Mixed species diets enhance the growth of two rocky intertidal herbivores

Seaweeds provide food and shelter for countless species of invertebrates. Many studies show how particular species of seaweed facilitate particular invertebrates, but few examine the simultaneous effects of multiple seaweeds, despite the fact that algal composition and diversity are known to vary considerably. We conducted laboratory experiments to determine how algal species richness affected the consumption and growth of 2 common rocky shore herbivores: the turban snail Chlorostoma funebralis and the lined shore crab Pachygrapsus crassipes. For both herbivores, highest growth was achieved on a diet of mixed algal species, though this was greater than growth on the best single species only for C. funebralis. However, the herbivores differed in their growth on particular single species diets and consumed prey species at different rates. This suggests that, despite the similar boost in growth achieved by a diverse diet, the exact mechanism was not the same for the 2 species. The benefits were not due simply to inclusion of the best single food species in the mixed diet because consumption of these highest quality foods was lower in mixed than in single species treatments and contributed only 50% of dietary intake in mixed diet treatments. It seems likely that complementary nutritional quality, chemistry, or morphology among prey species contributed to the positive effects of diet species richness on herbivore per formance. Similar findings for taxonomically and ecologically distinct herbivores suggest that such effects of diet species richness on consumer performance may be widespread among marine generalist herbivores.