Social evolution: Diverse divisions of labor in trematode parasites.

Some animal societies solve problems, like foraging or defense, by cooperatively dividing labor. A new discovery highlights that trematodes are unique in forming different societies at multiple parts of their complex life cycle with distinct divisions of labor to solve different problems.

EXPERIMENTAL INFECTIONS WITH EUHAPLORCHIS CALIFORNIENSIS AND A SMALL CYATHOCOTYLID INCREASE CONSPICUOUS BEHAVIORS IN CALIFORNIA KILLIFISH (FUNDULUS PARVIPINNIS).

Some parasites manipulate their host's phenotype to enhance predation rates by the next host in the parasite's life cycle. Our understanding of this parasite-increased trophic transmission is often stymied by study-design challenges. A recurring difficulty has been obtaining uninfected hosts with a coevolutionary history with the parasites, and conducting experimental infections that mimic natural processes. In 1996, Lafferty and Morris provided what has become a classic example of parasite-increased trophic transmission; they reported a positive association between the intensity of a brain-infecting trematode (Euhaplorchis californiensis) in naturally infected California killifish (Fundulus parvipinnis) and the frequency of conspicuous behaviors, which was thought to explain the documented 10-30× increase in predation by the final host birds. Here, we address the primary gap in that study by using experimental infections to assess the causality of E. californiensis infection for increased conspicuous behaviors in F. parvipinnis. We hatched and reared uninfected F. parvipinnis from a population co-occurring with E. californiensis, and infected them 1-2 times/week over half their life span with E. californiensis and a small cyathocotylid trematode (SMCY) that targets the host's muscle tissue. At 3 time points throughout the hosts' lives, we quantified several conspicuous behaviors: contorting, darting, scratching, surfacing, and vertical positioning relative to the water's surface. Euhaplorchis californiensis and SMCY infection caused 1.8- and 2.5-fold overall increases in conspicuous behaviors, respectively. Each parasite was also associated with increases in specific conspicuous behaviors, particularly 1.9- and 1.4-fold more darting. These experimental findings help solidify E. californiensis-F. parvipinnis as a classic example of behavioral manipulation. Yet our findings for E. californiensis infection-induced behavioral change were less consistent and strong than those previously documented. We discuss potential explanations for this discrepancy, particularly the idea that behavioral manipulation may be most apparent when fish are actively attacked by predators. Our findings concerning the other studied trematode species, SMCY, highlight that trophically transmitted parasites infecting various host tissues are known to be associated with conspicuous behaviors, reinforcing calls for research examining how communities of trophically transmitted parasites influence host behavior.

Let's restart formally naming 'larval' trematodes.

Arguably the most unique biological features of trematode parasites involve their clonal parthenitae and cercariae. These life stages are biologically fascinating, medically and scientifically important, and often studied for years, lacking knowledge of their corresponding sexual adult stages. But sexual adults are the focus of trematode species-level taxonomy, partially explaining the relative neglect of documenting the diversity of parthenitae and cercariae and why researchers who do describe them give them only provisional names. Provisional names are unregulated, unstable, often ambiguous, and, I argue, often unnecessary. I suggest that we recommence formally naming parthenitae and cercariae using an improved naming scheme. The scheme should permit us to reap the benefits of formal nomenclature and thereby enhance research involving these diverse and important parasites.

Potential for Emergence of Foodborne Trematodiases Transmitted by an Introduced Snail (Melanoides tuberculata) in California and Elsewhere in the United States.

We document that 3 human-infectious trematodes and their introduced first intermediate host snail (Melanoides tuberculata) are widespread throughout southern California. We surveyed 41 fishing localities, 19 of which harbored snails infected with zoonotic trematodes. Two of the parasites, Haplorchis pumilio and Centrocestus formosanus, are fishborne intestinal trematodes recognized as being important human pathogens in other areas of the world; the third, Philophthalmus gralli, can infect the human eye. An additional 5 species detected infecting M. tuberculata are likely of little direct threat to people; however, they may be recently introduced to the Americas, highlighting the risk that additional pathogenic trematodes transmitted by the snail in its native range could be introduced to the United States. The current, possible human-infection risk in California clarifies the need to consider the introduced snail and its parasites from a public health perspective anywhere in the United States the snail has been introduced.

