Amphipods on seaweeds: partners or pests?

Area of Science:

• Marine Biology
• Ecology
• Invertebrate Zoology

Background:

• Herbivorous marine amphipods are recognized for grazing on algae, potentially benefiting host macrophytes by limiting epiphytic overgrowth.
• In North Carolina, USA, amphipods constitute 97% of macroscopic animals on Sargassum filipendula, with populations peaking from late winter to early summer.

Purpose of the Study:

• To investigate the species-specific impacts of common phytal amphipods on the growth of Sargassum seaweed and its associated epiphytes.
• To determine if amphipod grazing on epiphytes translates to improved host seaweed fitness.

Main Methods:

• Outdoor tank experiments were conducted to assess the effects of individual amphipod species on Sargassum and its epiphytes.
• Laboratory feeding trials examined the dietary preferences of key amphipod species when offered both Sargassum and common epiphytes.

Main Results:

• Three amphipod species (Ampithoe marcuzii, Caprella penantis, Jassa falcata) significantly reduced epiphyte growth on Sargassum.
• Ampithoe marcuzii consumed Sargassum, reducing its mass by 11% in one experiment, while preferring host algae over epiphytes in lab trials.
• Caprella penantis and Jassa falcata reduced epiphytes without negatively impacting Sargassum growth.

Conclusions:

• The trophic diversity of seaweed-associated amphipods means their impact on host fitness varies significantly by species.
• Ampithoe marcuzii can be detrimental to Sargassum by directly consuming it, whereas Caprella penantis and Jassa falcata appear beneficial by controlling epiphytes.