Evaluating Substrate Suitability for Larval Settlement of the Tube-Building Polychaete Lanice conchilega in Relation to Hydrodynamic Modification

Lanice conchilega is a tube-building polychaete that is known as an ecosystem engineer because of its ability to change sediment structure, stabilise the seabed, and increase biodiversity in benthic communities. However, habitat degradation, bottom trawling, coastal development, and climate-induced changes in sediment dynamics are posing a growing threat to Lanice populations, resulting in a decrease in their natural beds. In order to restore Lanice conchilega reefs and maintain their ecological roles in coastal environments, restoration and habitat improvement initiatives are crucial. The introduction of substrates at appropriate sites to act as a base for larval settlement is a crucial step in the restoration of benthic organisms (such as oysters and mussels). The present approach to identifying suitable substrates for Lanice conchilega restoration is based on the flow reduction patterns observed within natural Lanice patches, which are known to stabilise sediment and create raised mounds. It is hypothesised that similar flow manipulation processes can be achieved using artificial substrates to promote larval settlement. Thus, identifying substrates with optimal properties, such as those that reduce flow, stabilise sediment, and provide sheltered microhabitats, is critical for facilitating Lanice conchilega larval settlement and growth. To address this, we evaluated 31 substrates with varying material, texture, thickness, and structural complexity to determine their impact on hydrodynamics and settlement potential.

Experimental trials were carried out in a small Kriesel tank to determine how these substrates alter water flow and affect larval settlement. A sand-filled gutter was placed in the tank's centre to serve as a test platform for individual substrates in controlled hydrodynamic conditions. Substrate performance was evaluated using two complementary parameters: (1) hydrodynamic behaviour, which describes how surface structures influence near-bed velocity and turbulence, and (2) capture efficiency, which is measured by the settlement/capture of both living larvae and pellet mimics. Flow velocity was measured using a Vectrino acoustic Doppler system for fine-scale near-bed data and Particle Tracking Velocimetry (PTV) for three-dimensional flow visualization. Capture of mimic particles was recorded at two-minute intervals over a 12-minute period, and a linear model (Capture rate ~ Time) was applied to derive the catchability slope as an indicator of substrate efficiency. Based on hydrodynamic data and mimic capture results, some of the best-performing substrates were selected for further testing with living larvae in 10-day trials to assess and verify the settlement success of Lanice conchilega larvae on different substrates.

Results indicate that substrate structure directly governs local hydrodynamics. While thick substrates with additional tube-like 3D structure (such as BESE and rope/tube materials) decreased flow and produced calmer microenvironments, thin or two-dimensional substrates (such as control sand and Basaltex) showed higher near-bed velocities and turbulence kinetic energy (TKE). Catchability analysis revealed that intermediate mesh size and thickness (e.g., BAS grid, BAS rough, geotextile, and shell-mixed types) provided the highest settlement rates, suggesting that optimal settlement occurs under moderate hydrodynamic conditions in the kreisel tank set-up. High-velocity regimes probably enhanced larval encounters but hindered attachment, while very low velocities limited delivery despite offering stable conditions once settled. Further experiments with living larvae were tested on two substrates (BESE and Basaltex grid) against control sand showed low survival rates, preventing definitive conclusions about substrate preference; however, the experimental setup proved effective for future testing.
Overall, these results show that the success of larval capture and retention is determined by hydrodynamic modification caused by substrate morphology. Balancing flow energy and surface complexity provides valuable insights for habitat engineering and restoration of Lanice conchilega populations.