Research Article
Records of terrestrial earthworms in a Canadian river as possible evidence of non-native dispersion
https://doi.org/10.21203/rs.3.rs-7882383/v1
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Rivers and streams have been hypothesized to be corridors of earthworm dispersal through watersheds, but little direct evidence of their dispersal within water columns has been observed. Here we report captures of at least two living, non-native earthworm species from instream drift-netting in a large order river of the Northern Great Plains.
Aporrectodea
Aquatic-terrestrial interactions
Drift Nets
Eiseniella tetraedra
Hydrochory
Invasion dynamics
Lumbricidae
Saskatchewan River System
Non-native earthworms have successfully colonized watersheds throughout North America to the extent that they now dominate the soil invertebrate communities of many riparian ecosystems (Hendrix et al. 2008). However, many early studies of their distribution have suggested that such a widespread extent in North America is not explained by passive or anthropogenic dispersal mechanisms alone, and that these mechanisms do not account for their presence in pristine and isolated watersheds (Costello et al. 2011; Paudel et al. 2016).
Earthworm dispersal is considered to be slow (e.g., 5–10 m per year) (Hale et al. 2005) and is cited as an explanation for why much of North America was earthworm-free after the Wisconsin Glaciation ~ 10,000 BP (Reynolds 1995). Since European colonization of North America though, the movement of soil through human activities is thought to have spread non-native species across the continent (Bohlen et al. 2004; Callaham et al. 2006; Dymond et al. 1997; Edwards 2004; Keller et al. 2007; Marinissen and van den Bosch 1992). However, movement in stream and river flow has been suggested and hypothesized for some time (e.g., Bouché 1972), and laboratory studies have demonstrated that many species can survive and remain reproductively viable underwater (Roots 1956; Turner 2000; Zorn et al. 2008). Previous aquatic benthic investigations have also noted terrestrial earthworms in stream benthos (Ward 1976) and cocoons in stream drift (Schwert and Dance 1979), and Costello et al. (2011) even demonstrated the ability for non-native earthworms to survive in Alaskan streams under field conditions. Despite an exceptional tolerance for submersion in water and evidence for their establishment associated with riparian zones along waterways (e.g., Costello et al. 2011), observations of hydrochory as a means for live earthworm dispersal has not been documented in detail. Here we report the collection of at least two living earthworm species (Aporrectodea spp., Eiseniella tetraedra [Savigny, 1826], non-Eiseniella Lumbricidae) in a large North American River system throughout the water column.
Site description
Our study was conducted in the South and North Saskatchewan Rivers near their confluence in Saskatchewan (Fig. 1). Both rivers belong to the Saskatchewan River system which begins in the Rocky Mountains to the west and discharges into Lake Winnipeg in the east. The mean daily discharge at the study site (Fig. 2) was sourced from the closest upstream Water Survey of Canada (WSC) hydrometric gauges on the South Saskatchewan River at Saskatoon, Saskatchewan, (WSC Station 05HG001) and on the North Saskatchewan River at Prince Albert, Saskatchewan (WSC Station 05GG001; Environment and Climate Change Canada 2024). Both rivers flow through the Prairie Ecozone for the majority of their length, but transition to the Boreal Plains ecozone at their confluence where the current study was conducted.
Drift net sampling and active search
Drift nets were originally deployed with the objective of capturing lake sturgeon eggs or larvae downstream of suspected spawning locations in May and June of 2022–2024; however, we encountered numerous living earthworms in drift net samples, which are the focus of the current study. The drift nets consisted of 900-µm mesh material, a removable cod end for collecting samples, and a conical aluminum cone of either 20.3 cm or 35.6 cm outside diameter. Each drift net was attached to an airline cable with a plastic coating, with each end attached to either a buoy or anchor. The drift net was attached closer to the buoy to collect samples near the water surface (~ 30 cm from surface) or to the anchor to collect samples above the river bottom (~ 30 cm from benthos). All traps in the South Saskatchewan River were set against the benthos due to the shallow depth at survey locations (< 1 m). Multiple drift nets were installed in a line perpendicular to the flow at each site, varying sampling between the top and bottom. The drift nets were deployed in the evening (~ 19:00), left overnight, and retrieved the following morning (~ 08:00). Net sets were usually deployed in 5 day/4 night intervals and occasionally extended to ~ 14 day intervals during high flow events. (see Fig. 2 for pattern of drift net sampling each year).
