Analysis of the Annelida Families in Four Contrast Coastal Defence Structures (CDS) in Kuala Nerus, Terengganu, Malaysia

Coastal defence structures (CDS) play a vital role in stabilizing navigation channels, sustaining deepened shipping routes, reducing wave and current actions, and ultimately mitigating erosion. However, their ecological effects, particularly on the Annelida communities, remain uncertain. This study examines the CDS impacts on Annelida communities in Kuala Nerus, Terengganu. A nine-month sampling period was conducted at four CDS stations- groin (St. 1), semi-enclosed jetty type breakwater (St. 2), parallel breakwater without tombolo (St. 3), parallel breakwater with tombolo (St. 4)- along with a control station, encompassing sheltered and exposed areas. Triplicate sediment samples were collected using a Ponar grab for biological and ecological analysis. A total of 1,152 Annelid worms from 31 families were identified. Station 1 presented the highest Annelida density (3,695 ind./m²), followed by Station 2 (1,550.72 ind./m²) and Station 4 (1,413.04 ind./m²). Oweniidae dominated the community (42.88%), followed by Spionidae (8.85%) and Dorvilleidae (8.33%). Sheltered stations exhibited higher densities (2526.570 ind./m2 ± 1976.25 ind./m2). The study area is characterised by sandy sediments with stable (consistent and balanced levels) total organic matter (TOM) and heavy metal contents, with minimal spatial variation. The heavy metal levels were within the permissible limits of the Upper Continental Crust (UCC). Current speed and wave height were lower at sheltered sites, while the control station recorded the greatest depth. The presence of the CDS provided favourable conditions for the annelida communities, which were primarily influenced by soft-bottom characteristics and food availability (represented in this study as TOM), with hydrodynamic stress acting as a limiting factor in exposed areas. The CDS altered the hydrodynamics, modified sediment composition, and elevated TOM and heavy metal concentrations, resulting in a higher abundance of deposit-feeding annelids near the structures. These findings highlight the role of CDS in shaping annelid communities in coastal environments.