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Resetarits EJ, Torchin ME, Hechinger RF. Social trematode parasites increase standing army size in areas of greater invasion threat. Biol Lett 2020; 16:20190765. [PMID: 32097594 PMCID: PMC7058954 DOI: 10.1098/rsbl.2019.0765] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2019] [Accepted: 01/31/2020] [Indexed: 11/12/2022] Open
Abstract
Organisms or societies are resource limited, causing important trade-offs between reproduction and defence. Given such trade-offs, optimal allocation theory predicts that, for animal societies with a soldier caste, allocation to soldiers should reflect local external threats. Although both threat intensity and soldier allocation can vary widely in nature, we currently lack strong evidence that spatial variation in threat can drive the corresponding variation in soldier allocation. The diverse guild of trematode parasites of the California horn snail provides a useful system to address this problem. Several of these species form colonies in their hosts with a reproductive division of labour including a soldier caste. Soldiers are non-reproductive and specialized in defence, attacking and killing invading parasites. We quantified invasion threat and soldier allocation for 168 trematode colonies belonging to six species at 26 sites spread among 10 estuaries in temperate and tropical regions. Spatial variation in invasion threat was matched as predicted by the relative number of soldiers for multiple parasite species. Soldier allocation correlated with invasion threat at fine spatial scales, suggesting that allocation is at least partly inducible. These results may represent the first clear documentation of a spatial correlation between allocation to any type of caste and a biotic selective agent.
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Affiliation(s)
- Emlyn J. Resetarits
- Center for the Ecology of Infectious Disease, Odum School of Ecology, University of Georgia, Athens, GA, USA
- Department of Integrative Biology, University of Texas, Austin, TX, USA
- Smithsonian Tropical Research Institute, Balboa, Ancon, Republic of Panama
| | - Mark E. Torchin
- Smithsonian Tropical Research Institute, Balboa, Ancon, Republic of Panama
- Marine Science Institute, University of California, Santa Barbara, CA, USA
| | - Ryan F. Hechinger
- Scripps Institution of Oceanography, Marine Biology Research Division, University of California-San Diego, La Jolla, CA, USA
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Buck JC, Hechinger RF, Wood AC, Stewart TE, Kuris AM, Lafferty KD. Host density increases parasite recruitment but decreases host risk in a snail-trematode system. Ecology 2017; 98:2029-2038. [PMID: 28518406 DOI: 10.1002/ecy.1905] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/03/2017] [Revised: 05/01/2017] [Accepted: 05/03/2017] [Indexed: 11/09/2022]
Abstract
Most species aggregate in local patches. High host density in patches increases contact rate between hosts and parasites, increasing parasite transmission success. At the same time, for environmentally transmitted parasites, high host density can decrease infection risk to individual hosts, because infective stages are divided among all hosts in a patch, leading to safety in numbers. We tested these predictions using the California horn snail, Cerithideopsis californica (=Cerithidea californica), which is the first intermediate host for at least 19 digenean trematode species in California estuaries. Snails become infected by ingesting trematode eggs or through penetration by free-swimming miracidia that hatch from trematode eggs deposited with final-host (bird or mammal) feces. This complex life cycle decouples infective-stage production from transmission, raising the possibility of an inverse relationship between host density and infection risk at local scales. In a field survey, higher snail density was associated with increased trematode (infected snail) density, but decreased trematode prevalence, consistent with either safety in numbers, parasitic castration, or both. To determine the extent to which safety in numbers drove the negative snail-density-trematode-prevalence association, we manipulated uninfected snail density in 83 cages at eight sites within Carpinteria Salt Marsh (California, USA). At each site, we quantified snail density and used data on final-host (bird and raccoon) distributions to control for between-site variation in infective-stage supply. After three months, overall trematode infections per cage increased with snail biomass density. For egg-transmitted trematodes, per-snail infection risk decreased with snail biomass density in the cage and surrounding area, whereas per-snail infection risk did not decrease for miracidium-transmitted trematodes. Furthermore, both trematode recruitment and infection risk increased with infective-stage input, but this was significant only for miracidium-transmitted species. A model parameterized with our experimental results and snail densities from 524 field transects estimated that safety in numbers, when combined with patchy host density, halved per capita infection risk in this snail population. We conclude that, depending on transmission mode, host density can enhance parasite recruitment and reduce per capita infection risk.
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Affiliation(s)
- J C Buck
- Marine Science Institute, University of California, Santa Barbara, Santa Barbara, California, 93106, USA.,Ecology, Evolution and Marine Biology, University of California, Santa Barbara, Santa Barbara, California, 93106, USA
| | - R F Hechinger
- Marine Biological Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, 92093, USA
| | - A C Wood
- Marine Science Institute, University of California, Santa Barbara, Santa Barbara, California, 93106, USA
| | - T E Stewart
- Program in Ecology, Evolution and Conservation Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801, USA
| | - A M Kuris
- Marine Science Institute, University of California, Santa Barbara, Santa Barbara, California, 93106, USA.,Ecology, Evolution and Marine Biology, University of California, Santa Barbara, Santa Barbara, California, 93106, USA
| | - K D Lafferty
- U.S. Geological Survey, Western Ecological Research Center, Marine Science Institute, University of California Santa Barbara, Santa Barbara, California, 93106, USA
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