Oyster species of the sub tropical coast of Pakistan...

Indian Journal of Marine Sciences Vol. 31(2), June 2002, pp. 108-118

Oyster species of the sub tropical coast of Pakistan (northern Arabian Sea) Ghazala Siddiqui & Muzammil Ahmed

Centre of Excellence in Marine Biology University of Karachi, Karachi-75270, Pakistan [ E-mail: [email protected] ]

Received 11 April 2001, revised 1 January 2002

In the present study 9 species of oysters belonging to three genera (Crassostrea, Saccostrea and Ostrea) were identified on the basis of their conchological and malacological features. These included Crassostrea gryphoides, C. madrasensis, C. belcheri, C. glomerata, Saccostrea cucullata, S echinata, Ostrea nomades, O. folium and O. cristagalli. Two species Ostrea nomades and O. cristagalli which are being reported for the first time from Pakistan. Crassostrea gryphoides and C. rivularis from Hub River Delta were earlier treated as separate species have been assigned to a single species C. gryphoides. Crassostrea gryphoides and C. madrasensis, which occur in the same habitat and show marked similarity in their external shell morphology, but differ in the colouration of their adductor muscle scar, are treated here as separate species.

[ Key words : Taxonomy, oyster, Crassostrea, Saccostrea, Ostrea ]

The coast of Pakistan, which is confined to the north-ern Arabian Sea, lies outside the tropics, as its south-eastern boundary is only a few miles short of Tropic of Cancer. The southeastern extremity touches the India with whom Pakistan shares a more or less com-mon oyster fauna.

The information regarding the taxonomy of Paki-stani and Indian oysters is based on the several re-ports1-23. From Pakistan, 4-10 species of oysters were recognized and were placed in genus Ostrea24-26. Ahmed27 updated the nomenclature and placed the oysters under two genera, Crassostrea and Ostrea, recognizing six species of Crassostrea, (C. rivularis, C. viriginica, C. gryphoides, C. tuberculata, C. glomerata and C. quercina) and one species of Ostrea, (O. folium). Later C. gryphoides and C. rivu-laris were placed in single species C. rivularis; and C. glomerata and C. tuberculata in the genus Saccos-trea.28 Ahmed et al.29 from Gwadar (Balochistan coast) included the tuberculated oysters like C. tuber-culata and C. echinata in the genus Saccostrea. The present paper reports the presence of nine species of oysters by attempting to update their nomenclature.

Materials and Methods Oysters were collected from three localities, i.e,

Hub River Delta, Keamari Seawall (Karachi Harbour) and Buleji. From Hub Delta the collections were made from August 1992 to December 1993, from Keamari the samples were obtained from October

1992 to December 1993. Collections from Buleji rocky ledge were made from July 1992 to December 1993. From Buleji sampling was also done in April 1994. The oysters were also collected from Paradise Point, Karachi from January 1992 to May 1992.

The collection sites are shown in Fig. 1. Hub Delta is mostly sandy but the mouth of river forms sandy-cum-muddy platforms, which are fairly easy to walk

Fig. 1 ⎯ Collection sites* of oysters along Sindh and Balochistan Coasts. (♣ Crassostrea gryphoides; C. madrasensis;

