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NUMBER 147, 12 pages 12 June 2022
BISHOP MUSEUM PRESS
HONOLULU
BISHOP MUSEUM
OCCASIONAL PAPERS
Range extension of the endemic teRRestRial isopod
Hawaiioscia rapui Reveals the dispeRsal potential of
the genus acRoss the south pacific
J. Judson wynne, stefano taiti, sebastían yancovic pakarati &
alma carolina castillo-truJillo
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Cover photo: Hawaiioscia rapui Taiti & Wynne, 2015 from Moto Motiro Hiva (top) and Rapa Nui (bottom).
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Range extension of the endemic terrestrial isopod
Hawaiioscia rapui reveals the dispersal potential
of the genus across the South Pacific
J. JUDSON WYNNE*
Department of Biological Sciences and Center for Adaptable Western Landscapes, Northern
Arizona University, Flagstaff, Arizona, USA
STEFANO TAITI
Istituto di Ricerca sugli Ecosistemi Terrestri CNR-IRET, Museo di Storia Naturale, Sezione di
Zoologia, Sesto Fiorentino (Florence), Italy
SEBASTIÁN YANCOVIC PAKARATI
Laboratorio de Socioecosistemas, Departamento de Ecología, Universidad Autónoma de Madrid,
Madrid, Spain; Consejo Asesor de Monumentos Nacionales de Chile - Rapa Nui, Chile; Manu
Project, Rapa Nui, Chile
ALMA CAROLINA CASTILLO-TRUJILLO
Woods Hole Oceanographic Institution, Falmouth, Massachusetts, USA
Abstract. Hawaiioscia rapui Taiti & Wynne, 2015 was first described from two caves on
Rapa Nui and considered a potential island endemic and disturbance relict (i.e., an
organism that becomes a relict species due to anthropogenic activities). As this species
was not subterranean-adapted, it may have had an island-wide distribution prior to the
arrival of the ancient Polynesians to Rapa Nui. We report new records for Hawaiioscia
rapui beyond its type locality. These findings extend this animal’s range to the closest
neighboring island, Motu Motiro Hiva (MMH), 414 km east by northeast of Rapa Nui. We
also report information on this animal’s natural history, discuss potential dispersal
mechanisms, identify research needs, and provide strategies for management. Our
discovery further underscores that MMH likely harbors a unique and highly adapted
halophilic endemic arthropod community. Conservation policies will be required to
prevent alien species introductions; additionally, an inventory and monitoring program
should be considered to develop science-based strategies to manage the island’s
ecosystem and species most effectively.
Keywords: Canoe Bug Hypothesis, rafting, marine littoral, Polynesia
INTRODUCTION
The genus Hawaiioscia (Family Philosciidae) was first described to accommodate four
subterranean-adapted species discovered in caves on the Hawaiian Islands (Taiti &
Howarth 1997). Each was a short-range endemic species with each species detected
within an individual cave; these four species were described from coastal Kauai, Maui,
Molokai, and Oahu. Nearly two decades later, Hawaiioscia rapui Taiti & Wynne, 2015
was described from two caves on Rapa Nui (Easter Island). Initially propounded as an
Wynne, J.J. et al. Range extension of the endemic terrestrial isopod
Hawaiioscia rapui reveals the dispersal potential of the genus across the
South Pacific. Bishop Museum Occasional Papers 147: 1–12 (2022).
ISSN (online) 2376-3191
Published online: 12 June 2022
* Correspondence: jut.wynne@nau.edu
island endemic and disturbance relict (i.e., a species with a relictual distribution due to
anthropogenic activities), this terrestrial isopod was believed to be restricted to caves due
to extensive surface disturbance (Taiti & Wynne 2015; Wynne et al. 2014). As this species
was not subterranean-adapted, the authors posited it may have had an island-wide
distribution prior to the arrival of the ancient Polynesians to Rapa Nui (Wynne et al. 2014,
2016). Taiti et al. (2018) described the epigean Hawaiioscia nicoyaensis Taiti,
Montesanto & Vargas 2018, from coastal Central America on Pita Playa, Costa
Rica. These six species and the findings reported herein set the stage for exploring
how their ancestral species may have colonized the South Pacific, as well as to
postulate how closely these species may be related genetically to one another.
