Introduction

Constituting more than 50% of living vertebrates, fi shes are known for sexual plasticity and genetic diversity. The root causes for genetic diversity have been traced to the following (Kah and Dufour, 2011): 1. Consequent to the third genome doubling after the emergence of teleost fi shes (see Pandian, 2011), the availability of duplicate sequences for evolution of new functions (Steinke et al., 2006), 2. The relatively higher frequency of gene linkage distruption leading to faster evolution of protein-gene coding sequences (Ravi and Venkatesh, 2008), and rapid radiation and speciation (e.g., the African cichlids, see Pandian, 2012) and 3. Remarkable capacity to innovate unique reproductive strategies due to sexual plasticity (see Pandian, 2010) at both the brain (e.g., cyp19a1b) and gonad (cyp19a1a, see also Pandian, 2012). Despite the enormity of genetic diversity and reproductive strategy, the regulatory mechanisms of endocrine sex differentiation have remained highly conserved (Oliveira and Goncalves, 2008). With the advent of molecular biology, molecular endocrinology has provided considerable insights into the regulatory mechanisms of endocrine sex differentiation. Nevertheless, fi sh farming and monosex aquaculture encounter many problems. Captive rearing of fish in farms has led to reproductive dysfunction; males produce less and low quality milt (Rurangwa et al., 2004), and females fail to ovulate and/or spawn (Zohar and Mylonas, 2001). The objective of this book is to comprehensively synthesize relevant information on the role of endocrines and behavior on sex differentiation in fi shes and explore the possibilities of providing a remedy to the problems encountered by aquaculturists.