Oysters fall under the family Ostreidae, Filibrachia, Bivalvia. More than 100 species and three genera have been indentified. Nevertheless, their taxonomic position has not been clearly established.
Oysters are included in three genera: Ostrea, Craassostrea, and Pycnodonta (Ranson, 1950, 1960). The classification is based on the form and structure of larval shell, mode of reproduction, life history, adult shell, morphology and foot shape.
Korea has five species of oysters - four of genus Crassostrea, namely, Crassostrea gigas, C. nippona, C. echinata and C. rivularia, and one of genus Ostrea. Ostrea denselamellosa. The major species are C. gigas, C. nippona and O. denselamellosa. The characteristics of these three species are as follows:
This species is relatively big and covered with scale-like shells. Oysters of this genus have small or big umboral cavities without chomata. Adductor muscle scar is on the opposite side of the abdominal region along with the central part of the shell. The inner side of the shell is covered with an ivory-like material known as conchiolin. Tentacles of the inner fold vary in size and are the longest among oysters. The auricles of the heart are connected to the anterior end of the shell.
Figure 2–1. Soft parts of Crassostrea sp. Shape of the mantle tentacles: A, tentacles of inner fold; B, mid-tentacles and C, outer tentacles; AD, adductor muscle; L, labial palps; V, ventricle; Au, auricle; AP, anus and anal pailla.
a. Crassostrea gigas (Figure 2–2)
Shell
This species is relatively big and irregular in shape. It is long and oval. Both shells are concave but the left one is more concave than the right. Radial ribs are on both shells starting from the umbo. Attached line of both shells and commissural shelf is wavy and narrow. There is no chomata. Adductor muscle scar is reniform or kidney-shaped. Dorso-anterior border is concave. Umboral cavity is commonly deep. The outer side of the shell is whitish yellow and has brown radial rib. Its inner side is white and partially whitish milky color. Muscle scar is of polished white color or light yellow. The muscle scar of right shell is sometimes violet.
Soft Part
Tentacles of the mantle's inner fold are conical and their length is almost four times their width. Tentacles of the middle fold have two layers, inner and outer. Inner layer tentacles are round conical and their length is three to five times their width. Outer layer ones are club-like. All of the tentacles are ivory or whitish yellow with brown or black spots. Gill is ivory in color and the filament number is 13 ± 2. Heart is ivory or whitish yellow. The auricles of the heart are light brown. Rectum is black. The coloration of soft parts of this species is the darkest among genus Crassostrea.
b. Crassostrea nippona (Figure 2–3)
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| Figure 2–2. Internal anatomy of C. gigas. | Figure 2–3. Internal anatomy of C. nippona. |
| A, anus; AD, adductor muscle; G, gill; L, labial palps; M, mouth; ME, mantle edge P, pericardium; R, rectum; S, stomach. | A, anus; AD, adductor muscle; G, gills; L, labial palps; M, mouth; ME, mantle edge; P, pericardium; R, rectum; S, stomach. |
Shell
This species is relatively big and its shape varies from oval to round. The left shell is flat or slightly concave while the right is concave. The right shell has wavy growth scales, growth lines and a big commissural shelf without radial rib and commissural plication. This species has a small umboral cavity without chomata. Adductor muscle is reniform. Dorsal border is concave.
External coloration of both shells is while. Growth line is dark brown and its basic part is light brown. Left shell is light violet and part of the inner side is milky white. Muscle scar is polished white with violet yellow and has growth line.
When the shell is cut across its dorso-ventral side, there are some concave chambers on the right shell. Left shell consists of whitish ivory sediments. The cutting line is white. Light violet pigments are sometimes found on some parts of the left shell.
Soft Part
The soft part of this species is similar to that of C. gigas, but different from that of genus Crassostrea when analyzed under electrophoresis (Torigae and Inaba, 1975).
The species of this genus are relatively medium or small in size and have thin, dense and leaf-like growth scales. Commissural plications are very gentle in slope. Radial ribs are very weak or sometimes absent. Umboral cavity is small. Adductor muscle scar is reniform and placed on the central part of the shell. The number of chomata varies from a few in some species to many in others. Both shells are concave and consist of whitish ivory sediments.
Tentacles of the middle fold have two to four layers. Its anal pailla is longer than that of genus Crassostra (Figure 2-1). Anus is placed on abdominal part.
Figure 2–4. Soft parts of Ostrea sp. Shape of the mantle tentacles: A, tentacles of inner fold; B, mid-tentacles; C, C, outer tentacles; AD, adductor muscle; L, labial palps; V, ventricle; Au, auricle; AP, anus and anal pailla.
a. Ostrea denselamellosa (Figure 2–5)
Shell
The shell of this species is medium in size and round in shape. Both shells are concave, but the right one is flatter than the left. Both shells have radial ribs. The ribs of the right shell are covered with many fragile growth scales while the left has none. Commissural shelf is narrow. This species has no chomata, or, if it has, limited to both sides of the ligament. Adductor muscle scar is reniform and placed on concave dorso-anterior border. Umboral cavity is small.
Outer side of the shell is unpolished white with light yellow spots while conchiolin is dark brown. Inner side is polished white with light brown or gray spots. Adductor muscle scar is polished white with light brown growth line. When the shell is cut across its dorso-ventral axis, concave chambers can be seen. Both shells have whitish ivory sediments.
Soft Parts
Tentacles of the inner fold of mantle are club-like and their length is two times their width. Tentacles of the middle fold have five layers, i.e inner and outer. Inner ones are club-like and bead-like in one layer. Outer ones are club-like in four layers. Lengths of the middle fold tentacles are 1.5 to 2 times their width.
All tentacles are whitish yellow with some black spots. Gills are ivory or ivory with gray. Normally, the number of gill filament is 13 ± 2. Anal pailla is slightly long.
Fig. 2.5. Internal anatomy of C. denselamellosa A, anus; AD, adductor muscle; G. gill; L, labial palp; M, mouth; ME, mantle edge; K, pricardium; R. rectum; S, stomach.