Here we discuss a range extension of Hawaiioscia rapui to Motu Motiro Hiva
(MMH; Salas y Gómez Island), Chile, 414 km east by northeast of Rapa Nui (Fig. 1).
These findings are based upon a morphological examination of specimens collected
in August of 2016. Additionally, we disclose a new littoral location for H. rapui on
Rapa Nui, provide some notes on its ecology, and investigate potential dispersal
mechanisms leading to its arrival on MMH. Importantly, we also discuss research needs
aimed toward collecting the information necessary to best manage H. rapui and the
broader arthropod community on MMH, as well as to provide some recommendations to
help ensure the long-term persist-ence of the arthropod community on the island.
METHODS AND FINDINGS
Methods
For arthropod sampling on Motu Motiro Hiva, on 23 August 2016, two individuals sam-
pled two areas for approximately 30 minutes each (for a total of 2 person hours of search-
ing). They examined vegetation, soil, and underneath rocks. Arthropods were hand col-
lected with forceps and watercolor paintbrushes. Additional details on collection and cura-
tion may be found via Hershauer et al. (2020).
On Rapa Nui, rocky coasts and beaches were sampled from 5 July to 1 September
2016. Both the rocky coastline and beach cove of Ovahe were sampled. The sandy cove
was sampled by examining detrital bands at the high tide boundary, examining decompos-
ing algae and animals washed ashore, and by searching within the beach-vegetation
boundary. In rocky areas, observers searched for and collected arthropods using a timed
direct intuitive search approach within and beneath rocks and within rocky crevices.
Observers used aspirators, watercolor paintbrushes, and forceps to collect arthropods.
Refer to Wynne et al. (2016) for additional details.
For both locations, all specimens were placed directly into vials with 95% ethanol.
Range extension
In their paper chronicling the first arthropod survey on MMH, Hershauer et al. (2020) pre-
liminarily identified two terrestrial isopods as different morphospecies (Halophilosciidae?
sp. 1 and Halophilosciidae? sp. 2). At the time, a species level identification was not
believed to be possible due to the poor preservation condition of most specimens. In email
correspondence with the second author (ST), the lead author suggested one of the mor-
phospecies was potentially Hawaiioscia rapui; however, upon examining images of one
of the specimens, ST intimated the specimens probably represented at least one morphos-
pecies of the family Halophilosciidae—as the specimens were collected in a halophilous
BISHOP MUSEUM OCCASIONAL PAPERS: No. 147, 2022
2
environment. ST recently examined all the terrestrial isopod specimens (n = 9) represent-
ing the potential two morphospecies identified by Hershauer et al. (2020). He determined
all were H. rapui. While the authors referred to the morphospecies designations as ques-
tionable (denoted by question marks following the family name), this misidentification
underscores why caution should be exercised when identifying certain taxonomic groups
(in the case of terrestrial Isopoda) via photointerpretation. In most cases, important taxo-
nomic characters cannot be sufficiently resolved resulting in questionable identifications.
All specimens were identified using the species description for H. rapui and the taxo-
nomic key provided in Taiti & Wynne (2015). As the only character(s) requiring measure-
ments was the length of the habitus, data for individuals from the MMH and Rapa Nui pop-
ulations (Fig. 2) are provided. To date, only 19 specimens were available for this species.
We acknowledge this represents a small number of specimens (nine for MMH, six individ-
uals from Rapa Nui caves (Taiti & Wynne 2015), and four specimens from the north shore
of Rapa Nui). However, because information is limited for this relatively new species, we
felt it was incumbent upon us to present all the available data.
Of the nine specimens from MMH (1 male and 8 females), only three were fully adult.
The remaining specimens were quite small (e.g., ≤4 mm); therefore, we did not include
these measurements. For the adult MMH specimens, the length of the male was 4.5 mm,
while the maximum length for female adults was 6.8 mm. For the six Rapa Nui cave spec-
imens, maximum length was 7.5 mm for both males and females (Taiti & Wynne 2015).
Additionally, four individuals belonging to Hawaiioscia rapui (e.g., Fig. 3C) were col-
lected from littoral habitats of Ovahe beach on the north shore of Rapa Nui. Maximum
length for these specimens was comparable to those reported by Taiti & Wynne (2015); 4.5
mm (♂) and 6 mm (♀) in length with two individuals ≤ 4 mm in length. Incidentally, this
species was not detected during the coastal cliff sampling effort (Wynne, unpublished data).
