Basic biology and ecology of pearl oysters
Pearl oysters are members of the phylum Mollusca and belong to the class Bivalvia. Bivalve mollusks are distinguished by having two shells (two valves), a soft body with a small foot, a byssal gland and paired gills. Although the common name of “pearl oyster” suggests a close relationship with other types of oysters, pearl oysters are actually a distinct genus from the edible oysters, Crassostrea and Ostrea and have important anatomical and behavioral differences. The internal anatomy of a pearl oyster is shown in Figure 3.
Figure 3. Internal anatomy of the pearl oyster (modified from George, 1978).
Pearl oysters are most commonly found in tropical areas. The Black-Lip pearl oyster (Pinctada
margaritifera) is widely distributed throughout the tropic Indo-Pacific area (Figure 4). There are several subspecies and strains of P. margaritifera, including the Hawaiian strain, P. margaritifera galstofii, and the closely related P. mazatlantica, which was considered a strain of the Black-Lip for many years. Black-Lip pearl oysters are generally found in areas where water temperatures range from 25 to 30°C. Below 23°C, tropical species of pearl oysters stop breeding and may die. While pearl oysters can tolerate a range of salinities, they are most common in water with high salinities (around 33 ppt). They appear to grow best in clear waters that are free of large amounts of sediment since pearl oysters may have difficulty feeding in turbid water.
The pearling boom of the late 1800s and early 1900s, they were extremely common and are reported to have thickly covered large areas of reef, including inter-tidal areas. They are now rare in many places as a result of overfishing, and are most common at depths below 60 ft. Pearl oysters are gregarious, meaning that they tend to be found in groups, both as juveniles and adults. Pearl oysters are protandric hemaphrodites, which means that most are first male, then female. The male phase usually occurs during the first 2-3 years of life, with the change to the female phase in later years. Pearl oysters have been reported to live as long as 25 years. Pearl oysters reproduce by releasing millions of eggs or sperm into the water column, where fertilization occurs randomly. In less than 24 hours, the fertilized egg develops into a trocophore larva, a free-swimming organism (Figure 5).
The larvae remain suspended in the water column for 2-3 weeks before undergoing metamorphosis, changing into an attached juvenile “spat.” Shortly before metamorphosis, the larva develops an enlarged foot and an eye-spot. The foot remains after metamorphosis, and the young spat retains the ability to move about for several months even after it attaches itself to a hard substrate. Pearl oysters can attach and reattach themselves using the byssus.
Pearl oysters feed on small algae found in the water column. The gills in bivalves are large, and tiny hair-like cilia on the gills are used to remove small particles from the water. Both adults and larvae feed on algae and other small organisms. Clear tropical waters contain limited amounts of algae. Therefore, a large amount of water must be filtered daily in order for the pearl oyster to obtain sufficient food. This is the reason that importance is placed on not crowding pearl oysters on the farm and for keeping the shells clean of organisms that compete for food.
Article source: The Basic Methods of Pearl Farming, Author: A Layman’s ManualMaria Haws, Ph.D. (Director, Pearl Research and Training Program, Pacific Aquaculture and Coastal Resources Center, University of Hawaii at Hilo, Hilo, HI 96720 USA, Center for Tropical and Subtropical Aquaculture, Publication No. 127, March 2002)
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