C. belcheri; C. glomerata; ♠ Saccostrea. echinata; ◗ S. cucullata; ▼ Ostrea nomades; O. folium; O. cristagalli).

Siddiqui & Ahmed : Oysters of Pakistan

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on. The delta has been cut off now from the main river due to the recent construction of Hub Dam. The seawater now extends two km inland from the Sea. The beds of C. gryphoides and C. madrasensis are now virtually on the verge of disappearance from here. The salinity here ranges from 32 to 40 ‰. Buleji rocky ledge is a triangular platform extending into the open Arabian Sea. It is uneven in profile, with its right flank (facing the sea) lying a bit low and facing maximum wave action. In this wave swept part a luxuriant growth of seaweeds and zooanthid coelen-trates was observed. The middle part of this ledge is elevated at places or lies low at other parts making it a somewhat backwater-like protected environment. The left margin of the ledge has much less wave action than the right one. In the main body of the triangular ledge exist small and large pools of water in which occur cerithid snails in abundance as well as floating algae and pelagic sea life. In the small shallow pools of the low tide area occurs the small larviparous oys-ter O. nomades. This oyster also occurs on the margin of the pools covered with a film of silt which blends with the substratum below. The salinity here mostly in the range of 35 to 40 ‰. The Keamari seawall is an artificial breakwater wall, about 1.5 km long, extend-ing into the open Arabian Sea from Karachi Harbour (Fig. 1). It is directly affected by oil pollution emanat-ing from the oil terminal located in Karachi Harbour. Water is relatively clear and unturbid and salinities are generally in excess of 35 ‰ for much of the year. Paradise Point is a wave-lashed rocky shore luxuriant in algal growth, many species of gastropds, zooanthid and coelentartes are found here. The water is clear and the salinity ranges between 35 to 40 ‰. Saccos-trea cucullata is found blended with the substratum at a tidal height of 2-3 m. Coast of Gwadar is a T-shaped formation. Different species of gastropods, bivalves, crustaceans, coelentrates, polychaetes, echinoderms, tunicates and seaweeds are found abundantly on East and West Bays of Gwadar. The West Bay is a unique shore with four species of oysters occurring on the same boulders29. The oyster species occur at tidal height of 1.5 to 2.5 m.

The oysters were measured to the nearest millime-ter by the method described by Galtsoff30 (Fig. 2A). In different sizes of Crassostrea gryphoides and C. madrasensis the hinge size was also measured from the tip to the centre of the hinge axis. The curvature of hinge to the left or right was also noted. The species were identified with the help of the reports of: Awati & Rai11, Durve18, Patel & Jetani20,21, Mane28 ,

Ahmed27, Thomson31 and Stenzel32. Other publications were also consulted33, 34 for the sake of comparison.

The identifications of the oyster species were based on the conchological characters i.e; morphology of the shell, its colour and luster, absence or presence of denticles and shape, colour and position of adductor muscle scar. Some malacological features like pres-ence or absence of promyal chamber, colour of the mantle folds and tentacles of the mantle folds were also examined. The morphological features used for the identification of the oyster species are shown in Fig. 2 B, C. All specimens were examined alive ex-cept where indicated otherwise.

Results Phylum : Mollusca Class : Bivalvia Subclass : Lamellibranchia Order : Anisomyaria Family : Ostreidae Genus : Crassostrea

Genus: Crassostrea (Sacco) Description of the genus as given by Thomson31 is

as follows . “Shell elongate, very variable shape even within the species; lower valve deep, cup-shaped, re-cessed under hinge; the muscle scar displaced both towards the dorso-lateral edge and towards the lip; upper valve opercular, flat except sometimes upraised at the rim, may or may not have a row of denticles along the rim laterally from the hinge. A promyal chamber on the right side. Non-incubatory”.

Fig. 2 ⎯ A) Diagram showing the regions where height and length were measured. (After Galtsoff, 1964). B) Morphological features used for the identification of oyster species, and. C) A portion of mantle fold showing middle and inner mantle folds.

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Species: Crassostrea gryphoides (Schlotheim) (Fig. 3)

Synonyms: Ostrea gryphoides Awati & Rai11, Rai12, Durve17 Kazmi24, Ranjha25, Hasan26, Vreden-burg35; Crassostrea gryphoides Durve18, Patel & Jet-ani21, Ahmed27, Rao36 ; Crassostrea rivularis Asif28; Crassostrea cattukensis Nagabhushanam & Mane22 .

In this study 685 oysters were examined from Hub River delta (size 12.9 – 260 mm ). Irrespective of the size of its available specimens this species showed elongated shells, anterior regions narrow and posterior region broad; lower valve slightly concave, heavy and thick specially below the hinge area; upper valve flat and less thick, forming a lid over the lower valve; oysters from 41 to 240 mm size showed hinge area long with a groove curving to the left in 68 % and to the right in 32 % of the oysters examined; a well marked recess below the hinge area present in all the specimens, the hinge area gradually increases with increase in shell size in different sizes of oysters ex-amined. Variations in shell shape in some oysters were likely due to overcrowding and orientation. Promyal chamber present; colour of the inner mantle fold in 97 % specimens pigmented black and with two rows of tentacles; in the remaining oysters it was yel-low; tentacles of the middle mantle fold in 77 % specimens were yellow and in the remaining black pigmented.

