Embryonic and Larval Development of Freshwater Fish

Abstract

During the past few years the natural population of the freshwater fishes has been rapidly declining due to various man-made and natural causes. According to IUCN (2000), among 266 species, 14 are going to be extinct, condition of 12 has been severely deteriorated and 28 of them are critically endangered. Moreover, the fish are also under threat due to drying up of the low lying areas and indiscriminate use of fertilizers and pesticides. There is no sufficient information on the early development of the freshwater fishes. So it is necessary to undertake proper study to characterize its various stages of embryonic and larval development to understand the biological clock and cultural techniques of these species (Rahman et al, 2009). Embryonic studies support phylogenetic development by presenting supportive proofs to determine an organism's ancestral forms. For example, it describes evolutionary development by explaining many issues like gill cleft in the lower vertebrates (fish) which is seen in almost all mammalian embryos in early developmental stages. In addition, this period of fish life is also used in various experimental studies; especially in aquaculture as well as toxicological studies (Rahman et al, 2009). Life starts with the unification of male and female gametes. As soon as the egg is fertilized by a sperm, the zygote is formed and embryonic development starts and ends up at hatching. The hatchlings further undergo organogenesis and appear as like as their parents, thus end the larval stages. Egg development in the ovary is maternally derived and is predetermined in the ovary but its genetics complex is determined at the very instant of fertilization (Rahman et al, 2009). In this section, embryonic and larval developments of freshwater fish were examined. Female fish used for egg production and the hormones using for the egg production mentioned in the text, and then fertilization phase which is the first foundation of the creature and some environmental factors affecting this process are described. In this context, after the fertilization formation of freshwater fish, the embryonic developmental stage, fertilized egg, cleavage, morula, blastula, gastrula, embryonic body formation, optic vesicle and auditory vesicle formation, blastopore closing, tail formation and hatching stages were examined. In the period of larval development after hatching, until the end of the yolk sac absorption period (pre-larvae) and subsequently until the end of metamorphosis (postlarval) formations are shown.