Northeastern Naturalist Vol. 25, No. 1 J.E. Rettig, G.R. Smith, G. Eng-Surowiac, D. Mirzashvili, M. Smyk1, Ma. Jones, and J. Hollis 2018 117 2018 NORTHEASTERN NATURALIST 25(1):117–122 Consumption of Invasive Western Mosquitofish Fry by Adult Conspecifics and Native Crayfish Jessica E. Rettig1,*, Geoffrey R. Smith1, Genevieve Eng-Surowiac1, Davit Mirzashvili1, Mallory Smyk1, Maggie Jones1, and Jeremy Hollis1 Abstract - The absence of predators is often invoked to explain the ability of non-native species to successfully invade a habitat, however native species can control or regulate populations of invasive species through predation. To better understand the regulation of invasive Gambusia affinis (Western Mosquitofish), we conducted a laboratory experiment to examine the potential for native Cambarus thomai (Little Brown Mudbug) and adult conspecifics to consume Western Mosquitofish fry. On average, female Western Mosquitofish consumed nearly 3 times the number of fry in 24 h than males. Little Brown Mudbug, consumed some of the Western Mosquitofish fry, but the resulting mosquitofish mortality was not significantly higher than the control (which had 100% survivorship). Our results show that cannibalism is a potentially significant source of mortality in Western Mosquitofish, and thus may be a factor involved in the regulation of their population dynamics. However, native Little Brown Mudbug, while they do consume mosquitofish fry, are probably not a major source of mosquitofish mortality in nature. Introduction The ability of non-native species to successfully invade a habitat is often thought to be related to an absence of predators; however, some studies have shown that native species can control or regulate populations of invasive species through predation (Coccia et al. 2014, deRivera et al. 2005, Weis 2011). It is also possible that self-regulation, perhaps in the form of cannibalism, may limit some populations of invasive species (e.g., Lohrer and Whitlatch 2002). Cannibalism in invasive populations can also be more prevalent in invasive populations than in populations of the same species in its native range (Tayeh et al. 2014). The presence of crayfish, both native and non-native, can negatively impact fish populations (see reviews Dorn and Mittelbach 1999, Reynolds 2011). Some crayfish are known to consume fish eggs (Findlay et al. 2015, Karjalainen et al. 2015, Morse et al. 2013), fish fry (Edmonds et al. 2011, Mueller et al. 2006, Renai and Gherardi 2004, Rubin and Svensson 1993), and even small adult fish (Guan and Wiles 1997, Ilhéu et al. 2007, Thomas and Taylor 2013). However, crayfish do not eat the fry of all fish species (Shu 1995). Native crayfish may be a potential predator on invasive fish, especially smaller-bodied fish, but few studies have examined the predatory impacts of crayfish in their native ranges (McCarthy et al. 2006 ). To better understand the factors that might affect populations of the invasive Gambusia affinis (Baird & Girard) (Western Mosquitofish), we conducted a 1Department of Biology, Denison University, Granville, OH 43023. *Corresponding author - rettig@denison.edu. Manuscript Editor: David Yozzo Northeastern Naturalist 118 J.E. Rettig, G.R. Smith, G. Eng-Surowiac, D. Mirzashvili, M. Smyk1, Ma. Jones, and J. Hollis 2018 Vol. 25, No. 1 laboratory experiment to examine the potential for both native crayfish Cambarus thomai Jezerinac (Little Brown Mudbug) and adult Western Mosquitofish to consume Western Mosquitofish fry. Little Brown Mudbug is an omnivorous, burrowing crayfish (Jezerinac et al. 1995, Loughman and Simon 2011), and thus, might be expected to have a minimal effect on fish relative to other crayfish. However, we have observed Little Brown Mudbug in the same habitats as Western Mosquitofish, and therefore, it may be an opportunistic predator on these fish and their fry. Given the strong negative effects that Western Mosquitofish can have on native communities (Pyke 2008), knowledge of predation on this species may help us to understand what factors may limit or affect their populations. Methods We employed dipnets or Gee’s® Minnow traps (Tackle Factory, Fillmore, NY) to collect adult male and female Western Mosquitofish and adult, inter-molt Little Brown Mudbugs