Introduction

Parasites make up about 40% of the earth's biodiversity and parasitism constitutes the most common type of ecological interaction [1]. Because of their effects on host population dynamics, parasites directly or indirectly influence the dynamics and structure of ecological communities [2]. One aspect of host-parasite ecology that has received considerable recent attention is the extent to which host-parasite interactions both influence and are influenced by the spread of invasive species [3], [2].

Parasites can influence the success of introduced species relative to native species in multiple ways. The effects of host-specialists are easiest to predict and have received the most attention as components of “enemy release” [4], [5]: introduced species likely leave behind specialist parasites from their native range and are likely avoided, at least initially, by specialist parasites in the introduced range due to a lack of shared evolutionary history. The dynamics involving introduced hosts and generalist parasites are more difficult to predict. Exotic hosts may introduce generalist parasites to which they have high resistance but to which native species have limited or no resistance [6], [7]. Invasive species may also be more or less susceptible to generalist parasites in the introduced range, influencing the dynamics between hosts and parasites as well as the transmission of any disease-causing organisms transmitted by generalist parasites [8], [9], [10]. For example, one longitudinal study discovered that ticks more heavily infest introduced chipmunks than native voles [11]. However, some other studies of wild-caught hosts revealed that non-native species tend to have fewer generalist parasites (external crustaceans and internal helminths) than their sympatric, native counterparts [12], [13].

Gnathiid isopods (Crustacea) are generalist ectoparasites of fish hosts in both temperate and tropical oceans [14], [15]. They are unusual among parasites of fishes in that only the larvae are parasitic (protelean parasitism). The larval phase consists of three stages (instars), ranging from approximately 0.5–3 mm in length that emerge from the substratum mostly at night and dawn and find host fish. When engorged on blood and body fluids, they return to the substratum and molt into the next larval stage. After the final blood meal, third stage larvae metamorphose into adult males or females that live in the benthos and do not feed [14], [15].

Gnathiids are of particular ecological importance on tropical reefs where they are an abundant crustacean parasite infesting a wide range of fish hosts [16], [17], [18]. They transmit protozoan blood parasites [19], [20], [21], reduce hematocrit [22] and can even be a direct cause of death in heavily-infected and/or small hosts [23], [24], [25]. Gnathiids are also the primary food item of cleaner mutualists such as cleaner wrasses in the Indo-Pacific [26] and cleaner gobies in the Caribbean [27], [28] and they influence the time hosts spend interacting with cleaning organisms [29], [30]. At sites on the Great Barrier Reef, removal of the cleaner wrasse Labroides dimidiatus, a main predator of gnathiids, has been shown to reduce growth and abundance of at least one host species [31]. Thus, the degree of susceptibility to gnathiids can have significant fitness consequences for reef-associated fishes, and can therefore influence the success of an introduced species in the coral-reef ecosystem.

While various species of marine fishes have been introduced by human activities to new habitats worldwide, our understanding of the role of parasites in these invasions lags far behind freshwater systems [32]. One marine fish in particular, the Indo-Pacific red lionfish (Pterois volitans), was listed in a recent review as one of the top 15 global conservation issues [33]. Since their accidental or intentional release from aquaria off the coast of Florida in the mid-1980s, lionfish have undergone a population explosion and now range throughout the western tropical and subtropical Atlantic, Caribbean, and Gulf of Mexico [34], [35]. The invasion has significantly affected native coral-reef ecosystems, mainly via predation of a broad diversity of juvenile fishes and crustaceans [36], [37], causing substantial reductions in the recruitment and abundance of reef fishes [38], [39], [40] and also negatively affecting native piscivorous predators via both predation of juveniles and possible competition with adults [41].

Greater local density (>390 fish/ha vs. 6 fish/ha) and maximum size (45 cm total length TL vs 38 cm TL) of lionfish in the invaded Atlantic relative to their native Pacific [42], [43] suggests some level of ecological release from natural control mechanisms, such as competition, predation, and parasitism [43], [44], [45], and the apparent inability of native reef communities to provide natural enemies that can offset invading lionfish indicates a lack of biotic resistance to the invasion [46].

