canarium, as with other Strombus species, is colonial and abundant
wherever it occurs. The species normally associated with sandy mud bottoms
and seagrass beds (Abbott, 1960; Erlambang and Siregar, 1995; Zaidi et
al., 2005). It was the most abundant herbivorous mollusc within the
study site and might contribute to the maintenance and well being of the
seagrass bed ecosystem (Zaidi et al., 2005). The species was highly
prized as seafood in Malaysia and other areas within the region (Chuang,
1961; Erlambang and Siregar, 1995; Zaidi et al., 2005).
The gleaning activity has been continued for ages and very seasonal in
nature. Each year the species appeared in large number during breeding
season, which starts from late November to early March (Zaidi et al.,
2005; Japar Sidik et al., 2006), which the locals take full advantage
by harvesting them. There was currently no regulation in Strombus
fishery activity, which prompts the current study. Preliminary surveys
in local market found very wide range of conch age was harvested (personal
observations), from juvenile to reproductively active females and to very
The objective of this study is to determine the minimum shell-length
at which they matured and become reproductively active. In addition the
minimum length at which sexual character appeared was also investigated.
The information is very important for better understanding and better
management of this commercially important fishery species.
Study sites and sample collection: The study was conducted at
the Merambong Shoal (01° 19.979’N, 103° 35.965’E)
in Johor Straits, Malaysia (Fig. 1), from January to
June 2005. It is a subtidal shoal that was only accessible during period
of extreme low tide. Sample collections were conducted using transect
line approach where all conch within 2 m areas of a 50 m transect line
were collected. In laboratory the shells were cleaned and fouling organisms
were scrapped-off and the animals were separated from their shell.
West Johor Straits, Malaysia showing
the sampling site i.e., Merambong Shoal (encircle), which was only
exposed during extreme low tide period
Ontogenetic development of sex characters: Shell-length and lip
thickness of juvenile of various length sizes were measured and the presence
of its external reproductive genitalia i.e. the verge in males and egg
groove in females, were noted. Individuals were then group into 1 mm length
classes and the proportions of sexed animals against unidentified sex
were determined for both sexes. A logistic-curve was then fitted to the
data using the software ORIGIN® (Microcal Software Inc.), using the
where, Pi is the probability that individual-i can be sexed.
A and B are coefficients describing the intercept and slope of the logistic-curve
and Li is independent variable (growth in length) of individual-i. The
non-linear curve fitting was conducted using Levenberg-Marquardt least-squares
method, incorporated in the ORIGIN® software package. The SX50,
which indicated the probability individuals could be sexed is 0.5, was
then determined for male and female S. canarium.
Sexual maturity: The juvenile and adult stages of S. canarium
are clearly defined through the thickened and flared shell-lip of the
latter. However, the onset of maturity and rates at which they matured
are still not known. Therefore maturity stages were determined, which
were based on: (1) Observation of penis development in males and egg groove
in females; (2) Flaring and thickening of the lips; (3) Macroscopic observation
on gonad coloration and appearance (e.g., vesicle seminalis in males)
under dissecting microscope and (4) Microscopic observation on gonad of
maturing or newly matured individuals, using compound microscope.
Maturity determination and analyses were then conducted based on shell-length
and shell-lip thickness, two main shell characters that are highly correlated
with maturity of this species (Abbott, 1960; Appeldoorn, 1988). The proportion
of mature animals in each size class was computed and a logistic-curve
was fitted, again using the Eq. 1 but Pi
is the probability that individual-i is mature; A and B are coefficients
describing the intercept and slope of the curve and Li is growth
in length or shell-lip thickness of individual-i. The onset of maturity
was then expressed as SL50 and LIP50, which indicated
the probability of an individual being mature is 0.5, at the respective
Shell-Length (SL) and shell-lip thickness (LIP) values. The non-linear
curve fitting was conducted using Levenberg-Marquardt least-squares method.
General anatomy of the gonad: At macroscopic levels, gonad of
the adult S. canarium can be differentiated between males and females.
Testes and seminal vesicle were clearly visible in males, while the ovary,
oviduct and fallopian tube present in females (Fig. 2A,
Strombus canarium gonad:
(A) immature male; (B) Immature female; (C) Mature male; (D) Mature
female. Scale bar = 1 cm
the male and female gonads were almost identical during juvenile stage
(Fig. 2C, D), which appeared as thin,
pale-brown tissues overlaying the digestive gland. There was subsequent
change in color of the gonad as they matured: from blackish to cream or
pale yellow to brownish yellow or bright orange at mature stage.
Ontogenetic development of sex characters: The minimum shell-length
that the sex characters appeared was at 30 mm shell-length for male and
32 mm shell-length for female and both sexes can definitely be determined
when S. canarium attained more than 41 mm shell-length. Using the
logistic curve analysis the SX50 (where the probability individuals
can be sexed is 0.5) was at 38.33 ± 0.41 mm (logistic curve analysis:
χ2 = 0.011, R2 = 0.94, N = 406) for male and
at 37.15 ± 0.31 mm (logistic curve analysis: χ2
= 0.0064, R2 = 0.97, N = 569) for female (Fig.