Habitat: This species occurs at about 0-tide level in the creeks, estuaries, backwaters and muddy low tidal areas of Sindh and Balochistan. At Hub River Delta where live oyster reefs were found fifteen to twenty years ago37, oysters are now on the verge of extinc-tion, since mostly oyster boxes can be seen there. This may have been partly due to the result of the construc-tion of Hub Dam or due to the gradual eleva-

tion/uplifting of the oyster habitat out of water. The coast of Balochistan, in Pakistan, is well-known for the uplifting of its terrain so that 10.26-12.82 m high cliffs stand at the back of the tidal zone29 bearing a rich record of fossil marine fauna on top38.

Remarks: Earlier Asif28 reidentified this species as C. rivularis from Sonari (Balochistan) near Hub River Delta and indicated that the older specimen of C. rivularis resemble the characteristics described for C. gryphoides27. We find that all these oysters irre-spective of their sizes belonged to the species C. gry-phoides which were earlier identified as O. gry-phoides (=C. gryphoides)11,17 and subsequently C. gryphoides27. We are of the opinion that C. rivu-laris is an altogether different species, that occurs in Japan (Numachi39) and not in Pakistan. Hence the oyster specimens which were earlier identified as C. rivularis by previous authors certainly belong to C. gryphoides.

Species: Crassostrea madrasensis (Preston) (Fig. 4 A,B)

Synonyms: Ostrea cucullata Hornell4; Ostrea ma-drasensis Preston3, Awati & Rai11, Durve17, Ranjah28; Ostrea virginiana Prashad10, Moses40; Crassostrea madrasensis, Durve18 Rao19, Patel & Jetani21; Crassostrea virginica, Ahmed27; Crassostrea cat-tukensis, Nagabhushanam & Mane22 , Stenzel32.

In this study 47 oysters were examined from Hub River Delta (size 35-180 mm). This species shows close resemblance to C. gryphoides (Table 1) in shell form, except that the inner margins of both valves and adductor muscle scar are of purple colour and the recess beneath the hinge is more well marked and more pronounced than in C. gryphoides. The hinge area is like that of C. gryphoides, being long, with a

Table 1 ⎯ Comparison of the shell characteristics of two species of Pakistani oysters (Crassostrea gryphoides and Crassostrea madrasensis) with the American oyster Crassostrea virginica.

Shell Species characteristics C. gryphoides C. madrasensis C. virginica

Upper valve Flat, lid like, about equal to lower valve

Flat, lid like about equal to lower valve Flat, lid like, smaller than the lower valve

Lower valve Thick, heavy, concave, deeply recessed beneath the hinge area.

Thick, heavy, concave, more deeply recessed beneath the hinge area. Blotches of purple colour present to-wards the margins of the valve.

Thin, deeply cupped.

Hinge Long, deeply grooved, curved to right or left.

Long, deeply grooved curved to right or left.

Short, deeply grooved curved to left.

Adductor muscle scar

White, roundish or bean-shaped, dorsally placed.

Purple, roundish or bean-shaped, dor-sally placed.

Purple, reniform, posterio-dorsally placed.


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Fig. 3 ⎯ A) Crassostrea gryphoides (a) outer view of upper valve (b) inner view of lower valve. B) C. gryphoides: lateral view. C) C. gryphoides: (2) C. madrasensis: (a) outer view of upper valve (b) inner view of lower valve. D) C. gryphoides: morphological variation in shell. E) C. gryphoides: inner view of shell.


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deep groove, and curved to left in many more specimens (65 %) than to right (35 %). The hinge area gradually increases with shell size. In the soft parts, the promyal chamber is present and the colour of the inner mantle fold in 85 % specimens was pigmented black with two rows of tentacles while in the remaining specimens it was yellow. Tentacles of the middle mantle fold in 80% individuals were yellow and in the remaining 20 % were black.

Habitat: Same as C. gryphoides in the creeks of Sindh and Balochistan.

