SELECTIVE HERBIVORY BY AMPHIPODS INHABITING THE BROWN ALGA ZONARIA-ANGUSTATA

The feeding preferences of small, invertebrate herbivores inhabiting the brown alga Zonaria angustata were examined. The 2 most common herbivores, the amphipods Tethygeneia sp. and Hyale rubra, consumed Z angustata, strongly preferring the meristematic region at the apex of each branch. Amphipod preference for new growth and the extent of damage to plants in the field suggest that they may cause considerable damage to Z. angustata. In contrast to the common preconceptions about the diets of such herbivores, amphipods did not consistently prefer to consume the most abundant epiphytes of Z. angustata ahead of the host macroalga itself. Z. angustata was not a homogeneous food source for amphipods. Amphipod feeding was negatively correlated with the density of phlorotannin-containing physodes in, but not with toughness of, the tissues at the apex of each branch. This correlation indicated that the pattern of damage to Z. angustata was most likely a result of within-plant variation in the amount of secondary chemicals. This variation occurred over only a few millimetres but influenced amphipod feeding due to their ability to select between algal tissues at a very small scale.