Southeastern Naturalist 383 J.C. Nifong and R.H. Lowers 22001177 SOUTHEASTERN NATURALIST 1V6o(3l.) :1368,3 N–3o9. 63 Reciprocal Intraguild Predation between Alligator mississippiensis (American Alligator) and Elasmobranchii in the Southeastern United States James C. Nifong1,* and Russell H. Lowers2 Abstract - The food habits and predatory interactions of Alligator mississippiensis (American Alligator) have been thoroughly studied within populations inhabiting inland freshwater ecosystems; however, it is increasingly evident that coastal populations habitually forage in estuarine and nearshore marine ecosystems inhabited by other top predators. While few studies have been performed, data reported thus far from marine-foraging populations indicate individuals chiefly consume small-bodied prey such as crustaceans, fish, and wading birds. Nonetheless, capture and consumption of large-bodied marine prey such as multiple species of sea turtles and a single species of Elasmobranchii (sharks and rays) have been documented. Here, we examine evidence regarding reciprocal intraguild predation between American Alligators and elasmobranchs. We provide the first evidence of American Alligator depredation of 4 Elasmobranchii species and review putative evidence for Elasmobranchii depredation of American Alligators. We discuss the ecological significance of these interactions, draw comparisons to similar interactions experienced by other crocodilians, and recommend further avenues for research on the subject. Background The predatory interactions and food habits of Alligator mississippiensis Daudin (American Alligator) have been thoroughly studied across multiple populations inhabiting various inland freshwater ecosystems (i.e., lakes, rivers, and marshes) throughout their native range in the southeastern coastal plain of the United States (e.g., Delany and Abercrombie 1986, Delany et al. 1999, Rice 2004, Wolfe et al. 1987). Documentation and characterization of the food habitats and predatory interactions of coastal-inhabiting American Alligator populations, which have the ability to cross-ecosystem forage in nearshore marine and estuarine ecosystems, however, was until recently, marginalized due to the lack of salt-secreting glands in alligatorids and the associated paradigm that American Alligators do not habitually use these ecosystems (Boggs et al. 2016, Chabreck 1971, Gabrey 2010, McNease and Joanen 1977, Nifong 2016, Nifong and Silliman 2013, Nifong et al. 2012, Rosenblatt et al. 2015, Tamarack 1989, Tellez and Nifong 2014, Wheatley 2010). American Alligators, while lacking salt secreting-glands (Taplin et al. 1982), are now known to frequent brackish and estuarine (5–25 ppt salinity) habitats as well as perform repetitive long-term (days to weeks) travel into marine (> 25 ppt salinity) 1Kansas Cooperative Fish and Wildlife Research Unit, Division of Biology, Kansas State University, 104 Ackert Hall, Manhattan, KS 66506. 2Integrated Mission Support Services, Mail code IM-300, Kennedy Space Center, FL 32899. *Corresponding author - jcnifong@ ksu.edu. Manuscript Editor: Michael Cove Southeastern Naturalist J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 384 habitats (Elsey 2005, Mazzotti and Dunson 1989, Nifong and Silliman 2017, Nifong et al. 2015, Rosenblatt and Heithaus 2011, Tamarack 1989). The reported findings from stomach-content studies and observations made within estuarine and marine ecosystems indicate that American Alligators primarily feed on crustaceans (i.e., crabs and shrimp), Limulus polyphemus (L.) (Atlantic Horseshoe Crab), small fish and minnows (i.e., Fundulidae [mud minnows], Cyprinidae [top minnows], Mugil cephalus L. [Striped Mullet], etc.), small mammals (i.e., rodents, Neovison vison Schreber, [American Mink], Procyon lotor L. [Raccoon], etc.), and wading birds (Chabreck 1971; Gabrey 2010; Nifong 2016; Nifong et al. 2012, 2015; Rosenblatt et al. 2015; Tamarack 1989; Valentine et al. 1972). In addition, albeit reported less frequently, coastal-inhabiting American Alligators are documented to consume larger-bodied marine prey. For example, Nifong et al. (2011) reported 6 accounts of scavenging and putative predation on 2 sea turtle species, Caretta caretta L. (Loggerhead Sea Turtle) and Chelonia mydas L. (Green Sea Turtle), by American Alligators along the southeastern US Atlantic coast. In terms of the depredation of Elasmobranchii, the only published evidence was reported by Tamarack (1993), who provided putative evidence (embedded spines in the jaw and head) of Dasyatis americana Hildebrand and Schroeder (Southern Stingray) depredation by a 314-cm total length (TL) male and a 158-cm TL