THE INDO-PACIFIC PEN SHELL ATRINA VEXILLUM IN SOUTH-EASTERN SIAM BAY (SOUTH CHINA SEA)

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were compromised but greater reduction in its lower temperature limit was observed,
i.e., 5.6ºC (salinity 0) and 5.1ºC (salinity 10). The ability to withstand cold and hot
conditions at both high and low salinities could explain its wide latitudinal distribution,
although such physiological capabilities remain to be elucidated for this species. High
tolerance to extremes in salinity and temperature also explains its considerable biofoul-
ing persistence and survival in the face of rapid changes in the environment.
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Effect of salinity on lower and upper thermal survival limits of Mytilopsis sallei (Dreissenidae), based
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(A) mean lower temperature survival limits; (B) mean upper temperature survival limits.
THE INDO-PACIFIC PEN SHELL ATRINA VEXILLUM
IN SOUTH-EASTERN SIAM BAY (SOUTH CHINA SEA)
Alla V. Silina
A.V. Zhirmunsky Institute of Marine Biology, Far East Branch,
Russian Academy of Sciences, Vladivostok 690041, Russia
e-mail: allasilina@mail.ru
At the south-eastern coasts of Siam Bay (=the Gulf of Thailand) (southwest
Vietnam), one species of the genus Atrina (Pinnidae) was found, namely, the flag pen
shell Atrina vexillum (Born, 1778). It is characterized by elongated, wedge-shaped shell
(Fig. 1). It attains big size and usually reaches 30 cm (maximum 48 cm) (Poutiers, 1998).
Along with all species of the family Pinnidae, A. vexillum is a sessile semi-infaunal sus-
pension-feeder and it lives vertically embedded in the bottom sediments, usually mud
or muddy sand, anchoring by net of byssus threads. It is a tropical species distributed in

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the Indo-Pacific region from east-
ern Africa to eastern Polynesia, in
western, south-western and central
Pacific; north to Japan and Hawaii,
and south to Australia. It is a com-
mercial species.
The decline in A. vexillum
populations threatened 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 A. vexillum inhabited Siam Bay are limited. The present study is a contribution to the
knowledge of A. vexillum ecology, growth, and structures of the populations inhabiting
the southwestern Vietnam coasts. The hypotheses of differences in growth and popula-
tion structure of A. vexillum were investigated according to sub-areas in eastern Siam
Bay characterized by different environmental factors.
The study area of south-eastern Siam Bay was subdivided in three sub-areas.
The first sub-area (S1) was identified near the coasts of Phu Quoc Island which is the
nearest (among the studied sub-areas) to the top of the bay and nearby to the coastline of
continent. The second sub-area (S2) was at the coasts of An Thoi Islands located at the
south of Phu Quoc Island not far from the continent. The third sub-area (S3) was situated
near Tho Tu Island, in the open part of the bay, nearby the boundary of the Siam Bay.
Specimens of A. vexillum were sampled in 1986. In each sub-area, different aspects of
population demography were studied (i.e., size structure, age structure and growth).
A comparison between the populations inhabiting these sub-areas was carried
out and we found important differences between them. Population structure varied
with sub-areas and habitat types. At open S3, maximum and mean (±SE) size was
the highest (shell length (L), 320 mm, 272.7±13.2 mm, nonmetering the very large
specimen that had shell 438 mm). At S2 situated between S1 and S3, mean L was
171.8±4.5 mm, and max L was equal 224 mm. At S3, the most deeply embedded
within the bay, L did not exceed 182 mm, mean L=166.7±9.5 mm. The significant
differences between all mean values of shell length were revealed by t-test (P<0.01).
Length frequency of A. vexillum was wide-ranging; nevertheless, the specimens with a
shell length of 170–190 and 290–310 mm were the most frequently found at S1 and S3
sub-areas, respectively. At S2, the size structure of A. vexillum population was bimodal
with two peaks for 150–170 and 170–190 mm shell length.
At S2, the wet total weight varied from 82 to 500 g (236±18 g on average), the
soft tissue wet weight was 15–105 g (41±3 g), and wet weight of the posterior adductor
muscle was 5–30 g (13±1 g). The shell height varied from 85 to 140 mm (116±3 mm),
Fig. 1. Flag pen shell Atrina vexillum. The outer shell surface
with the elementary growth layers.

