MATERIALS AND METHODS

Sample collection: Stocks of harpacticoid copepods used in this study were collected from the sea grass patch at estuaries of Merchang, Terengganu (5°2.260’N, 103°17.821’E) in the South China Sea on 16 January 2007. Meiobenthic samples were collected using the 62 μm plankton net (Somerfield et al., 2005), where the net was towed on the surface of sediment at seagrass patch. The samples were then placed in aquarium containing aerated seawater and immediately transported to the Biodiversity Laboratory, Universiti Malaysia Terengganu (UMT), Malaysia.

Culture of harpacticoid copepod: Harpacticoid copepods were reared in static (batch) cultures in 20 mL culture medium since 2007 (Rippingale and Payne, 2001). For the present study, seawater was filtered with GFC membrane filter and then autoclaved for 15 min. Culture was initiated by the single gravid female of harpacticoid copepod. The copepods were fed with 1 mL of baker’s yeast (0.02 g L^-1) daily (Nanton and Castell, 1998).Temperature was maintained at 25±1°C and salinity was at 27±1 ppt. Once the copepods produced the eggs, the hatched nauplii were counted. Then the female copepod was transferred into a new culture medium. Total number of copepods were counted under dissection microscope. The observations were carried out each day for 40 days.

Data analysis: Accidentally two culture vessels were found to support the same species of R. knoxi after identification. Thus data for both cultures were included in this report. Life table parameters were calculated using standard life table methods following Ricklefs (1990). Fertility at age x (m_x) was the average number of births per female of age x, during the time interval x to x+1. Net reproductive rate, Ro, represented the total expected number of offspring produced by a female during her lifetime. The calculation for Ro as follows:

Mean generation time (T) is the time required for a population to increase by the factor Ro. It is derived from the equation:

RESULTS

The life-history parameters including generation time, T (days) for three species of harpacticoids cultured under laboratory condition (temperature 25±1°C and salinity 27±1 ppt) for 40 days are given in Table 1. P. oculata showed the longer mean generation time (17.19±4.74 days) compared to other two species. The mean generation time of S. knabeni and R. knoxi were 10.19±3.51 days and 8.93±1.00 days respectively. The net reproductive rate (Ro) was higher in P. oculata (100.00±28.28) followed by R. knoxi (22.50±20.51) and S. knabeni (13.00±9.90). The Intrinsic growth rate (r) was higher in R. knoxi (0.58±0.18) followed by S. knabeni (0.50±0.22) and P. oculata (0.45±0.13).

Naupliar stages for all three species were completed within two days in the second generation though it took between two and seven days in the first generation.

Table 1:	Life-history parameters of P. oculata, S. knabeni and R. knoxi under laboratory condition
Values are Mean±SD, Ro: Σlxmx, T: Σxlxmx/Ro, r: lnRo/T

Fig. 1:	Population of the first generation of P. oculata, S. knabeni and R. knoxi, under laboratory condition

Fig. 2:	Population of the second generation of P. oculata, S. knabeni and R. knoxi, under laboratory condition

Copepodites also showed the same trend and took only the maximum of four days to complete the stages in the second compared to between six and 10 days in the first generation. Despite that, the duration for female to become adult and gravid was more or less the same between the first and second generation. Overall the time required for all the developmental stages to grow up to adult stage has notably reduced from first generation to the second (Fig. 1, 2).

DISCUSSION

Copepod production and their aquaculture nutritional values were extensively discussed by Stottrup (2000) and their suitability as a larval feed was studied (McKinnon et al., 2003; Gopakumar and Santhosi, 2009). In this study, three harpacticoid species (P. oculata, S. knabeni and R. knoxi) were observed to have a short generation time and the population growth seemed to be quickly adapted to laboratory culture in the second generation. Recent studies on the effect of food, environmental stability and temperature suggested that they could play an important role in affecting the generation time of copepod (Zaleha and Busra, 2012). Generation time of R. knoxi was considered short compared to the other two species in this study and others species in previous study. Similar results were noted by Rhodes (2004) who observed a generation time of 10-12 days for Nitokra lacustris. On the other hand Amphiascus tenuiremis has a generation time of 16 to 17 days (Chandler et al., 2004) when cultured under laboratory condition. Harpacticoids with short generation time proved their suitability for mass culture. This was found in Tisbe biminiensis and Tigriopus japonicus (Pinto et al., 2001). The longer generation time in most of the life stage period in the first generation might be caused by the physiological adjustment to the culture condition. The past moving water and tidal change from the South China Sea influenced the harpacticoids in the area (Zaleha et al., 2006) and naturally they need to adjust their physiology in response to the calm environment in the culture bottle. Some species collected from the same vegetative area including Paralaophonte octavia and Longipedia sp. (Nurul Huda and Zaleha, 2005) failed to survive the same laboratory condition.

Nauplii stage is an important first day feed for some fish larvae (Stottrup and Norsker, 1997). Larval stages of shrimp such as Litopenaeus vannamei was also reported to prefer the naupliar over the copepodite stage of harpacticoid (De Lima and Souza-Santos, 2007). The short life stage for the species found in this study (2 days) needs an efficient collecting or harvesting technique if they are planned to be used as live feed in aquaculture. Culture technique by Sun and Fleeger (1995) for negative phototaxis species might be a good example. Nevertheless, further study of phototaxism of the present species need to be carried out to facilitate the culture technique design.

CONCLUSION

Of the three cultured species, R. knoxi showed its potential as a good candidate for mass culture due to their short generation time (8.93±1.00 days). On the other hand, P. oculata showed more stable and high population starting from the first generation thus also indicates its commercial potential. The Naupliar and copepodite stage took longer time during the first generation in all studied species. The finding on the longer time taken by the harpacticoids to stay in their juvenile stages as shown in this study could be further investigated as to be manipulated for hatchery industry. Further research is also important to design a technique to sample the different life stage of the population.

ACKNOWLEDGMENT

The authors are indebted to ABI-MOSTI, Malaysia for the e-science research grant (07-05-ABI-AB005) on ‘Mass Fry Production Technology of Grouper (Epinephelus sp.): Production of Live Feeds’.