Remarks: Crassostrea madrasensis was found in the same habitat and at the same tidal level as C. gry-phoides. It occurs in the Hub Delta but in much small numbers (6 %) and cannot be separated from C. gry-phoides on the basis of its external shell morphology. Internal morphology of its shells differs, with respect to the purpleness of the adductor muscle scar and in-ner surface of the shells (Table 1). Earlier workers had suggested that O. madrasensis is closely related to the American oyster Ostrea virginiana (=Crassostrea virginica)5,11,35. Prashad10 separated the Indian and American species and assigned the Indian and Suma-tran forms to Ostrea gryphoides. Ahmed27 identified this species from Pakistan as C. virginica, but consid-ers it an error now (in this paper).

During the present study the specimens of C. ma-drasensis from the Hub Delta were compared with specimens of the American oyster C. virginica ob-tained from USA by Ahmed and were found different (Table 1). Crassostrea madrasensis resembles C. gry-phoides closely and would be inseparable on the basis of the outer appearance and morphology of their shells. It is probable, however, that C. madrasensis found in Pakistani waters may simply be a morpho-logical variant of C. gryphoides. Durve17 suggested that the slight variation in the shape of shell and the adductor muscle scar of the two species may be ra-cially or environmentally influenced. However, been able to separate the two species on the basis of their protein band pattern (paper in preparation).

Species Crassostrea belcheri (Sowerby) (Fig 4 C):

Synonyms: Ostrea discoidea Rai12; Ostrea belcheri Awati & Rai11, Kazmi24, Hasan26; Crassostrea belcheri Patel & Jetani21, Chen41, Kong & Luh42.

In this study 233 specimens were examined from Keamari seawall ( size 9-171 mm). The shell is large, more or less rounded to oval in shape, and foliaceous, thick, lower valve flat, upper valve convex, dome

shaped; hinge broad, forming a small beak at the ante-rior end, ligament brown in colour; cavity beneath the hinge very shallow; anterior end of the shell is some-what narrow compared to the posterior side, ventral margin slightly expanded. The exterior colouration of the shell is dull green. Internally the shell is white and sometimes nacreous with no denticulations; upper shell margin has a narrow band of greenish brown colour. The adductor muscle scar is white being oblong lunate towards the hinge side; slightly dis-placed dorsally. A promyal chamber is present. The colour of the inner mantle fold is black. The mantle bears two rows of tentacles; the middle folds being usually yellow, sometimes black pigmented.

Habitat: This species was found on stones of the Keamari seawall, semi-protected habitat, at tidal heights of about 0-0.9 m near the mouth of the Manora channel in Karachi Harbour. The open Ara-bian Sea begins at a distance of about 1 km from here. This species seems to be tolerant of the polluted envi-ronment of the channel. This species was earlier re-ported from the littoral zones of Karachi, northern bank of Korangi creek and Mekran coasts11,24,26.

Remarks: The shell characteristics are similar to those described by earlier workers11,20,21, but the pur-plish green scales reported by them do not occur here in this oyster from Keamari seawall. The upper sur-face of the right convex valve of this species always presents a dull greenish appearance.

Species: Crassostrea glomerata (Gould) (Fig 4D):

Synonyms: Saxostrea amasa Iredale43; Crassostrea amasa Iredale (in Carreon33); Ostrea glomerata Awati & Rai11; Hasan26; Saccostrea glomerata Asif 28; Crassostrea glomerata Ahmed27.

In this study 50 specimens were examined from Karachi (size 40-70 mm). The shell is dull brown, tainted with purple; internally the shell is white with purple margins. In most of the specimens a character-istic purple band is present around the margins of the lower valves, which have a deep upward ascending recess under the hinge. The lower valve is laminated with its margins sometimes projecting into spout-like processes. The upper valve is opercular, without den-ticulations and slightly smaller than the lower valve. The adductor muscle scar is purple, located dorsally, bean shaped and with a slight concavity facing the hinge area. The hinge is narrow, grooved and forms a small beak.

Habitat: This species usually occurs in protected environment, at about 1.5-3.0 m tidal height27. It can


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Fig. 4 ⎯ A,B) Crassostrea madrasensis: morphological variations in shell form. (a) upper valve (b) lower valve. C) Crassostrea belcheri : (a) inner view of lower valve; (b) outer view of upper valve. D) Crassostrea glomerata : (a) inner view of upper valve (b) inner view of lower valve.


