Sloping Shores: Unveiling the Impact of Rocky Coastline Topography on Mussels and Sea Stars

Abstract

Rocky coastlines, with their jagged crevices and myriad boulders of all shapes and sizes, compose a rugged landscape that provides living space for an incredibly diverse array of organisms. Coastal habitats also provide a wealth of important ecosystem services, such as sediment exchange (Dethier et al., 2016), nutrient cycling, and water filtration (Jaramillo et al., 2021). Furthermore, they serve as natural barriers against storms, provide space for recreational activities, attract eco-tourism, and provide habitat for commercially important species (Arkema et al., 2015). However, the stability and functionality of these ecosystems are increasingly threatened by anthropogenic climate change, overfishing, pollution, recreation pressures, and development (Hawkins et al., 2007; Kunze et al., 2021). Given the ongoing repercussions of climate change, it is imperative to comprehend the influence these stressors will have on the species that have outstanding impacts on the rest of their communities. This study focuses on mussels (Mytilus californianus & Mytilus trossulus) and sea stars (Pisaster ochraceus), both of which play critical roles in shaping the structure and diversity of intertidal ecosystems (Broitman et al., 2009; Paine 1966; Pain & Trimble 2004). As suspension feeders Mytilus spp. prey upon phytoplankton and are in turn preyed upon by P. ochraceus (Paine 1966). Thus, they play a crucial role in intertidal food webs by concentrating nutrients from the water column and making them available to predators occupying higher trophic levels (Blanchette et al., 2007; Pain 1966). Mytilus spp. can also exert negative influences on their surroundings because their dominance as space competitors can reduce biodiversity in their surroundings when they crowd out other species (Blanchette et al., 2007; Broitman et al., 2009; Paine 1966, Paine 2004). On the other hand, P. ochraceus is a keystone predator that performs a vital regulatory role in maintaining biodiversity by preventing the domination of Mytilus spp. (Paine 1966). By examining the influence of current environmental and geomorphic factors on these species, we can predict how they will respond to changing conditions and assess the potential cascading effects these responses will have throughout the intertidal. While the role of many factors, such as temperature (Blanchette et al., 2007; Broitman 2009; Kunze et al., 2021; Raymond et al., 2022), UV radiation (Burnaford & Vasquez 2008), and disease (Miner et al., 2018) in defining the ranges of these species have been extensively studied, the influence of local geomorphology remains less explored. Therefore, this report presents the findings of a survey that investigated the influence of topography on the distribution and abundance of Mytilus spp. and P. ochraceus. We predicted that Mytilus spp. would be more common on steeper slopes because studies have shown that bivalves are susceptible to desiccation and heat stress (Broitman et al., 2009; Raymond et al., 2022), and steeper slopes may provide more refuge from the sun than a flat slope. Additionally, waves breaking on a vertical rock face may produce more spray that can help cool Mytilus spp. during low tide, while simultaneously providing more opportunities for filter feeding (Blanchette et al., 2007) Finally, we hypothesized that P. ochraceus would be drawn to these steeper areas because of their predilection for feeding upon Mytilus spp. (Paine 1966; Paine & Trimble 2004), and because previous studies have demonstrated that P. ochraceus avoids photosynthetically active solar radiation by sheltering in shaded areas (Burnaford & Vasquez 2008). Overall, the results of our study did not suggest that slope was correlated to Mytilus spp. distribution patterns, but higher abundances were found on steeper slopes.