First records of the pitcher plant mite Sarraceniopus gibsoni (Nesbitt, 1954) (Astigmata: Histiostomatidae) in Europe

The carnivorous pitcher plant Sarracenia purpurea has been introduced across Europe from North America. Thus far, whether the inquiline species intimately associated with S. purpurea have also been introduced had not been studied. Sarraceniopus gibsoni is the obligate mite species found in S. purpurea and is ubiquitous within the pitchers in North America. Three sites in Britain and Ireland were sampled. Two of the three sites were confirmed to support S. gibsoni populations, one site in Ireland and one in England. The presence of S. gibsoni suggests that living plants of S. purpurea had been introduced to the sites hosting mite populations.


Introduction
Pitcher plant mites in the genus Sarraceniopus (Astigmata: Histiostomatidae) are obligate inhabitants of plants in Sarraceniaceae.Sarraceniaceae are carnivorous plants with tubular shaped leaves called "pitchers" that are adapted to attract, trap, digest, and absorb nutrients from a wide variety of prey animals, usually arthropods (Naczi 2018).Despite the effectiveness of these traps, a small number of arthropod species are able to inhabit and thrive in the fluid within the pitchers (Addicott 1974;Heard 1994;Bledzki and Ellison 2003).Sarraceniopus mites complete their life cycles only within Sarraceniaceae pitchers and are part of the detritivore food web facilitating prey "digestion" by feeding on bacteria and particulate organic matter from captured prey (Baiser et al. 2013).Only for very brief periods, to facilitate dispersal, do the mites ever leave the pitcher (Naczi 2018).
Sarraceniopus gibsoni (Nesbitt, 1954) is an obligate associate of Sarracenia purpurea L. living in its rain-filled pitchers (Naczi 2018).Nesbitt (1954) described the species, as Zwickia gibsoni, from collections he made from inside S. purpurea pitchers in Nova Scotia, Ontario, and Québec, Canada.Later, Fashing and OConnor (1984) described Sarraceniopus as a new genus for histiostomatid mites living inside pitchers of Sarraceniaceae and transferred the species to the genus as Sarraceniopus gibsoni.Other obligate arthropod associates of Sarracenia purpurea are aquatic larvae of the pitcher-plant mosquito Wyeomyia smithii (Diptera: Culicidae), the pitcher plant midge Metriocnemus knabi (Diptera: Chironomidae) and the sarcophagid fly Fletcherimyia fletcheri (Diptera: Sarcophagidae) (Addicott 1974;Heard 1994;Bledzki and Ellison 2003).Sarracenia purpurea is native to the eastern U.S.A. and much of Canada.Within this range, S. gibsoni is ubiquitous in S. purpurea, at landscape, population, plant, and pitcher scales (Buckley et al. 2010).Sarraceniopus gibsoni is usually abundant in S. purpurea pitchers.Given their ubiquity and abundance, it is likely that the mites have a significant impact on the food web within the fluid of S. purpurea.Sarraceniopus gibsoni may be a mutualist, as documented for other arthropod symbionts of S. purpurea (Bradshaw and Creelman 1984).Other arthropod symbionts typically feed on the fragmented prey, accelerating nutrient digestion by the host.Sarraceniopus gibsoni appears to do the same, which is hypothesized to make nutrients more readily available to the host plant.
Sarracenia purpurea has been deliberately introduced at over 100 sites across North West Europe (including Sweden, Switzerland, France, Germany, UK and Ireland), and at a small number of sites in Japan and New Zealand (Adlassnig et al. 2010;Pyšek et al. 2012).On some of these sites S. purpurea has become invasive reducing the abundance of native vascular plants and bryophytes (Walker 2014).The commercial availability of S. purpurea has increased in Europe within recent years, with it now being readily available within the horticulture trade, including garden centers.This makes probability of introduction into the wild even greater.
The extent to which S. gibsoni has been unintentionally introduced alongside S. purpurea is currently unknown.Other species of Sarraceniopus are confirmed as introduced in Japan in unidentified, cultivated Sarracenia hybrids (Tagami 2004).Given the ubiquity of S. gibsoni in S. purpurea pitchers in its native range, understanding whether the mite is present within the pitchers in its non-native range is important for understanding pitcher plant ecology, and how its ecologic roles may be modified when introduced outside of its native range, especially in the areas where it may become invasive.Sarracenia purpurea is known in some situations to have been deliberately introduced into Europe as living plants, and in other situations from seed (Wulf 2008).Introduction of living plants presents the potential for the unintentional co-introduction of S. gibsoni, but such a possibility has yet to be confirmed.The aim of the current study was to determine whether S. gibsoni is present in non-native, European populations of S. purpurea.