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Fig. 5 ⎯ A) Saccostrea echinata: (a) outer view of upper valve; (b) inner view of lower valve; (c) inner view of upper valve; (d) whole shell. B) Saccostrea cucullata:: outer (a) and inner (b) view of upper valve; (c)inner view of lower valve. C) Saccostrea cucullata: (a) inner view of upper valve (b) inner view of lower valve. D) Ostrea nomades: (a) outer view of upper valve; (b)inner view of upper valve: (c) inner view of lower valve. E) Ostrea folium: (a) inner view of lower valve. (b) inner view of upper valve. F) Ostrea cristagalli: (a) inner view of upper valve. (b) inner view of lower valve.


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occasionally be found in semi-protected conditions also but was never observed in exposed environment with heavy wave action. It occurs near high water mark, in the backwaters and creeks forming only a single layer of oysters, not allowing a second layer to develop on top. This species forms a narrow 0.6-0.9 m wide band if growing on a vertical substratum. It is widely distributed on the coast of Sindh and Balochis-tan11,25,26,27,28.

Remarks: This species closely resembles its de-scription given by Awati & Rai11. Asif 28 assigned this species to the genus Saccostrea. He also reported in it the presence of highly developed chomata. Awati & Rai11, Ahmed27, Ansari & Ahmed44 and this study did not find any denticulation along the margins of the shell in any specimens. Therefore, following the char-acteristics of the genus Saccostrea given by Ahmed27

and Stenzel32 this species cannot be assigned to the genus Saccostrea.

Phylum: Mollusca Class: Bivalvia Subclass: Lamellibranchia Order: Anisomyaria Family: Ostreidae Genus: Saccostrea

Genus Saccostrea (Dolfuss & Dautzenberg)

Description of the genus as reported by Stenzel32.“Small to medium-sized, outline variable, but divisible into normal ostreiform ecomorphs with spatulate or vertical-oval to irregular form and ab-normal rudistiform ecomorphs with small, operculi-form right valve (upper valve) and large, slender conical to cornocupia-like. Left valve (lower valve) carrying very high, slender ligamental area, and has large attachment area, and grows preferentially on bare rock surfaces or mangroves, carries many rough growth squamae and 10 to 30 rough irregular rounded dichotomous radial folds extending into small pro-truding lobes. Umbonal cavity in average very deep. Both valves with strong chomata, which commonly encircle entire valve. Promyal chamber is present. Non-incubatory”.

Species: Saccostrea echinata (Quoy & Gaimard) (Fig 5 A) Synonyms: Ostrea mytiloides Cox45; Ostrea echi-

nata Quoy & Gaimard46; Ostrea spinosa Deshayes47; Ostrea mordox Cox45; Ostrea nigromarginata Saville-Kent48, Hedley49 (non Sowerby50); Saxostrea gradiva

Iredale43; Crassostrea echinata Quoy & Gaimard46, Thomson31.

In this study 2 specimens were examined from Gwadar West Bay (size 83-85 mm). Shell inequivalve, somewhat oval in outline, lower valve moderately deep, rim of the lower valve with 12 to 14 regular crenulations, mostly on the posteroventral side. The upper valve is rather flat, but with a central hump be-tween hinge and outer lip; rim upraised; lamellar growth lacking. Externally valve is whitish gray with a black band inside the upraised rim; internally the colour is white with smudges of dirty yellow and a purplish black band present on the margin, particu-larly well marked in the upper valve. In the two specimens examined the shell near the margins is pearly white. The adductor muscle scar is dorsally placed, white, and excavated on the hinge side. Inter-nally the upper valve bears a marginal row of denti-cles which are variably developed, being 38-61 on the ventral side and 31-42 on the dorsal side. Hinge plate broad tapering towards the anterior side.

Habitat: This species was found attached to rocks at a tidal height of about 1.5 m. This species was found on the West Bay of Gwadar, Balochistan29.

