Shellfish Breeding and Hatchery Management (2+1)

21.2.3 Materials used in the spat collection

Although, spawning of tropical oysters occurs throughout the year, the peak is invariably during the onset of rains or at the end of the dry season. In the case of Pinctada fucata, the peak spawning has been recorded during April – May & August - September.

After a brief period of planktonic life, the larvae begin to settle on a hard substratum and at this stage they are called "spat". In the natural environment these spats can be collected with the help of spat collectors. The abundance of the larvae before their settlement can be confirmed by periodic plankton collections in the seasons mentioned before. In order to avoid biofouling in spat collectors, the man made collectors should be suspended at the appropriate times otherwise the surface of the spat collectors will be preoccupied either by silt deposits or by foulers like seaweeds and barnacles. Suspension of spat collectors below racks and trays where oysters are grown may help in effective utilization of the natural seed supply.

The following materials are used for the spat collection:

Lime-coated tiles:

Semicylindrical roofing lites (24x15cm size) are first dipped in lime solution (12.5kg of lime dissolved in 25 l of water would be sufficient for 500 tiles) and are dried under shade. After drying, the tiles are given a second coating with lime-mortar (30kg of lime + 50kg of find sand and 40l of water). These dried tiles are placed in rectangular trays (90x60x15 cm) in the rack at 50no/tray in such away that the concave sides of the tiles face dewnwards. These tiles are periodically examined for the settlement of oyster spats. The spats that are settled in the tiles are scraped off after they reach a size of about 2 cm for the tranfer to trays set on racks.

Oyster shell strings:

About 20 oyster shells with a central opening in each (made with a punching hammer) tied in a 1.5 m long No.10 galvanized wire are used for the collection of spats. Each unit is called as ’ren’ and 100 such rens may be hung horizontally from a rack,

Coconut shell string:

Tar coated and centre punched 10 coconut shells are held together by a 3 mm synthetic rope of 1.5m length. Between two shells, an interspace of 10-15 cm may be allowed. The strings are held horizantally on a rack and kept in the spat settlement area.

Strips of corrugated roofing asbestos:

Lime coated 5-7 corrugated asbestos strips of 30x20 cm size may be alternately arranged one above the other and are tied with a 3 mm synthetic rope in such a way that every unit can be hung easily from a rack.

Asbestos sheet:

Strips of asbestos sheet of size 128 cm are given lime coating. After drying, the strips are placed in grooves of palmyra reepers which are tied to 2m long poles installed in shallow areas.

Among the various types of spat collectors, the strips of corrugated roofing asbestos have been proved to be useful in the collection of large number of spats.

Rope collectors:

The rope collectors consist of a 9mm main rope made of synthetic material in which bunches of untwisted nylon filaments are inserted at intervals of 25cm. One end of the main rope is firmly tied on the wooden pole of the raft and the other end is tied to the granite stone weighing around 5kg. When the rope is suspended from the raft the filaments from the bunch spread in the entire water column from surface to bottom. The synthetic ropes and flamentous bunches are of three colours, viz., blue, green and yellow.

Synthetic filamentous spindle:

Bunches of synthetic monofilaments of 6 m length secured at both ends in the form of spindle was suspended vertically into the water column. The spindle was supported by main rope in the middle, one end was tied on the wooden pole of the raft and the other end in a 5kg of granite stone. A good spread of monofilament was noticed in the entire water column commencity from surface to bottom.

Facundity, Incubation & Hatching

Fecundity is generally estimated as the number of eggs carried externally. Facundity varies greatly to between species and within species among size classes. The range of clutch size are as follows: metapnephrops andamanicus 6000-1400, N norvegcus, 900-6000, H. gammarus, 5000-17,000 H. americanus,

10% per month in N.Norvegicus and a small egg loss over the incubation period in P.cygnus. The number of eggs in a brood is not as important to the overall biology of the animal as is the valative fecundity. This is developed from data concerning brood size, egg loss, number of broods per year, and proportion of the population represented by each size class of funales. Beory (1971) estimated that by far the highest fecunclity was achieved by medium size P-homarus rubellus. The relative reproductive contribution of the very large fanales was diminished by the small proportion that they comprise of the population, although they breed upto four times per season and carry many more eggs than chair smaller counterparts.

The incubation period of the eggs is temperature dependent, but at least for H.americanus, it is also dependents, it is also dependent on the age of embryos at the time of exposure to a given temperature. The early embryos develop faster at a given temperature may synchronize hatching to a limited period, even when egg extrusion is staggered in different individuals.

Hatching in all species studied occurs at night, & may occur over several nights. In fact, paterson (1969) reported days. In homarus, hatching lasts from 2-3 days to 2-6 weeks and nephrops relcases larwae over a period of several days. The process of hatching in all the Nephropidal seems to be similar. Internal water pressure is thought to burst the outer egg membrnes independently of any action by the female. Larwal release occurs when the female assumes the hatching posture described by Templeman (1937) for H.americanus. The female assumes a tail-up posture, standing on the tips of her walking legs, and beats the pleopds rapidly, releasing several hundred larval at a time. The violent pleopod action may initiate ecdysis while the prelarvae are still enveloped in the embryonic cuticle. larval release appears to be under endogenous control, since shaking of the female does not induce it, although such motion appears to be similar to female hotching behaviour. In H. gammarus, larwal release occurs almost entirely in the first few hours of darkness, and there appears to be an endogenous rhythm associated with the release, since the temporal darkness. H. americanus is not as closely lied to the light dark cycle, although the kanority of its larval release also occurs in short bursts rather than continuously, Ennis()1975 pointed out the surviva rate, relative to preclation, of rleasing larwae in small batches, usually at night, over a long period of time.