Hydrodynamic Interactions Between Macroalgae and Their Epibionts

Abstract

Macroalgae provide surfaces where other organisms live. Unlike organisms on rigid substrata, epibionts on host macroalgae sit on flexible surfaces that bend, stretch, and move in turbulent water currents and waves. We used blade-like red algae, Mazzaella splendens, and encrusting bryozoans, Membranipora membranacea, to investigate the biomechanical and hydrodynamic effects of encrusting epibionts on macroalgae, and of flexible hosts on epibiotic bryozoans. Passive flapping by algae in wave-driven ambient flow enhanced renewal of water near hosts and epibionts. Wave exposure and the presence of a surrounding canopy of flexible algae altered the locations along algal blades where bryozoans encountered the highest time-averaged boundary shear velocities. Hydrodynamic forces on flexible algae moving back-and-forth with the water were lower in waves than in unidirectional flow. Bryozoan epibionts increased hydrodynamic forces on host algae by affecting their reconfiguration in moving water. Encrusting bryozoans increased the flexural stiffness of algal blades, but the elastic modulus, extensibility, and strength of blade tissue was unaffected by bryozoan epibionts. Algal blades were more extensible and stronger than bryozoans, so bryozoans fractured or popped off stretched algae. Algae in rapid-flow habitats had few epibionts, and encrusted algae transplanted from a protected to a wave-exposed habitat lost their epibionts.

Ecological biomechanics of damage to macroalgae

“…Dislodgement by holdfast detachment often leads to mortality ( Koehl and Wainwright, 1977 ; Seymour et al, 1989 ). Stipe breakage removes photosynthetically-active blade tissue ( Figure 1D ; Santelices et al, 1980 ; Biedka et al, 1987 ; Carrington, 1990 ; Shaughnessy et al, 1996 ; Bell, 1999 ; Stewart, 2006b ), but does not necessarily cause mortality if drifting thalli survive or if new fronds grow from the holdfast ( Lubchenco, 1980 ; Stewart, 2006b ; Loffler et al, 2018 ; Burnett and Koehl, 2020 ; Koehl and Daniel, 2022 ). Biomass loss when blades are damaged is small compared to biomass loss when holdfasts or stipes are broken ( Figures 1D , E ; Johnson and Mann, 1986 ; Padilla, 1993 ; de Bettignies et al, 2013b ).…”

“…Epibionts are organisms living on surfaces of other organisms. Some epibionts on macroalgae are herbivores, but many others do not consume host tissue (see examples in Figures 2C,D ; e.g., algae, bryozoans, hydroids, tube worms, barnacles; Koehl and Daniel, 2022 ). Increased pH under attached epibionts damages host tissues ( Wahl, 1989 ; Harder, 2009 ), as do anchoring hooks of epibionts ( da Gama et al, 2014 ).…”

“…Increased pH under attached epibionts damages host tissues ( Wahl, 1989 ; Harder, 2009 ), as do anchoring hooks of epibionts ( da Gama et al, 2014 ). Epibionts also damage macroalgae by increasing hydrodynamic forces that their hosts bear by enlarging the structure exposed to ambient flow ( Anderson and Martone, 2014 ), or by stiffening the host, thereby interfering with its reconfiguration by moving water ( Koehl and Daniel, 2022 ).…”

“…ESF is the ratio of the season-dependent stress required to break a macroalga to the maximum flow-induced stress it experiences in its habitat during that season ( Johnson and Koehl, 1994 ). Many macroalgae have high ESF’s during the calm summer growth and reproduction season, but low ESF’s during winter, as they accumulate damage and experience storms ( Johnson and Koehl, 1994 ; Koehl and Daniel, 2022 ). Some species develop similar ESF’s in rapid-flow habitats as in calm sites by increasing tissue strength or cross-sectional area of support structures, and/or by growing into low-drag morphologies ( Johnson and Koehl, 1994 ; Sirison and Burnett, 2020 ; Koehl and Daniel, 2022 ).…”

“…10.3389/fpls.2022.981904 Frontiers in Plant Science 04 frontiersin.org structure exposed to ambient flow(Anderson and Martone, 2014), or by stiffening the host, thereby interfering with its reconfiguration by moving water(Koehl and Daniel, 2022).…”