We report that 3 human-infecting trematodes and their introduced intermediate host snail are widespread in southern California freshwater fishing localities. Eating undercooked or underfrozen fish is the way people get infected by 2 of the parasite species, which are recognized as important human pathogens in other areas of the world. We also found 5 non­human-infectious trematodes carried by the snail that may be cointroduced, highlighting the possibility that other dangerous pathogens transmitted by the snail where it is native could arrive later or already be present in the United States. The common presence of the human-infecting fishborne trematodes at fishing localities, the widespread popularity of eating uncooked fish (eg, as sashimi, sushi, poke, or ceviche), and the potential for additional human-infecting trematodes to also be introduced, all justify consideration of the introduced snail and its parasites from a public health perspective in California and other areas in the United States where the parasites or the host snail have already been reported.

Lynnia grapsolytica n. gen, n. sp. (Ciliophora: Apostomatida: Colliniidae), a Deadly Blood Parasite of Crabs with a Novel Pseudocytopharynx.

We describe a new genus and species of blood-dwelling apostome ciliate, Lynnia grapsolytica n. gen., n. sp. (Apostomatida: Colliniidae). A distinct kinety "hook" pattern on the tomite's posterior ventral face, coupled with its marine habitat and use of a decapod host, readily distinguishes this ciliate from all known colliniids. We detected the parasite in ~12% of Pachygrapsus crassipes (Brachyura: Grapsidae) crabs in a California estuary and confirmed its presence at a Baja California rocky intertidal site. As existing methods failed to adequately stain this ciliate, we developed a new miniaturized silver carbonate impregnation staining method that produced excellent somatic and nuclear stains in all five observed cell types. A possibly unique trait is the active invagination of the tropho-tomont's anterior to form a temporary "pseudocytopharynx," likely used for feeding. Histological examination revealed that the ciliate invaded and damaged skeletal muscle, the heart, connective tissues, and gonads. Survivorship analysis indicated that infected crabs experienced 2.6 times greater daily mortality than uninfected crabs. Laboratory and field experimental infection attempts failed, suggesting a complex life cycle with outside-host development. Phylogenetic analysis at the 18S and COI loci confirmed the ciliate's placement in the Colliniidae. We emend the diagnosis of Family Colliniidae.

Brain-encysting trematodes (Euhaplorchis californiensis) decrease raphe serotonergic activity in California killifish (Fundulus parvipinnis).

Modulation of brain serotonin (5-HT) signalling is associated with parasite-induced changes in host behaviour, potentially increasing parasite transmission to predatory final hosts. Such alterations could have substantial impact on host physiology and behaviour, as 5-HT serves multiple roles in neuroendocrine regulation. These effects, however, remain insufficiently understood, as parasites have been associated with both increased and decreased serotonergic activity. Here, we investigated effects of trematode Euhaplorchis californiensis metacercariae on post-stress serotonergic activity in the intermediate host California killifish (Fundulus parvipinnis). This parasite is associated with conspicuous behaviour and increased predation of killifish by avian end-hosts, as well as inhibition of post-stress raphe 5-HT activity. Until now, laboratory studies have only been able to achieve parasite densities (parasites/unit host body mass) well below those occurring in nature. Using laboratory infections yielding ecologically relevant parasite loads, we show that serotonergic activity indeed decreased with increasing parasite density, an association likely indicating changes in 5-HT neurotransmission while available transmitter stores remain constant. Contrary to most observations in the literature, 5-HT activity increased with body mass in infected fish, indicating that relationships between parasite load and body mass may in many cases be a real underlying factor for physiological correlates of body size. Our results suggest that parasites are capable of influencing brain serotonergic activity, which could have far-reaching effects beyond the neurophysiological parameters investigated here.

Regional Distribution of a Brain-Encysting Parasite Provides Insight on Parasite-Induced Host Behavioral Manipulation.