Refer to Wynne et al. (2016) for details on coastal and cliff sampling, as well as the project’s
broader scope.
We emphasize that because isopods molt as they mature, these measurements may be
relative.
3
Figure 1. Present distribution of Hawaiioscia rapui Taiti & Wynne 2015 in the Easter Island
Province, Chile. Yellow circles denote the locations where the species was first discovered (Taiti &
Wynne 2014), while red triangles demarcate the range extensions to Ovahe Beach and Motu Motiro
Hiva. Map is not to scale.
Wynne et al. — Pacific Hawaiioscia rapui
A halophilic species
The discovery of Hawaiioscia rapui on MMH (Fig. 3A, B) and along the Rapa Nui coast
(Fig. 3C) has yielded additional insights into its autecology. Moto Motiro Hiva is a small
island (~2.5 km²) with the highest elevations reaching ~30 m above sea level; as a result,
the entire island is incessantly showered by saltwater and salt spray. Additionally, Rapa
Nui coastal beach habitats are subjected to the same littoral environmental conditions.
Thus, H. rapui must be salt tolerant, and should be considered a littoral halophilic species.
Prior to these findings, H. nicoyaensis was the only species in the genus considered a
marine littoral species (Taiti et al. 2018). For the two caves where this species was initially
discovered, one was a coastal cave, and the other cave was ~1.2 km from the coast
(Wynne et al. 2014)—thus, both cave entrances are also exposed continuously to salt
spray, while the deeper cave environments are expected to be more insulated from surface
conditions (Fig. 3D, E).
BISHOP MUSEUM OCCASIONAL PAPERS: No. 147, 2022
4
Figure 2. Hawaiioscia rapui Taiti & Wynne 2015. Dorsal views of female specimens from [A] Moto
Motiro Hiva and [B] Rapa Nu i, southeastern-most Polynesia, Chile. Scale bar is for both individuals.
DISCUSSION
Potential dispersal mechanisms
Concerning the arrival of Hawaiioscia rapui to MMH, we examined the most probable
dispersal mechanisms: (1) rafting on flotsam (Thiel & Gutow 2005) from Rapa Nui to
MMH (or perhaps the inverse) and (2) anthropogenic-assisted dispersal between the two
islands. Importantly, terrestrial isopods are not known to disperse via phoresy on pelagic
birds (Wynne et al. 2014), nor have they been observed dispersing by direct flotation on
the open ocean (e.g., Peck 1994).
Dispersal of plants and animals across the open ocean in the direction of prevailing
ocean currents is well-documented (de Queiroz 2005, Gillespie et al. 2012, Gressitt 1961,
Jokiel 1990, Peck 1994). For this to occur, prevailing currents should flow in the general
direction of the perceived dispersal route. Ocean circulation patterns between Rapa Nui and
MMH are highly complex (Bertola et al. 2020, Chaigneau & Pizarro 2005, Moraga et al.
1999, Qiu & Chen 2004) and generally unfavorable for dispersal between Rapa Nui and
MMH. The geostrophic flow pattern near Rapa Nui is predominantly north to northwesterly
(Bertola et al. 2020; Chaigneau & Pizarro 2005). However, the Ekman currents are season-
ally variable due to winds, and thus, could provide favorable eastward flow conditions for
dispersal from Rapa Nui to MMH (Chaigneau & Pizarro 2005; Martinez et al. 2009; Thiel
et al. 2021). Moreover, eastward current reversals can occur for days to weeks due to storm-
generated swells originating off Antarctica (Snodgrass et al. 1966), local storms (e.g.,
Gressitt 1961), and potentially tsunamis (e.g., Carlton et al. 2017); these phenomena would
produce shifts in ocean currents favoring dispersal from Rapa Nui to MMH.
