Hydrodynamic mechanisms of larval transport and settlement in oscillatory boundary layer flows

Larval settlement in flow at the benthic surface is a critical part of life for a vast array of marine invertebrates. Understanding the mechanisms behind this settlement is important for many applications, including the crucial process of coral reef restoration, yet it is poorly understood and difficult to observe in nature. There have been studies investigating biological, chemical and physical cues, which help to identify what attracts larvae to settlement locations or makes them want to settle. However, there is still a limited understanding of the physical mechanisms behind larval settlement, especially in realistic reef flow conditions. Here, we use experiments and simulations to advance our understanding of the physical mechanisms underlying larval settlement in oscillatory boundary layer flows. Using a custom oscillatory flume, we find that substrates with millimeter-scale ridges increase coral larval settlement by over 2$\times$ compared to flat substrates in reef flow conditions. Ridged substrates can generate vortices and create low flow speed regions that affect the transport and settlement of coral larvae. Agent-based larval modeling confirms that generated vortices transport larvae closer to the surface and trap them in the gaps until lower flow speeds in the gaps allow the larvae to swim to the surface and settle. Ridged substrates with smaller scale ridges close to the larval body size do not create the same large vortices, and therefore have lower settlement similar to a flat surface in reef flow. Using the larval model to investigate larger parameter spaces than would be practical with experiments alone, we examined the effects of changing topography and larval characteristics on larval transport and settlement. Over a range of rectangular ridged topographies at the millimeter length scale, optimal high settlement topographies produce a high averaged vertical velocity variance in the bulk flow, indicating that vertical larval movement to benthic surfaces is dominated by passive transport driven by recirculatory flow structures. We found larval settlement to be positively correlated with mean vertical velocity variance, and settlement on complex multiscale roughness substrates was quantitatively similar to the simple rectangular models with similar mean vertical velocity variance. Exploring a range of larval characteristics showed that larval settlement was sensitive to swimming speed changes in some flow regimes and larval shape in others. The presence of high flow speed transport barriers in some regimes, which impede larval transport to the surface and settlement, selected for faster swimming larvae that could better cross them. Larval aspect ratio affected the alignment of larvae with the local flow direction, affecting transport through flow structures. Together, these findings reveal how substrates can be designed with surface features to promote larval settlement and be tailored to select for specific species in natural reef flow conditions independent of biological or chemical cues. While hydrodynamic effects on larval settlement alone are important to understand, chemical settlement cues are used in many larval experiments and projects and their influence on flow-larval interactions must be identified. As a first step to investigating how chemical-mediated larval behavior changes settlement in boundary layer flow, we performed chemotaxis experiments with a known organic cue, crustose coralline algae (CCA), and a variety of inorganic cues to quantify their effects. In addition to overall attractive or repulsive responses to chemical cues, individual behavioral and morphological changes were examined. Larvae changed their swimming speed and turning frequency in magnesium, calcium, and CCA, as well as changing shape in calcium and CCA. Directions to expand on this work include adding newly quantified larval behaviors to the agent-based larval model and applications for anti-fouling and microplastics research.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Gysbers, Daniel

Contributors dc:contributor

• Juarez, Gabriel
• Noronha-Hostler, Jacquelyn
• Espinoza-Marzal, Rosa
• Gruebele, Martin

• Boundary Layer
• Transport
• Coral
• Topography
• Benthic
• Larval Settlement

Statement dc:rights

• Copyright 2024 Daniel Gysbers

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/127232

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F127232