Inadequate supply of spawners remains as one of the major constraints in the development of the shrimp farming industry especially for species such as the tiger shrimp, Penaeus monodon. Recent success in eyestalk ablation techniques has greatly enhanced constant supply of spawners through ablating captive broodstock to maturation. The results have not been consistent and further research refinement is needed before the techniques can be disseminated. At the present time, the supply of broodstocks and spawners still depend on captures from the wild.
Upon arrival in the hatchery, the broodstock are first acclimatized in holding tanks for 4–7 days. The holding tanks should be big enough to provide proper space and aeration. 60% of the water in the tanks is changed daily. Once the animals have recovered from transport stress, they are induced to molt by manipulating salinity of water in the holding tank. The salinity is decreased by about 4–5 pptfor two days and then increased to the normal salinity of the seawater. Majority of shrimps willmolt after changing salinity. Mating occurs during this time when the females are newly molted. The shrimps are then ablated 2–3 days after molting or when the shell is completely hardened.
Table 6. Comparison of the egg development of three species of shrimp.
| Development | Time after spawning | ||
| P. semisulcatus | P. monodon | P. merguiensis | |
| 2 cells | 40 min | 40 min | 40 min |
| 4 cells | 1 hr 20 min | 1 hr | 50 min |
| 8 cells | 1 hr 20 min | 1 hr 10 min | 1 hr 10 min |
| 16 cells | 1 hr 50 min | 1 hr 25 min | 1 hr 25 min |
| 32 cells | 2 hr | 1 hr 35 min | 1 hr 50 min |
| 64 cells | 2 hr 20 min | 1 hr 35 min | 1 hr 55 min |
| 128 cells | 2 hr 40 min | 2 hr 05 min | 2 hr 20 min |
| 2nd antenna | 4 hr | 3 hr 50 min | 4 hr |
| 3rd antenna | 5 hr 40 min | 5 hr 50 min | 5 hr |
| 1st antenna | 7 hr 20 min | 6 hr 50 min | 6 hr |
| hatch out | 18 hr | 15 hr | 12 hr |
Table 7. Comparison of the chronological development of three species of shrimp.
| Larval stages | Days after spawning | ||
| P. semisulcatus | P. monodon | P. merguiensis | |
| Nauplius 1 | -/18 | -/15 | -/12 |
| Nauplius 2 | -/22 | -/20 | -/20 |
| Nauplius 3 | 1/4 | 1/2 | 1/- |
| Nauplius 4 | 1/10 | 1/8 | 1/6 |
| Nauplius 5 | 1/16 | 1/14 | 1/12 |
| Nauplius 6 | 1/22 | 1/20 | 2/10 |
| Protozoea 1 | 2/22 | 2/16 | 2/10 |
| Protozoea 2 | 4/16 | 4/4 | 3/-10 |
| Protozoea 3 | 6/1 | 5/- | 5/12 |
| Mysis 1 | 8/- | 8/4 | 7/4 |
| Mysis 2 | 9/6 | 9/16 | 8/4 |
| Mysis 3 | 10/12 | 10/4 | 9/4 |
| Post larvae | 12/- | 11/6 | 10/4 |
Table 8. Ages and average body length of P. monodon larvae.
| Mean Body Length (mm) | Days after hatch out (Age) | |
| Nauplius 1 | 0.32 | 15 hours |
| Nauplius 2 | 0.35 | 20 hours |
| Nauplius 3 | 0.39 | 1 day-2 hrs |
| Nauplius 4 | 0.40 | 1 day-8 hrs |
| Nauplius 5 | 0.41 | 1 day-14 hrs |
| Nauplius 6 | 0.54 | 1 day-20 hrs |
| Protozoea 1 | 1.05 | 2 days-16 hrs |
| Protozoea 2 | 1.9 | 4 days-4 hrs |
| Protozoea 3 | 3.2 | 6 days |
| Mysis 1 | 3.8 | 7 days-4 hrs |
| Mysis 2 | 4.3 | 8 days-16 hrs |
| Mysis 3 | 4.5 | 9 days-4 hrs |
| Postlarvae 1 | 5.2 | 10 days-20 hrs |
| Postlarvae 5 | 8 | 16 days |
| Postlarvae 15 | 12 | 26 days |
| Postlarvae 20 | 18 | 30 days |
Only suitable broodstocks are used for eyestalk ablation. The criteria are:
complete appendages (Fig. 19A)
presence of spermatophore in the thelycum of females (Fig. 19 B)
size should at least be 100 gm.
Presently, the most practical way of inducing maturation is by unilateral ablation of either right or left eyestalk of the female. Ablation is done by using a razor blade to cut/open the eye, then squeezing out the eyestalk from the base to the tip with the thumb and forefinger (Fig. 20) or using the fingers alone to break and squeeze the eye. The ablated animals are stocked in maturation tanks at a density of 5–6 per square meter and a sex ratio of one male to one female.
The broodstocks are fed with squid, mussel or cockle meat or pellet feeds at the rate of 10% of total biomass. The water in the tank is allowed to flow through continuously or changed daily at 60–70% of total capacity.
Gonadal development of an ablated female is checked 3–5 days after ablation while checking for gravid females is carried out every other day.
Sampling and checking are done at night or at any time if the tanks are sufficiently covered and kept dark. During sampling, an underwater flashlight, tied to a pole is held close to the shrimp so that the light strikes perpendicularly on the dorsal part of the body where the ovaries are located. Water in the maturation tank can be lowered to 30 cm to allow the worker to get inside for closer observation. Only gravid females with stages III or IV ovaries are collected and transferred to spawning tanks.
Fig. 19. A. Shrimp with complete appendages
B. Thelycum and spermatophore
Fig. 20. Eyestalk ablation
