Abstract
The fish Sarpa salpa (L.) is one of the main macroherbivores in the western Mediterranean. Through direct and indirect mechanisms, this herbivore can exert significant control on the structure and functional dynamics of seagrass beds and macroalgae. Past research has suggested nutritional quality of their diet influences S. salpa herbivory, with the fish feeding more intensively and exerting greater top down control on macrophytes with higher internal nutrient contents. However recent findings have questioned this notion and shown that herbivores do not preferentially feed on macrophytes with higher nutrient contents, but rather feed on a wide variety of them with no apparent selectivity. To contribute to this debate, we conducted a field fertilization experiment where we enriched leaves of the seagrass Posidonia oceanica, a staple diet for S. salpa, and examined the response by the herbivore. These responses included quantification of leaf consumption in fertilized and non-fertilized/control plots within the bed, and food choice assays where fertilized and non-fertilized/control leaves were simultaneously offered to the herbivore. Despite the duration of leaf exposure to herbivores (30 days) and abundant schools of S. salpa observed around the plots, leaf consumption was generally low in the plots examined. Consumption was not higher on fertilized than on non-fertilized leaves. Food choice experiments did not show strong evidence for selectivity of enriched leaves. These results add to a recent body of work reporting a broad generalist feeding behavior by S. salpa with no clear selectivity for seagrass with higher nutrient content. In concert, this and other studies suggest S. salpa is often generalist consumers not only dictated by diet nutrient content but by complex interactions between other traits of nutritional quality, habitat heterogeneity within their ample foraging area, and responses to predation risk.
Highlights
In the Western Mediterranean, seagrass meadows are dominated by Posidonia oceanica (L.) Delile [1,2] where the fish Sarpa salpa (L.) and the sea urchin Paracentrotus lividus (Lam.) are the two main macroherbivores [3]
Some of the factors involved in S. salpa herbivory include; macrophyte availability and accessibility, habitat heterogeneity, nutritional quality, human pressure on herbivore populations, herbivore recruitment, predation and patterns of movement [11,12,13]
We hypothesized that fertilization would increase leaf growth rates and decrease C:N ratios, and that S. salpa will respond by selecting fertilized shoots over controls; displaying higher pressure over these nutritional enriched shoots and recording higher consumption rates in the marine protected areas (MPA) due to fishing protection
Summary
In the Western Mediterranean, seagrass meadows are dominated by Posidonia oceanica (L.) Delile [1,2] where the fish Sarpa salpa (L.) and the sea urchin Paracentrotus lividus (Lam.) are the two main macroherbivores [3] Herbivory rates on this seagrass species are extremely variable, ranging from 2–57% of P. oceanica leaf productivity, according to the available literature [4,5]. Since high nutrient levels and grazing pressure can depress leaf production, it is important to expand upon the knowledge of what role nutrients serve in driving herbivore feeding preferences This would help us to better understand the variability of herbivory and how it may affect the distribution of seagrass species. We hypothesized that fertilization would increase leaf growth rates and decrease C:N ratios, and that S. salpa will respond by selecting fertilized shoots over controls; displaying higher pressure over these nutritional enriched shoots and recording higher consumption rates in the MPA due to fishing protection
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