Multiple mutualists provide complementary benefits to their seaweed host

Although the importance of mutualisms in structuring communities is becoming increasingly appreciated, relatively little is known about the degree to which potential mutualists are interchangeable with respect to the services they provide their host. We examined the degree to which potential mutualists of a seaweed provide complementary vs. redundant benefits to their host to begin to assess the effects of mutualist diversity on host performance. The gastropods Anachis lafresnayi (Costoanachis lafresnayi) and Mitrella lunata (Astyris lunata) are common associates of the red alga Chondrus crispus in subtidal locations in southern New England. Field surveys showed that where these snails were rare, Chondrus was sparse and covered with a heavy growth of sessile invertebrates, but where snails were common, Chondrus was abundant and free from fouling overgrowth. In manipulative field experiments, plants with no snails had the most overgrowth (measured both as percent cover and total mass) and lost 25% of their mass in five weeks. Plants with either species of snail alone were also heavily fouled and lost mass. Those with only Mitrella were overgrown by solitary ascidians, whereas those with only Anachis were overgrown by bryozoans, suggesting that both snails remove epibionts, but they differ in the suite of epibionts they are capable of removing. As a result, only Chondrus with both species of snail present remained unfouled and gained mass over the course of the experiment. Field tethering experiments showed that both snail species can gain a refuge from crab predation by associating with Chondrus. Our results show that these two snail species are not interchangeable and they provide complementary benefits to their host because they differ in their prey preferences. If this type of differentiation in resource use often mediates coexistence among mutualists, then complementary effects of mutualists on their hosts may be common.