Some parasite species alter the behavior of intermediate hosts to promote transmission to the next host in the parasite's life cycle. This is the case for Euhaplorchis californiensis, a brain-encysting trematode parasite that causes behavioral changes in the California killifish (Fundulus parvipinnis). These manipulations increase predation by the parasite's final host, piscivorous marsh birds. The mechanisms by which E. californiensis achieves this manipulation remain poorly understood. As E. californiensis cysts reside on the surface of the killifish's brain, discerning regional differences in parasite distribution could indicate mechanisms for host control. In this study, we developed a method for repeated experimental infections. In addition, we measured brain-region specific density using a novel methodology to locate and quantify parasite infection. We show that E. californiensis cysts are non-randomly distributed on the fish brain, aggregating on the diencephalon/mesencephalon region (a brain area involved in controlling reproduction and stress coping) and the rhombencephalon (an area involved in controlling locomotion and basal physiology). Determining causal mechanisms behind this pattern of localization will guide future research examining the neurological mechanisms of parasite-induced host manipulation. These findings suggest that parasites are likely targeting the reproductive, monoaminergic, and locomotor systems to achieve host behavioral manipulation.

Social trematode parasites increase standing army size in areas of greater invasion threat.

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.

Parasitic nematodes of marine fishes from Palmyra Atoll, East Indo-Pacific, including a new species of Spinitectus (Nematoda, Cystidicolidae).

Here, we present the results of a taxonomic survey of the nematodes parasitizing fishes from the lagoon flats of Palmyra Atoll, Eastern Indo-Pacific. We performed quantitative parasitological surveys of 653 individual fish from each of the 44 species using the intertidal sand flats that border the atoll's lagoon. We provide morphological descriptions, prevalence, and mean intensities of the recovered seven species of adult nematode (Pulchrascaris chiloscyllii, Capillariidae gen. sp., Cucullanus bourdini, Cucullanus oceaniensis, Pseudascarophis sp., Spinitectus (Paraspinitectus) palmyraensissp. nov., Philometra pellucida) and three larval stages (Pulchrascaris sp., Hysterothylacium sp., Cucullanus sp.). We recorded: Pulchrascaris chiloscyllii from Carcharhinus melanopterus; Capillariidae gen. sp. from Chaetodon lunula, Lutjanus fulvus, and Ellochelon vaigiensis; Cucullanus bourdini from Arothron hispidus; Cucullanus oceaniensis from Abudefduf sordidus; Pseudascarophis sp. from Chaetodon auriga, Chaetodon lunula, and Mulloidichthys flavolineatus; Spinitectus (Paraspinitectus) palmyraensissp. nov. from Albula glossodonta; Philometra pellucida from Arothron hispidus; and three larval forms, Pulchrascaris sp. from Acanthurus triostegus, Acanthurus xanthopterus, Rhinecanthus aculeatus, Platybelone argalus, Carangoides ferdau, Carangoides orthogrammus, Caranx ignobilis, Caranx melampygus, Caranx papuensis, Chaetodon auriga, Chanos chanos, Amblygobius phalaena, Asterropteryx semipunctata, Valencienea sexguttata, Kyphosus cinerascens, Lutjanus fulvus, Lutjanus monostigma, Ellochelon vaigiensis, Mulloidichthys flavolineatus, Upeneus taeniopterus, Gymnothorax pictus, Abudefduf septemfasciatus, Abudefduf sordidus, and Stegastes nigricans; Hysterothylacium sp. type MD from Acanthurus triostegus, Carangoides ferdau, Chaetodon lunula, Chanos chanos, Kyphosus cinerascens, Abudefduf sordidus, and Arothron hispidus; and Cucullanus sp. from Caranx ignobilis. Spinitectus (Paraspinitectus) palmyraensissp. nov. (Cystidicolidae) is described from the intestine of roundjaw bonefish Albula glossodonta. All the nematode species reported in this study represent new geographical records. We discuss how our survey findings compare to other areas of the Indo-Pacific, and the way the relatively numerical dominance of trophically transmitted larval stages likely reflect the intact food web of Palmyra Atoll, which includes a large biomass of large-bodied top predator sharks and ray-finned fishes.

Metabolic theory of ecology successfully predicts distinct scaling of ectoparasite load on hosts.

The impacts of parasites on hosts and the role that parasites play in ecosystems must be underlain by the load of parasites in individual hosts. To help explain and predict parasite load across a broad range of species, quantitative theory has been developed based on fundamental relationships between organism size, temperature and metabolic rate. Here, we elaborate on an aspect of that 'scaling theory for parasitism', and test a previously unexplored prediction, using new data for total ectoparasite load from 263 wild birds of 42 species. We reveal that, despite the expected substantial variation in parasite load among individual hosts, (i) the theory successfully predicts the distinct increase of ectoparasite load with host body size, indicating the importance of geometric scaling constraints on access to host resources, (ii) ectoparasite load appears ultimately limited by access-not to host space-but to host energy, and (iii) there is a currency-dependent shift in taxonomic dominance of parasite load on larger birds. Hence, these results reveal a seemingly new macroecological pattern, underscore the utility of energy flux as a currency for parasitism and highlight the promise of using scaling theory to provide baseline expectations for parasite load for a diversity of host species.

