Grazing Impact Of, and Indirect Interactions Between Mesograzers Associated With Kelp (Laminaria Digitata)1

Abstract

Little is known about the indirect effects of nonlethal grazing impacts in mesograzer-seaweed interactions. Using laboratory experiments, the effect of grazing by the seasonally abundant kelp-associated gastropod Lacuna vincta on subsequent kelp consumption by one kelp-associated (Idotea granulosa) and one nonassociated species of isopod (I. emarginata) was determined. Measurements of the toughness and elemental composition of different parts of the sporophyte of Laminaria digitata (Huds.) J. V. Lamour., as well as grazer-induced changes in the palatability of the blade, were conducted to explore possible mechanisms of indirect effects. In situ grazing pressure was the highest between July and September, with the blade being the preferred part of the kelp sporophyte, despite missing differences in the elemental composition among kelp parts. The laboratory experiments supported our hypotheses in that kelp consumption by both species of isopods was lower on intact than on L. vinctadamaged areas of the blade. This pattern was not caused by grazing-induced changes in blade palatability. Instead, the observed increase in isopod consumption following grazing by L. vincta resulted more likely from the combined effects of a reduction in the toughness of L. vincta-damaged kelp blades and some unknown gastropod cue(s). These results suggest that kelp-associated and nonassociated mesograzers may benefit from the nonlethal grazing impact of L. vincta due to changes in physical traits of the seaweed. Thus, the nonlethal grazing impact by one species of mesograzer can positively modify the trophic interactions between kelp and other potential competitors, suggesting that the interactions among mesograzers might be more complex than previously assumed.

Spatial and temporal variation in grazing damage by the gastropod Lacuna vincta in Nova Scotian kelp beds

“…As we predicted, grazing damage was generally lowest in the basal section of blades, especially during the seasonal peak in kelp growth between February and May (Mann 1972, Chapman & Craigie 1977 13: 163-173, 2011 associated with the degradation of aged tissues (Norderhaug et al 2003, Molis et al 2010. We found no evidence to support our prediction that encrustation by the invasive bryozoan Membranipora membranacea limits grazing damage by Lacuna vincta on kelp blades.…”

“…Structural properties and nutritional quality of blades can also vary seasonally with changes in water temperature (Sjotun et al 1996, Abdullah & Fredriksen 2004. For example, warming temperatures can increase grazing damage by accelerating degradation of older tissues at the distal ends of blades (Rothausier et al 2009), which reduces toughness and increases palatability (Molis et al 2010).…”

“…Structural properties and nutritional quality of blades can also vary seasonally with changes in water temperature (Sjotun et al 1996, Abdullah & Fredriksen 2004. For example, warming temperatures can increase grazing damage by accelerating degradation of older tissues at the distal ends of blades (Rothausier et al 2009), which reduces toughness and increases palatability (Molis et al 2010).The process of blade elongation, and variation in chemical or structural characteristics of tissues along kelp blades may influence the distribution of grazing damage on individual thalli. As new blade tissue is produced at the basal meristem, existing tissue continually progresses towards the distal end of the blade, where it fragments or gradually erodes.…”

The responses of brown macroalgae to environmental change from local to global scales: direct versus ecologically mediated effects

“…2; see B5). Moreover, feeding scars in the thallus surface of tough seaweeds may facilitate consumption by grazing species that have difficulties in penetrating undamaged thallus parts (Molis et al 2010). Epibionts may enhance or reduce the consumption of the host alga depending on the identity of the grazer (Karez et al 2000) and the epibionts ("n" in Fig.…”

Grazer Functional Roles, Induced Defenses, and Indirect Interactions: Implications for Eelgrass Restoration in San Francisco Bay

“…Such a trait-mediated indirect interaction between grazers through grazer-induced changes in plant traits has not previously been observed in a seagrass system; however, this phenomenon may be common in competitive interactions between mandibulate insect species on land plants [3,68]. In addition, there are a growing number of examples of plant trait-mediated indirect competitive interactions among grazers in marine systems, including on cordgrass in salt marshes (congeneric planthoppers [69] or gastropods and planthoppers [70]), and on several species of brown algae (gastropods and isopods [53,71], isopods and amphipods [72], and congeneric gastropods [22]). …”