Biology and Ecology of Dog Conch (Strombus Canarium Linnaeus, 1758) (Gastropoda: Strombidae) From Merambong Shoal, Johor Straits, Malaysia

Strombus canarium Linnaeus, 1758, locally known as „siput gonggong‟ is a prosobranch gastropod from the family Strombidae that was highly associated with seagrass bed ecosystem. The main objectives of this study were to investigate and document the life history characteristics, biology, and ecology of the species, which are currently not well established. This study has been conducted at Merambong Shoal, Johor Straits, Malaysia. Sample collections were conducted using a belt transect method where all individuals within the transect area were collected. The population dynamic was analyzed using both the Electronic Length Frequency Analysis (ELEFAN) and Length-based Fish Stock Assessment (LFSA) approaches, incorporated in the FiSAT software package. Apart from field observation, field experimentation and laboratory analyses, laboratory culture experiment (from egg mass to juvenile) was also conducted, using a standard larval culture protocol. Strombus canarium produced a long gelatinous tube of egg strand in the form of an egg mass. It took between 4.5 to 5.5 days of incubation before hatching, and the planktotrophic larvae spent between 18 to 24 days in the planktonic stage before metamorphosed. Metamorphic cue associated with bio-active materials from sediment taken from its nursery habitat showed the strongest effects compared to sediment conditioned seawater, seagrass detritus, seagrass detritus leachate, Potassium Chloride (KCL) and -aminobutyric acid (GABA). Changes of behavioral patterns related to metamorphosis were described, which can be divided into 3 phases: initiation, actual metamorphosis, and completion, resulted in completely camouflaged juveniles. In their natural habitat, juveniles can be found at specific localities (nurseries), characterized by low to medium density Halophila bed; high sediment organic content; high sorting coefficient, and low mean grain size. The adult population was widely distributed, but present in local colonies, and was highly seasonal in abundance. Based on the gonadosomatic index (GSI), combined with other biological, ecological and behavioral observations, it could be concluded that the species was highly seasonal in reproduction, which starts from November to late March. The population showed sexual polymorphisms where 3 different sexual morphs present i.e., males, normal females, and imposex females. Imposex was characterized by superimposition of male sexual characters on female. The imposex females showed significantly larger and heavier shell, and higher degree of outer-lip flaring compared to males and normal females. Imposex severity was defined as Stage 0 without the male genital system; Stage 1 with the appearance of rudimentary penis, but without penis duct; Stage 2 with simple prong of penis and penis duct, but without accessory; and Stage 3 with male penis complete with accessories. Development of other male characters as outlined in the general scheme of imposex evolution in prosobranchs was never observed. The percentage incidence of imposex (%I) was lower compared to other gastropod within the study areas, and there was no case of female sterility observed. In population dynamic studies, a total of 766 males and 1322 females have been sampled from January to December 2005. Growth parameter estimation using both the ELEFAN-I and LFSA methods showed higher asymptotic length (L∞) value in females compared to the males, but there was no significant difference in growth constant (K) and age at zero-length (to) of both sexes. The mean growth rate during the first year growth was estimated at 2.66 ± 0.16 mm/month for male and 4.59 ± 0.59 mm/month for female, which was comparable with the actual growth recorded using mark-recapture technique. The growth performance index (φ‟) was slightly lower in male (φ‟=3.48) compared to the female (φ‟=3.81), but both was well within the values reported for other marine gastropods. Recruitment patterns, together with observations on spawning and copulation suggest that the species was highly synchronized (seasonal) in reproduction. The production and biomass was also seasonal, significantly higher during the wet monsoon season compared to the dry season. In conclusions, many aspects of life history, biology, and ecology of S. canarium had been established. However more studies are urgently needed for better understanding and better management of the species, and for advancement of the species into mariculture

Age, growth, mortality and population structure of Strombus canarium (Gastropoda: Strombidae): variations in male and female sub-populations.

The age, growth, mortality and population structure of Strombus canarium Linnaeus, 1758 were examined in the Johor Straits, Malaysia from January to December 2005. A total of 2088 conchs were sampled where females were more abundant than males with monthly sex ratio of 1.72±0.17 (N = 12). The estimated growth parameter showed higher asymptotic length (L∞) and growth coefficient (K) in females (L∞ = 70.20 mm, K = 1.50 year-1) compared with the males (L∞ = 69.30 mm, K = 1.20 year-1). This resulted in better overall growth performance of females (φ’ = 3.81) compared with the males (φ’ = 3.48). The growth pattern of females and males showed positive allometric nature of growth (b>3, p<0.05), with estimated maximum life span of 2.0 and 2.5 year, respectively. The recruitment pattern was continuous, displaying only a single major peak event per year. The estimated natural mortality rate (M) was 0.95 year-1 in females and 0.86 year-1 in males, while the total mortality rate (Z) was 2.56 and 2.72 year-1, respectively. The fishing mortality rate (F) was 1.61 year-1 in females and 1.86 year-1 in males, which were higher than the natural mortality rates, thus indicating an unbalanced position of the stock. In addition, the exploitation rate (E) was higher than the maximum allowable limit of exploitation (EMSY), which was a further indication of overexploitation. For sustainable utilization of the resource, serious efforts should immediately be taken in reducing the exploitation rates of S. canarium in the study area

Sexual maturity and sex determination in Strombus canarium Linnaeus, 1758 (Gastropoda: Strombidae).

