By bending their body to reduce drag forces, Garden Eels, fish that feed while being anchored to the bottom, effectively feed on drifting zooplankton even when currents are strong. AbstractA major challenge faced by sessile animals that feed in the flow is to maintain effective feeding postures while enduring hydrodynamic forces.Garden eels exhibit an exceptional lifestyle: feeding on drifting zooplankton while being "anchored" in a burrow they dig in the sand. Using underwater observations, sampling and 3-D video recording, we measured the feeding rates and characterized feeding postures of garden eels under a wide range of current speeds. We show that the eels behaviorally resolve the tradeoff between adverse biomechanical forces and beneficial fluxes of food by modulating their body postures according to current speeds. In doing so, the eels substantially reduce drag forces when currents are strong, yet keep their head well above bottom in order to effectively feed under conditions of high prey fluxes. Those abilities allowed garden eels to become one of the rare oceanic fishes that live in sandy, predation-rich habitats and feed on zooplankton while being attached to the bottom.

Turbulent boundary layers, created by flws interacting with the rough topography of coral reefs, are important to many aspects of reef function and health (Davis et al., 2021). For example, turbulent boundary layers are important to coral health by supporting nutrient uptake (Lowe & Falter, 2015). For an unstratified, turbulent flow over a rough bottom, the boundary layer takes the form of the law-of-the-wall (Pope, 2012):where u(z) is the horizontal velocity at height z above the bed, u * is the friction velocity, z 0 is the roughness height, d is the displacement height, and we adopt the von Kármán constant κ = 0.41 following Nepf (2011). The friction velocity can also be calculated directly from the Reynolds stresses at the bed Abstract Coral reefs are hydrodynamically rough, creating turbulent boundary layers that transport and mix various scalars that impact reef processes and also can be used to monitor reef health. Often reef boundary layer characteristics derived from a single instrument are assumed to accurately represent the study site. This approach relies on two assumptions: first, that the boundary layer is relatively homogeneous across the area of interest, and second, that two instruments displaced in space or with different spatiotemporal resolution would produce similar results when sampling the same flow. We deployed four velocimeters over a 15 × 20 m reef at 10 m depth in the Chagos Archipelago. The site had a 1 m tidal range, and waves were primarily locally generated wind waves with H rms < 0.5 m. Depth-averaged currents were typically 0.2 m/s. Friction velocities derived directly from Reynolds stress measurements by fitting the law of the wall show agreement between instruments (pairwise coefficients of determination R 2 ranged from 0.53 to 0.86). Thus, the boundary layer appears to be spatially homogeneous, at least at the scale of our array, and it appears that in the present case friction velocities from one instrument are indeed generally representative of the site. We calculate drag coefficients using curve-fitting and Structure-from-Motion photogrammetry, and while we find general agreement between estimates one instrument in particular produces drag coefficients an order of magnitude larger in comparison. Hence, some variability between instruments was observed, notably when high-resolution instruments measured localized flow features.

Coral reef sessile organisms inhabiting cryptic spaces and cavities of the reef matrix perform vital and varied functional roles but are often understudied in comparison to those on exposed surfaces. Here, we assess the composition of cryptobenthic taxa from three remote tropical reef sites (Central Indian Ocean) alongside a suite of in situ environmental parameters to determine if, or how, significant patterns of diversity are shaped by local abiotic factors. To achieve this, we carried out a point-count analysis of autonomous reef monitoring structure (ARMS) plate images and employed in situ instrumentation to recover long-term (12 months) profiles of flow velocity, wave heights, temperature, dissolved oxygen, and salinity, and short-term (3 weeks) profiles of light and pH. We recovered distinct environmental profiles between sampling sites and observed that ocean-facing reefs experienced frequent but short-lived cooling internal wave events and that these were key in shaping in situ temperature variability. By comparing temperature and wave height profiles recovered using in situ loggers with ex situ models, we discovered that global satellite products either failed to recover site-specific profiles or both over- and underestimated actual in situ conditions. We found that site choice and recruitment plate face (top or bottom) significantly impacted the percentage cover of bryozoans, gastropods, soft and calcified tube worms, as well as crustose coralline algae (CCA) and fleshy red, brown, and green encrusting macroalgae on ARMS. We observed significant correlations between the abundance of bryozoans, CCA, and colonial tunicates with lower mean temperature and higher mean dissolved oxygen profiles observed across sites. Red and brown encrusting macroalgae abundance correlated significantly with medium-to-high flow velocities and wave height profiles, as well as higher pH and dissolved oxygen. This study provides the first insight into cryptobenthic communities in the Chagos Archipelago marine-protected area and adds to our limited understanding of tropical reef sessile communities and their associations with environmental parameters in this region. With climate change accelerating the decline of reef ecosystems, integrating analyses of cryptobenthic organisms and in situ physicochemical factors are needed to understand how reef communities, if any, may withstand the impacts of climate change.