If rafting did occur from Rapa Nui to MMH, this most likely transpired when the
palm-dominated shrub forest on the island was largely intact (i.e., prior to or during the
formative stages of Polynesian settlement; Wynne et al. 2014). During this time, palms
and/or brambles of vegetation were available to be set adrift during intense inclement
weather. Today, most of the vegetation on Rapa Nui is characterized as a low-lying inva-
sive shrub-grassland association. Moreover, we deem dispersal via rafting from MMH to
Rapa Nui to be improbable; in a historical sense, “rafting” is predicated upon plants and
animals rafting on vegetation debris. Plant diversity on the MMH is limited to three suc-
culent and one spleenwort species (Vilina & Gazitua 1999). Thus, as rafting material is
largely absent on this island, dispersal to Rapa Nui via this mechanism seems unlikely.
Concerning human-assisted dispersal, alien arthropod species populations on Rapa Nui
date back to some of the earliest natural history investigations (e.g., Fuentes 1914;
Olalquiaga Faure 1946). The composition of the arthropod community, predominated by
alien species, is attributed to a long history of merchant ship traffic to the island. In recent
times, a steady influx of alien species continues to arrive on Rapa Nui as stowaways on sup-
ply ships primarily hailing from mainland Chile. While it is plausible H. rapui may have
actively dispersed from Rapa Nui to MMH with contemporary mariners, MMH is a small
uninhabited island and is not a safe harbor for maritime traffic. Thus, it is not a routine
stopover point for merchant traffic between Rapa Nui and mainland Chile. We surmise that
a contemporary human-assisted colonization event from Rapa Nui to MMH is possible, but
not probable.
However, a growing body of evidence supports the idea that ancient Polynesians
reached South America and/or interacted with South American indigenous groups. For
example, contact with coastal Native American tribes in present-day Chile is inferred
Wynne et al. — Pacific Hawaiioscia rapui 5
from the bones of the Polynesian chicken in the archaeological record (Storey et al. 2007).
Additionally, next generation genetic analysis has revealed “pre-European contact”
Native American ancestry in eastern Polynesian groups predating the settlement of Rapa
Nui (Ioannidis et al. 2020).
As most endemic arthropod populations can be presumed to have been comparative-
ly robust prior to the arrival of Europeans and during the formative years of the Rapanui
civilization, H. rapui was likely far more common on the island historically. Thus, it could
have been inadvertently collected in the soil of “canoe plants” (plants transported through-
out the South Pacific in gourds for food, medicine, materials for dwellings and canoes,
and other purposes; see Whistler 2009) and transported to MMH by the ancient
Polynesians (Edwards 1928; Wynne et al. 2014). Wynne et al. (2014) referred to this con-
cept as the “Canoe Bug Hypothesis”.
Subsequently, both active and passive dispersal from Rapa Nui to MMH are possible.
Because ocean currents are weak and variable between Rapa Nui and MMH, rafting
would have been possible only when native vegetation was available as rafting material
and weather conditions and current reversals favorably influenced ocean currents toward
MMH. Additionally, the ancient Polynesians, European mariners, and contemporary
BISHOP MUSEUM OCCASIONAL PAPERS: No. 147, 2022
6
Figure 3. Surface and cave habitats where Hawaiioscia rapui was detected. Motu Motiro Hiva is rep-
resented in the top two panels—driftwood (i.e., potential rafting flotsam) featured in [A] and the
lighthouse in the foreground of [B] provides scale. [C] Ovahe beach cove, north shore, Rapa Nui with
four people at far-left center for scale. Cave habitats on Rapa Nui presently consist of the coastal
cave (Q15-056 cave) [D] and a cave 1.2 km inland (Q15-076-078 cave) [E]; refer to Taiti & Wynne
(2015) for details.
Chileans have traversed the waters between the two islands for centuries—with the afore-
mentioned caveats provided. Subsequently, H. rapui likely dispersed to MMH via rafting
or with the assistance of the ancient Polynesians (or perhaps historically by the
Europeans).
Future research and conservation needs
We enumerate several research areas essential to advancing our knowledge of both H.
rapui and the genus (and perhaps its allies). Firstly, what is the degree of genetic connec-
tivity between the Rapa Nui and MMH populations? If there is evidence of divergence,
can we establish a time when these two lineages began to diverge (i.e., when did H. rapui
first colonize MMH—or perhaps Rapa Nui)? To address this question, additional speci-
mens should be collected from both islands for genetic and molecular clock analysis.