female Alligator captured near St. Catherines Island, GA (31.624372°N, 81.152887°W; Fig. 1). Here we present the first records of American Alligators consuming 4 species of Elasmobranchii, Ginglymostoma cirratum (Bonnaterre) (Nurse Shark), Sphyrna tiburo (L.) (Bonnethead Shark), Negaprion brevirostris (Poey) (Lemon Shark), and Dasyatis sabina (Lesueur) (Atlantic Stingray). We then review observations and putative evidence for depredation of American Alligators by Elasmobranchii. We discuss the ecological significance of these interactions, draw comparisons to the documented interactions of other crocodilian species, and recommend further avenues for research on the subject. Depredation of Elasmobranchii by American Alligators On Wassaw Island, GA (31.880737°N, 80.968109°W; Fig. 1), a marine turtle researcher (M. Frick, University of Florida, Gainesville, FL, unpubl. data) observed several instances of elasmobranch depredation by American Alligators. Specifically, on 21 June 1997 during sea turtle nesting surveys along the oceanfront beaches (>25 ppt) of Wassaw Island, an adult American Alligator (~250 cm TL, sex unknown) was observed attacking and consuming a Bonnethead Shark. Later, on 7 July 1999, the same researcher observed a sub-adult (~160 cm TL) American Alligator capture and consume a juvenile Lemon Shark in the surf zone. No photographs were obtained. On 2 August 2003, a United States Fish and Wildlife Service (USFWS) staff member observed and photographed an American Alligator (~250 cm TL) capturing, manipulating, and consuming a Nurse Shark (~100 cm TL) (Fig. 2) in an estuarine (10–35 ppt) mangrove swamp (26.471047°N, 82.151627°W; Fig. 1) within the Ding Darling National Wildlife Refuge in Sanibel, FL. Southeastern Naturalist 385 J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 On 19 September 2006 at 1528, a naturalist (J. Cooke, St. Marks, FL) while viewing foraging wading birds, observed and photographed an adult American Alligator, presumably male based on body size (~300 cm TL), capture, manipulate, and consume an adult Bonnethead Shark (Fig. 3) in a tidal salt marsh (15–35 ppt) Figure 1. Map of the southeastern United States. Circles containing a crocodile symbol are localities where American Alligators acted as predators of Elasmobranchii (n = 7 records) and circles containing a fish symbol are locations of historical accounts describing Elasmobranchii depredation of American Alligators (n = 3 records). Figure 2: Photographs of an adult American Alligator capturing (left) and manipulating (right) a Nurse Shark prior to consumption in a estuarine mangrove swamp within the Ding Darling National Wildlife Refuge in Sanibel, FL. Photographs courtesy of USFWS. Southeastern Naturalist J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 386 of St. Marks, FL (30.156747°N, 84.219081°W; Fig. 1). The observer documented the event through a series of 12 photographs. The final set of accounts were recorded in estuarine habitats (15–25 ppt) surrounding Cape Canaveral, FL (28.512411°N, 80.607415°W; Fig. 1) from 3 October 2008 to 13 January 2011. First, on 3 October 2008 during routine captures performed for ecological research, an adult female American Alligator (236 cm TL) was captured while holding a juvenile Atlantic Stingray in her jaws (28.57385°N, 80.58896°W; Fig. 4). During capture, the female released the stingray and researchers were unable to recover the remains. Subsequent captures in this general location yielded 2 other American Alligators, a female (240 cm TL, captured on 10 November 2010 at 28.61438°N, 80.66576°W; Fig. 1) and a male (295 cm TL, captured on 13 January 2011 at 28.59127°N, 80.68087°W), with stingray barbs (unidentified species, likely from Atlantic Stingrays) embedded in their jaw and neck (Fig. 5), presumably received while attacking and manipulating stingrays prior to their consumption. Depredation of American Alligators by Elasmobranchii The only reports of Elasmobranchii depredation of American Alligators originate from a series of eye-witness accounts published in newspapers and magazines in the late 1800s. First, on 5 October 1877 the sports magazine The Fishing Gazette Figure 3. Photographs of an ~3 m TL male American Alligator capturing (top-left), manipulating (top-right and bottom-left), and consuming (bottom-right) an adult Bonnethead Shark in salt marsh near St. Marks, FL. Photographs © Judy Cooke. Southeastern Naturalist 387 J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 published an article entitled “Alligator and Shark Fight”, recounting the observations of an epic skirmish between American Alligators and sharks (unknown species) in a tidal inlet