The goal of our study was to test the susceptibility of lionfish to parasitic gnathiid isopods at sites in both their native Pacific and introduced Atlantic ranges, relative to other species and under natural conditions. Disentangling the effects of exposure of hosts due to differences in encounter rates and other factors that influence the probability of infestation is essential for understanding variation in the susceptibility of hosts to ectoparasites. Most studies of components of host susceptibility to ectoparasites in terrestrial [47], [48], [49], [50], [51] and aquatic environments [52], [53], [54], [55], [56], [57] are based solely on either cultured, laboratory-reared parasites, or are observational studies of parasites found on wild-caught hosts. While these studies are instructive, they do not control for differences in host habitat utilization, and often fail to control for temporal variation in the activity of ectoparasites. To our knowledge, ours is the first field experimental study to compare the susceptibility to generalist parasites of invasive and native hosts in either the invaded or native range.

Materials and Methods

Results

Introduced range of lionfish

At the Bahamas site, all 13 grunts were infested with gnathiids (range  = 1–8, mean intensity  = 3, SD = 2.45), compared with only 1 of 11 lionfish (9.1% - infested with 1 gnathiid). This inter-species difference was highly significant (Fisher Exact Test, p<0.001). Similarly, at the Cayman Islands site, 12 of 15 (80.0%) grunts were infested (range  = 1–23, mean intensity  = 6, SD = 6), compared with only 4 of 15 lionfish (26.7% - infested with 1–3 gnathiids each). This difference was also highly significant (Fisher Exact Test, p = 0.009).

At the Virgin Islands sites, 34 of 42 grunts (81.0%) were infested with gnathiids (range  = 1–222, mean intensity  = 57.68, SD = 71.58), compared with 12 of 28 lionfish (42.9%; range  = 1–23, mean intensity  = 5.92, SD = 6.36). Overall, lionfish had significantly fewer gnathiids than French grunt (F_1,62 = 5.24, p = 0.026). When compared with 7 other ecologically similar Caribbean species at the Virgin Islands sites, lionfish also had the lowest gnathiid loads (F = 10.88, df = 7,168, p<0.001; Fig. 1).

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Figure 1. Box and whisker plot of infestation of gnathiid isopods on red lionfish, Pterois volitans, compared with ecologically similar, native tropical western Atlantic species.

Levels of infestation are expressed as residuals from regression of gnathiid loads against host fish surface area and the local abundance of gnathiids as measured by the average gnathiid density on 5 French grunt “standards” placed near each host fish. Data for Caribbean species are grouped by families, with species codes given in parentheses: Haemulidae: Hs =  Haemulon sciurus (n = 24); Lutjanidae: La =  Lutjanus apodus (n = 29), Ls = L. synagris (n = 14); Serranidae: Eg =  Epinephelus guttatus (n = 26); Holocentridae: Hr =  Holocentrus rufus (n = 32); Ha =  Holocentrus adscensionis (n = 20); Scorpaenidae: Sp =  Scorpaena plumieri (n = 7); Pterois volitans (n = 30). Dashed line  =  mean, solid line  =  median, outliers are shown as single points.

https://doi.org/10.1371/journal.pone.0095854.g001

Native range of lionfish

Less than half (43.8%) of the 16 lionfish had gnathiids following dawn retrieval. Six of these had five or fewer, although one had 20 (range  = 1–20, mean intensity 4.86, SD = 6.81). In contrast, 21 of the 22 (95.5%) individuals among comparison species had gnathiids (range  = 1–50, mean intensity  = 11.32, SD = 12.21, Fig. 2). Overall gnathiid loads differed significantly among species (F = 5.37, df = 3,33, p = 0.004), with lionfish being lowest (Fig. 2).

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Figure 2. Box and whisker plot of the number of gnathiids on lionfish (n = 16) off Silliman Beach, Negros, Philippines, in comparison to native species of three other families of carnivorous reef fish (n≥6).

Dashed line  =  mean, solid line  =  median, outliers are shown as single points. Note that median for lionfish is zero.

https://doi.org/10.1371/journal.pone.0095854.g002

Discussion

At two invaded localities (the Bahamas and Cayman Islands), most red lionfish did not become infested with gnathiids, whereas most individuals of the native comparison species (haemulid grunts) did. This result could be due in part to the apparently low overall gnathiid abundance at those sites. However, even when the experiment was repeated at invaded sites with much higher abundances of gnathiids (Virgin Islands), the results were similar; a higher proportion of lionfish became infested with at least one gnathiid, but densities were far lower than on native grunts.