The number of drift nets (i.e., sampling effort) varied between rivers. In the North Saskatchewan River, sites NSR Bravo and NSR Delta typically had 11 nets deployed each night, but only SSR Charlie was regularly sampled on the South Saskatchewan River with 14 nets (Fig. 1). The remaining sites were occasionally sampled through 2022–2024 depending on available resources. Detailed descriptions of drift netting can be found in Water Security Agency report on lake sturgeon spawn assessment (Water Security Agency 2025). The drift nets were 'swirled' in the water to release the material attached to the mesh's inside and concentrate the sample within the cod end. After the cod ends were removed, the samples were immediately sorted and any earthworms encountered were placed into glass vials with 95% ethanol.
Additionally, opportunistic earthworm surveys on the riverbank were conducted at two locations (Forks Camp and Boatlaunch, Fig. 1) to add some insight into what earthworms may be living along the river system. At the Forks Camp site shallow holes (~ 30 cm x 30 cm) were dug on five occasions in the riparian zone along the river, and all encountered earthworms were retained for identification. On 10 June, 2024, a collection of earthworms was made from specimens observed to be eroding from the bank and being washed into the river during a rain event. Finally, as earthworms were being used as bait in an angling component of the Lake Sturgeon spawn study, three individuals were retained to be identified from the large stock of store-bought earthworms.
Identifications
Adult earthworms were identified to the lowest possible taxonomic level using the taxonomic key from Reynolds (2022a). Aporrectodea species belonging to Aporrectodea turgida, Aporrectodea tuberculata, and Aporrectodea trapezoides were grouped as “Aporrectodea sp.” as these species are difficult to discern and are both taxonomically and ecologically similar (Pérez-Losada 2009). Juvenile Eiseniella tetraedra were identified to species, however other sexually immature earthworms could only be identified to family and were grouped together as “non-Eiseniella Lumbricidae.”
Molecular identification techniques were attempted for specimens that could not be identified to species based on morphology. Extraction and quantification of DNA was performed with E.Z.N.A. Tissue Extraction Kit and Qubit 4 Fluorometer respectively following manufacturer’s instructions. Universal primers (LCO1490/HCO2198) described in Folmer et al. (2004) were used to amplify a region of mitochondrial cytochrome c oxidase I gene. A PCR program of 4 min at 94°C, 35 cycles of 50s at 94°C, 30 s at 49°C and 60 s at 72°C, and finally 3 min at 72°C on an Applied Biosystems Veriti Thermal Cycler. After samples failed to amplify under these conditions, PCR was attempted again with annealing temperatures of 50°C, and 52°C but again failed. Control samples and lumbricid samples from other studies extracted and amplified in the same run amplified successfully suggesting these samples may have been contaminated with a compound inhibiting the amplification reaction and thus could not be identified using these methods.
Analysis
We used logistic regression to test whether mean daily flow influenced the occurrence (presence/absence) of earthworms in drift net catches. We standardized our flow variable by subtracting the mean and dividing by the standard deviation and fitted flow models for the North and South Saskatchewan Rivers separately using the forms as follows:
North Saskatchewan River
\(\:\text{L}\text{O}\text{G}\text{I}\text{T}\left(p\right)\) = -4.745 = (0.00427 • NSR Flow)
South Saskatchewan River
\(\:\text{L}\text{O}\text{G}\text{I}\text{T}\left(p\right)\) = 0.762 - (0.30340 • SSR Flow)
Whereby NSR Flow and SSR Flow are the mean daily flow in the North Saskatchewan River sites and South Saskatchewan River sites, respectively. Analyses were performed in R version 4.5.1 using the pscl and sjPlot statistical packages to conduct the logit model and calculate effect sizes (Odds ratios) and associated confidence intervals (R Core Team 2025).
