PEARL OYSTER CULTURE
Excerpts from a Blacklip Pearl Oyster Culture Manual of Dr. Rick Braley 1998
The pearl oyster industry traditionally relied upon spat collection in the field to supply its needs for farm requirements. As the industry has expanded the need for a more predictable source of the spat has emerged. Likewise, the thought of controlling some desirable characteristics of the pearl oysters has resulted in the setup of hatchery and nursery facilities. The hatchery and nursery will give the farmer a predictable supply of spat and will allow greater manipulation of genetic traits. This is the road to proper animal husbandry.
Dr. Rick Braley has prepared plans to setup a blacklip pearl oyster hatchery and nursery at Fakareva Atoll, Tuamuotu, French Polynesia and redesigned and setup an existing hatchery in Penrhyn Atoll, Cook islands as well as trained staff to operate the two facilites. Aquasearch works on-site with the technicians to assure the technical knowledge has been successfully transferred.
Every hatchery and nursery has site specific conditions which require fine-tuning of the initiating protocol used at the facility. This development of the specific protocol for a hatchery will take place as the start-up begins and as spat are produced. In the end, the protocol for the given hatchery will be the most efficient possible for that hatchery.
Good observation and thoroughness are necessary to fine-tune the initiating protocol. The hatchery/nursery staff must exhibit these qualities in their work. It is the intent of Dr Braley that the start-up and training stage of this new hatchery and nursery will be greatly facilitated by the information contained in a manual which is prepared for each hatchery.
Central to larval culture is the availability of good quality broodstock. Broodstock Pinctada margaritifera may be obtained from shell that have settled on collectors set in the lagoons of atolls or in some cases from wildstock shell. Freshly collected shell which are potential broodstock should be assessed for DVM (Dorso-Ventral Measurement) size and shape of the shell.
If the shell looks suitable for broodstock use, the shell should be drilled in the appropriate place and hung on chaplets from a longline or they may be placed into 8-pocket panel nets and suspended from a longline. They should remain on the longline at least 4-6 months before they may be used during a spawning for the hatchery. This allows time for the shell to acclimate to the lagoon condition and for gametogenesis to proceed toward ripe gametes.
Another very suitable source of broodstock are seeded shell which are already hanging on longlines. Although the farmer must be careful not to have excessive handling of these seeded shell, it is normal in most places that cleaning of these shell is carried out on a regular basis to reduce fouling organisms growing on the outside of the shell. Cleaning of these shells can be combined with their use as broodstock for a planned spawning in the hatchery.
Large quantities of good quality eggs can be obtained by this method because shells which have been hanging on chaplets and longlines generally are in good condition and release gametes readily upon handling. The numbers of shell being cleaned are generally large, so there is no problem with obtaining sufficient eggs for stocking into the hatching tanks.
Other than the thermal method of spawning stimulation, chemical methods can be used to induce spawning. These include different concentrations [1.532, 3,064, or 6.128 millimolars] of hydrogen peroxide either in normal seawater or alkaline seawater (pH 9.1).
The pH media can be prepared using Tris buffer or Sodium hydroxide pellets. The Pearl Oyster Farming and Pearl Culture Manual in India (Central Marine Fisheries Research Institute at Tuticorin, India, published February 1981), stated that when inducing spawning chemically "A pH value of 9.0 in the case of Tris buffer and 9.5 in NaOH gives 78.6% and 68.4% of spawning, respectively."
Further, they say, "Injection of 0.2 ml of N/10 ammonium hydroxide (NH4)H) solution into the aductor muscle of the pearl oyster results in 48% spawning."
It should be noted here that serotonin-induced spawning such as used with giant clams (1-4 ml of 2 millimolar serotonin solution) may also be a possibility with pearl oysters. However, information from the James Cook University Blacklip Pearl Oyster Project indicates that serotonin is not so effective with pearl oysters as it is with giant clams or other bivalves.
Although initial spawnings may utilise relatively large numbers of broodstock that will not be identified as the parents, eventually the hatchery will be used for crosses between broodstock shell with desirable traits. In this case, the broodstock should be kept on tagged chaplets or 8-pocket panel nets so that the parents can be identified. DVM measurements would also be recorded at each spawning date that the shells are used as broodstock.
Do pearl oysters which produce good quality, round pearls at the first harvest possess a genetic trait for round pearls? This question could be tested by using these shell for a special spawning after they have been re-seeded a second time and had 4-6 months rest in the lagoon. The growth, survival, and general development of their offspring would be carefully recorded up until the time they were old enough for their first seeding. The result will then come out in the first harvest. There are other characteristics of the shell shape, the nacre, etc. which are likewise important for genetic manipulation.
Larval Rearing in the Hatchery
A most important factor in larval rearing success is cleanliness. It is essential that egg and sperm collection materials have been chlorine-cleaned and are stored dry for use during a spawning and during the larval cycle.
The following details on the development of embryos and larvae are taken from the Pearl Oyster Farming and Pearl Oyster Culture Manual (Central Marine Fisheries Research Institute at Tuticorin, India, published February 1981):
Early development and larval rearing:
The first cell division is seen 45 minutes after fertilisation resulting in the formation of a micromere and a macromere. The polar body is placed at the cleavage furrow. During the second cleavage the micromere divides into two and the macromere divides unequally into a micromere and macromere. The stage with three micromeres and a macromere is called Trefoil stage. The macromere does not take part in further divisions. Micromeres divide repeatedly thus becoming smaller and smaller and passing through 8-cell, and so on util the morula stage. Each micromere develops a small cilium which helps in the movement of the embryo.
