Spatial scale and structure of complex life cycle trematode parasite communities in streams

Abstract

By considering the role of site-level factors and dispersal, metacommunity concepts have advanced our understanding of the processes that structure ecological communities. In dendritic systems, like streams and rivers, these processes may be impacted by network connectivity and unidirectional current. Streams and rivers are central to the dispersal of many pathogens, including parasites with complex, multi-host life cycles. Patterns in parasite distribution and diversity are often driven by host dispersal. We conducted two studies at different spatial scales (within and across stream networks) to investigate the importance of local and regional processes that structure trematode (parasitic flatworms) communities in streams. First, we examined trematode communities in first-intermediate host snails (Elimia proxima) in a survey of Appalachian headwater streams within the Upper New River Basin to assess regional turnover in community structure. We analyzed trematode communities based on both morphotype (visual identification) and haplotype (molecular identification), as cryptic diversity in larval trematodes could mask important community-level variation. Second, we examined communities at multiple sites (headwaters and main stem) within a stream network to assess potential roles of network position and downstream drift. Across stream networks, we found a broad scale spatial pattern in morphotype- and haplotype-defined communities due to regional turnover in the dominant parasite type. This pattern was correlated with elevation, but not with any other environmental factors. Additionally, we found evidence of multiple species within morphotypes, and greater genetic diversity in parasites with hosts limited to in-stream dispersal. Within network parasite prevalence, for at least some parasite taxa, was related to several site-level factors (elevation, snail density and stream depth), and total prevalence decreased from headwaters to main stem. Variation in the distribution and diversity of parasites at the regional scale may reflect differences in the abilities of hosts to disperse across the landscape. Within a stream network, species-environment relationships may counter the effects of downstream dispersal on community structure.

The role of small‐scale environmental gradients on trematode infection

“…For example, Theron et al (1978) sampled along a small canal in Guadeloupe and found that infection prevalence with Schistosoma mansoni in Biomphalaria glabrata snails increased downstream. However, other studies have found the opposite trend, with higher parasitism upstream in headwater sites (Zemmer et al, 2020), although the mechanism for this is unclear. Flow may explain some of the variation in infection prevalence in our system since flow rate was unidirectional (albeit seemingly slow), and may have caused eggs or miracidia (that infect snails) to accumulate downstream.…”

Underreported and taxonomically problematic: characterization of sanguinicolid larvae from freshwater limpets (Burnupiidae), with comments on the phylogeny and intermediate hosts of sanguinicolids

“…alias ‘ Sanguinicola ’ sp. from the USA (Zemmer et al ., 2020) formed a strongly supported subclade with P . occidentalis , indicating that it is a species of Pseudosanguinicola .…”

“…from Vietnam; Pseudosanguinicola occidentalis Warren and Bullard, 2023 and unidentified species from the USA (Warren et al ., 2023; Warren and Bullard, 2023). In addition, DNA data are available for cercariae of 12 sanguinicolids from freshwater gastropods from Australia, East Africa, Poland and USA (Olson et al ., 2003; Brant et al ., 2006; Zemmer et al ., 2020; Preston et al ., 2021; Cutmore et al ., 2023). However, the identities of some of the cercariae whose sequences are available remain uncertain.…”

Trematode genetic patterns at host individual and population scales provide insights about infection mechanisms

“…It may also occur through human-mediated movements, although no information is available regarding this possibility. Therefore, it is more logical to expect a correlation with the population genetic structure of seabass, the most mobile host in the trematode life cycle (Zemmer et al ., 2020). Two distinct genetic population units, one in the Atlantic area and another in the Mediterranean, have been previously identified for seabass (Souche et al ., 2015), with a slight genetic differentiation in the Atlantic area observed south of the Strait of Gibraltar (Morocco) attributed to a hybrid zone between 2 evolutionary lineages (Lemaire et al ., 2005; Vandeputte et al ., 2019).…”