near Jupiter, Florida (26.946583° N, 80.080181°W; Fig. 1). Figure 4. Photographs of an adult American Alligator captured while biting an Atlantic Stingray in the upper Banana River Estuary of Cape Canaveral, FL. Photographs © R.H. Lowers. Figure 5. Photographs of an adult American Alligator captured (top-left) with stingray barb lodged in the dorsal region of the neck (top-left and top-right, white arrow indicates location of the barb). Researchers removed the barb (bottom-left), measuring ~9 cm (bottom-right). Photographs © R.H. Lowers. Southeastern Naturalist J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 388 The observers claim a large number of American Alligators (~500 individuals) congregated within a bight in the inlet to take advantage of schools of fish trapped there by a strong flood tide (Alligators and ... 1877). A few days following this feeding aggregation, the combination of a strong northwest wind and high tide caused the water and the engorged American Alligators trapped in the bight to flow out into the main channel of the inlet. It was said that in the inlet a mass school of sharks (hundreds) had congregated, likely drawn by the smell of blood and the feeding activities of the Alligators. Once in the inlet, the Alligators were attacked by the awaiting sharks. Quoting from an eye-witness statement “…[the] sharks and alligators rise on the crest of the waves and fight like dogs”. In the days following the skirmish, carried by strong currents, numerous carcasses of both American Alligators and sharks were reported to have washed ashore along beaches extending to Cape Malabar, FL (nearly 130 km to the north). Second, on 14 May 1884 the periodical The Palatka Daily News published an account (unknown author) describing a mortal contest between a shark (300 cm TL, species not reported) and a American Alligator (210 cm TL) at East Pass near Pilot Cove, FL (30.395858°N, 86.519194°W; Fig. 1). Described therein, following multiple attempts by the shark, a mortal bite was delivered to the thoracic region of the American Alligator, severing the individual in 2 portions. Then the shark was then observed consuming one half of the American Alligator remains (A shark ... 1884). The next account was published in 1888 (unknown publisher and unknown author) in an article entitled “Sharks and Alligators: Furious duel on the coast of Florida”, which describes a lengthy, larger-than-life clash between 5 or 6 American Alligators (the largest reported to be 465 cm TL, was likely an exaggeration) and a comparable number of sharks (body size and species not reported), witnessed by a group of fisherman in the tidal Indian River, FL (28.567348°N, 80.762268°W; Fig. 1). Observers noted that during the altercation the sharks removed the forelimbs as well as portions of the tail from several of the alligators involved in the skirmish and that several of the sharks as well as alligators suffered mortal injuries. Furthermore, the author(s) of this article report interactions such as this were commonly observed during this period (Sharks and ... 1888). Discussion Crocodilians and elasmobranchs have shared foraging grounds since the appearance of Crocodylia in the Mesozoic era (~80 Ma), and evidence from the fossil record suggest these top predators have historically engaged in predatory exchanges with one another (Hill et al. 2015). Here, we discuss contemporary evidence regarding reciprocal intraguild predation between American Alligators and elasmobranchs in the southeastern United States. Recent observations expand the number of elasmobranch species depredated by American Alligators (increase from 1 to 5 species) as well as significantly extends the geographic range over which these interactions are known to occur. Southeastern Naturalist 389 J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 Depredation of Elasmobranchii by American Alligators American Alligators are the most abundant large-bodied top predator in the southeastern United States and inhabit a wide range of aquatic habitats from freshwater lakes and rivers to estuaries and nearshore marine habitats (Conant and Collins 1998, Nifong et al. 2015). The food habits and predatory interactions of American Alligators, while thoroughly studied in freshwater systems, have received comparatively little attention in estuarine and marine systems (e.g., Nifong 2016, Rosenblatt et al. 2015, Wheatley 2010). Observations reported here expand the occurrence of Elasmobranchii in the American Alligator diet by 4 species and dramatically extend the geographic range (Fig. 1) over which these interactions have been observed (i.e., one barrier island in Georgia to multiple