3, 4, respectively).
The verge of a matured male consists of a grooved stalk with extended
prongs at terminal. Important features of a matured S. canarium
verge include: auxiliary prongs
Logistic curve plot of sex determination
in male S. canarium. P = probability of successful sex determination;
SL = shell-length (mm)
(anterior and lateral), accessory pad and keel. Figure
5 showed a schematic diagram of a typical adult verge and
Logistic curve plot of sex determination
in female S. canarium. P = probability of successful sex
determination; SL = shell-length (mm)
||Schematic line drawing of a matured S. canarium verge. A
= verge total length; B = keel length; C = auxiliary prong length;
D = stalk width; E = keel width
measurements that were used to investigate variability in the verge structure
and to compare with other conch species. For a narcotized (MgCl2)
adult, S. canarium verge length ranged from 20.18 to 31.51 mm;
stalk width ranged from 2.2 to 3.72 mm; keel length from 7.93 to 13.96;
penis keel width from 4.88 to 9.76 mm; accessory pad width from 1.43 to
4.07 mm and the main auxiliary prong length from 3.45 to 7.28 mm (Table
Sexual maturity: Male conch matured at lower shell-length and
lower shell-lip thickness compared with females. The SL50 (length
at which the probability of individuals are matured is 0.5) for males
was 54.14 ± 0.86 mm (logistic-curve analysis: χ2
=0.021, R2 = 0.81, N = 257) lengths (Fig. 6),
whilst SL50 for female was 58.51 ± 1.02 mm lengths (logistic-curve
analysis: χ2 = 0.028, R2 = 0.74, N = 497) (Fig.
7). The LIP50 value on the other hand was about 0.69 ±
0.0003 mm (logistic-curve analysis:
||Logistic curve plot of percentage of mature S. canarium males
for the respective shell-length class. Abbreviations: P, probability
of mature males; SL, shell-length (mm)
||Logistic curve plot of percentage of mature females for the respective
shell-length class. Abbreviations: P, probability of mature males;
SL, shell-length (mm)
||Morphometric measurement of verge characters of adult S. canarium
χ2 = 0.00002, R2 = 0.99, N = 257) for males
(Fig. 8), compared with 0.80 ± 0.014 mm (logistic-curve
analysis: χ2 = 0.01, R2 = 0.95, N = 497) in
females (Fig. 9)
||Logistic curve plot of probability of mature males for the respective
lip thick class. Abbreviations: P, probability of mature males; LIP,
shell-lip thickness (mm).
||Logistic curve plot of probability of mature females for the respective
lip thick class. Abbreviations: P, probability of mature females;
LIP, shell-lip thickness (mm)
The general anatomy of S. canarium was strikingly similar to other
Strombus species except for the verge structure. For the first
time detailed examination has been conducted on the morphology of S.
canarium verge. Abbott (1960) only briefly described the verge as
simple, with broad swollen distal end. This study however found the verge
was rather complex, with few extensions and prongs, which make it unique
from that of other Strombus species present within the study area,
i.e., Strombus marginatus, Strombus urceus and Strombus
||Schematic line drawing of Strombus verge, drawn from fresh
specimen taken from Merambong Shoal: A. S. canarium; B. S.
marginatus; C. S. urceus; D. S. vittatus. Scale
bar = 1 mm
(Fig. 10). Important characters that differentiate
S. canarium verge from other Strombus species were the presence
of an elongated auxiliary prong, an extended accessory pad and a much
wider keel section.
Sex characters first appeared at lower shell-length in males than in
females. Male conch also matured at lower shell-length than the females.
The findings indicate that sexual dimorphisms occurred in the species
and it happens very early in their life stage. Polymorphism has indeed
been reported in few other Strombus species (Kuwamura et al.,
1983; Reed, 1992, 1993a, b, 1995). Recently Zaidi et al. (2008)
showed clear sexual dimorphism within S. canarium population, which
however were only apparent when the conch reached adult stage where flaring
and thickening of shell-lip was prominent.
The current study was generally in agreement with previous studies on
other Strombus species where males are smaller than females, e.g.
Abbott (1949) on S. gibberulus gibbosus Linnaeus, 1758, Randall
(1964) on S. gigas Linnaeus, 1758, Kuwamura et al. (1983),
Reed (1995) on S. luhuanus Linnaeus, 1758 and Reed (1993b) on S.
pugilis Linnaeus, 1758. Zaidi et al. (2008) conclude this condition
as a general characteristic of Strombidae.
The verge structure of S. canarium is unique for the species,
clearly different compared with other Strombus present in the study
area. The sexual characters of S. canarium appeared at lower shell-length
in males compared to females. The males also matured at lower shell-length
than females. The findings indicate that sexual dimorphisms occurred very
early in their life history, even before the ornamentation of the shells
The authors would like
to thanks the deanery and staffs of Biology Department, Faculty of Science,
UPM and School of Environmental and Natural Resource Science, UKM for
technical support and laboratory facilities provided. Special thanks to
the Ministry of Science, Technology and Environment (MOSTE), Malaysia,
for the research grant (04-01-02-SF0124) and JPA Malaysia for the scholarship
awards, which make this study possible.