A.V. Zhirmunsky Institute of Marine Biology, Far Eastern Branch of the Russian Academy of Sciences, Vladivostok 690059, Russia
All species of the family Pinnidae Leach, 1819 (Bivalvia) are sessile semi-infaunal suspension feeders and live vertically embedded in the bottom sediments, usually in mud or muddy sand, anchor- ing by net of byssus threads (Yonge, 1953; Purchon, Purchon, 1981). They are commercial species.
The large posterior adductor muscle is used as dainty food, and the shell is utilized to carve decorative ornaments. Beautiful black pearls are sometimes produced by the pinnids. Because it attains a large size, pinnids are probably one of the most economically important members of the family in the Indo- West Pacific (Poutiers, 1998).
However, little is known about ecology of pinnids. Only Pinna nobilis and Pinna bicolor from the Midetteranean Sea are well studied. For other pinnids, usually only synonymy was referred (e.g., Hedley, 1924; Yonge, 1953; Rosewater, 1961; Purchon, Purchon, 1981; Abbott, Dance, 1982;
Bernard et al., 1993; Huber, 2010; etc.). The decline in pinnids’ populations is observed for the last decades due to various pollutants, sea resorts, anchoring of yachts, as well as unauthorized fishing.
Despite the need for conservation, knowledge of the ecology and monitoring of the structure and condition of the main populations of pinnids inhabiting the western part of the South China Sea is especially limited.
The family Pinnidae consists of the single genus Pinna Linne, 1758, which comprises three subgenera Atrina Gray, 1842; Pinna Linnaeus, 1758 and Streptopinna von Martens, 1880 (OBIS Indo-Pacific Molluscan Database 2006). At the coasts of southern Vietnam, two Pinnidae species are found. They are Atrina vexillum (Born, 1778) (=Pinna nigra Dillwyn, 1817; Pinna nigra Chemnitz, 1785; Pinna vexillum Born, 1778; Atrina nigra Okan and Hosgor, 2009) and Pinna trigonium Dunker, 1852 (=Pinna fumata Reeve, 1858; Pinna philippinensis Reeve, 1858) (Silina, 2011a). The shells of the Pinnidae bivalves are very large and always triangular in outline, tapering to a point anterior (Yonge, 1953) (Figs. 1, 2).
It is poorly known in world literature about P. trigonium and there is a knowledge gap about their distribution near Vietnam. Studies about biology and ecology of this species are few (Yonge, 1953; Malakhov et al., 1985). As a rule, pinnids live vertically embedded in the ground, however, P. trigonium was found not only in the ground, but also among the biofouling of the sunken vessel (Silina, 2010).
P. trigonium was found in Nha Trang Bay (6–10 m of depth) and in the eastern Gulf of Thailand (3–12 m). Near the coasts of Nha Trang, P. trigonium was collected and compared in 1980, 1984 and 1986. P. trigonium population was not numerous in 1980–1986 in Nha Trang Bay, which is under anthropogenic pressure. Now this pinnid is not found near the coasts of Nha Trang. Here, the mean value of shell length of this bivalve was 315±22 mm in 1980–1984, but in 1986 it was only 207±20 mm
Alla V. Silina
(Silina, 2010). In the Gulf of Thailand in 1986, specimens of P. trigonium were sampled near the coasts of An Thoi Archipelago, mainly near Hon Vang Island (Echo Island) (6–7 m depth), Hon Roi (Lamp Island), and Hon May Rut Island (Cold Cloud Island) (3–12 m). In the Gulf of Thailand, mean and maximum parameters of P. trigonium individuals were higher then those of specimens from Nha Trang Bay. At the coasts of An Thoi Archipelago, the total wet weight of P. trigonium varied from 262 to 1590 g (559±118 g on the average), wet weight of the posterior adductor muscle was 7–25 g (15±2 g) and shell length varied from 249 to 377 mm (303±15 mm) (Silina, 2010).
A. vexillum is a cosmopolitan tropical species distributed in Indo-Pacific region from East Africa to eastern Polynesia; Western Pacific, South-Western Pacific, Central Pacific; north to Japan and Hawaii, and south to Australia (Hedley, 1924; Habe, 1968; Morris, Purchon, 1981; Abbott, Dance, 1982; Bernard et al., 1993; Huber, 2010). Common name of A. vexillum is Indo-Pacific pen shell. A shell of A. vexillum reaches large size and usually attains 30 cm (maximum to 48 cm) (Poutiers, 1998).
Specimens of A. vexillum were found in 1986 in the eastern Gulf of Thailand, near the coasts of Phu Quoc Island and An Thoi Archipelago located in the inner part of the Gulf, and near Tho Tu Island situated in the outer part of the Gulf.
In 2010, A. vexillum was not regis- tered near both Phu Quoc Island and An Thoi Archipelago (Latypov, Selin, 2011). Population structure varied with areas and habitat types.
In the outer area of the Gulf, shell length (mean±S.E.) was the highest, 272.7±13.2 mm. In the inner area of the Gulf, shell length did not exceed 182 mm (166.7±9.5 mm).
Shell length frequency of A. vexil- lum populations was wide-ranging;
nevertheless, the specimens with a shell length of 170–190 and 290–
310 mm were the most frequently found in the inner and outer areas, respectively. In the population lived at the coasts of An Thoi Archipelago, the total wet weight of A. vexillum varied from 82 to 500 g (236±18 g) and the wet weight of the posterior adductor muscle was 5–30 g (13±1 g) (Silina, 2011b).