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and shell width was 27–49 mm (40±1 mm). Tissue wet weight was 10.7–25.0% of total
wet weight, that is lower than for Pinna nobilis (Garcia-March et al., 2007), but higher
than for Pinna trigonium (=P. fumata) (Silina, 2010) and the wet weight of the poste-
rior adductor muscle was 3.1–7.7% of total wet weight and 19.0–37.0% of tissue wet
weight, that is higher than for P. nobilis and P. trigonium (Garcia-March et al., 2007;
Silina, 2010).
The growth of wet total weight (Wtotal, g), wet soft tissue (Wsoft tissues, g), wet muscle
(Wmuscle, g) and shell weights (Wshell, g) was not isometric with shell length (L, cm)
increase, as the exponents were many less than 3:
Wtotal = 0.160L2.560, R = 0.820;
Wsoft tissues = 0.020L2.674, R = 0.852;
Wmuscle = 0.010L2.498, R = 0.794;
Wshell = 0.130L2.466, R = 0.846.
Classical allometric model is not supported for other Pinnidae species too
(Rabaoui et al., 2007; Silina, 2010). Besides, the linear parameters of the A. vexillum also
did not change isometrically during their life, as the exponents were many less than 1:
H = 1.582L0.704, R = 0.791, where H is shell height, cm;
D = 0.698L0.614, R = 0.628, where D is shell width, cm.
It means that shell of the A. vexillum becomes more prolonged and less convex
(gibbous) with increase of its age.
The outer shell layer of the individuals of family Pinnidae is calcitic prismatic that
is contribute to appearance of elementary growth layers on the outer surface of the shell
(Fig. 1). Previously, we have revealed that the width of the elementary growth layers of
P. trigonium varied accordingly to seasons with periods of slow growth rates during late
autumn – early spring and high growth rates during spring–autumn (Dorofeeva et al.,
1987). It allows determining the age of each individual by counting the quantity of shell
portions with thin elementary
growth layers. Additionally, there
were the annual rings on the mus-
cle mark (track) on the inner shell
surface of A. vexillum; they cor-
responded to the quantity of shell
portions with thin elementary
growth layers. For the individuals
sampled in February, the muscle
marks were ended with such ring;
thus, the ring was formed during
the winter (Fig. 2). Butler and
Srewster (1979) also have used
such rings on the muscle mark
for age determination of Pinna
Fig 2. The inner surface of the shell of 2.5-year-old Atrina
vexillum sampled in February. The muscle mark is ended with
the winter annual ring. 1w, 2w and 3w are winter rings.

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bicolor. It was revealed that growth rates varied with shell size, with a peak at the first
year of A. vexillum life, followed by a sharp decline.
The A. vexillum individuals inhabited S1 had the slowest growth rates and up to
an age of 3.5–5.5 years their shells were smaller than shells of specimens from other
studied populations in Siam Bay. Growth rates were higher at S2 than at S1, but statisti-
cally significantly (P<0.05–0.01, for ages 4.5 and 5.5 years old, t-test) slower than at
open S3 (see Table).
Growth rates of the flag pen shell Atrina vexillum at the different sub-areas of the south-eastern Siam Bay
Age, years
An Thoi Islands Tho Tu Island
N, ind. Length, mm N, ind. Length, mm
2.5 1 134 0 –
3.5 8 159.6±6.0 1 195
4.5 11 187.3±7.2 3 231.3±8.7
5.5 3 203.3±2.3 3 288.0±9.3
6.5 0 – 0 –
7.5 0 – 4 311.8±5.0
Thus, it was revealed that the farther to the bay (to the direction of its top), the
growth rates of A. vexillum is getting slower and life span is shorter. The explanations
for this phenomenon may be as follows. It is known that the water salinity of Siam Bay
<31.26‰ (30.80‰ on average), and the bay is under influence ofrunoff of rivers.Mem-
bers of the family Pinnidae is intolerant to the water salinity differentials. However,
at open S3, the water salinity is about 32.0‰ as well as in the open part of the South
China Sea. Moreover, concentration of suspended particular matter (SPM) increases
from open S3 to S2 and farther to S1 from 0.69 mg/l to 1.26 mg/l (Report, 1986). All
species of suspension filter feeding pinnids are sensitive to high concentration of SPM
that adversely affects filtering apparatus, feeding and respiration of mollusk. Besides,
concentration of suspended organic carbon also increases from 17% (S3) to 31% (S1) of
dry suspended matter. Concentration of chlorophyll «a» is 0.25–0.29 µg/l in the water
at open S3, but it is 0.57 and 0.94–1.22 µg/l at S2 and S3, respectively. However, water
temperature increases in this direction from 25.3–26.2ºC to 27–28.2ºC in February
(Report, 1986). Therefore, oxygen concentration decreases in the direction of S3–S1
due to high level of organic matter decomposition, especially at high water temperature.
Thus, it is 5.31 ml/l in the water of open S3 and 5.09 ml/l at S2 and S1 (Report, 1986).
Low oxygen concentration also adversely influences the respiration and condition of
the studied bivalve species and, in turn, its growth rates and life span. The results con-
tribute to increase the knowledge of population ecology of A. vexillum and to provide
useful information for implementing conservation policies.

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