Potential effects of kelp harvesting on kelp-associated macroinvertebrates - changes in the level of phlorotannins may affect grazers' distribution

Kelps are foundation species present on shallow rocky costs around the world. They form a dominant habitat, the kelp forest, which supports a great variety of invertebrate and fish species, due to their high productivity and capacity to function as a spawning and nursery grounds (Steneck et al., 2002). However, intense grazing can severely compromise kelp abundance and function (Scheibling et al., 1999). The gastropods are benthic marine herbivores, whose grazing activity is capable of affecting the growth and reproductive abilities of individual macroalgal species and even the composition and density of macroalgal communities (Carney et al. 2005). The marine snail Tegula tridentata is a common organism inhabiting the kelp forest of Lessonia trabeculata, reaching densities that may range from 4.2 ind.10 m-2 (Villegas et al. 2008) to > 280 ind.m-2 (Gaymer in Perreault et al., 2014). Kelps contain phlorotannins - polyphenolic compounds located in intracellular vesicles called physodes, which function as herbivore deterrents, digestive inhibitors, antibacterial agents, protectors against UV radiation and have also an active role in cell-wall formation. This chemical compound can be released into the surrounding water either directly via exudation or indirectly via erosion or shedding of macroscopic parts (Jennings and Steinberg, 1994). Several studies have measured the exudation rates of algae in the laboratory or in the field. Although the studies focusing on algae exudation where mainly done with intertidal species (which are subject to immersion/emersion cycles that increase the release of pholorotannins), there are studies focusing on subtidal kelp species like the Ecklonia radiata where the average release rate was 5.5 µg g (dry wt)-1 h-1 (Jennings and Steinberg, 1994). Chile is the most important producer of kelp from natural populations in the world. The increasing market demand on this resource is prompting a clear trend of harvesting increase, particularly in the central coast of the country. The study conducted by Vasquez and Santelices (1990) showed that reductions in kelp density from 3 to 0.5 m-2 (by removal of the entire kelp plants) resulted in significant increases in the density of the herbivore sea urchin Tetrapygus niger and the herbivore snail Tegula tridentata. This result was attributed to the change in the plant-spacing pattern that promoted the exposure of the plants and substratum to increased grazing effects, creating barren grounds that persisted for several years, unless the grazers were removed. A recent study by Subida et al (in preparation) held in central Chile, showed a four-time increase in the numbers of T. tridentata after commercial harvesting of Lessonia trabeculata. In the present study I hypothesize that the exudation of phlorotannins by L. trabeculata can significantly increase the concentration of these compounds in the surrounding seawater and that this concentration will be higher in kelp beds with higher kelp densities. I further hypothesize that Tegula tridentata responds to changes in the concentration of phlorotannins in the seawater, by moving towards harvested kelp beds, which, due to a reduction in the density of adult kelp sporophytes exuding metabolites, show lower concentration of these compounds in the surrounding water. The objective of the study is twofold: to determine i) if the phlorotannins exuded by L. trabeculata can significantly alter the chemical composition of the surrounding seawater, and ii) if the herbivore gastropod Tegula tridentata responds to this alteration with changes in its mobility behavior.

ISCED Categories

The highlighted icons, represent the fields of education (in compliance with ISCED Classification) engaged during this course/programme.

0511 - Biology", "0521 - Ecology