Remarks: Saccostrea echinata is the second most abundant species on the West bay of Gwadar29. and S. echinata examined during the present study showed close resemblance to C. echinata from Aus-tralia31. Thomson31 however, reported morphological variation in this species from the sheltered conditions such as a slightly folded lip with vague sinulations and lamellar growth fringes on the lower valve.

Species: Saccostrea cucullata (Born) (Fig. 5 B,C)

Synonyms: Ostrea tuberculata Lamarck51; Lamy52; Ostrea cucullata Hornell6, Awati & Rai11, Kazmi24, Ranjah25 Lamarck51; Crassostrea tuberculata Ah-med27, Thomson31, Carreon33 Lamarck51; Crassostrea cucullata Patel & Jetani20,21; Saccostrea cucullata Rao19, Ahmed53.

In this study 302 specimens were examined from Paradise Point (size 32-82 mm). The shell is small and subtrigonal, brownish, tainted with purple colour towards the margins and internally the colour of the shell is yellowish with a purple band near the mar-gins. It is distinctly fluted and terminates into sharp crenulations. The upper valve is flat forming a lid, crenulated at the margin with distinct anachomata (tubercles or teeth) extending posteriorly almost two thirds from the hinge. The lower valve is moderately deep with a large recess under the hinge; the margin is


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upraised, fluted, the folds giving rise to sharp crenula-tions. The adductor muscle scar is distinct on the up-per valve, and is variably coloured from white to dark purple; on the lower valve the scar is less distinct, a bit excavated on the hinge side and greatly displaced towards the postero-dorsal side. The colour of the ad-ductor muscle scar sometimes varies on both the valves.

Habitat: This species was found firmly attached to the rocks, almost blending with them completely and is extremely hard to detach from the substrate. It was found at a tidal height of 2-3 m on the wave-lashed rocky shores54 and is never seen in protected envi-ronment. It remains out of water for very long periods of time getting submerged for 4-5 hours per day or less. This species has been reported from the wave-lashed rocky shores of Sindh and from the semi-protected/exposed rocky beach at Jiwani and Gwadar, Balochistan29.

Phylum : Mollusca Class : Bivalvia Subclass : Lamellibranchia Order : Anisomyaria Family : Ostreidae Genus : Ostrea

Genus Ostrea (Linnaeus)

Description of the genus as listed by Thomson31. “Shell subcircular, relatively constant in shape within the species; lower valve shallow, not recessed under the hinge, muscle scar subcentral, upper valve may be flat and opercular, or domed and subequal to the lower, and it may or may not have denticles along the margin laterally to the hinge which fit into pits in the lower valve. Incubatory forms”.

Species: Ostrea nomades (Iredale) (Fig. 5 D)

Synonyms: Ostrea crenulifera (Saville-Kent as in Thomson31); Ostrea nomades Iredale43, Thomson31.

In this study 673 specimens were examined from Buleji, and 402 specimens (size 26-97 mm) from Keamari seawall were studied. The lower valve is flat, it appears slightly concave internally and its rim being upraised and folded. The upper valve is convex, folded near the rim where it fits into the broad undula-tions of the lower valve. Externally the shell is green-ish white, with a few individuals showing dull yellow coloration. The internal coloration of the shell is opal-escent greenish white without any well-marked rim. The adductor muscle scar is large and green, more or

less rounded, situated more towards dorsal side, and is green in colour. Anachomata are of small size present almost throughout the edge of the shell, with corre-sponding catachomata (pits) in the lower valve.

A promyal chamber is absent. The colour of the mantle and gills is greenish white. The mantle folds are generally without any pigmentation but pale yel-low pigment may be present in many specimens.

Habitat: This oyster was found on rocky platforms at a tidal range of about 0-0.5 m. At Buleji it occurs in and outside of tide pools on the rocky ledge in the low tide zone and on Keamari seawall.

Remarks: This species is being reported for the first time from the coast of Pakistan. The characteristic features of the shell of this species collected from Ka-rachi in the present study closely resemble those given for Ostrea nomades from Australia31.

Species: Ostrea folium (Linnaeus) (Fig. 5E):

Synonyms: Ostrea folium Thomson31; Dendostrea folium Sowerby50, Harry34; Crassostrea folium, Rao19, Rao36.