A. vexillum individuals inha- bited the inner part of the gulf had slower growth rates than the speci- mens from the outer area of the gulf.
The local differences in growth rates Fig. 2. Pen shell Pinna trigonium.
Fig. 1. Flag pen shell Atrina vexillum with epibionts on its shell.
Bivalve pinnid populations
of A. vexillum indicate that the molluscs are under different levels of environment pressure in the areas studied. The water temperature, concentration of suspended organic carbon, and the concentration of chlorophyll “a” increase from the outer to inner locations, but oxygen concentration decreases in this direction due to the high level of organic matter decomposition. So, in the inner area, A. vexillum is subject for the whole set of unfavourable environmental factors.
During the last decades, studied pinnid populations have declined drastically due to increasing anthropogenic pressure. In the turbid waters of Nha Trang Bay, P. trigonium was practically petered out. Due to rise in tourist industry in the studied part of the Gulf of Thailand, especially on the Phu Quoc Island, pinnid populations that lived near the coasts of Phu Quoc Island and adjacent An Thoi Archipelago are the most vulnerable. These 15 islands and islets can be visited by boats, and it’s a fine area for sightseeing, fishing, swimming and snorkelling. In addition, pinnids are important component of the soft sediment communities of sublittoral zone. They are large bivalves that, alive and died, provide shelter for motile species and hard substrate for settling larval invertebrates and egg-laying (Fig. 1). Sometimes, for instance, in St. Joseph Bay, Florida, pen shells Atrina rigida are the most abundant source of hard substrate. Therefore, pinnids are need in their conservation and implementa- tion of sustainable resource management in the studied areas.
Acknowledgements
This work was supported by grant from the Far Eastern Branch of the Russian Academy of Sciences (no 12-I-DBS-09).
References
Abbott R.T., Dance S.P. 1982. Compendium of Seashells. New York: E.P. Dutton Inc. 311 p.
Bernard F.R., Cai Y.Y., Morton B. 1993. Catalogue of the Living Marine Bivalve Molluscs of China. Hong Kong: Hong Kong University Press. 121 p.
Habe T. 1968. Shells of the Western Pacific in Color. Vol. 2. Japan. Osaka: Hoikusha Publishing Co. 233 p.
Hedley C. 1924. A revision of the Australian Pinnidae // Records of the Australian Museum. V. 14, N 3.
P. 141–153.
Huber M. 2010. Compendium of Bivalves. A Full-Color Guide to 3,300 of the World’s Marine Bivalves.
A Status on Bivalvia after 250 years of Research. Hackenheim: ConchBooks. 901 p.
Latypov Yu.Ya., Selin N.I. 2011. Current status of coral reefs of islands in the Gulf of Siam and southern Vietnam // Biologiya Morya, Vladivostok. V. 37, N 4. P. 246–253.
Malakhov V.V., Yakovlev Yu.M., Blinov S.V. 1985. Studies of the biology of commercial bivalve mollusks during IX cruise of the R/V “Berill” in the coastal waters of South Vietnam // Soviet Journal of Marine Biology.
N 3. P. 71–74.
Morris S., Purchon R.D. 1981. The marine shelled mollusca of West Malaysia and Singapore. Part 3. Bivalvia //
Journal of Molluscan Studies. V. 47, N 3. P. 322–327.
Poutiers J.M. 1998. Bivalves FAO Species Identification Guide for Fishery Purposes. The Living Marine Resources of the Western Central Pacific. Vol. 1: Seaweeds, Corals, Bivalves and Gastropods. Rome:
FAO. 686 p.
Purchon R.D., Purchon D.E.A. 1981. The marine shelled mollusca of West Malaysia and Singapore. Part 1.
General introduction and an account of the collecting stations // Journal of Molluscan Studies. V. 47, N 3.
P. 290–312.
Alla V. Silina
Rosewater J. 1961. The family Pinnidae in the Indo-Pacific // Indo-Pacific Mollusca. V. 1, N 4. P. 175–226.
Silina A.V. 2010. Spatial and temporal changes in the population structure of bivalve Pinna fumata at the South Vietnam // Proceedings of the International Conference “Marine Biodiversity of East Asian Seas: Sta- tus, Challenges and Sustainable Development”, Nha Trang, Vietnam, December 6−7, 2010. Nha Trang.
P. 98–100.
Silina A.V. 2011a. Comparison of bivalves of family Pinnidae from southern Vietnam: a morphometric approach // Korean Journal of Malacology. V. 27, N 4. P. 345–351.
Silina A.V. 2011b. The Indo-Pacific pen shell Atrina vexillum in south-eastern Siam Bay (South China Sea) //
Proceedings of the International Conference “Coastal Marine Biodiversity and Bioresources of Vietnam and Adjacent Areas to the South China Sea”, Nha Trang, Vietnam, November 24−25, 2011. Vladivostok:
Dalnauka. P. 12–16.
Yonge C.M. 1953. Form and habit in Pinna carnea Gmellin // Phylosophical Transactions of the Royal Society of London. Ser. B: Biological Sciences. V. 237, N 648. P. 335–374.
Marine Ecosystems under the Global Change in the Northwestern Pacific