from Olde Minnow Pond on the Denison University Biological Reserve, Granville, Licking County, OH, during the summer of 2014. Olde Minnow Pond is a small pond (0.60 ha) in which Western Mosquitofish was the only fish present at the time of this study (J.E. Rettig and G.R. Smith, unpubl. data). Descriptions of Olde Minnow Pond can be found in Schultz and Mick (1998) and Surace and Smith (2016). To obtain fry, we maintained 4 pregnant female Western Mosquitofish in aquaria until they gave birth. We separated from females within 12 h of parturition and maintained the fry in aerated aquaria until used in the experiment. We conducted our experiments in small plastic containers (18 cm W x 31 cm L x 12 cm H) filled with 5 L of tapwater that had been aged for 24 h. We assigned each container to 1 of the 4 treatments: control (no predator), male Western Mosquitofish (2–3 cm TL), female Western Mosquitofish (4.5–5.5 cm TL), or Little Brown Mudbugs (5–6 cm TL) and placed a single individual of the appropriate predator in each non-control treatment container. No refugia were provided in the experimental containers. We withheld food from predators for 24 h prior to the trials. We used partitions to visually isolate each container. We replicated treatments 15 times over 3 separate trial days with 5 replicates each day. To examine consumption rates, we placed 10 fry into each container and counted remaining fry after 24 h with the assumption that any missing fry had been consumed. We conducted the experiment at 21 °C and under a 12h:12h light:dark cycle. We employed a non-parametric Kruskal–Wallis test to examine the effects of predator treatment on the proportion of Western Mosquitofish fry consumed after 24 h. We used pairwise Wilcoxon tests to determine which treatments were significantly different. We conducted all analyses in JMP 10.0 (SAS Institute, Inc., Cary, NC) for all statistical tests. Results There was a significant effect of predator treatment on the proportion of fry consumed after 24 h (Fig. 1; H3 = 44.88, P < 0.0001). Female Western Mosquitofish Northeastern Naturalist Vol. 25, No. 1 J.E. Rettig, G.R. Smith, G. Eng-Surowiac, D. Mirzashvili, M. Smyk1, Ma. Jones, and J. Hollis 2018 119 treatments had the highest proportion of fry consumed, followed by male Western Mosquitofish (Fig. 1). Control and Little Brown Mudbug treatments did not differ from each other, and both had significantly lower proportions of fry consumed after 24 h than either Western Mosquitofish treatment (Fig. 1). We found that some fry had been consumed in the Little Brown Mudbug treatments; 100% of fry were recovered in all control treatments. Discussion We found that adult Western Mosquitofish were substantial potential predators of conspecific fry. In particular, female Western Mosquitofish consumed an average of almost 3 times the number of fry in 24 h than did males. The native crayfish, Little Brown Mudbug, consumed an average of 2% of the fry, but the number consumed resulted in a mosquitofish fry mortality that was not significantly different than the control (which had 100% survivorship). Our results show that cannibalism is a potentially significant source of mortality for Western Mosquitofish fry, although the laboratory conditions may overestimate the extent of cannibalism in nature (see Meffe 1986). The high proportions of fry eaten in 24 h by adult mosquitofish in our experiment, especially by the females, indicates the possibility that cannibalism could impact population dynamics in Western Mosquitofish. Our documentation of cannibalism is not unexpected. Previous studies have demonstrated that cannibalism exists in Gambusia (mosquitofish) (Dionne 1985, Hubbs 1991, Nesbit and Meffe 1993). In addition, our results suggest that female Western Mosquitofish are greater consumers of conspecific fry than males. This pattern of greater cannibalism by females has been demonstrated in Western Mosquitofish (Hubbs 1991) and Gambusia holbrooki Girard (Eastern Mosquitofish) (Nesbit and Meffe 1993). The larger size of females may facilitate consumption of the fry, as has been shown in the consumption of larval Siphateles bicolor mohavensis Girard (Mohave Tui Chub) (Henkanaththegedara and Stockwell 2013). Females Figure 1. Mean (± 1 