Small burrowing amphipods cause major damage in a large kelp

Abstract

Large herbivores such as sea urchins and fish consume a high proportion of benthic primary production and frequently control the biomass of marine macrophytes. By contrast, small mesograzers, including gastropods and peracarid crustaceans, are abundant on seaweeds but have low per capita feeding rates and their impacts on marine macrophytes are difficult to predict. To quantify how mesograzers can affect macrophytes, we examined feeding damage by the herbivorous amphipods Sunamphitoe lessoniophila and Bircenna sp., which construct burrows in the stipes of subtidal individuals of the kelp Lessonia berteroana in northern-central Chile, southeast Pacific. Infested stipes showed a characteristic sequence of progressive tissue degeneration. The composition of the amphipod assemblages inside the burrows varied between the different stages of infestation of the burrows. Aggregations of grazers within burrows and microhabitat preference of the amphipods result in localized feeding, leading to stipe breakage and loss of substantial algal biomass. The estimated loss of biomass of single stipes varied between 1 and 77%. For the local kelp population, the amphipods caused an estimated loss of biomass of 24-44%. Consequently, small herbivores can cause considerable damage to large kelp species if their feeding activity is concentrated on structurally valuable algal tissue.

Keywords: Ampithoidae; Phaeophyceae; algal detritus; kelp; mesograzers; plant–herbivore interaction.

Figure 1.

(a) Number of burrows within amphipod domiciles of different infestation stages (see drawings on top of the figure and electronic supplementary material, figure S1) in stipes of L. berteroana from Playa Blanca collected in 2011 and 2014. (b) Number of individuals of S. lessoniophila and Bircenna sp. in domiciles (including single burrows and conglomerates) of different infestation stages in stipes of L. berteroana. Data from the sampling years 2011 and 2014 were combined because the pattern was similar in both years. The dot with a number above it represents an outlier at 117 individuals per domicile, which lies outside the scale of the ordinate.

Figure 2.

Composition of assemblages of S. lessoniophila inside individual burrows in stipes of subtidal individuals of L. berteroana (n = 20). Assemblages from conglomerates were not analysed. Numbering of the burrows is done posterior to sample analysis and only for illustrative purposes.

Figure 3.

Vertical distribution of amphipod domiciles (n = 312; columns—left ordinate) on the internode levels along stipes (n = 98) of subtidal individuals of L. berteroana from Playa Blanca and vertical percentage distribution of L. berteroana biomass (circles—right ordinate) within the stipes.

Figure 4.

(a) Estimated maximal potential loss of biomass (%) in stipes of L. berteroana due to stipe breakages clearly induced by amphipod domiciles and due to all breakages. (b) Observed (triangles) biomass of stipes of L. berteroana from Playa Blanca (including undamaged and damaged stipes) and expected (circles) biomass of undamaged stipes (including undamaged stipes and damaged stipes with the lost biomass being reconstructed). The average (± s.e.m.) biomass was reconstructed for stipes of different size (no. of internode levels) with damage induced by amphipod domiciles (filled circles) and with damage due to all breakages (including breakages due to amphipod domiciles and breakages due to unknown reasons—open circles). n = 40.