Hermaphrodites and parasitism: size-specific female reproduction drives infection by an ephemeral parasitic castrator.

Sex can influence patterns of parasitism because males and females can differ in encounter with, and susceptibility to, parasites. We investigate an isopod parasite (Hemioniscus balani) that consumes ovarian fluid, blocking female function of its barnacle host, a simultaneous hermaphrodite. As a hermaphrodite, sex is fluid, and individuals may allocate energy differentially to male versus female reproduction. We predicted the relationship between barnacle size and female reproductive function influences the distribution of parasites within barnacle populations. We surveyed 12 populations spanning ~400 km of coastline of southern California and found intermediate-sized barnacles where most likely to be actively functioning as females. While it is unclear why larger individuals are less likely to be actively reproducing as females, we suggest this reduced likelihood is driven by increased investment in male reproductive effort at larger sizes. The female function-size relationship was mirrored by the relationship between size and parasitism. We suggest parasitism by Hemioniscus balani imposes a cost to female function, reinforcing the lack of investment in female function by the largest individuals. Within the subset of suitable (=female) hosts, infection probability increased with size. Hence, the distribution of female function, combined with selection for larger hosts, primarily dictated patterns of infection.

Parasitic copepods (Crustacea, Hexanauplia) on fishes from the lagoon flats of Palmyra Atoll, Central Pacific.

We surveyed copepods parasitic on the fishes at Palmyra, a remote atoll in the Central Indo-Pacific faunal region. In total, we collected 849 individual fish, representing 44 species, from the intertidal lagoon flats at Palmyra and recovered 17 parasitic copepod species. The parasitic copepods were: Orbitacolaxwilliamsi on Mulloidichthysflavolineatus; Anuretesserratus on Acanthurusxanthopterus; Caligusconfusus on Carangoidesferdau, Carangoidesorthogrammus, Caranxignobilis, Caranxmelampygus, and Caranxpapuensis; Caliguskapuhili on Chaetodonauriga and Chaetodonlunula; Caliguslaticaudus on Rhinecanthusaculeatus, Pseudobalistesflavimarginatus, M.flavolineatus, Upeneustaeniopterus, Chrysipteraglauca, and Epinephalusmerra; Caligusmutabilis on Lutjanusfulvus and Lutjanusmonostigma; Caligusrandalli on C.ignobilis; Caligus sp. on L.fulvus; Caritusserratus on Chanoschanos; Lepeophtheiruslewisi on A.xanthopterus; Lepeophtheirusuluus on C.ignobilis; Dissonussimilis on Arothronhispidus; Nemesis sp. on Carcharhinusmelanopterus; Hatschekialongiabdominalis on A.hispidus; Hatschekiabicaudata on Chaetodonauriga and Chaetodonlunula; Kroyerialongicauda on C.melanopterus and Lernanthropus sp. on Kyphosuscinerascens. All copepod species reported here have been previously reported from the Indo-Pacific but represent new geographical records for Palmyra, demonstrating large-scale parasite dispersion strategies.

Guide to the trematodes (Platyhelminthes) that infect the California horn snail (Cerithideopsis californica: Potamididae: Gastropoda) as first intermediate host.

The California horn snail, Cerithideopsis californica, lives in estuarine habitats from California (USA) in North America to Piura (Peru) in South America. Throughout this range, the snail serves as first intermediate host for a diverse guild of digenean trematodes. These parasites are used in teaching laboratories, and have been subject to a large amount of taxonomic, biological, evolutionary, and ecological research. Despite the abundance of research on these trematodes, we lack a satisfactory guide to these parasites. This manuscript treats the 19 trematode species that we are currently able to distinguish morphologically. I provide taxonomic affinities, information on second intermediate host use, an identification key focused on cercaria traits, information and a key for regressed infections lacking cercariae, information on early infections, and species accounts. The species accounts present photographs, additional diagnostic information, taxonomic notes, information concerning cryptic species, and connections to relevant literature. The primary aim of this manuscript is to facilitate research on this trematode guild by serving as an identification tool, and by providing background information, including highlighting gaps in our knowledge.