locations along Florida’s coast into the Gulf of Mexico). Although these observations are limited, depredation of Elasmobranchii is likely ubiquitous across the range of coastalinhabiting American Alligators. These predation events potentially represent a significant source of mortality for juvenile and small-bodied elasmobranchs that occur in estuaries and nearshore marine habitats frequented by American Alligators, as these areas are known to serve as nursery grounds for multiple Elasmobranchii species (Curtis et al. 2011, Knip et al. 2010, Reyier et al. 2008). Furthermore, findings from a semiochemical bioassay study indicate juveniles of at least one Elasmobranchii species, the Lemon Shark, and likely others are not chemically aware of American Alligators, whereas chemical exudates produced by Crocodylus acutus Cuvier (American Crocodile) elicited responses (i.e., tonic immobility reversal) in 53–80% of the trials depending on the concentration of exudates applied (Rasmussen and Schmidt 1992). Thus, unsuspecting elasmobranchs may be more susceptible to depredation by marine-foraging American Alligators as compared to species of true crocodiles (Crocodylidae). Certain elasmobranchs are euryhaline, and thus are not restricted to estuarine and nearshore marine habitats. They have the ability to travel into freshwater systems for considerable distances and extended periods of time. For example, the euryhaline Atlantic Stingray (a documented prey species of American Alligators) is well known to occur in freshwater rivers throughout its’ range and has been documented to maintain permanent freshwater populations in the St. Johns River, FL (Johnson and Snelson 1996, Piermarini and Evans 1998). Similarly, Carcharhinus leucas (Müller and Henle) (Bull Shark) are well known to travel considerable distances upstream in freshwater rivers, some establishing landlocked populations (Montoya and Thorson 1982, Thorson et al. 1973). In estuaries of the Florida Everglades, the distribution of immature Bull Sharks overlaps with marine-foraging American Alligators (Rosenblatt and Heithaus 2011, Simpfendorfer et al. 2005). While no evidence exists, given the overlap in habitats used, large American Alligators foraging in these estuaries potentially consume immature Bull Sharks to some extent. The absence of direct evidence for Elasmobranchii consumption from stomach- content studies performed on coastal American Alligator populations is likely driven by several factors. First, the majority of alligators subject to stomachSoutheastern Naturalist J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 390 content examination from coastal populations have been less than 250 cm TL, and based on the reports of American Alligator predation on Elasmobranchii, it seems that generally larger adult individuals (i.e., >250 cm TL) more frequently capture and consume Elasmobranchii. Second, given the highly acidic gastric environment maintained by alligators (pH ≤ 2; Coulson and Hernandez 1964), digestion of Elasmobranchii tissues (excluding teeth and placoid scales) in the stomach is likely to be very fast (1–3 days), and therefore researchers have a very short window of opportunity to recover evidence, especially if only muscle and organ tissues are consumed. Moreover, the small size of Elasmobranchii placoid scales may prevent their collection in a sample depending on the size mesh used and also their detection even when present, unless viewed under high magnification (≥ 25X). Future stomach-content studies performed on coastal American Alligator populations should attempt to include larger (TL > 250 cm) individuals and use small (≤1 mm) mesh sieves to collect stomach effluent. In addition, contents should be carefully inspected under high magnification to aid in the detection of pl acoid scales. More broadly, considering the 24 recognized species of crocodilians that inhabit tropical ecosystems worldwide, 5 other species attain large body sizes and frequent estuarine and nearshore marine habitats where there is increased potential to interact with Elasmobranchii taxa. Specifically, the species Crocodylus acutus Cuvier (American Crocodile), C. moreletii Duméril and Bibron (Morelet’s Crocodile), C. porosus Schneider (Estuarine Crocodile), C. palustris Lesson (Mugger Crocodile), and C. niloticus Laurenti (Nile Crocodile) are potential predators of Elasmobranchii. Both historic and contemporary records exist documenting depredation of multiple Elasmobranchii taxa by the aforementioned crocodile species. Perhaps the most commonly reported are interactions between large (>300 cm TL) Estuarine Crocodiles and various Elasmobranchii taxa along the