The study was conducted at the Merambong Shoal, Johor Straits, Malaysia. Strombus canarium was the most abundant herbivorous mollusc within the study site, highly prized as seafood and contributes to the economics of the locals. Ontogenetic development of sex characters and sexual maturity were determined via logistic curve analysis, using shell-length and/or lip thickness as predictors. The minimum shell-length at which sex characters appeared was at 30 mm in males and 32 mm in females and both sexes can definitely be determined at more than 41 mm shell-length. The SX50 (probability of individuals can be sexed is 0.5) was at 38.33+0.41 mm for male and at 37.15+0.31 mm for female. The SL„ (length at which the probability of individuals are matured is 0.5) was at 54.14+0.86 mm shell-lengths for males and at 58.51+1.02 nun shell-lengths for females. The LIP„ (lip thickness at which the probability of individuals are matured is 0.5) values on the other hand was about 0.69+0.0003 and 0.80+0.014 mm for males and females respectively. The findings indicate that sexual dimorphisms occurred at very early stage in S. canarium life histor

Description and evaluation of imposex in Strombus canarium Linnaeus, 1758 (Gastropoda, Strombidae): A potential bio-indicator of tributyltin pollution.

Strombus canarium Linnaeus, 1758 is an important gastropod species within the study area and was traditionally collected for food by the locals. The objective of the present study is to assess the incidence of imposex and its severity in this species. Adult conchs were sampled during their main reproductive period, from October 2005 to January 2006, at Sungai Pulai estuary, Johor Straits, Malaysia. A total of 32.81% of adult females showed imposex characteristics, with varying degrees of severity though. The relative penis size (RPS) index ranged from 1.74 to 33.29 (mean = 13.40 ± 2.27, n=21), while the relative penis length (RPL) index ranged from 6.28 to 55.19 (mean = 25.83 ± 3.33, n=21). The use of vas deferens sequence (VDS) index was however cannot be applied as the presence of egg groove obscured any vas deferens development in affected females. Sequence of imposex (male penis) development in female conch, from merely a small stump to an advance male penis homologous was therefore carefully analyzed and described, and an alternative imposex classification scheme was proposed. S. canarium can be a good indicator for monitoring of organotin pollution within the study area. However, more studies are needed in order to further develop and test its validity and application, such as its correlation with levels of pollutant within the tissues and the environment, as well as its application on other Strombus species

Diet composition of cobia fish, Rachycentron canadum (Linnaeus, 1766) from northeastern waters of Peninsular Malaysia

A study was conducted to investigate the food and feeding habits of cobia, Rachycentron canadum from the northeastern waters of Peninsular Malaysia. Stomach content of 82 R. canadum fish were analyzed with reference to gender and size classes. Fish was the most dominant diet component for R. canadum with % IRI value of 95.3%, followed by squid (3.1%), crab (1.5%), and shrimp (0.07%). Fish was also predominant regardless of gender and size classes of the R. canadum in the study area. Shrimp was not taken by the male, but found in females with sizes ranged between 80 and 120 cm TL. Rachycentron canadum that was larger than 120 cm TL also did not prefer crab as part of its diet component. Their feeding intensity increased with size accordingly that was indicated by the increase in the percentage of active feeder. This research finding will contribute to the development of R. canadum culture in Malaysia

Habitat preference and usage of Strombus canarium Linnaeus, 1758 (Gastropoda: Strombidae) in Malaysian seagrass beds.

Despite their economic importance, the ecology of Strombus canarium is poorly understood and factors influencing their habitat preferences remain largely unexplained. The species was reported as highly associated with seagrass bed ecosystems, but their distribution and specific preferences within the habitat remain unknown. Determining the mechanisms that regulate conch distribution and abundance is of fundamental importance for management of the species. In this study habitat preference and usage of conch in their natural habitat were investigated. The population was patchily distributed and present in local colonies that comprised of mixed age groups. There was high spatial variation in conch abundance, where multivariate analysis (PCA) showed high preferences for microhabitat with mixed seagrasses dominated by Halophila spp. Other important habitat characteristics preferred were high sediment organic content (% LOI), high sediment sorting (φ), and low mean sediment particulate size. Areas with dense Enhalus acoroides meadows, though in many reports have been linked with the species, were surprisingly the least preferred microhabitat. It is concluded that the preference towards specific microhabitat within the seagrass bed is associated with their feeding, and intra-specific interactions among individuals