SE) proportion of Western Mosquitofish (Gambusia affinis) fry consumed after 24 h in 4 predator treatments. Means sharing the same letter are not significantly different (pairwise Wilcoxon tests: P < 0.05). Northeastern Naturalist 120 J.E. Rettig, G.R. Smith, G. Eng-Surowiac, D. Mirzashvili, M. Smyk1, Ma. Jones, and J. Hollis 2018 Vol. 25, No. 1 also attack prey at higher rates than do males (Arrington et al. 2009, Blanco et al. 2004, Shankuntala 1977); thus, both size and attack rate may explain the higher rate of cannibalism in female Western Mosquitofish compared to males. Our experiment shows that Little Brown Mudbugs are able to consume Western Mosquitofish fry, although the consumption rate is low and mosquitofish fry survival was not significantly different from the control. To our knowledge, this is the first demonstration that Little Brown Mudbug potentially consumes fish fry. However, other species of crayfish have been observed to consume fish fry (Edmonds et al. 2011, Mueller et al. 2006, Renai and Gherardi 2004), and Procambarus clarkii Girard (Red Swamp Crayfish) can consume adult mosquitofish (Gutiérrez-Yurrita et al. 1998, Ilhéu et al. 2007). Thus, it is possible that Little Brown Mudbug may consume some Western Mosquitofish fry in natural ecosystems. However, it seems unlikely that they would be capable of controlling Western Mosquitofish populations via consumption of fry, given the low consumption rate in the laboratory which likely overestimates the natural consumption rate due to the simple environment without refugia used in the experiment and the burrowing habit of Little Brown Mudbug (Jezerinac et al. 1995, Loughman and Simon 2011). We observed Little Brown Mudbugs in the same minnow traps as adult Western Mosquitofish, suggesting that the 2 species may spatially overlap in the pond. Further experiments considering the interaction of adult Western Mosquitofish and Little Brown Mudbug might be useful to examine whether larger adults of the former may be susceptible to predation or damage by the latter. It would also be informative to examine the potential for other native crayfish, particularly species that may be more likely to interact with the fish (e.g., non-burrowing species), to consume invasive Western Mosquitofish. In conclusion, our experiment demonstrated that cannibalism, especially by female Western Mosquitofish, could be a substantial source of mortality for conspecific fry. In contrast, the low consumption of Western Mosquitofish fry by the native Little Brown Mudbug, suggests it is unlikely to be a major source of mortality for Western Mosquitofish fry in nature. It is unclear what the long-term population consequences of cannibalism are on Western Mosquitofish populations, but our results, and those of others (e.g., Meffe and Crump 1987), suggest future investigations into the role that cannibalism plays in the population biology of Western Mosquitofish would be fruitful in understanding the population dynamics of this invasive species. For example, future research might explore cannibalism in natural populations and in mesocosms to examine the conditions under which cannibalism likely occurs with high enough frequency to affect population growth. Experiments where alternative prey are provided would also help address the preference for cannibalism relative to consumption of other species. In addition, modelling studies that examine the role of cannibalism in the population dynamics of mosquitofish could contribute to our understanding of the sensitivity of mosquitofish populations to cannibalism, especially if the models consider both negative and positive effects (e.g., increased energy for adult mosquitofish) of cannibalism on population dynamics. Northeastern Naturalist Vol. 25, No. 1 J.E. Rettig, G.R. Smith, G. Eng-Surowiac, D. Mirzashvili, M. Smyk1, Ma. Jones, and J. Hollis 2018 121 Acknowledgments We thank the Anderson Endowment of Denison University for financial support. G.W. Stocker identified the crayfish predators used in our study. This experiment was approved by the Denison University IACUC. The comments of 2 anonymous reviewers helped improve the manuscript. Literature Cited Arrington, J.J., K.R.J. Thaman, J.E. 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