Parasite and host biomass and reproductive output in barnacle populations in the rocky intertidal zone.

The rocky intertidal zone has a long history of ecological study with barnacles frequently serving as a model system to explore foundational theories. Parasites are often ignored in community ecology studies, and this particularly holds for true for the rocky intertidal zone. We explore the role of the isopod parasite, Hemioniscus balani, on its host, the acorn barnacle, Chthamalus fissus. We use the currencies of biomass and reproduction measured at the individual level, then applied to the population level, to evaluate the importance of this parasite to barnacle populations. We found H. balani can comprise substantial biomass in 'apparent' barnacle populations, sometimes even equaling barnacle biomass. Additionally, parasite reproduction sometimes matched barnacle reproduction. Thus, parasites divert substantial energy flow from the barnacle population and to near-shore communities in the form of parasite larvae. Parasites appeared to decrease barnacle reproduction per area. Potentially, this parasite may control barnacle populations, depending on the extent to which heavily infected barnacle populations contribute to barnacle populations at larger scales. These findings regarding the importance of a particular parasite for host population dynamics in this well studied ecosystem call for the integration of disease dynamics into community ecological studies of the rocky intertidal zone.

Sea-level rise, habitat loss, and potential extirpation of a salt marsh specialist bird in urbanized landscapes.

Sea-level rise (SLR) impacts on intertidal habitat depend on coastal topology, accretion, and constraints from surrounding development. Such habitat changes might affect species like Belding's savannah sparrows (Passerculus sandwichensis beldingi; BSSP), which live in high-elevation salt marsh in the Southern California Bight. To predict how BSSP habitat might change under various SLR scenarios, we first constructed a suitability model by matching bird observations with elevation. We then mapped current BSSP breeding and foraging habitat at six estuarine sites by applying the elevation-suitability model to digital elevation models. To estimate changes in digital elevation models under different SLR scenarios, we used a site-specific, one-dimensional elevation model (wetland accretion rate model of ecosystem resilience). We then applied our elevation-suitability model to the projected digital elevation models. The resulting maps suggest that suitable breeding and foraging habitat could decline as increased inundation converts middle- and high-elevation suitable habitat to mudflat and subtidal zones. As a result, the highest SLR scenario predicted that no suitable breeding or foraging habitat would remain at any site by 2100 and 2110. Removing development constraints to facilitate landward migration of high salt marsh, or redistributing dredge spoils to replace submerged habitat, might create future high salt marsh habitat, thereby reducing extirpation risk for BSSP in southern California.

Monogenea of fishes from the lagoon flats of Palmyra Atoll in the Central Pacific.

A survey of the monogeneans of fishes from the lagoon flats of Palmyra Atoll detected 16 species already reported from the Indo-West Pacific faunal region. A total of 653 individual fish from 44 species were collected from the sand flats bordering the lagoon of the atoll. Eighteen species of fish were infected with monogeneans. The monogenean species recovered were: Benedenia hawaiiensis on Acanthurus xanthopterus, Chaetodon auriga, Chaetodon lunula, Mulloidichthys flavolineatus, Pseudobalistes flavimarginatus and Rhinecanthus aculeatus; Ancyrocephalus ornatus on Arothron hispidus; Euryhaliotrema annulocirrus on Chaetodon auriga and Chaetodon lunula; Euryhaliotrema chrysotaeniae on Lutjanus fulvus; Euryhaliotrema grandis on Chaetodon auriga and Chaetodon lunula; Haliotrema acanthuri on Acanthurus triostegus; Haliotrema aurigae on Chaetodon auriga and Chaetodon lunula; Haliotrema dempsteri on Acanthurus xanthopterus; Haliotrema minutospirale on Mulloidichthys flavolineatus; Haliotrematoides patellacirrus on Lutjanus monostigma; Neohaliotrema bombini on Abudefduf septemfasciatus and Abudefduf sordidus; Acleotrema girellae and Acleotrema parastromatei on Kyphosus cinerascens; Cemocotylella elongata on Caranx ignobilis, Caranx melampygus and Caranx papuensis; Metamicrocotyla macracantha on Crenimugil crenilabris; and Pseudopterinotrema albulae on Albula glossodonta. All these monogenean-host combinations represent new geographical records. The monogenean species composition of the Palmyra Atoll is similar to that of the Hawaiian Islands. However, the number of species recovered was lower compared with other localities within the Indo-West Pacific, perhaps due to the geographical isolation of Palmyra Atoll.