From 2022-2024, we recovered 36 living earthworms in the drift nets, represented by at least two taxa: Aporrectodea sp., Aporrectodea sp. (sensu tuberculata or turgida), non-Eiseniella Lumbricidae, and Eiseniella tetraedra (Table 1). All 18 specimens of E. tetraedra were recovered from nets in the South Saskatchewan River (Table 1). In contrast, all nine specimens of Aporrectodea sp. were collected from nets in the North Saskatchewan River. Two and six specimens of non-Eiseniella Lumbricidae, were collected from the South and North Saskatchewan Rivers, respectively (Table 1). All individuals recovered from the drift nets were active in the samples and no deceased earthworms were encountered. Although most earthworms were recovered from nets set near or on the bottom of the rivers, one non-Eiseniella Lumbricidae and two Aporrectodea sp. individuals were captured in drift nets set close (~ 30 cm depth) to the river surface in the North Saskatchewan River (NSR Bravo, Table 1) on 26 June, 2022. We estimate depth at this time and site to likely be ~1.5 m, suggesting the earthworms were being transported downstream high in the water column.
Earthworms recovered from active searches of the terrestrial environment at the Forks Camp and Boatlaunch near drift net sites reflected the taxa found in the North Saskatchewan River drift nets (Table 1). During a heavy rainstorm on 10 June, 2024, 40 individuals of Aporrectodea sp. were observed at the boat launch location, washing down an eroding rivulet into the mainstem Saskatchewan River, approximately 2 km downstream of the confluence of North and South Saskatchewan Rivers (Table 1). As store-bought earthworms were commonly used as bait for a Lake Sturgeon angling component of the spawning study, three of these bait earthworms were identified but found to be a different earthworm than was found in the drift nets, Lumbricus terrestris Linnaeus, 1758 (Table 1).
In 2022, a total of 266 net sets were conducted at SSR Charlie, 4 at SSR Bravo, and 461 at NSR Bravo (see Figure 1). In 2023, a total of 336 net sets were conducted at SSR Charlie, 6 at SSR Bravo, and 10 at SSR Alpha, while 308 net sets were conducted at NSR Bravo, 219 at NSR Delta, 22 at NSR Foxtrot, and 16 at NSR Charlie. Drift net effort in 2024 was highest at SSR Charlie with 140 net sets followed by NSR Bravo with 118 net sets, NSR Delta with 85 net sets, and 8 net sets at NSR Charlie. In total, the South Saskatchewan River site at SSR Charlie had a rate of 0.027 earthworms/trap night, and the North Saskatchewan River sites at NSR Bravo and NSR Delta had rates of 0.012 and 0.010 earthworms/trap night, respectively.
The North Saskatchewan River has a highly variable flow regime relative to the South Saskatchewan River, due to operation of Gardiner Dam upstream on the latter river (see Shook and Pomeroy 2016). The North Saskatchewan River had two pronounced flow events in 2022 and 2023 but had no distinct freshet-driven flow event in 2024. Despite this, precipitation events in late May 2024 led to an increase in flow on 14 May that gradually declined through to the end of the monitoring period in late June (Fig. 2), and across all three years there is an association of earthworm capture in the North Saskatchewan River during or shortly after these increases in flow (Fig. 2). Overall, the probability of catching earthworms in our drift nets was positively related to mean daily flows in the North Saskatchewan River (n = 88, p = 0.013, Odds ratio: 2.48 [1.22 – 5.21]) and unrelated to flow for drift nets in the South Saskatchewan River (n = 89, p = 0.376, Odds ratio: 0.74 [0.34 – 1.40). Furthermore, earthworm catches in the North Saskatchewan River only occurred during higher flow events, whereas catches were more consistent across generally lower flow conditions in the South Saskatchewan River (Fig. 3).
