Food webs and fishing affect parasitism of the sea urchinEucidaris galapagensisin the Galápagos

Abstract

In the Galápagos Islands, two eulimid snails parasitize the common pencil sea urchin, Eucidaris galapagensis. Past work in the Galápagos suggests that fishing reduces lobster and fish densities and, due to this relaxation of predation pressure, indirectly increases urchin densities, creating the potential for complex indirect interactions between fishing and parasitic snails. To measure indirect effects of fishing on these parasitic snails, we investigated the spatial relationships among urchins, parasitic snails, commensal crabs, and large urchin predators (hogfish and lobsters). Parasitic snails had higher densities at sites where urchins were abundant, probably due to increased resource availability. Commensal crabs that shelter under urchin spines, particularly the endemic Mithrax nodosus, preyed on the parasitic snails in aquaria, and snails were less abundant at field sites where these crabs were common. In aquaria, hogfish and lobsters readily ate crabs, but crabs were protected from predation under urchin spines, leading to a facultative mutualism between commensal crabs and urchins. In the field, fishing appeared to indirectly increase the abundance of urchins and their commensal crabs by reducing predation pressure from fish and lobsters. Fished sites had fewer snails per urchin, probably due to increased predation from commensal crabs. However, because fished sites also tended to have more urchins, there was no significant net effect of fishing on the number of snails per square meter. These results suggest that fishing can have complex indirect effects on parasites by altering food webs.

Fishing drives declines in fish parasite diversity and has variable effects on parasite abundance

“…In the Atlantic lowlands of Costa Rica, McKenzie (2007) found that, among three species of amphibian hosts collected from clear-cut cattle pasture and forest habitats, six metazoan parasite species were more abundant in pasture than forest, two species were more abundant in forest than pasture, and 13 species did 3 Present address: Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Colorado 80309 USA. Some have found higher parasite abundance and diversity in reserves than in open-access areas (Sasal et al 1996, Bartoli et al 2005, Loot et al 2005, Wood et al 2013, while others have found higher parasite abundance in open-access areas (Sonnenholzner et al 2011), or have failed to find significant differences in abundance and diversity between reserve and openaccess areas (Ternengo et al 2009). Aeby et al (2011) found that, while prevalence of growth anomalies (a tumor-like disease of unknown etiology) of Porites spp.…”

Marine protected areas facilitate parasite populations among four fished host species of central Chile

“…In the Galapagos, removal of large fish predators reduces predation pressure on crabs that live commensally with sea urchins. Because those crabs eat the eulimid snail parasites of their urchin hosts, the resulting increase in commensal crabs reduces the epidemiological abundance of eulimid snails parasitic on sea urchins in open‐access relative to protected areas (Sonnenholzner, Lafferty & Ladah ). This variety of results – in which fishing can increase, decrease or not affect parasite abundance – highlights the potential complexity of effects of fishing on parasites.…”

How have fisheries affected parasite communities?

“…;Lafferty et al 2008b;Freeman and MacDiarmid, 2009;Ternengo et al 2009;Marzoug et al 2012;Sonnenholzner et al 2011;Sala-Bozano et al 2012;Wootton et al 2012;Wood et al 2013).…”