Environment not ‘nativeness’ dictates reproductive trait shifts in Ponto‐Caspian gobies

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Amongst the many theories explaining success of introduced species (e.g., the ‘enemy release theory’, ‘invasional meltdown’ theory), that of ‘alternative ontogenies and invasive potential’ (henceforth AOIP) has recently been put forward as regards invasional success of, amongst others, fish. The theory of alternative ontogenies (Balon 2004) suggests that environmental cues, acting mostly during early ontogeny, can shift an individual (or cohort, as members of each cohort often encounter identical conditions) along a generalised-specialised r-K (or altricial-precocial) gradient of life-history strategies. Several recent studies have widened this theory in relation to invasive biology, resulting in the AOIP theory (e.g., Z ahorsk a et al. 2013; Horkov a & Kov a c 2014). This predicts that (i) the invasive potential of a species is supported by its life-history plasticity, and (ii) that individuals in a recently established invasive population will shift to more altricial strategies (Z ahorsk a et al. 2013). The latter hypothesis predicts higher fecundity, lower size-at-maturity and smaller oocytes in non-native populations (Horkov a & Kov a c 2014). Data from both native and non-native ranges are needed to verify this hypothesis. Such comparative data are still lacking for most invasive species (Parker et al. 2013), however, despite the huge benefits of such comparisons to fundamental hypotheses and principles of invasion biology. One such ‘blank spot’ relates to reproductive traits in invasive Ponto-Caspian gobiids. Five PontoCaspian gobiid species (monkey goby Neogobius fluviatilis, racer goby Neogobius gymnotrachelus, bighead goby Neogobius kessleri, western tubenose goby Proterorhinus semilunaris and the round goby Neogobius melanostomus (Pallas, 1814)) have greatly increased their ranges over recent decades, having spread across several major European river basins, including those of the Danube (beyond their original range), Rhine and Vistula (see Roche et al. 2013 for a review). The aim of this study is to provide missing comparative reproduction data for invasive Ponto-Caspian gobies and to interpret this data in the context of AOIP. The two most commonly used reproductive trait measurements [i.e., absolute fecundity (FA), sizeat-maturity] were compared between native and nonnative populations to test whether gobies in recently invaded areas do indeed shift to a more altricial strategy. To avoid bias connected with using native-area data from published literature [which frequently describe fish sampled from marine environments where growth of gobies is accelerated (Corkum et al. 2004)], we sampled both native and non-native freshwater populations of three goby species (monkey, round and bighead goby). In each case, samples were taken from a single population. Furthermore, a single analyst determined the reproductive traits using identical methodology. Native goby populations were sampled from the Bulgarian stretch of the Danube near the town of Vidin in April 2005 (round and bighead gobies at river km 780–781 and monkey gobies at rkm 790; Table S1, Fig. 1). Non-native populations were sampled from the German Rhine near the town of Rees in April 2011 (rkm 831 for monkey gobies and rkm 844–845 for round and bighead gobies; Table S1, Fig. 1). All non-native populations were at an early stage of settlement; that is, less than five generations from first record in the Rhine (Roche et al. 2013), and both native and non-native populations were sampled before their spawning seasons. Estimated relative abundance at native and non-native sites was 0.3 and 0.58 fish m 1 of shoreline surveyed for round goby,