under consideration. The unlikeliness of pleiotropy may explain why A. paraguayensis has not become an ovophilic mouthbrooder, i.e. is not optimally designed.