Frontispiece. A male from a breeding pair of discus fish (Symphysodon spp.) providing produced mucus as a source of nutrition to offspring. Parental care and the development of the parent offspring conflict in discus fish (Symphysodon spp.) by Jonathan Buckley A thesis submitted to Plymouth University in partial fulfilment for the degree of DOCTOR OF PHILOSOPHY School of Biomedical and Biological Sciences October 2011 i Copyright Statement This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without the author's prior consent. Jonathan Buckley October 2011 ii Parental care and the development of the parent offspring conflict in discus fish (Symphysodon spp.) Jonathan Buckley Abstract Parental care has evolved across the animal kingdom to increase the probability of offspring surviving in an environment fraught with danger. While parental care is common among mammals and birds, it is relatively rare in fish with the vast majority of fish showing no form of parental care at all, whilst those that do, often just provide parental care to developing eggs pre-hatch. The provision of parental care in discus fish (Symphysodon spp.) is, therefore, interesting in that parents provide mucus to offspring as a source of nutrition during the first few weeks of care. In mammals this post-birth provision of parental care can lead to the development of the parent offspring conflict. It is, however, possible that this conflict is also present in discus fish. This thesis examines both the interesting parental care strategy of discus fish along with the potential for the parent offspring conflict to develop. To examine the dynamics of parental care in discus fish, a range of behavioural and mucus composition studies were carried out. The analysis of mucus revealed that similar to mammals, parents provided offspring with an initial high quantity of nutritional and non-nutritional factors including antibodies (IgM), essential ions and hormones. Behavioural studies also revealed that initially parents were highly diligent in providing care to offspring but that after two weeks of care, the behaviour of parents changed making it harder for offspring to obtain mucus. At this point a weaning period was initiated where offspring began spending less time with parents and more time foraging for external food sources. The initiation of this weaning period suggests the presence of the parent offspring conflict and indicates that a point is reached where the energetic demands of offspring are too great and that energy is better invested in to future offspring. Research into the bite size and feeding rate of fry suggest that during the weaning period fry could demand excessive amounts of mucus, which may be energetically unsustainable leading to the observed offspring avoidance behaviour of parents. As parental care behaviour is known to be intimately associated with mate choice, mate choice behaviour was also assessed in discus fish with the hypothesis that the ability to provide mucus would be selected for by prospective mates. My dietary experiment, which examined the effect of dietary protein on an individual’s ability to mate, did not influence mucus quality or mating ability. The mate choice experiment, however, did reveal the importance of hierarchies in discus fish, indicating that dominant individuals were significantly more likely to pair than subordinates. This is similar to that observed in closely related cichlids where the ability to be dominant and protect a territory was indicative of the ability to successfully raise offspring. In conclusion, the parental care behaviour of discus fish appears to share more similarities with that seen in mammals than that observed in fish. The implications of these findings indicate that parental care in discus fish could be a new model of parent offspring conflict hitherto unseen in fish which could ultimately help our understanding of the evolution of parental care in fish. iii List of contents Abstract.......................................................................................................................... iii List of figures................................................................................................................. xi List of tables................................................................................................................. xxi Acknowledgements..................................................................................................... xxii Chapter 1. Parental care and the development of conflict in an Amazonian cichlid: a review. 1.1 Abstract................................................................................................................... 2 1.2 Introduction............................................................................................................. 3 1.3 Nutritional/Non-nutritional composition of mucus................................................. 5 1.3.1 Nutritional factors.................................................................................... 6 1.3.2 Immune factors...................................................................................... 10 1.3.3 Hormones............................................................................................... 13 1.3.4 Ions........................................................................................................ 18 1.4 Wild versus aquarium bred discus........................................................................ 20 1.5 Environmental contaminants................................................................................. 21 1.6 Conflict associated with parental care................................................................... 25 1.6.1 Parent –Offspring conflict: offspring solicitation of care...................... 26 1.6.2 Offspring-Offspring conflict.................................................................. 31 1.6.3 Parent-Parent conflict............................................................................ 32 1.7 Conclusions........................................................................................................... 35 1.8 Project aims and objectives................................................................................... 36 iv Chapter 2. Biparental mucus feeding in an Amazonian cichlid, a unique example of parent offspring conflict 2.1 Abstract................................................................................................................. 38 2.2 Introduction........................................................................................................... 39 2.3 Materials and methods.......................................................................................... 44 2.3.1 Experimental fish and husbandry.......................................................... 44 2.3.2 Behavioural observations...................................................................... 44 2.3.2.1 Distribution of parental care...................................................... 45 2.3.2.2 Bite rate..................................................................................... 45 2.3.3 Mucus physiology methods................................................................... 45 2.3.3.1 Mucus sampling......................................................................... 45 2.3.3.2 Bradford protein assay............................................................... 46 2.3.3.3 Chloride assay............................................................................ 46 2.3.3.4 Ions............................................................................................ 47 2.3.3.5 Antibody (IgM) detection and validation.................................. 47 2.3.3.6 Cortisol...................................................................................... 64 2.3.4 Statistical analysis.................................................................................. 64 2.3.4.1 Physiology................................................................................. 64 2.3.4.2 Behaviour................................................................................... 64 2.4 Results................................................................................................................... 65 2.4.1 Time on parent....................................................................................... 65 2.4.2 Change in parental duties...................................................................... 65 2.4.3 Bite rate................................................................................................. 66 2.4.4 Mucus IgM concentration...................................................................... 68 2.4.5 Mucus total protein concentration......................................................... 69 v 2.4.6 Mucus ion composition......................................................................... 70 2.4.7 Mucus cortisol concentration................................................................. 71 2.5 Discussion............................................................................................................. 74 2.6 Conclusion............................................................................................................. 79 Chapter 3. Comparative analysis between wild and aquarium bred discus 3.1 Abstract...................................................................................................................