oa Preliminary investigations into the reproductive biology, in vitro fertilization and laboratory culturing of the Qatari Pearl Oyster (Pinctada radiata)

The marine species Pinctada radiata is the most abundant pearl oyster found in Qatari waters; however, little is known about its reproductive biology, ecological importance and life cycle. This species has been harvested for many centuries primarily for natural pearls but also for edible flesh and lustrous shell. Wild stocks of P. radiata are now significantly threatened in the Arabian Gulf region as a result of both natural (e.g. extreme salinity, high temperatures, high evaporation rates and low flushing rates) and anthropogenic factors (e.g. rapid development of coastline areas, overfishing and heavy exploitation). Nonetheless, pearl oysters are a dominant component of the benthic community of the Qatari Coastal waters and are important ecologically as suspension feeders, as food and habitat for other animals, and as structural components of the substratum. Oysters filter prodigious volumes of water, capturing particles down to 5 microns in size and ingesting algae, zooplankton, bacteria and detritus. They are therefore exposed to both dissolved and suspended contaminants. Consequently, they are likely to be excellent indicators of potential contaminants in the Arabian Gulf and can be used for risk assessment and monitoring as part of an effective environmental management program.

Prior to using the pearl oysters in toxicity testing, we investigated the reproductive biology of P. radiata and its in vitro fertilization. The oysters were collected from intertidal habitat at the Qatari coast and cultured in flow-through systems at a density of 2-3 oysters/L. They were divided into three equal groups and were placed every day into a feeding tank for 2 hours. One group was fed with a combination of two microalgae species and rice powder, the second group was fed the microalgae alone and the third group was fed rice alone. After 30 days, 3 oysters were taken from each group to check if maturation was achieved. Eggs from mature individuals were kept in different concentrations of ammonia solution for up to 2 hours to prime them for fertilization. Next, matured eggs and sperms were mixed at different ratios. The fertilized embryos were kept at 27°C and salinity of 40 PSU. Embryo development was followed under a light microscope.

Successful fertilization was achieved with gametes obtained from oysters fed with mixtures of algae and rice (80%), rice alone (60%) and microalgae alone (20%). Fertilization occurred in treatments with 2% ammonia solution and a ratio of 8000:2000 sperm: egg. All stages of larval development were identified (in time) and noted for protocol development. Early development followed the typical marine bivalve pattern of trochophore, D-stage veliger, umbo stage, eye-spot stage, pediveliger, metamorphosis and newly settled spat. This took 3–4 weeks in total, depending on culture condition and food types. The successful in vitro culture and fertilization can provide both embryos and adults which can be used as indicator species to carry out toxicity studies. Future work will focus on examining the sensitivity of the P. radiata to contaminants of concern in the Arabian Gulf region. These efforts are important for assessing environmental risk in the Arabian Gulf and providing science-based tools for making management decisions.