Materials and methods
Three populations of S. purpurea were chosen across Great Britain and Ireland to provide a wide geographical spread: one in northern England (Cumbria), one in southern England (Dorset) and one in the Republic of Ireland (Offaly) (Figure 1, Table 1).These populations were chosen as S. purpurea was introduced more than twenty years ago making the populations well established and regenerating within each site (Walker 2014).Sampling was undertaken during August 2019.Within each site 20 pitchers (from separate rosettes) were sampled.The sampled pitchers were the present year's new pitchers.Only fully mature pitchers were sampled for there to have been time for colonisation of pitchers by arthropod associates.Cumbria and Offaly's pitchers were sampled along a transect across  (Hijmans et al. 2005).
the entire population, roughly a 30 m and 100 m transect respectively.Following removal of the majority of the population for site management purposes, the site in Dorset is small (c. 100 plants) and restricted to permanent monitoring plots.Sampling was therefore undertaken randomly within five separate square plots.Pitchers were individually removed and liquid from each individual pitcher was poured into a 50 ml falcon tube and preserved by adding equal amounts (to the individual pitcher fluid) of 70% ethanol.Samples were assessed to determine presence or absence of mites in a laboratory using a Zeiss dissecting microscope (40x magnification).Where mites were present, a subset from each population were sent to New York Botanical Garden for confirmation of identification of S. gibsoni.Key identification characteristics for S. gibsoni are claws elongate (> 30% length of adjacent tarsi), palp lacking setae, palpal solenidion rigid, and chelicerae short (about as long as tibia I), in all feeding (non-deutonymphal) stages of the life cycle (Nesbitt 1954;Fashing and OConnor 1984; Figure 2).
For identification, mites were cleared with Nesbitt's fluid, mounted in Hoyer's medium on microscope slides, and studied with light microscopy (Walter and Krantz 2009).Slide-mounted voucher specimens of Sarraceniopus gibsoni are deposited in the collection of the Division of Invertebrates of The Natural History Museum, London and in the collection of the second author.

Results
We confirmed the presence of S. gibsoni within S. purpurea pitchers in two of the three sites sampled.These were the sites in Dorset UK and in Offaly Central Ireland (Figure 3).Despite a comprehensive search of material sampled from pitchers growing at the Cumbria UK site, no S. gibsoni were recorded in any of the 20 samples collected and examined.We, therefore, consider it likely that S. gibsoni is absent from this site.
Sarraceniopus gibsoni were recorded in all pitchers examined from Offaly and Dorset, and abundance within individual pitchers was typically high (> 20 at Offaly and > 50 individuals per pitcher at Dorset when arthropod prey remains were present in the fluid).Individuals were observed clinging to the surface of arthropod remains and freely within the pitcher fluid.Given the nature of sampling in the field, some individuals almost certainly remained within the pitcher after sampling and therefore the abundances recorded are a minimum estimate of the population within each pitcher.

Discussion
These are the first records of S. gibsoni from Europe and the first record of any obligate arthropod associate of S. purpurea in Europe.The presence or absence of obligate species in S. purpurea pitchers is important because of the role they can play in the community structure within the food web (Buckley et al. 2010).Due to how abundant S. gibsoni is in pitchers in the native range it is suspected these mites play an important role within the pitcher food web and impact nutrient availability, although this is not yet confirmed.Prey nutrient uptake is an important component of S. purpurea ecology and may contribute to the invasive nature of the plant in some situations when introduced.Knowing where obligate associates are present or absent is therefore important and provides a more comprehensive understanding of S. purpurea introductions.
The null record for the Cumbrian site presents an interesting opportunity to advance understanding of the functional role of obligate species in-situ.Sarraceniopus gibsoni is abundant and ubiquitous at all landscape scales in its native range (Buckley et al. 2010) and is predicted to have an important impact on the carnivorous function of S. purpurea.The presence of S. gibsoni may also reveal information about the history of S. purpurea introductions.For example, it is highly likely that S. gibsoni will be introduced every time that S. purpurea is introduced using whole/living plant material.As a result, where S. gibsoni is present we predict that the S. purpurea introduction was from at least one whole plant, with or without seed introductions as well.In contrast, the absence of S. gibsoni from a site where S. purpurea has been introduced potentially indicates the introduction was from seed.However, it was previously thought the Cumbria population was introduced from plant material.It is recorded that many populations of S. purpurea in Ireland (including the site we sampled), have been established from translocated plants originating from one main site (Taggart et al. 1990).This suggests that S. gibsoni may be widespread in Irish populations.There are, however, other hypotheses of how S. gibsoni has been introduced to these European sites.As there has not been a full survey of these sites for S. gibsoni it may be that S. gibsoni is present within similar environments (such as tree holes) and the pitchers and then colonised rather than introduced with the plants themselves.It may be that S. gibsoni populations may exist elsewhere in ornamental or laboratory plant populations and disperse via phorsey into the established S. purpurea populations.While we cannot yet rule out these possibilities for S. gibsoni introduction, we expect this mite species is more likely to have been introduced with S. purpurea.
To fully understand the biogeography of S. gibsoni within Europe a more comprehensive sampling campaign is required.This would be facilitated through the use of DNA metabarcoding, once a reference library for S. gibsoni is available.We predict that S. gibsoni is widespread across S. purpurea populations in Europe, indicating naturalisation facilitated by the naturalisation of a non-native host species.

Figure 1 .
Figure 1.Pictures showing the three sampled populations of S. purpurea across Great Britain and Ireland.A -population located in Cumbria in the UK, B -population located in county Offaly in central Ireland, and C -population located in Dorset England (photo credit: Ellen Goddard).

Figure 3 .
Figure 3. Map showing sites sampled across Great Britain and Ireland.Filled circles represent sites where S. gibsoni was confirmed as present.Circle with a cross represents the site at which we did not detect S. gibsoni.

Table 1 .
Site descriptions of the three sampled sites.