Although the taxonomy and distribution of echinoderms in the Galápagos
Islands are well known, their biological interactions with parasitism are not.
Echinoderms are intensively parasitized by eulimid gastropods (Pearse and
Cameron 1991), but there has been little research about the eastern Pacific eulimid
species and their ecology (Skoglung 2004). Sonnenholzner and Molina (2005)
provided information on the sea urchin–eulimids host–parasite relationship along
the southern and western regions of the Galápagos Islands. A total of 1,855
specimens of E. galapagensis were examined. Of them, 925 individuals were
mainly affected by Sabinella shaskyi. The maximum number of parasites in a host
was 63 (both adult and juvenile parasites), seven on the same spine. Between 25 %
and 42 % of the host spines were affected. Eucidaris galapagensis (n = 930) had
another parasite, Pelseneeria sp., which crawled freely on the host test and was
commonly found attached but not on or inside the spines as S. shaskyi was.
Pelseneeria sp. affected the gonads of the host in weight (a low gonad index) and
color (female gonads were pale). The infestation reached high percentages (95 %)
and the maximum number of parasites on the same host was five (adult parasites
only). At night, snails occasionally occurred on the substrate near sea urchins
(J. Sonnenholzner, pers. obs.), suggesting that snails may migrate between sea
urchins. It is unknown how S. shaskyi affects urchins at the population level, but
loss and reduction of spine number (Metz 1994; Sonnenholzner and Molina 2005)
would appear to increase susceptibility to predation and energetic costs for spine
repair (Heatfield 1971; Denny and Gaylord 1996).
Parasites dominate food web links (Lafferty et al. 2006). For this reason,
Sonnenholzner et al. (2011) took advantage of the variation in fishing pressure to
investigate complex indirect effects on two eulimid snails that parasitize the most
abundant sea urchin E. galapagensis in the Galápagos. Past work in the Galápagos
suggests that fishing reduces lobster and fish densities and this relaxation of predation pressure indirectly increases sea urchin densities, creating the potential for
complex indirect interactions between fishing and parasitic snails. To measure
indirect effects of fishing on these parasitic snails, the spatial relationships among
sea urchins, parasitic snails, commensal crabs, and large sea urchin predators
(hogfish and lobsters) was investigated. Parasitic snails had higher densities at sites
where sea urchins were abundant, probably due to increased resource availability.
Commensal crabs that shelter under sea urchin spines, particularly the endemic
Mithraculus nodosus, preyed on the parasitic snails in aquaria. Snails were less
abundant at field sites where these crabs were common. In aquaria, hogfish and
lobsters readily ate crabs, but crabs under sea urchin spines were protected from
predation. This would lead to a facultative mutualism between commensal crabs
and sea urchins. In the field, fishing appeared to indirectly increase the abundance
of sea urchins and their commensal crabs by reducing predation pressure from fish
and lobsters. Fished sites had fewer snails per sea urchin, probably due to
increased predation from commensal crabs. However, because fished sites also
tended to have more sea urchins, there was no significant net effect of fishing on
the number of snails per square meter. These results suggest fishing can have
complex indirect effects on parasites by altering food webs.
214
J. Sonnenholzner et al.
Islands are well known, their biological interactions with parasitism are not.
Echinoderms are intensively parasitized by eulimid gastropods (Pearse and
Cameron 1991), but there has been little research about the eastern Pacific eulimid
species and their ecology (Skoglung 2004). Sonnenholzner and Molina (2005)
provided information on the sea urchin–eulimids host–parasite relationship along
the southern and western regions of the Galápagos Islands. A total of 1,855
specimens of E. galapagensis were examined. Of them, 925 individuals were
mainly affected by Sabinella shaskyi. The maximum number of parasites in a host
was 63 (both adult and juvenile parasites), seven on the same spine. Between 25 %
and 42 % of the host spines were affected. Eucidaris galapagensis (n = 930) had
another parasite, Pelseneeria sp., which crawled freely on the host test and was
commonly found attached but not on or inside the spines as S. shaskyi was.
Pelseneeria sp. affected the gonads of the host in weight (a low gonad index) and
color (female gonads were pale). The infestation reached high percentages (95 %)
and the maximum number of parasites on the same host was five (adult parasites
only). At night, snails occasionally occurred on the substrate near sea urchins
(J. Sonnenholzner, pers. obs.), suggesting that snails may migrate between sea
urchins. It is unknown how S. shaskyi affects urchins at the population level, but
loss and reduction of spine number (Metz 1994; Sonnenholzner and Molina 2005)
would appear to increase susceptibility to predation and energetic costs for spine
repair (Heatfield 1971; Denny and Gaylord 1996).
Parasites dominate food web links (Lafferty et al. 2006). For this reason,
Sonnenholzner et al. (2011) took advantage of the variation in fishing pressure to
investigate complex indirect effects on two eulimid snails that parasitize the most
abundant sea urchin E. galapagensis in the Galápagos. Past work in the Galápagos
suggests that fishing reduces lobster and fish densities and this relaxation of predation pressure indirectly increases sea urchin densities, creating the potential for
complex indirect interactions between fishing and parasitic snails. To measure
indirect effects of fishing on these parasitic snails, the spatial relationships among
sea urchins, parasitic snails, commensal crabs, and large sea urchin predators
(hogfish and lobsters) was investigated. Parasitic snails had higher densities at sites
where sea urchins were abundant, probably due to increased resource availability.
Commensal crabs that shelter under sea urchin spines, particularly the endemic
Mithraculus nodosus, preyed on the parasitic snails in aquaria. Snails were less
abundant at field sites where these crabs were common. In aquaria, hogfish and
lobsters readily ate crabs, but crabs under sea urchin spines were protected from
predation. This would lead to a facultative mutualism between commensal crabs
and sea urchins. In the field, fishing appeared to indirectly increase the abundance
of sea urchins and their commensal crabs by reducing predation pressure from fish
and lobsters. Fished sites had fewer snails per sea urchin, probably due to
increased predation from commensal crabs. However, because fished sites also
tended to have more sea urchins, there was no significant net effect of fishing on
the number of snails per square meter. These results suggest fishing can have
complex indirect effects on parasites by altering food webs.
214
J. Sonnenholzner et al.