In this study the preserved samples (10 specimens of size 45-77 mm) collected by Ahmed et al.29 were examined from West Bay of Gwadar, Balochistan. The external coloration of the shell is bronze pink/red. Internally the shell is opalescent pinkish to copper-coloured, the left valve being slightly concave with a variably folded rim showing weak crenulations. The upper valve is slightly concave with weakly upraised rim. The hinge plate is narrow. The ligament is brown with a dark purple patch on either side. The adductor muscle scar is dorsally displaced, and is white to bronze coloured, reniform, and slightly excavated on the hinge side. Promyal chamber is absent.

Habitat: This species was found on small boulders on the West Bay of Gwadar, on the coast of Balochistan.

Remarks: Thomson31 described various morpho-logical variations of its shell related to habitat: strong flutations in valves, which were loosely attached to the substratum in the intertidal zone, but found low degree of flutations which occurred in submerged conditions or in the oysters, which were firmly at-tached to the substratum. The shells of this oyster available to us for study show fewer crenulations and foldings compared to those described by Thomson31 . The absence of a promyal chamber and the presence of sperm balls in the histological sections55 qualify this species for inclusion in the genus Ostrea.


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Species: Ostrea cristagalli (Linnaeus ) (Fig. 5F)

Synonyms: Mytilus cristagalli Linnaeus56; Ostrea cristagalli Lamarck51; Lopha crystagalli Linnaeus56; Wada57; Torigoe58; Harry34; Crassostrea crystagalli, Rao19, Patel & Jetani20,21, Rao36.

In this study preserved samples (2 specimens of size 49 and 67 mm) of Ostrea cristagalli collected by Ahmed et al.29 were examined from West Bay of Gwadar, Balochistan. The external colouration of the two shells is light brown. Inner surface of the shell is nacreous and exhibits various shades of brown. The shells are rounded, lanceolate, and have radial folds feebly developed. Shell margin is deeply crenulated, and shows a band of large number of minute tubercles (or pustules) all along the entire margin of the upper valve. Hinge is broad, with its ligament being of dark brown colour. Adductor muscle scar is oblong and brown in colour, excavated on the hinge side and is located more towards the dorsal side. Promyal cham-ber is absent.

Habitat: The specimens were found attached to rocks in West Bay of Gwadar, Balochistan. Rao19 found it attached to stones or dead corals.

Remarks: There is the presence of a series of small round granules (minute tubercles or pustules) all along the inner margin of the upper shell valve. Patel & Jetani 20,21 also reported the occurrence of such granules in their specimens collected from the Gulf of Kutch. Stenzel 32 identified this species as Lopha cristagalli, having sharp radial plicae, which are not present in the specimens at hand. Stenzel32, however, reported the presence of a band of tubercle-shaped anachomata all along the margin of the right valve in the Indo-Pacific specimen obtained from Guam.

Hornell5,7 identified this species as Ostrea cucul-lata. Satyamurti15 described C. cucullata as C. cristagalli. This species, however, stands out from all other Ostrea as Lopha cristagalli, owing to the presence of the 2-3 rows of a large number of minute granule-like tubercles (pustules ⎯ a term used by Stenzel32) all along the margin of the right valve of the shell. These tubercles, which are barely visible to the naked eye, do not have corresponding pits in the lower valve, and differ considerably in form and number from the anachomata present in oysters of the genus Saccostrea (such as S. cucullata ) and Ostrea. We have found Ostrea-like eggs and sperm balls in the histological sections of the gonads of this species, which is indicative of the fact that it is a species of the larviparous oyster genus Ostrea55.

The present study shows the occurrence of nine species of oysters namely, Crassostrea gryphoides, C. madrasensis, C. belcheri, C. glomerata, Saccostrea cucullata, S. echinata, Ostrea nomades, O. folium and O. cristagalli on the coast of Pakistan. These belong to the genera Ostrea, Crassostrea and Saccostrea. Crassostrea gryphoides has been found to be the prin-cipal edible oyster species of Pakistan. Crassostrea gryphoides and C. madrasensis are considered here as separate species. Ostrea nomades and O. cristagalli have been reported for the first time from Pakistan. References

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