Prominent grazing rates and feeding preferences of an abundant exotic benthic herbivore in the western Mediterranean Sea

Exotic herbivores can exert profound impacts on terrestrial communities,
but their ecological effects on marine habitats are not sufficiently
quantified. The exotic crab Percnon gibbesi, which is spreading throughout
the Mediterranean Sea, grazes almost exclusively on benthic macrophytes,
providing an opportunity to study the potential impacts of herbivores in
the marine realm. Here, we first quantified the abundance of P. gibbesi in
the Balearic Islands in 2023 (67 individuals 100 m-2), which was
approximately 33 times greater than that recorded previously. We then
performed a feeding preference experiment using common native and invasive
macroalgae from the Mediterranean Sea. The per capita grazing rate of P.
gibbesi (average 3.83 WW g crab -1 day-1) was higher than that recorded
for most native herbivorous species in the Mediterranean. The estimated
daily grazing rates for P. gibbesi average 23.98 kg WW macroalgae ha-1
d-1, value that corresponds to an average of 5.58% of the total macroalgae
production in this area. Our experiment revealed clear preferences of P.
gibbesi for three species of macroalgae, which were not explained by the
nutritional content. Overall, our results indicate that the ecological
impacts of this exotic herbivore on Mediterranean communities could be
substantial and, until now, unreported.