Effects of temperature and salinity on Penaues monodon eggs incubation time and hatching
The fertilized eggs spawned by female prawn were spherical translucent with a narrow perivitelline space (Zacharia and Kakati 2002, Crisp et al. 2016, Crip et al. 2017, Plate 9). Embryo development initiated immediately after spawning and developing nauplii almost completely filled the cavity within the egg. The incubation time of P.monodon eggs embryo development took 8-15 hours depending on the temperature and salinity and hatched out as first non-feeing nauplius larvae. In crustacean, reproduction usually is dependent on environmental factors and commonly occurs in narrow range than those required for growth and survival of the adults (Preston 1985). The egg incubation time of P. monodon species is mainly dependent on quality of environmental factors. Temperature and salinity are two most essential environmental parameters that affect hatching time and hatching rate of Penaeids Spawning, hatching and larval development usually occurs in marine waters were temperature and salinity are relatively stable with no considerable fluctuations (Aktas and Çavdar 2012). Under this circumstance, variation in water temperature and salinity have an impact on P. monodon egg incubation time and hatching success. The environmental factors, temperature and salinity optimum for egg incubation time and hatching rate are species specific and are usually determined by each species (Preston 1985).
Early studies on P. monodon suggested to have requisite egg incubation temperature between 260C to 290C (Primavera 1985). The latter research revealed that an increase in temperature from 230C to 280C or 330C decreased the incubation period but had no effect on hatching rate (Aktas and Çavdar 2012). In this study higher temperature-salinity combination 350C and 40ppt reduced P. monodon egg incubation time and hatching percentage than combinations of 270C and 30ppt temperatures and 310C and 35ppt salinities which had long incubation time and high mean hatching percentage. Though high salinity reduced incubation time, the effect of temperature on the same was more pronounced. Short incubation time in high salinity may be due to absorption of salt which creates more internal pressure that facilitates easy rapture of egg membrane (Soudarapandian 2008). This coincide with the study on the effect of salinity and osmolarity on incubation and hatching of fresh water Macrobrachium malcolmsonii in which the incubation time was 14days in fresh water but reduced to 11days with addition of brackish water in 7ppt (Soudarapandian 2008). A salinity of 35ppt also was reported as optimal for hatching success for P. monodon (Nisa and Ahmed 2000).
Temperature being one of the abiotic environmental factors is mostly important in regulating egg development for several crustacean species (Choy 1991, Zeng et al. 1991, Arshad et al. 2006). Incubation time of P. monodon eggs in this study was principally determined by temperature. Temperature 270C had the longer incubation period in all salinities than the rest temperatures 310C and 350C. However, hatching rate of P. monodon eggs was higher in 310C than in 270C and 350C. This is due to the fact that lower temperature reduces the rate of egg development in crustaceans as embryo metabolic rate is decreased and high temperature increase egg development rate due to increase in metabolic rate at temperatures (Robertson and Kruger 1994). The results agree with the study on Metapenaeus monoceros in which the incubation time was shorter at 320C (11.2 hours) than at 280C (14 hours) or 240C (17.2 hours) (Aktas and Çavdar 2012). Furthermore, the findings on Penaues semisulcatus indicated to have longer incubation time at 240C (17.5 hours) than at 280C (14.5 hours) or 320C (11.5 hours) (Aktas et al. 2004).
According to Reyes (1985) incubation period of P. monodon in temperatures 230C, 280C and 330C was 22, 16 and 14 hours respectively. However highest hatch rate of the same species was obtained at 230C and 33ppt, 300C and 30ppt (Reyes 1985, Ch and Shailender 2013). Study on effect of temperature on the embryonic development, morphometrics and survival of Macrobachium idella iddella showed decrease in incubation time in high temperature 330C (241hours) and increased incubation period at low temperature 260C (302hours) (Soundarapandian et al. 2014). Nevertheless, the fresh water prawn Macrobrachium rosenbergii revealed that incubation period was significantly affected by temperature. The longest mean incubation period of M. rosenbergii of 23days was recorded at the lowest tested 240C temperature while the shortest period of 17 days was reported at 340C temperature (Habashy and Hassan 2011). The study on egg incubation time of Portunus pelagicus, blue swimming crab decreased from 8.33 to 6.67 days with increase in temperature from 280C to 320C (Arshad et al. 2006). Based on results, the ideal incubation temperature range is 270C-310C and salinity 30ppt-35ppt for P. monodon. Temperature above 310C and salinity above 35ppt is unsuitable for incubation of P. monodon eggs. Consequently, salinity and temperature at a certain limit increases the incubation period and hatching.