The embryo is ball-like with transparent cells and a blastocoel. The embryos lift themselves in the water column and congregate at the surface. The floating embryos are siphoned out to clean containers and the residues at the bottom, containing broken tissues, undeveloped embryos, unfertilised eggs, sperm, etc. are discarded. Reorientation of cells starts and the blastocoel and blastopore are formed. The blastula stage is reached 5 hours after fertilisation.
Gastrulation takes place by epiboly. The cells convolute and differentiate into different dermal layers. The archenteron is formed. The embryo is bean-shaped as there is convolution of cells. The gastrula exhibits negative phototropism. The stage is reached in 7 hours.
The minute cilia present in the gastrula stage disappear and the pre-oral and post-oral tufts of cilia develop, thus marking antero-posterior differentiation of the embryo. A single apical flagellum is developed at the anterior side. The anterior portion of the larva is broader while the posterior end is tapering like an inverted triangle. The movement of the larva is affected by the propulsive movement of the flagellum. The dorsal ectodermal cells secrete the embryonic shell, known as the prodissoconch I.
A definite ‘D’ shape is obtained by the secretion of the prodissoconch I having a hinge line, mantle and rearrangement of the pre-oral tuft of cilia into a velum. The single flagellum, pre-oral and post-oral tufts of cilia disappear. The veliger larva (of Pinctada fucata) measures 67.5 um along the antero-posterior axis and 52.5 um along the dorso-ventral axis. This stage is reached in 20 hours.
Culture of blacklip pearl oyster (Pinctada margaritifera) larvae at James Cook University’s Orpheus Island Research Station [no. Queensland, Australia] resulted in larger larvae. Here, mean egg diameter was 61.03 + 0.04 um and the D-stage veliger stage was reached in 20-24 hours with a mean APM (antero-posterior measurement) of 82.09 + 1.37 um.
A Quick Reference to Feeding Schedules for Larvae and Spat are prepared in the manual. This useful reference should be placed on the Algal Lab wall and used daily during the larval feeding. Another part of the protocol shown in the manual are days of larval life shown against the columns of Stocking Density (of larvae), Algal Feed, Flushing requirements, Draindown requirements, Microscope checks on size of larvae, % feeding, and Spat collectors. This table will allow a quick visual check over the larval cycle by technicians to see what is required.
Technician Duty Schedule
Along with the above tables which help the technicians to keep up with the protocol, there need to be forms for Weekly Duty Schedules for:
The Hatchery Phase,
The Nursery Stage, and,
The Algal Production.
These schedules list the most important duties that the technicians need to do over a weekly period. Some duties are required daily whilst others are required less regularly.
ABOVE: Indonesian silver pearls produced by Pinctada maxima.
ABOVE: Indonesian gold pearls produced by Pinctada maxima.
ABOVE: Blacklip pearl oysters (Pinctada margaritifera) from nature (top) and from hatchery culture (bottom), Penrhyn, Cook Islands.
ABOVE: Hatchery cultured blacklip pearl oysters in a recirculation experiment at Aquasearch lab, Qld., Australia.
ABOVE: Silver-lip pearl oyster spat (Pinctada maxima) on rope collectors.
ABOVE: 29 day old plantigrades / spat of the siver-lip pearl oyster.
ABOVE: Oyster life cycle. Source: Wikipedia
ABOVE: Outer shell of a pearl oyster
Land Nursery Culture of Spat
The manual discusses settling materials to use for late stage larvae and the potential positive effects of conditioning the collector materials. Spat can be left on their collectors until they are large enough to be safely removed and placed into trays with the appropriate size mesh to retain the spat. The minimum protocol required in the raceways is discussed in the manual.
The usual food of bivalve larvae such as blacklip pearl oysters is unicellular algae ranging from 2-10 microns (um). Generally, it is wise to be careful of feeding new veliger larvae with too much unicellular algae as the gut may only just be in the process of completion and the possibility exists of the gut becoming plugged up with algal cells that can not be completely digested. They are ready to feed by the end of the day that they first become veliger larvae. The protocol on the feeding density over days of the larval cycle are shown in the manual.
Whether a hatchery is located in a temperate or tropical area, the monospecific unicellular algal cultures are required for larval rearing needs. A considerable amount of time is needed to set up and maintain these cultures. The trained technicians handling the algae cultures must keep careful attention to detail and hygiene. The manual shows the steps involved in starting with stock cultures of unicellular algae (= microalgae) to large mass cultures of 60-250-L. The f/2 medium is one of the standard microalgal culture mediums in use around the world. This will be the medium to be used at hatchery for cultures from 50 ml flasks to the 250-L cylinder cultures, but where the budget is restricted Aquasearch has it's own cheap medium which works nearly as well as the f/2 medium.
The stocks must be cared for because all the cultures come from the stocks. It should be noted here that sodium metasilicate is only added to the media which will be used to grow diatoms in.
Diatoms have an outside shell (like a jewelry box that fits neatly into top and bottom) made of silicate, so this material becomes limiting in dense cultures.
Mass microalgal cultures that will be grown in the outdoor algal culture area will grow well with other culture media that are tested for mass culture. These media are cheaper than f/2 and quite suitable for the large volumes of algae being grown to feed spat in the land nursery raceways.