Pearl Oyster

Seed production in P. fucata

Site selection

The prime requirement for site selection to construct bivalve hatchery is good quality seawater free from pollutants, suspended particles and silt. The site of drawal of water may be from rocky, coralline or sand mixed area. The intake point should preferable be away from any kind of industrial/domestic sewage. River mouths should be avoided, as dilution would become a problem during monsoon. Proximity to the natural resources and farm site and logistics are also to be considered.

Water intake and distribution

Water intake point should be beyond low water mark. Water is collected in a well through PVC pipeline (15 cm dia.) by gravitational force/pumping. The well acts as a sump cum sedimentation tank. Sedimentation tanks can be rectangle in shape. The water that is drained into the sedimentation tank is allowed to stand for some time for settling the heavy particles. Then the supernatant water is pumped into the biological filter. The biological filter is made of coarse river sand, pebbles and charcoal and the water allowed to pass through it is then collected in a sump and then pumped into overhead tank. From overhead tank, the water is drawn through PVC pipelines to the hatchery and subsequently passed through ultraviolet sterilizing chamber or cartridge filters (mesh size i.e., 10, 5, & 1 μm) to destroy bacterial load before being taken to the larval rearing tanks.

Air supply system

Air compressors or blowers are used to supply the required aeration to the tanks. Air compressors can be of Piston or Rotary vane type. Rotary van model is considered suitable because they give a high output at low pressure and are also less prone to mechanical failures. Air is compressed into a storage tank. The automatic cut off valves allows the compressor to rest for a while when the tank is full. The air is then passed through a series of filters to remove oil and moisture and supplied to the hatchery through PVC pipelines. Electrical air blowers can be twin lobe oil free type for the use in hatchery. However, the disadvantage of air blower is that it should run continuously as it doesn't have storage tanks and regulate airflow at the source.

Larval rearing tanks and sieves

Larval rearing is done in rectangular FRP tanks of assorted size. A hatchery should have various sized FRP tank i.e., 100, 500 and 1000 L for different uses. The FRP tanks could preferably be black in colour. Sieves made of Nylobolt cloths of different mesh size ranging from 20 to 250μm are essential for hatchery operations.

Collection and broodstock maintenance

Collection of brood pearl oysters is mostly done from the natural bed. A diver can collect oysters either by skin diving or using SCUBA (Self contained underwater breathing apparatus). SCUBA equipment enhances the diver's underwater stay for a longer period for collection. Collected oysters are beached, cleaned and stocked in rearing cages and maintained in the farm. In order to get spawners throughout the year, the brood oysters are kept in conditioning room at temperature less than 25^oC and fed with mixed culture of algae containing mostly Chaetoceros spp.atarateof4.0L/oyster/dayintworations. Thematureoysterscanbekeptforprolongedperiodunder25-27oC and can be used for induced spawning purposes.

Natural spawning : Most of the oysters collected during the spawning season (June to August & November to January) would be sexually ripe and may spawn naturally in the hatchery.

Induced spawning : In the event of negative response from the oysters, induction of various kinds can be resorted to make the oysters spawn.

Chemical induction : Hydrogen peroxide (H2O2), Tris buffer (Hydroxymethyl amino methane), Sodium hydroxide (Na OH) and Ammonium hydroxide (NH4 OH) are found to have some effect on the oysters and can induce them to spawn with some degree of success.

Thermal induction : The most preferred and suitable technique for inducing pearl oysters to spawn is thermal induction. As the term implies, the oysters are kept in room temperature of less than 25 oC in a conditioning room for some time and quickly changed to water having 5 to 6 oC higher temperature. The change would stimulate the oysters to spawn.

Spawning : In all the cases, the male oyster responds to the induction and initiates spawning. The presence of sperms in the water column stimulates the females to respond within 30 minutes.

Developmental stages of P. fucata

Fertilization: Majority of the eggs released is pyriform in shape. A large clear germinal vesicle (nucleus) is distinctly seen. The yolk cytoplasm is heavily granulated and is opaque. Immediately after discharge, the eggs are fertilized and assume spherical shape and attain a size of about 50μ in diameter. During the process of fertilization, the first and second polar bodies are released.

Cleavage: 45 minutes after fertilization, the first cleavage begins and a micromere and macromere are formed. During the second cleavage the micromere divides into two and the macromere divides unequally into a micromere and a macromere. This stage with three micromere and a macromere is called Trefoil stage. Macromere become smaller and smaller in size after passing through eight, sixteen cell stage and so on to reach the morula stage. Cilium is formed in each of the micromeres and the rotary movement of the embryo develops.

Blastula: Blastula is reached 5 hrs after fertilization. In this stage, reorientation of cells results in formation of blastopore and blastocoel.

Gastrula: Gastrulation takes place by epiboly. The cells convolute and differentiate into different layers. The archenteron is formed and communicates to the exterior through blastopore. The embryo exhibits phototropism and it takes 7 hrs to reach the stage.

Trochopore: The early trochophore larva develops pre oral and post oral tufts of cilia thus marking antero posterior region of the embryo. A single apical flagellum is developed in the typical trochopore stage. The minute cilia noticed in the blastula stage disappear. A shell gland of the dorsal ectoderm secretes the prodissoconch I. Trochophore is attained in 10 hrs.

Veliger: Veligerstageisreachedat20hrsbytheformationofstraighthingeline,mantle,rearrangementofpre oral cilia into a velum and disappearance of the apical flagellum, pre oral and post oral ciliary bands. The larvae measure an average of 65μm antero posteriorly (APM) and 52.0μm dorso ventrally (DVM).

Umbo: The development of umbo stage is gradual. Typical clam shaped umbo is reached between 10-12 days measuring about 135 x 130μm. The shell valves are equal and mantle fold is developed.

Eyespot : Eyespot is developed on 15th day when the larvae are about 190 x 180μm and at the base of the foot primordium. Ctinidial ridges are also formed.

Pediveliger stage: The foot is developed on 18th day at a size of 200 x 190μm. The transitional stage from swimming to crawling phase. The larva has both velum and foot. The foot becomes functional later with the disappearance of velum. 2-4 gill filaments are seen in this stage.

Plantigrade: Plantigrade is reached on 20th day when the larvae measure 220 x 200μm. Labial palps and additional gill filaments develop. Shell growth is by the formation of a very thin, transparent, uniform conchiolin film around the globular shell margin except in the vertex of the umbo region. This is the beginning of the formation of the adult shell or the dissoconch.

Spat: The plantigrade transforms to a young spat. The hinge line, anterior and posterior auricles and the byssall notch assume specific shape. The left valve is slightly more concave than the right one. The spat attaches with the substratum with byssal thread. This stage is attained on 24th day and the size is 300μm.

Algal culture

Flagellates measuring less than 10μm are the ideal food for pearl oyster larvae. The haptophycean flagellate Isochrysis galbana is the most suited food and measures 7μm. Apart from this, Pavlova, Chromulina and Dicrateria spp have also been found to be good larval feed.

Feed volume estimate : Depending on the stocking density and stage of the larvae stocked in the larval rearing tanks the feed volume is worked out following the formulae.

No. of larvae x feed rate (cells/larvae)

—————————————————————— = ml

Cell concentration of algal culture

Larval rearing

Water exchange: During the course of larval rearing complete (100%) water exchange has to be done on alternate days. The larva in the tanks is drained through appropriate sieves made of nylobolt cloth of various mesh sizes to a container with filtered seawater. Later the tanks are cleaned and fresh filtered seawater is filled and the larvae transferred.

Larval density estimate

No. of larvae in sub sample X Volume of the sample = Larval density

Updated on Feb 2015