coasts of Australia. Webb and Manolis (1988) commented that large Estuarine Crocodiles in Northern Australia occasionally eat small sharks, but this claim was not substantiated with data or photographs. More recently, several online articles have been published describing and in some cases providing photographs of Estuarine Crocodiles capturing and consuming Bull Sharks (Hainke 2010, Keartes 2014). Magnusson (2015) reported the use of dead sharks (multiple taxa) as bait to lure Estuarine Crocodiles into traps. More recently, Morgan et al. (2017) examined scarring rates on juvenile Pristis pristis (L.) (Freshwater Sawfish) along the tidal Fitzroy River in Western Australia. They found nearly 60% of individuals studied had scarring from bite wounds, the majority determined to be from failed predation attempts by Crocodylus johnstoni Krefft (Freshwater Crocodile) and to a lesser extent Bull Sharks. Morgan et al. (2017) postulate that while most predatory attempts by Freshwater Crocodiles are likely unsuccessful, due to their small body size, attacks by the larger-bodied Estuarine Crocodile would probably prove fatal for juvenile Freshwater Sawfish migrating upstream from estuarine birthing grounds located in the lower portions of the river. In Florida, known nursery grounds of the critically endangered Pristis pectinata Latham (Smalltooth Sawfish) are frequented by large American Southeastern Naturalist 391 J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 Alligators, especially during the wet season (P. O’Donnel, Florida Department of Environmental Protection, Naples, FL, unpubl. data); given the propensity of predatory attacks by crocodiles on Freshwater Sawfish in Australia, further examination of the interactions between American Alligators and Smalltooth Sawfish is warranted because studies may identify an overlooked source of mortality for the juvenile life-stages of this critically endangered species. The Nile Crocodile has also been documented to consume sharks. Cott (1961) reported the stomach contents of single individual (unknown sex and body size) out of 32 individuals sampled from KwaZulu-Natal, South Africa (likely coastal locations), contained the remains of 2 unidentified shark species. While no records exist, it is reasonable to assume that both Mugger Crocodiles and the American Crocodiles consume Elasmobranchii to some degree. Mugger Crocodiles inhabit large rivers also frequented by Bull Sharks. American as well as Morelet’s Crocodiles inhabit estuarine and near-shore marine habitats known to serve as nursey grounds for multiple shark species in addition to stingray and skate species. Future stomach-content studies using the methods described for American Alligators will likely provide documentation of such interactions for these species. Depredation of American Alligators by Elasmobranchii Evidence for the depredation of American Alligators by Elasmobranchii was scant, only reported through eye-witness accounts published in a few newspaper and magazine articles of the late 1800s (which were most certainly embellished to some degree). During this period in time, it was reported these interactions were common, but it seems most were the result of random happenstances rather than directed prey pursuits by elasmobranchs. In the late 1800s, American Alligator populations were far greater in size than in recent history, and individuals likely traveled to estuarine and near-shore marine habitats more frequently and in greater numbers than they do today (Allen and Neill 1949). Two of the reported events were the product of feeding aggregations coinciding with extreme environmental conditions. Feeding aggregations are common behavior among crocodilians as well as sharks (Dinets 2015, Heupel and Simpfendorfer 2005), and these aggregations often coincide with prey migrations and various environmental extremes (i.e., drought, spring tides, floods, etc.). Thus, the reported observations, while likely embellished for theatrical effect, are plausible given the natural history and behavior of the species involved. Depredation and serious injury of other crocodilian species by Elasmobranchii are more frequently reported. Price (1877) described an altercation between what were likely Caiman crocodilus (L.) (Spectacled Caiman) and several sharks (unidentified species) in Manzanilla, Mexico. Like the events described in several accounts of American Alligator depredation, heavy rains had displaced a number of Spectacled Caiman from a coastal freshwater lake into a tidal river. Once in the river, the caiman were reportedly immediately attacked by sharks. Similar to accounts for alligators, a violent battle