Being Bullate in the Intertidal: A New Wrinkle on the Functional Morphology of Saccharina Sessilis

Saccharina sessilis, a dominant kelp of rocky intertidal shores of the northeastern Pacific, has a variable morphology with two common forms: bullate and strap-like. Bullate thalli have upright, rugose lamina that grow in a compact, cabbage-like habit. Strap-like thalli have smooth and long lamina that lie prostrate on the rocks when emersed at low tide. Previously, researchers have observed that morphological variation in kelps is associated with variation in water flow or wave energy, yielding characteristic 'wave-exposed' and 'wave-sheltered' forms. They have also argued that the bullate morphology is an adaptive response to growing in wave protected shores where boundary layer formation can limit nutrient and carbon dioxide supply for strap-like individuals, while bullae disrupt boundary layers. For Saccharina sessilis we observed the two morphologies varied systematically with tidal height on wave-exposed shorelines, with bullate thalli dominating high on the shore. Furthermore, the bullate thalli appeared to retain small pools of water in the bullae on their blades and in the rosette at the base of the thallus during low tide. In addition, we observed some thalli had an intermediate morphology with lamina of both types, suggesting the trait might be plastic and driven by changing environmental conditions; we called these thalli semi-bullate. We hypothesized that the bullate morphology in intertidal kelps functions to ameliorate emersion stresses during low tide, and allows Saccharina to persist in higher intertidal habitats than would be possible for strap-like thalli. We monitored the density of the three morphotypes across a tidal height gradient over time at a site in Northern California. In the high zone bullate Saccharina were the dominate morphotype in spring and summer when extreme low tides occur more frequently in the morning and sun exposure during emersion is greatest, while straplike morphotypes were rare or absent. In the low zone straplike thalli were present during all surveys. During the fall when day time emersion times are shorter and extreme low tides are more frequent in the afternoon and evening, the straplike morphotype dominated and no bullate or semibullate morphotypes were observed. We characterized the rugosity of the lamina and found that the more rugose lamina, characteristic of the bullate morphology, retained more water on the thallus surface when emersed. Furthermore, the more upright habit of the bullate thalli resulted in reduced light interception, conferring a potential physiological benefit during sunny low tides. Using a pulse-amplitude-modulation (PAM) fluorometer to measure two metrics of photosynthetic performance: maximum quantum yield (MQY or F IF m) and electron transport rate (ETR) to make in situ field measurements we found that high zone, bullate thalli also had greater MQY and greater ETR then the low zone, strap-like thalli during a sunny morning low tide, despite longer emersion times. We also observed that the bullate morphotype desiccated at a slower rate than the straplike morphotype. These differences were less pronounced on days that were not sunny and dry during the morning low tide. These results challenge a prior paradigm about the function of variable morphology in intertidal kelps, and contribute to our understanding of the physiological ecology Saccharina sessilis, and its scope for responding to climate change.