Reproductive physiology of Arapaima gigas (Schinz, 1822) and development of tools for broodstock management

Abstract

Arapaima gigas is the largest scaled freshwater fish in the world reaching over 250 kg. With growth rates of 10 kg+ within 12 months, A. gigas is considered as a promising candidate species for aquaculture development in South America. However, the lack of reproductive control in captivity is hindering the industry expansion. The work carried out in this doctoral thesis therefore aimed to better understand the species’ reproductive physiology, develop tools to identify gender and monitor gonad development, test hormonal therapies to induce ovulation and spawning and characterise the cephalic secretion for its potential roles in pheromone release and during parental care. Initially, a genomic study investigated the overall extent of polymorphism in A. gigas, which was found to be surprisingly low, with only 2.3 % of identified RAD-tags (135 bases long) containing SNPs. Then, a panel with 293 single nucleotide polymorphism (SNP) was used to characterise the genetic diversity and structure of a range of Amazon populations. Results revealed populations from the Amazon and Solimões appeared to be genetically different from the Araguaia population, while Tocantins population comprised individuals from both stocks. This data provided a tool for broodstock identification and future management. The PhD then aimed to evaluate the effects of slow-release mGnRH implants and different broodstock size pairings on maturation and spawning. Results showed that the implants stimulated the brain-pituitary-gonad axis resulting in increased plasma levels of testosterone (females) and 11-ketotestosterone in males, respectively regardless of pairing sizes. However, no spawning was observed. Results also showed the release of sex steroids with potential pheromonal action through the cephalic secretion, a biological fluid released from the adult head along the reproductive period. Thereafter, a non-surgical field endoscopy method was developed and validated for ovarian assessment and gender identification. The method was then used to describe the female gonopore and obtain biopsy of the ovary through cannulation which allowed the description of oogenesis in A. gigas. Importantly, oocytes obtained by cannulation confirmed that adult females under investigation were maturing with oocytes in final maturation stage but failed to ovulate/spawn. Another hormonal induction trial was therefore performed in which a combination of GnRHa (mGnRHa/sGnRHa) was used by injection to induce ovulation and spawning in selected maturing females with effects on oocyte maturation monitored post-induction through biopsy. However, this trial appeared to not be successful at inducing ovulation or spawning. Finally, the peptidome and proteome of the cephalic secretion was further characterised through the comparison between parental and non-parental fish. Results highlighted the complex role of this biological fluid including potential roles on the developing offspring during the parental care period. Overall, this doctoral thesis provided new basic and applied data on A. gigas reproduction and tools that can be used in future studies to better understand the environmental and hormonal control of oogenesis and spawning.