was said to last all day, though, in this case all Spectacled Caiman involved in the skirmish suffered mortality. In the July 1962 Southeastern Naturalist J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 392 issue of the periodical Louisiana Conservationist published by the Louisiana Wildlife and Fisheries Commission, the author of the article “What You Should Know about Sharks” describes several findings of crocodilian remains in the stomachs of large Galeocerdo cuvier (Péron and Lesueur) (Tiger Shark) captured in coastal waters of Indonesia and South Africa. Described therein, a Tiger Shark (~3 m TL) was captured somewhere off the coast of Indonesia with an entire Estuarine Crocodile (2 m TL) in its stomach. Similarly, off the coast of Durban, South Africa, a Tiger Shark (~4 m TL) contained the head and upper body portions of a Nile Crocodile along with the hind limbs of a Ovis aries L. (Domestic Sheep), 3 seagulls (undefined taxa), 2 cans of peas, and a cigarette tin. More recently, Somaweera et al. (2013) reported that hatchling and juvenile Estuarine Crocodiles as well as Freshwater Crocodiles inhabting tidal rivers in northern Australia are consumed by Tiger Sharks, Carcharhinus amboinensis (Müller and Henle) (Pigeye Shark), and Glyphis spp. (river sharks) as well as a large predatory fish, Lates calcarifer (Bloch) (Barramundi). Webb and Messel (1977) attributed numerous tail amputations observed in hatchling Estuarine Crocodiles to shark attacks. In addition, several web pages have published putative evidence for the predation of crocodiles by sharks. Nelson (2013) reported photographs and video detailing the discovery of the head of a Nile Crocodile (~3.5 m TL) on a beach in St. Lucia, South Africa. In light of the clean bite wound, apparent teeth marks, and the fact that poachers would have most certainly retained the skull as a trophy, it was postulated that only Carcharodon carcharias (L.) (Great White Shark) could have delivered such a mortal bite. In Columbia, Great White Sharks have been documented to prey upon adult American Crocodiles (Medem 1981), which attain similar body sizes to Nile Crocodiles. Both the Tiger Shark and Bull Shark (known crocodile predators) occur in coastal habitats frequented by American Alligators; these elasmobranchs can kill and consume crocodilians similar in body size to American Alligators. We suggest, stomach-content analysis of large-bodied individuals occurring sympatrically with American Alligators may help to assess the importance and prevalence of these interactions in the southeastern United States. Conclusion Crocodilians and elasmobranchs have shared foraging grounds for millions of years. While infrequent, predatory interactions between these taxa do occur. Here, we present novel observations of American Alligator depredation and putative predation of 4 elasmobranch species (i.e., Bonnet Head Shark, Lemon Shark, Nurse Shark, and Atlantic Stingray), increasing the number of known elasmobranch prey from 1 (i.e., Southern Stingray) to 5 species and significantly extending the geographical range over which these interactions are known to occur. The lack of evidence for these interactions is likely a product of the extreme gastric environment possessed by crocodilians and the failure of researchers to capture individuals within the narrow window of time post-consumption that consumed elasmobranch body portions are identifiable. Application of the techniques described herein should assist researchers in gathering additional evidence to understand the extent of these interactions. The near-shore marine and estuarine ecosystems where these Southeastern Naturalist 393 J.C. Nifong and R.H. Lowers 2017 Vol. 16, No. 3 interactions occur serve as nursey grounds for a variety of elasmobranchs including threatened and endangered species. Consumption of juveniles by American Alligators and other crocodilians worldwide likely represents an important and previously overlooked source of mortality for young and small-bodied elasmobranchs (Morgan et al. 2017). Quantification and incorporation of these interactions into predictive models will likely improve population-modeling efforts for certain elasmobranch taxa, especially threatened species that spatially overlap with crocodilians. Reciprocal depredation of American Alligators and other crocodilians by elasmobranchs appears to be of minor significance. However, multiple observations document these interactions globally, and knowledge of these interactions should be incorporated into our understanding of crocodilian natural history and ecology (Somaweera et al. 2013). 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