Final remarks and further research considerations
The cause of earthworm drift in the North and South Saskatchewan Rivers may, in part, be explained by the life histories of the earthworm species we collected. For example, E. tetraedra is a limicolous species that prefers wet environments (Reynolds 2022a), whereas members of the genus Aporrectodea, including A. trapezoides Orley, 1885, A. tuberculata (Eisen, 1874), and A. turgida (Eisen, 1873), are all endogeic, living in mineral soil layers. Based on these habitat preferences, it makes sense that E. tetraedra would enter the river under regular flow conditions as is characteristic of the South Saskatchewan River where there is little change in flow, while Aporrectodea species would be washed in during heavy rainfall and bank erosion and would be a characteristic of the highly variable North Saskatchewan River flows. Interestingly, although Lumbricus terrestris is a commonly used bait worm for fishing in both rivers it did not occur in the drift net collections or in collections on land in the area, suggesting that it may not tolerate submergence well nor have suitable habitat in the area., Although Roots (1956) has shown it can survive submersion for months in a controlled laboratory setting, Reynolds (1977) suggests it is a strictly terrestrial earthworm in the wild. Cameron et al. (2007) only found this species in abundance where it had been frequently dumped by anglers at boat launches and not as abundant in the forest surrounding waterbodies suggesting its dispersal and colonization by water is not very successful.
To our knowledge, this is the first record of E. tetraedra occurring in Saskatchewan (Reynolds 2018, Reynolds 2022b). In a summary of first recorded observations of earthworm species in North American jurisdictions, Reynolds (2018) lists E. tetraedra as having been recorded to the east and south in neighboring Manitoba, Canada, Montana, U.S.A, respectively. As the Saskatchewan River basin flows from Saskatchewan eastward to Manitoba and does not connect with Montana to the south, colonization of this area by drift from recorded locations is unlikely to have been their source.
Future studies of rivers as dispersal corridors for non-native earthworms should consider 1) how far earthworms are able to travel drifting with the current, and 2) how successful they are in establishing reproductive populations when they return to terrestrial habitats downstream. Our study provides evidence that at least two species of non-native earthworm taxa are using these corridors to move farther distances than they are capable of traveling in terrestrial environments and provides the first abundance estimates of earthworm taxa drifting in the water column. The distance earthworms drift and their success at colonizing downstream environments is still unresolved; however, we predict that parthenogenic species will be more successful at colonizing new habitats given that they can reproduce asexually and are less dependent on finding conspecifics in newly colonized areas.
Although our study focused on adult and juvenile earthworms in drift nets, it is also possible that earthworm cocoons may also be readily transported downstream. We frequently observed cocoons during sorting (I Phillips personal observation), however they were not enumerated. Further, the absence of particular species in the current study should not be an indication that they are not relocated through river conduits as they may not have been present in source populations.
The vast network of lakes and rivers in Canada may facilitate the spread of non-native earthworms into otherwise inaccessible regions, especially the north. Thus, hydrochory needs to be considered when predicting future spread and large-scale impacts of non-native earthworms, and in plans to limit the spread of invasive earthworms into new regions.
Acknowledgements We thank A Mills, R Chou, T Dumont, J Moskalyk for their field assistance in this project. Funding for the drift net sampling was provided by a Habitat Stewardship Program for Aquatic Species at Risk grant, a Saskatchewan Fish and Wildlife Development Fund grant, and financial assistance from SaskPower all to assess lake sturgeon spawning areas.
Author Contributions IP developed the study conception and design. SP conducted all earthworm identifications, IP, AB, and SP prepared the manuscript and figures.
Funding Funding for the drift net sampling was provided by a Habitat Stewardship Program for Aquatic Species at Risk grant, a Saskatchewan Fish and Wildlife Development Fund grant, and financial assistance from SaskPower all to assess lake sturgeon spawning areas. The authors received no grants, funds, or other sources of support during the preparation of this manuscript.
Data Availability Statement The datasets generated during the current study are all presented in the tables and text of the manuscript.
Conflict of interest The authors declare that we have no conflict of interest to disclose.
Table 1 is available in the Supplementary Files section.
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