More broadly, how are the six sibling species of Hawaiioscia phylogenetically relat-
ed? As we have discussed, long-distance dispersal on prevailing ocean currents is well-
established (Bertola et al. 2020, de Queiroz 2005, Gillespie et al. 2012, Gressitt 1961,
7
Figure 4. Known locations for the genus Hawaiioscia: H. nicoyaensis (Costa Rica), the Hawaiʻi
group, and H. rapui (Rapa Nui (western-most location) and Moto Motiro Hiva). A generalized rep-
resentation of ocean currents modified from USASF (1943), Qiu and Chen (2004), and OSCAR data
(Earth & Space Research 2021). Based on prevailing patterns of ocean currents, the rationale that rep-
resentatives of the genus Hawaiioscia (or its ancestral lineage) may have originated from the
Neotropics and dispersed, via rafting, to Hawaiʻi, Rapa Nui, and possibly other littoral areas in the
South Pacific stands to reason. Hawaiian Islands not to scale.
Wynne et al. — Pacific Hawaiioscia rapui
Jokiel 1990, Peck 1994). An examination of ocean currents (Martinez et al. 2009, Qiu &
Chen 2004, USASF 1943) within the region where the six Hawaiioscia sibling species
occur revealed a strong probability that Hawaiioscia (or its ancestral lineage) originated
somewhere along the southern-most Central American (or perhaps the northern South
American) coast, and then rafted to Hawaiʻi, Rapa Nui, and likely points in between. If
correct, the Hawaiian and Rapanui/MMH species should be more closely related to the
Costa Rican species than to each other. Genetic analysis revealed the four subterranean-
adapted Hawaiioscia species from the Hawaiian Islands likely descended from one or
more littoral epigean species either not yet discovered or extinct (refer to Rivera et al.
2002). COI sequences for these species are available on GenBank. Thus, to test this
hypothesis and gain stronger inference regarding the phylogenetics of this group,
sequence data would be required for the Rapa Nui/MMH and Costa Rica populations.
Another intriguing question relates to the overall distribution of this halophilic
genus. In recent years, the distribution of Hawaiioscia has been expanded from the
Hawaiian Islands to three additional localities in the tropical Pacific (coastal Costa Rica,
Rapa Nui, and now Motu Motiro Hiva). Applying the rationale provided above, we
hypothesize this marine littoral group has radiated throughout the tropical Pacific Ocean.
This question may be addressed by conducting additional surveys along the Central and
South American coasts within the region conducive to dispersal via the South Pacific Gyre
and equatorial currents (refer to Fig. 4), including the coasts of the Desventuradas Islands,
the Juan Fernandez Islands, the Galápagos Islands, and greater Polynesia. Interestingly,
Taiti & Howarth (1997) considered dynamic boulder beaches, which were thought to
represent the ancestral habitat of epigean Hawaiioscia, to be among the least sampled
environments on oceanic islands largely due to the hazardous conditions. To ascertain
how other Hawaiioscia species are placed phylogenetically within the genus, as well as to
better chart the dispersal of this group across the southern Pacific Ocean, future workers
should collect specimens in preparation for genetic studies (i.e., preserved in 100% non-
denatured molecular grade ethanol and then appropriately stored).
Concerning the Rapa Nui population of H. rapui, this species was represented by low
numbers compared to alien isopod species (refer to Taiti & Wynne 2015)—specifically
Porcellio scaber Latreille, 1804, which was the most abundant terrestrial arthropod iden-
tified on Rapa Nui (Wynne et al. 2014). This was also the case when examining isopod
specimens collected from Ovahe Beach. We posit that alien isopod species, in particular
P. scaber, are likely exerting competitive pressure on native isopod populations (and
native arthropod species writ large). Conversely, while arthropod sampling on MMH was
limited (Hershauer et al. 2020), P. scaber is a rather conspicuous isopod and readily
detectable; neither it, nor the other alien isopod species known from Rapa Nui [refer to
Taiti & Wynne (2015) for the complete list] were detected during the 2016 sampling
effort. Although these data were limited and thus the results should be interpreted careful-
ly, the absence of alien isopod species (as well as other potential alien arthropod competi-
tors) suggests the MMH population (and the broader arthropod community) represent a
largely endemic and/or indigenous arthropod community.