Effect of temperature and salinity on survival, growth and development of P. monodon larvae
Salinity and temperature are two of the most important abiotic factors affecting the growth and survival of aquatic organisms (Kumlu et al. 2000). Larval stages of most penaeid prawn species occur in seawater and at stable water temperatures and salinities. Therefore, it is generally agreed that penaeid prawns are intolerance in significant fluctuating environmental conditions during their early developmental stages (Kumlu et al. 2000). Generally, it is accepted that temperature has a more pronounced effect on growth and survival of penaeids (Parado-Estepa 1998). High survival rates of P. monodon are important to be attained for successful rearing. Survival rate could be an indicator of the final product to be achieved in the which later will determine the final abundance of the prawn product to be harvested.
The life cycle of P. monodon is dependent on the environment factors especially at the early stages of development in which species embryonic and early larval development are much more attributed to the natural environments (Ch and Shailender 2013). Embryonic development usually depends on the quality of deposited egg yolk up to first nauplii larval stage. The capability the species to tolerate fluctuating conditions of environmental conditions of temperature and salinity is an indication of satisfactory completion of its life cycle. Penaeid species patterns of their development are closely related to members of the genus (Zacharia and Katati 2004, Ch and Shailender 2013). Development pattern of P. monodon were observed based on appearance of morphological features at each larval stage. Features in the present study observed in the larval stages were similar to that mentioned in P. monodon (Motoh 1985), P. merguiensis (Zacharia and Katati 2004) and in Metapenaeus dalli (Crisp et al. 2016, Crip et al. 2017). The growth and development of P. monodon larvae usually occurs by moultiung as that of other crustaceans. The most characteristic feature was the enormous growth taking place during the metamorphosis of nauplius to the first protozoeal stage. More moultng frequencies occur at the early stages resulting to vast growth and development following metamorphosis. The growth in length of P. monodon at this stage was high. Similar observations of maximum growth of the larva during the metamorphosis of the last nauplius into the first protozoea were reported in several penaeid species P. merguiensis (Zcharia and Kakati 2004), M. monoceros (Aktas and Çavdar 2012), P. monodon (Ch and Shailender 2013), M. rosenbergii (Mohanty et al. 2016), M. dalli (Crisp 2017), M. Idella Idella (Soudarapandian 2008).
Each species to have a unique range of salinities suitable for embryonic development and hatching, which bears no a priori relationship to the tolerance of adults or larvae (Preston 1985. Zacharia and Kakati 2004). The survival, growth and development of P. monodon at each stage varied according to temperature and salinity conditions in which the species were reared. The maximum larval survival rate was at 310C and 30ppt followed by 270C and 35ppt based on cumulative larval survival result. Other high tested temperature and salinities had low survival rate. This indicate that salinity had more effect on larval survival than temperature based on statistical analysis of the results. The result of the study is supported by studies on P. semisulcatus (Kumlu et al. 2000) P. merguiensis (Zacharia and Kakati 2004) and Ch and Shailender 2013) that reported the salinity to have caused more effect of survival than temperature. As temperature increased there was decrease in survival of P. monodon larvae. Combined effect of temperature and salinity showed interaction in which at later larval stages could worsen the surviving larvae (Ponce-Palafox et al. 1997, Kumlu et al. 2000). This also coincides to the study on M. monoceros that reported mortality to increase at relatively high temperatures (Aktas and Cavdar 2012). The study by Kumlu et al. (2000) reported temperatures above 300C were found to increase mortality in larval P. semisulcatus and study by Crisp et al. (2017) on Metapenaues dalli which had low survival at 32.60C and high in 29.40C. The Penaeid larvae body chemistry basically has their lethal limit, exceeding or below normal may have an impact on larval survival (Crisp et al. 2017). This fact corresponds to the suggestion that increase in salinity to a certain point, increase the moulting frequency and larval growth but reduce the survival of penaeid prawn because of low protein content deposited in the body tissues (Parado-Estepa, 1998, Kumlu et al. 2000). The P. monodon larvae survival rate cultured in 30ppt and 35ppt, at 270C and 310C were almost equal in all stages. The result also corroborates with the study on P. semisulcatus that indicated that the optimal salinity and temperature combination during the protozoea larval stages was 30ppt and 300C (Kumlu et al. 2000). In order to optimize rearing conditions of P. monodon conditions, mutual concessions should be made between growth and survival data (Staples and Heales1991). The consensus was made by calculating the performance index which was high in 310C and 30ppt followed by 270C and 35ppt. Based on the survival and growth results and the evidence from the performance index that as result of harmonized survival and growth data. It’s there suggested that the optimal temperature and salinity suitable for rearing of P. monodon larvae of the coast of Tanzania waters is between 270C-310C and 30ppt -35ppt respectively. The suggested range of temperature and salinity are basically those related to the oceanic conditions.