Reproductive Biology and Larval Rearing of Blue Swimming Crab, Portunus Pelagicus (Linnaeus, 1758)

Portunus pelagicus broodstock caught from the coastal region of Port Dickson, Negeri Sembilan, Malaysia in August 2004, and conditioned in the Hatchery Unit of Marine Research Station, UPM were used for the reproductive experiments including larval rearing studies. Various aspects of the reproductive biology of blue swimming crab, P. pelagicus were studied. Generally, sexual dimorphism and reproductive system of male and female blue swimming crab were observed to be similar to most other decapods crustaceans. The pubertal molt, the abdominal and gonopores structures of the female showed changes that are generally accepted as external morphological indications of sexual maturity. Unlike the female, the male showed pre-pubertal (loss of the attachment of the abdominal flap to the cephalothorax) rather than pubertal molt. The ovaries and testes were classified into five and three development stages respectively, and the ovarian histology of each stage was characterized. The ovarian stages correlated closely with the gonadosomatic index (GSI), the characteristics of ovarian histology and oviposition period. Fecundity estimates ranged from 148,897 to 835,401 eggs with brood size highly correlated to carapace width (CW), carapace length (CL), body weight (BW) and egg batch weight (EBW). The complete embryonic development of the blue swimming crab, P. pelagicus was described based on morphological features observed in live eggs. Periods of development were defined in sequence of 12 to 24 hours each and in relation to the time of embryonic development. Eight periods were described and illustrated for P. pelagicus. Embryonic development from recently spawned eggs (egg-mass appeared yellow and compact) to hatching lasted 8 days at 28-30oC. The larval stages included four zoeal stages and one megalopa. The megalopa molted to the first crab instar. The zoeae and megalopa were very similar to those of other portunids. The first and the second zoeal stages spanned 3-4 days each, the third and fourth stages were 2-3 days each, and the megalopa was 3-4 days. The first crab instar emerged 15- 18 days after hatching. The egg incubation period decreased exponentially from 8.33 to 6.67 days with the increase in temperature in the range 28-34oC. The best fertilization and hatching rates of eggs were obtained at temperature in the range 28-30oC and 28-32oC, respectively. Relationships between temperature, and incubation period, fertilization and hatching rate of eggs were found to be quadratic. The development duration and survival rate of the blue swimming crab larvae fed with tested diets were significantly higher compared to the control. Thirty percent of first zoea successfully developed to the first crab stage when fed with the Artemia nauplii alone whereas, 15.56% molted from the first zoea to megalopa when fed with the combination diets containing Nannochloropsis oculata + Artemia nauplii (15.56%) and N. oculata + rotifers Branchionus plicatilis + Artemia nauplii (5.56%). The molting of the first zoea to third zoea (22.67%) was achieved through the single use of rotifers and larval development from first zoea to second zoea (10.00%) was achieved by using N. oculata + rotifers B. plicatilis. No development beyond pre-metamorphic first zoea was observed when the larvae were fed solely with N. oculata diet but the results showed they survived longer compared to the control. A continuous fluorescent light of 3000-3500 lux supported the highest mean survival (27.78%) from hatching to successful metamorphosis of first crab instar. The first zoeal stages reared under 1000-1500 lux did not reach the third zoeal stages. All larvae died after 14 days. The influence of temperature was more evident on the survival rate and larval development duration during the first crab stages (C1) (P<0.05). The highest and lowest larval survival rates until C1 stages were at 30oC (36.67%) and 34oC (12.22%) respectively. The second best survival rate of C1 (31.11%) was obtained at 28oC. The shortest larval development (3.67-4.00 days) occurred at 30-34oC. At 26oC and 28oC larval development of C1 stage took around 5.67 days (P>0.05). In order to achieve optimal growth and survival in larviculture, blue swimming crab larvae is recommended to be reared at salinities in the range of 28-30 ppt. The development duration of C1 was relatively shorter (3.33-4.00 days) at all salinity levels. The relationships between salinity, and successful metamorphosis and development duration of C1 were quadratic. A high stocking density (40-60 larvae/L) consistently produced low successful metamorphic development throughout the experimental period resulting in only 19.11-20.00% final survivals. The low stocking density levels (20-30 larvae/L) tested in this study resulted in only 22.67-23.11%, which were not significantly different (P>0.05) among the five treatments. The relationships between stocking density, and larval development duration and successful metamorphosis of C1 were linear and quadratic respectively.

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