Development and growth of larvae of the dog conch, Strombus canarium (Mollusca: Gastropoda), in the laboratory

Strombus canarium egg masses used in this study were collected while still underneath spawning females, and embryonic and post-hatching larval development was observed in the laboratory. Strombus canarium larvae were reared at 200 larvae/L in 2-L containers with 0.22 μm filtered seawater medium at a salinity of 30 ± 1 PSU, and fed a single algal species, Isochrysis galbana, at 1000 cells/ml. Fecundity was estimated at 48,745 ± 877 to 93,643 ± 1685 (n = 10) eggs/egg mass. The incubation time Fecundity was estimated at 48,745 ± 877 to 93,643 ± 1685 (n = 10) eggs/egg mass. The incubation time 93.34% ± 1.68% hatched, n = 3). The larvae have 2 velar lobes and 1.5 shell whorls at the time of hatching, with an average shell length of 216.77 ± 5.72 μm (n = 10). Based on prominent larval characters and visible morphological features, S. canarium larvae can be assigned to 4 different development stages, i.e. stages I, II, morphological features, S. canarium larvae can be assigned to 4 different development stages, i.e. stages I, II, hatching, and only metamorphosed when settlement cues (sediments from its natural habitat and 15 mM KCl) were introduced. They showed a short period of metamorphic competence, and no spontaneous metamorphosis was observed. Morphological changes and larval behaviors at the onset and during metamorphosis are also described

Seasonal variation in growth and survival of Strombus canarium (Linnaeus, 1758) larvae.

This study was conducted to analyze variation in Strombus canarium larvae development, growth and survivals when cultured during wet (main reproductive period) and dry seasons. Larvae were reared at 200 larvae L(-1) in filtered seawater (0.22 microm) and fed with Isochrysis galbana at 1000 cells mL(-1) ad libitum. The culture environment was maintained at 29 +/- 1 degrees C, salinity of 30 +/- 1 PSU and photoperiod of 12:12 light dark cycle. Growth of the larvae was described on a length-at-age basis using the modified Gompertz regression. There was high correlation in shell length-at-age relationship for both wet season (R2 = 0.99) and dry season (R2 = 0.98) culture experiments. The maximal growth rate (M) and survival rate (S) were higher for larvae cultured during wet season (M = 62.44 microm day(-1), S = 14.36-2.31%), compared with dry season (M = 43.05 microm day(-1), S = 5 +/- 1.15%). The maximal attainable larval size (a) was however lower during wet season (950.19 +/- 66.93 microm shell length) compared with dry season (1343.05 +/- 586.51 microm shell length), which might be due to significantly low larvae density in the latter. Further studies are needed to investigate variation in bio-chemical composition of the egg mass, which was suggested as the main reason for the differences

On the biology and basic characteristics of the population dynamic of the dog conch, Strombus canarium Linnaeus, 1758 (Strombidae)

Strombus canarium Linnaeus, 1758 is among the important gastropod fishery within the western Johor Straits, Malaysia. In this study, systematic random samplings using belt transect were conducted from January to December 2005. Shell-length data were grouped into different sexes and length-classes of 2 mm intervals. Overall ratio of males to females was 1:1.73, which was not significantly deviated from the expected 1:1 (P > 0.05). The population structure was analyzed using the Length-based Fish Stock Assessment (LFSA) protocol in FiSAT (FAO-ICLARM Stock Assessment Tools) software package. Both males and females were highly polymodal in monthly length-frequency distributions. Sexual dimorphism was evident within the population. Specific cohort growth parameters found higher asymptotic length (L∞) and growth constant (K) in females compared to the males. The mean L∞ was 62.90 ± 3.68 mm in males and 69.73 ± 0.80 mm in females, while the mean K was 1.10 ± 0.15 yr–1 in males and 1.40 ± 0.10 yr–1 in females. The higher growth rates and faster growth to marketable size, combined with all year round recruitment indicated great potential for introduction into aquaculture. However, further studies are greatly needed in assessing actual growth and other parameters of this important gastropod species

Species description and distribution of Strombus (Mollusca, Strombidae) in Johor Straits and its surrounding areas

A total of 230 individuals of Strombus were sampled at various locations along the Johor Straits, Malaysia. There were four species of Strombus present in the study areas i.e. Strombus canarium Linnaeus, 1758; Strombus urceus Linnaeus, 1758; Strombus marginatus subspecies succinctus Linnaeus, 1767; Strombus marginatus subspecies robustus Sowerby, 1874; and Strombus vittatus subspecies vittatus Linnaeus, 1758. Strombus canarium was the most common, widely distributed and most abundant, followed by S. urceus, while the others were only rarely found. Among the species Strombus marginatus and Strombus vittatus were two new distribution records for the Johor Straits. Since all Strombus were traditionally harvested and consumed by the locals since long ago, further studies are needed particularly regarding the population dynamics and fishery of the harvested species