102 / 2023-07-30 15:23:56

Community Assemblages and Trophodynamics of Macrobenthos in Blood Cockle Culture Mudflats in the Klang Strait, Selangor, Malaysia

Macrobenthos community,Bottom water parameters,Stable isotope analysis,Food web,Species interaction

Blood cockles (Tegillarca granosa) is one of the commercially important shelled molluscs for human consumption. Blood cockles are produced using semi-cultured methods, where naturally occurring spats are accumulated and sown on specific plots in mudflat area and allowed to grow on harvestable size. In Malaysia, production of cockles accounted 91 % of total shellfish production. However, the production of cockles has been unstable largely due to mass mortality and low spat supply. Blood cockles are part of the macrobenthos community with many species co-existing with them in the culture area. Despite numerous attempts to address issue of mass mortality of cockles, most studies focused on abiotic factors that relates to coastal pollution such as elevation of ammonia level, sedimentation and hypoxic condition. There is minimal literature available that address the biotic components such as species interactions (e.g., competition and predation among co-existing macrobenthos species) that relates to energy flow within the ecosystem that may affect the survivability of the cultured blood cockles. This study therefore, aims to address this knowledge gap. Monthly samplings of macrobenthos and measurements of bottom water parameters were performed from May 2022 to January 2023 at culture and non-culture sites at three locations in Selangor, Malaysia, namely Kuala Selangor, Assam Jawa and Bagan Sungai Buloh. Macrobenthos samples were enumerated to estimate their density, diversity and species compositions. Their carbon (C) and nitrogen (N) isotopic signatures were also determined using stable isotope analysis for tracing energy flow within the ecosystem. The mudflat ecosystem included in this study hosts at least 69 macrobenthos species, consisting of mainly bivalves (13 species), crustaceans (18), and gastropods (16), with deposit feeding/scavenging nassariid snails (Nassarius bellulus, N. jacksonianus and N. cf. olivaceus) and the hermit crab Diogenes moosai dominating the macrobenthos assemblages in terms of composition and frequency of occurrence (86 % to 100 % in most sites). The presence of dense filter/deposit feeding nassariids, hermit crabs as well as the cultured blood cockles may suggest the possibility of competition among them for space and food resources. In addition, the presence of predatory gastropods such as P. tigrina and I. malayensis may pose threat to blood cockles since these gastropods have been known to feed on other shelled molluscs. This was evidenced by the presence of bore hole on empty shells of the cockles. Overall, macrobenthos assemblages as well as water parameters were generally quite homogeneous across the mudflats, with small spatial variation due to distance from adjacent mangrove habitats and river mouth, effects of water depth, salinity, turbidity, DO content, tides, and seasonal changes. Stable isotope data revealed four trophic levels of the macrobenthic food web including mangroves, phytoplankton (in seston) and microphytobenthos (in sediment) basal sources at the first trophic level, followed by filter/deposit feeding species at the second level, and the omnivorous scavengers and predators occupying the third and fourth trophic levels. This study seeks to enhance the understanding on the species interactions and trophodynamics of macrobenthos associated with the cultured blood cockles. Thus, providing baseline information that may be useful for a more effective ecosystem management of the blood cockle culture areas.