Temporal Variations in Reproduction, Growth, Condition Index and Some Population Parameters Ofperna Viridis (Linnaeus 1758)

The aim of this study was to investigate the temporal reproductive cycle, growth rate, sex ratio, spat fall densities, condition index and population parameters of Perna viridis in Sebatu, Malacca. Condition Index (which relates tissue dry weight to shell volume), of the green mussel P. viridis was concurrently studied with the histological observations of the gonads. Monthly samples of P. viridis were collected from the same culture site. Variations in average monthly Condition Index ranged from 21.06 to 26.72 g/cm3. Rapid declining of the Condition Indices of the mussels was recorded in December, 2003 and January, 2004. This rapid declining indicates the spawning period which agrees with the results from the histological study. Studies on the gametogenesis, size at sexual differentiation, and size at first maturity of P. viridis from Sebatu, Malacca were performed using histological sections. More than three hundred specimens of P. viridis between 14.96 and 113.52 mm in total shell length were examined. No gonadal development was observed in individuals smaller than 17 mm. Sexual differentiations began at 18 mm with incipient acini formation. Specimens larger than 20.0 mm had well-developed gonads at different stages of maturation. Size at first maturity was smaller than previously reported at other localities. Active gametogenesis was recorded throughout the study period, with all stages of development usually occurring simultaneously within the same sample population. In the determination of gonadal index (GI), over 300 individuals were histologically prepared and analyzed. A clear fall in gonadal index was observed in the months of January and February, at this time most of the female were clearly spent. Their acini were almost empty of mature ova. Except of some residual ova, most of the surface area of the lumen of the acini were occupied with masses of cellular derbies. A second fall in GI with less intense was observed again during the months of September and October. Gonadal index results show that spawning event in P. viridis is year around except during the months of May until August. The release of gamete between sexes was fairly synchronized, no differences in GI were observed between the male and female. 1000 specimen of P. viridis were used for sex ratio. The number of visually identified male, female and undetermined by looking at the mantle was 524, 412 and 64, respectively. The male: female ratio was 0.79:1 by visual identification. 316 individuals were histologically identified for sex ratio. The number of identified male, female and undetermined individual was 161, 151 and 4. The histological male: female ratio was 0.94:1. Two hermaphrodites specimen were recorded during the months of June and September, 2004. Both specimens had the occurrence of the male and female gametes on only one side of the mantle. No trace of hermaphroditism was observed neither in the second side of the mantle nor the mesosoma. Spat fall study of P. viridis was carried out from February, 2004 until December 2004 on collector ropes suspended from the same culture site. Spat fall of the mussels was active during the entire study period with two peaks, one main peak during the months of March to June and a second peak during the months of August to December. Average, maximum and minimum settlement densities were 11,389, 29,788 and 1649 spat per meter respectively. Maximum settlement densities were counted during the month of April, while the minimum settlement densities were counted during the month of August. Population parameters like asymptotic length (L∞), growth co-efficient (K), mortalities, exploitation level (E) and recruitment pattern of green mussel P. viridis from the same site have been studied. The study was carried out using the length frequency based analysis of FiSAT software to evaluate the growth parameters, mortality rates and exploitation level. Asymptotic length (L∞) was 102.38 mm and growth co-efficient (K) was estimated at 1.50 yr-1. Total mortality was 1.44 yr-1 for P. viridis. Natural mortality (M) and fishing mortality (F) were 1.67 yr-1 and 0.81 yr-1, respectively. Exploitation level (E) of P. viridis was 0.32 while the maximum allowable limit of exploitation (Emax) value was 0.43. The exploitation level (< 0.50) is below the optimum level which indicates the under fishing condition of P. viridis in the coast of Sebatu/ Malacca, Malaysia. Average values recorded for temperature, dissolved oxygen, pH, and salinity were 29.44 ºC, 6.06 mg/l, 7.92, 30.12 ppt respectively. Highest values recorded for temperature, dissolved oxygen, pH, and salinity were 32.17 ºC, 7.61 mg/l, 8.44, 33.26 ppt respectively. Lowest values recorded for temperature, dissolved oxygen, pH, and salinity were 27.82 ºC, 4.94 mg/l, 7.32, 27.93 ppt, respectively. Chlorophyll-a, and total suspended solids (TSS) ranged from 1.50 to 9.40 mg/l, and 0.0253 to 0.0733 g/l, respectively. Incidences of pea crab Arcotheres latissimus (Burger, 1895) infestation in P. viridis at Sebatu was monitored from December 2003 until December 2004. Maximum infestation frequency was observed during the month of June. Size ranged of infested P. viridis from 19.26 to 92.18 mm. Wet weight of infested P. viridis ranged from 0.17 to 10.41 g. Wet weight of pea crabs ranged from 0.009 to 0.55 g.

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