BULLETIN OF MARINE SCIENCE. 89(1):123–134. 2013 htp://dx.doi.org/10.5343/bms.2011.1124 123 Bulletn of Marine Science © 2013 Rosenstel School of Marine & Atmospheric Science of the University of Miami PARASITES IN MARINE FOOD WEBS Kevin D Lafferty ABSTRACT Most species interactions probably involve parasites. Tis review considers the extent to which marine ecologists should consider parasites to fully understand marine communities. Parasites are infuential parts of food webs in estuaries, temperate reefs, and coral reefs, but their ecological importance is seldom recognized. Tough difcult to observe, parasites can have substantial biomass, and they can be just as common as free-living consumers after controlling for body mass and trophic level. Parasites have direct impacts on the energetics of their hosts and some afect host behaviors, with ecosystem-level consequences. Although they cause disease, parasites are sensitive components of ecosystems. In particular, they sufer secondary extinctions due to biodiversity loss. Some parasites can also return to a system after habitat restoration. For these reasons, parasites can make good indicators of ecosystem integrity. Fishing can indirectly increase or decrease parasite populations and the efects of climate change on parasites are likely to be equally as complex. Food webs are ecological maps of species interactions. For instance, a recent in- vestigation found that in an estuarine food web there were 505 feeding interactions among 87 free-living species (Laferty et al. 2006). Te addition of 47 parasites to this food web revealed a further 615 host-parasite or parasite-parasite interactions. Furthermore, there were 1093 interactions where parasites were eaten by free-living species. Tis accounting indicates that parasites are frequent, if not dominant as- pects of species interactions in marine communities. In addition to acting as con- sumers or prey, parasites can alter predator-prey interactions, sometimes strongly, and with considerable consequences. Here, I review the literature to consider six questions about the importance of parasites in species interactions, using examples from marine systems. Te phrase “invisible fabric of nature” describes parasites well. Tey are small and often hidden within their hosts. As a result, discussion of infectious disease agents (typical parasites, pathogens, parasitic castrators, parasitoids, etc.) has traditionally been excluded from marine ecology courses. Nevertheless, close inspection of any marine species will typically reveal parasites. Tis raises questions about the de- gree to which ecologists should consider parasites. For instance, how many parasite species are there? What do they do to their hosts? How might parasites afect host populations? Are they abundant enough to afect energy fows through systems? Are parasites important parts of food webs? How do human impacts like fshing, climate change, species invasions, and habitat degradation afect the role of parasites in ma- rine systems? In this brief review, I discuss these questions, drawing heavily on work with several colleagues in a variety of marine systems. MOTE SYMPOSIUM INVITED PAPER