Two interpretations of these results are that gnathiids at these sites have a narrow range of hosts they can infest, and/or that grunts are otherwise particularly susceptible to infestation. We did not identify the species of gnathiid from hosts at the Cayman Islands site. However, at the Bahamas and Virgin Islands sites, we identified gnathiids as Gnathia marleyi [64], which has thus far been shown to feed on blood from 19 different Caribbean fish species. Among the 14 Caribbean fish species (from 8 families and 3 orders) compared by Coile and Sikkel [62] in the Virgin Islands, haemulid grunts and lutjanid snappers were the most susceptible to gnathiid infestation. By employing the same protocol as that study, using French grunts as a standard, allowed us to compare our results for lionfish with samples from their study and thus compare the susceptibility of lionfish to that of multiple native Caribbean species. Although we limited statistical comparison in this study to carnivorous species that share habitat with invasive lionfish (i.e., controlling for covariables), gnathiid loads for lionfish deployed in Lameshur Bay, St. John (where Coile and Sikkel [62] conducted their study) were also lower than all 14 native species examined in that study.

Combined, these results suggest that lionfish are not highly susceptible to infestation by generalist ectoparasitic gnathiids in the Atlantic. Is this resistance limited to Atlantic gnathiids, or even to a particular species of Atlantic gnathiid, or are lionfish relatively resistant to gnathiids in general? Our results from identical experiments in the native range of lionfish (the Philippines) are consistent with the latter hypothesis. Although our sampling in the native range was limited to one site (we also conducted a single preliminary trial of the experiment at a site in Guam, but no gnathiids were found on any host) and fewer comparison species (limited by permit restrictions), lionfish also had the lowest gnathiid loads among comparison species.

To our knowledge, there is only one published account of gnathiids infecting Pterois spp., which comes from the eastern coast of South Africa where 2 individuals of Gnathia pilosus were found infecting a single adult devil firefish, Pterois miles [65]. P. miles and P. volitans are sibling species and both inhabit the western Atlantic, although P. miles appears to be limited to the southeast coast of the United States [35]. Furthermore, cleaning symbiosis experts in the Pacific and Indian Oceans report that lionfish rarely visit cleaning stations (A. Grutter and R. Bshary, personal communications, respectively). Considering that gnathiids are a primary food source for cleaner species [27], [66], and that gnathiid burden is positively associated with host interaction with cleaners [29], [30], low visitation rates by lionfish to cleaning stations could be explained, at least in part, by low gnathiid loads in their native range.

The mechanism by which lionfish appear to be less susceptible to gnathiids is currently unknown. However, there are multiple mechanisms by which teleost fish may reduce their susceptibility to external infection that may vary among some combination of taxa, localities, and individuals. For example, fish hosts may secrete excess mucosal epithelium [67], shed their scales [68], or initiate coagulation in response to ectoparasite attachment [69]. Skin toxins may also affect the attachment of ectoparasites on fishes [70]. Lionfish do possess venom in the grooves of their dorsal, pelvic, and anal spines that likely deter predation by larger fishes [71], [72]. While the venom itself is not located on the skin or in the blood, the venom precursor is systemic (Wilcox and Hixon in revision). It would be worthwhile to determine whether this chemical deters ectoparasites.

Lionfish in the subtropical and tropical western Atlantic appear to have spread from Florida from introduced individuals native to the Philippines and Indonesia [73]. Although our findings in the Atlantic appear robust among sites, and consistent with our experiment at one native locality, it is conceivable that invasive lionfish from a different population and/or native or invasive lionfish at other localities might exhibit higher susceptibility. Furthermore, while our comparison species represented the most common ecologically similar and sympatric species, these species were from different taxonomic families, with one exception (see below). It is possible that lionfish may appear relatively more susceptible if they are compared with more phylogenetically similar species. The fact that a native tropical western Atlantic scorpionfish (S. plumieri), a sedentary species from the same family as lionfish, also exhibited relatively low susceptibility compared with other species may provide insights. While this is the most common Caribbean and tropical western Atlantic scorpaenid [74], it is nowhere locally abundant, and we found no other genera of scorpaenid fishes at our Philippines site.

