WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2013 41 ecosystems, conditioned by a high degree of instability in physical (temperature) and biological (food) conditions (Calbet and Alcaraz 1996). Acartia grani has been recorded in different areas of the Atlantic Ocean, such as the Canary Islands (Corral 1970, Vives 1982), the Western Mediterranean Sea (Guerrero and Rodríguez 1998, Saiz et al. 1998) and the Portuguese coast (Vilela 1972). This calanoid species presents good potential for use as live feed in aquaculture (Cunha et al. 2007) because it is one of the most abundant in oceanic systems and they are an important component in the diet of a great number of fish species. Preparation of Acartia Stock Solution Acartia grani eggs (Fig. 1) used in this study were from an “egg bank” that contained harvested and “cleaned” eggs that were stored in 1-µm filtered and UV-treated seawater in closed conical Falcon tubes at 4 C. Tube contents were rinsed on a mesh (55 µm) with distilled water to wash out accumulated feces stored with eggs (Fig. 2). This procedure does not affect the viability of unhatched eggs (Knuckey et al. 2005). Eggs were then transferred to a 500-mL beaker containing seawater. Five 1-mL samples were taken from the beaker using an automatic pipette (Pipetman – Gilson 1000 uL). The number of eggs in each sample was counted using a Sedgewick-Rafter counting cell and a microscope (Zeiss – Axioskop 2 Plus). The coefficient of variation of egg number among samples was less than 10 percent. The average number of eggs in samples was calculated and used to estimate egg concentration in the stock solution. Effect of Temperature on Acartia Hatching Rate Seawater (300 mL, 1-µm filtered, UV treated, salinity 38 ± 1 psu) was added to each of twelve 600-mL beakers. Each beaker was stocked with 150 A. grani eggs and incubated at four temperatures (18.0, 22.0, 24.0 and 28.0 ± 0.5° C) using water baths; each incubation temperature was replicated in three beakers. Throughout the experiment, great care was taken to ensure that the correct temperatures were maintained in all water baths and that all other conditions were similar in all replicates. Each beaker was exposed to continuous light (approximately 1200-1500 lux). Every two hours, all hatched nauplii were carefully removed from each beaker with a plastic pipette and discarded. This procedure was repeated every two hours, until each replicate reached a hatching rate of 50 percent. The time necessary to reach 50 percent hatching rate decreases with increasing temperature (Fig. 3). For copepods incubated at 28 C, the 50 percent threshold was reached within 24 hours, compared to 35 hours for copepods incubated at 18 C. A 50 percent egg hatching rate was achieved after 35 hours at all temperatures considered. Milione and Zeng (2007) tested hatching rates of Acartia sinjiensis (a species belonging to the same family as Acartia grani) at 28 C and obtained a mean hatching rate approximately of 35 percent within 48 h. In our study, the hatching rate at the same temperature was more than double (75 ± 5 percent) after 35 hours of incubation. Overall the effect of temperature on egg hatching rate are FIGURE 2. Parental culture of Acartia grani: (A) filter set and ultraviolet sterilizer; (B) stored eggs of Acartia grani stored at 4 C and ready for use; (C) 500-L production tank; (D) production tank with the first eggs on the bottom; (E) eggs and organic matter on the tank bottom; (F) egg collection system consisting of two sieves (200 and 55 µm) and squeegee siphon for cleaning. (CONTINUED ON PAGE 42)
RkJQdWJsaXNoZXIy MjExNDY=