As we now have two Hawaiioscia rapui populations on two distinct islands, we
could repopulate one island with individuals from the other population—albeit once the
question concerning genetic relatedness between the two island populations has been
resolved. Specifically, should one population become imperiled, the other may serve as a
BISHOP MUSEUM OCCASIONAL PAPERS: No. 147, 2022
8
source population for a captive breeding and reintroduction program (refer to Wynne et
al. 2014). To prepare for this potentiality, both populations should undergo population via-
bility analyses (Boyce 1992, Chaudhary & Oli 2020) to estimate both population sizes and
extirpation risks. These results would equip resource managers and conservation biolo-
gists with the information required to make informed and measured decisions concerning
a reintroduction program—should one become necessary. Given the Rapa Nui arthropod
community is dominated by alien species, it is reasonable to infer this population is more
likely to become imperiled than the MMH population.
Hawaiioscia rapui, previously known only from Rapa Nui, is now the second arthro-
pod species also considered endemic to MMH (as both islands comprise the Easter Island
Province). The first species, Ariadna motumotirohiva Giroti, Cotoras, Lazo & Brescovit,
2020, is a tube-web spider identified as a short-range endemic presently occurring solely
on MMH (Giroti et al. 2020). Given the limited distributions of both species and the
diminutive size of MMH, we recommend both taxa be considered species of management
concern. Surveys should be conducted to gather the much-needed information concerning
these species distributions and to obtain a baseline understanding of their population sizes.
Once done, both species could be assessed using IUCN (International Union for the
Conservation of Nature) Red List criteria (IUCN 2021) to determine their conservation
significance.
This exciting discovery underscores that MMH may harbor a unique and highly
adapted halophilic endemic arthropod community. However, before this can be estab-
lished, additional research will be required. Although this community has not been thor-
oughly inventoried, precautionary policies should be established to reduce the likelihood
of alien species introductions. One approach would be to regulate human visitation to the
island—at least until the arthropod community on Motu Motiro Hiva can be sufficiently
studied.
ACKNOWLEDGMENTS
For the Rapa Nui work, JJW extends his wholehearted gratitude to the field research team
(Francisco Ika, Pedro Lazo Hucke, Sergio Manuheuroroa, Lazaro Pakarati, Drew Bristow,
Rafael Rodríguez Brizuela, Eric Fies, Nicholas Glover, Walter Lynn Hicks, Dustin Kisner,
Ivory Marinakis, Benjamin Shipley, and Byron Yeager). Logistical support and permitting
was provided by CONAF-Parque Nacional Rapa Nui (Lillian Gonzales, Ninoska Hucke,
Michel Pate, Katherine Moreira, Andrea Valdez Riroroko, Raul Palominos, Christophe
Soon, Enrique Tucki, and Ramone Martinez Tepihe) and Consejo de Monumentos
Nacionales (Jimena Ramirez and Merahi Atam López). Hostel Vai Here (Soraya Laharoa,
Patricia Lillo Chinchilla, and Dale Simpson Jr.) and Hotel Tupa (Sergio Rapu Sr. and
Sergio Rapu Jr.) provided accommodations for the research team. The Fulbright Visiting
Scholars Program, CONAF-Parque Nacional Rapa Nui, and the National Speleological
Society’s International Exploration grant program supported this research. Sponsors
included Yale Cordage (Sarah Burr and Jamie Goddard) for rope, Act Safe (Britt Trude
Christensen) for the power ascender, and Rock Exotica (Brandon Lane) for climbing gear.
Rapa Nui fieldwork was recognized as an Explorers Club Flag Expedition. For the Moto
Motiro Hiva field research, SYP acknowledges: Edgardo Quezada V. from Servicio
Agricola y Ganadero (SAG), Oficina de Rapa Nui; Pedro Lazo Hucke with CONAF, Rapa
Nui; Violeta Producciones of Rapa Nui; the Chilean Navy and the AP 41 Aquiles crew;
Wynne et al. — Pacific Hawaiioscia rapui 9
Consejo Asesor de Monumentos Nacionales (CAMN), Secretaria Tecnica de Patrimonio
(STP) on Rapa Nui; and Sebastián Pakarati Trengove. We also thank Francis G. Howarth,
Jeremy Vandenberg and two anonymous reviewers for comments leading to the improve-
ment of this manuscript.
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