By using confined fish deployed in the field during the period of peak gnathiid activity, we found that both native and invasive red lionfish do not appear highly susceptible to generalist blood-feeding gnathiid isopods, compared with other common species in either their native Pacific or introduced Atlantic range. Previous studies that compared the generalist parasites infecting introduced vs. native host species have been observational [11], [12], [13], or conducted in the laboratory [12], and therefore cannot separate the relative contributions of exposure and susceptibility. While our study system is ecologically most similar to the tick-rodent system studied by Pisanu et al. [11], our results align more closely with Miller et al. [12] and Gendron et al. [13] where non-native species tended to have fewer generalist parasites (external crustaceans and internal helminths, respectively) than their sympatric, native counterparts. Time since introduction may also play a part in the relative susceptibility of invasive species to generalist parasites. Gendron et al. [13] revealed that more recently established populations of the invasive round goby (Neogobius menalostomus) in the Great Lakes-St. Lawrence River basin have fewer generalist helminths than older populations of invasive round goby. However, a study investigating the parasite fauna of peacock grouper (Cephalopholis argus) 50 years after their introduction to the Hawaiian archipelago, found that this invasive marine fish remains largely uninfected by native generalist and ubiquitous parasites [75]. Furthermore, in a study conducted almost 40 years after the appearance of rabbitfishes (Siganus rivulatus and S. luridus) off the Mediterranean coast of northern Africa, only eight cymothoid isopod individuals (generalist ectoparasites) were found on 375 rabbitfish [76]. Further studies should assess whether this pattern will exist in the decades following the establishment of invasive lionfish in the western Atlantic.

Considerable evidence suggests that introduced species tend to arrive in their new environments free from their natural parasites [3], [77], [78], which may enable them to more effectively establish and spread in their introduced habitat. However, generalist parasites might be able to exploit invasive species [79], which could reduce the fitness of infected individuals, thereby negatively affecting the invasive species population [80]. Our results demonstrate that lionfish are not highly susceptible to infestation by the most common and damaging generalist ectoparasite on coral reefs. The relatively low numbers of gnathiids infesting lionfish in both their native and invaded ranges suggest that these ectoparasites either are not highly attracted to or cannot effectively exploit lionfish as a host, and therefore cannot be expected to negatively affect populations of invasive lionfish. However, given that the susceptibility of lionfish to gnathiids is not zero, and that gnathiids transmit blood parasites, the possibility remains that, during the initial phases of their invasion, lionfish may have been vectors for blood parasites from their native range. Although fish imported through the aquarium trade are often treated for parasites before shipment and/or upon arrival, such treatments are applied inconsistently, depending on the collector or importer, and target external parasites. Indeed, protozoan blood parasites have been found in lionfish collected from some Atlantic localities [81]. Thus, future studies of parasites in lionfish should include assays for blood parasites.

Acknowledgments

For work in the Virgin Islands, we thank the staff of the Virgin Islands Environmental Resource Station (St. John) and the MacLean Marine Science Center (St. Thomas) for logistical support and use of their facilities. We also thank Bobby Vante and Joe Sellers for providing some of the lionfish, and John Artim, Elizabeth Brill, Will Jenkins, and Hunter Owen for assisting with field experiments. Marnya Ellis, Nissa Hughes, Kayla Parker, and Bryce Shirley assisted with literature and calculations of fish surface area, and Nico Smit assisted with gnathiid species identification. For work in the Bahamas, we thank the staff of the Perry Institute of Marine Science for logistic support, and Mark Albins, Tye Kindinger, Emily Pickering, Gabe Scheer, and Kristal Ambrose for assisting with the collection of data. For work in the Cayman Islands, we thank the staff of the Central Caribbean Marine Institute at the Little Cayman Research Center for their logistic support, and Tye Kindinger and Casey Benkwitt for their extensive help in the field. For work in the Philippines, we thank Hilconida Calumpong (Director) and the staff of the Silliman University Marine Laboratory for supporting our visit there. We also thank Dioscoro Inocencio, Tina Santos, Malatapay Bantay Dagat, and Hayahay, Dumaguete, for assistance with field work and obtaining fish in the Philippines, and Rene Abesamis, Renclar Jadloc and Jennifer McIlwain for assistance with permit processing. This is contribution number 105 from the Center for Environmental and Marine Science, University of the Virgin Islands, and contribution number 2014-12 from the Department of Biology, University of Hawai'i at Manoa.