Trematodes with a reproductive division of labour: heterophyids also have a soldier caste and early infections reveal how colonies become structured.

Recent findings have extended the documentation of complex sociality to the Platyhelminthes, describing the existence of a reproductive division of labour involving a soldier caste among the parthenitae of trematode parasites. However, all species examined to date occupy high positions in trematode interspecific dominance hierarchies and belong to two closely related families, the Echinostomatidae and the Philophthalmidae (Superfamily Echinostomatoidea). Further, the two species documented as lacking soldiers also belong to the Echinostomatidae. Here, we examine four species of intermediate dominance, all belonging to the family Heterophyidae (Superfamily Opisthorchioidea): Euhaplorchis californiensis, Phocitremoides ovale, Pygidiopsoides spindalis and Stictodora hancocki, all of which infect the California horn snail, Cerithideopsis californica (=Cerithidea californica). We quantify morphology, distribution and behaviour of rediae from fully developed colonies. We also provide information on colony structure for three developing heterophyid colonies to better understand colony development. We discuss the implications of our findings, particularly with respect to how they suggest alternatives to the conclusions of other researchers concerning the nature of trematode sociality. Our analyses of morphological, distributional and behavioural patterns of developed colonies indicate that these heterophyid trematodes have a non-reproductive caste whose function is defence of the colony from invading trematodes. Hence, a soldier caste occurs for species lower in dominance hierarchies than previously known, and is present in at least two superfamilies of digenean trematodes, suggesting that selection for a soldier caste may be much more common among the Trematoda than previously recognised.

The role of competition - colonization tradeoffs and spatial heterogeneity in promoting trematode coexistence.

Competition - colonization tradeoffs occur in many systems, and theory predicts that they can strongly promote species coexistence. However, there is little empirical evidence that observed competition- colonization tradeoffs are strong enough to maintain diversity in natural systems. This is due in part to a mismatch between theoretical assumptions and biological reality in some systems. We tested whether a competition - colonization tradeoff explains how a diverse trematode guild coexists in California horn snail populations, a system that meets the requisite criteria for the tradeoff to promote coexistence. A field experiment showed that subordinate trematode species tended to have higher colonization rates than dominant species. This tradeoff promoted coexistence in parameterized models but did not fully explain trematode diversity and abundance, suggesting a role of additional diversity maintenance mechanisms. Spatial heterogeneity is an alternative way to promote coexistence if it isolates competing species. We used scale transition theory to expand the competition - colonization tradeoff model to include spatial variation. The parameterized model showed that spatial variation in trematode prevalence did not isolate most species sufficiently to explain the overall high diversity, but could benefit some rare species. Together, the results suggest that several mechanisms combine to maintain diversity, even when a competition - colonization tradeoff occurs.

The role of competition--colonization tradeoffs and spatial heterogeneity in promoting trematode coexistence.

Competition - colonization tradeoffs occur in many systems, and theory predicts that they can strongly promote species coexistence. However, there is little empirical evidence that observed competition - colonization tradeoffs are strong enough to maintain diversity in natural systems. This is due in part to a mismatch between theoretical assumptions and biological reality in some systems. We tested whether a competition - colonization tradeoff explains how a diverse trematode guild coexists in California horn snail populations, a system that meets the requisite criteria for the tradeoff to promote coexistence. A field experiment showed that subordinate trematode species tended to have higher colonization rates than dominant species. This tradeoff promoted coexistence in parameterized models but did not fully explain trematode diversity and abundance, suggesting a role of additional diversity maintenance mechanisms. Spatial heterogeneity is an alternative way to promote coexistence if it isolates competing species. We used scale transition theory to expand the competition - colonization tradeoff model to include spatial variation. The parameterized model showed that spatial variation in trematode prevalence did not isolate most species sufficiently to explain the overall high diversity, but could benefit some rare species. Together, the results suggest that several mechanisms combine to maintain diversity, even when a competition - colonization tradeoff occurs.