DOI: 10.5061/dryad.dfn2z34wx
title: Social trematodes parasites increase standing army size in areas of greater invasion threat

description: 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 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 on defence, attacking and killing invading parasites. We
 quantified invasion threat and soldier allocation for 168 trematode
 colonies belonging to 6 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.
description: <i>Target species
 collection</i> We
 focused on six trematode species that have a soldier caste and infect
 California horn snails (Table 1) [16-18]. We collected over 300 snails
 from each of nine estuaries across California and two estuaries across
 Panama. We aimed to collect a minimum of 100 snails by hand during low
 tide from three sites per estuary. However, at one Panamanian estuary
 where snails were extremely patchy, we collected all snails from two
 sites, while at another, we opportunistically collected snails from eight
 sites to find our trematode species of interest. Additionally, in areas we
 expected to have low infection prevalence (given previous survey work), we
 collected more snails to get sufficient numbers of trematode colonies for
 dissection (Tables S3-S4). A total of 5520 snails were dissected to
 estimate infection prevalence, a proxy for invasion threat.
 <i>Identification,
 colony processing, index of invasion
 threat</i> In the
 laboratory, snail length was measured with Vernier callipers, processed
 following Torchin et al [25], and trematode species identified following
 Hechinger [31]. We assessed soldier allocation and deployment for
 trematodes colonies that were the only infection within a snail (i.e. no
 coinfection; Table 1). The deshelled snails were divided into three
 sections, and soldiers and reproductives were counted in each section
 (Figure 1B).             As a
 metric for invasion threat, we summed the prevalences (proportion of hosts
 infected) of all trematode species [32]. This metric provides the
 cumulative risk of invasion by any trematode species at that site. We
 calculated infection prevalence for each site and each estuary (pooling
 all snails) (Table S3-S4).
description: See Read_Me.doc files for important information about raw data
 tables.