# Influence of larval and juvenile life history on age at first maturity in two tropical amphidromous fish species

Raphaël Lagarde, Nils Teichert, Henri Grondin, Thomas Hue, Philippe

Gaudin, Dominique Ponton

► To cite this version:

Raphaël Lagarde, Nils Teichert, Henri Grondin, Thomas Hue, Philippe Gaudin, et al. Influence of larval and juvenile life history on age at first maturity in two tropical amphidromous fish species. Ecology of Freshwater Fish, 2020, 29 (1), pp.63-73. 10.1111/eff.12488. hal-02291840

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# Influence of larval and juvenile life history on age at first maturity in two tropical amphidromous fish species

Lagarde Raphaël <fnref n="1" />,<fnref n="2" />,<fnref n="3" />,<fnref n="4" />, Teichert Nils <fnref n="5" />, Grondin Henri <fnref n="4" />, Hue Thomas <fnref n="6" />, Gaudin Philippe <fnref n="7" />, Ponton Dominique <fnref n="3" />

## Abstract :

This study investigates how age at first maturity of two tropical amphidromous species Sicyopterus lagocephalus (Pallas, 1770) and Cotylopus acutipinnis (Guichenot, 1863) varies in relation to their larval and juvenile life history. Reproductive stage was estimated based on histological observation of ovaries of more than 200 females of each species caught monthly over 1 year. The age of fish was estimated by interpreting the daily increments deposited on otoliths during the oceanic larval phase, and the juvenile phase in the river. The age at first maturity was approximately 9 months for S. lagocephalus and 7 months for C. acutipinnis, corresponding to approximately 70-130 and 90-130 days after they returned to freshwater respectively. For both species, the time spent in freshwater before maturity was significantly influenced by the duration of the pelagic larval stage at sea (PLD) and the season of return in freshwater. Individuals with a long PLD, or returning in freshwater during the warmer season, maturated faster once in freshwater. This reproductive advantage may minimise the risk of extirpation due to catastrophic events at each generation and thus probably benefits amphidromous species living in very unpredictable tropical rivers.

**Keywords** : Cotylopus acutipinnis, Diadromy, Gobiidae, reproductive strategy, Sicyopterus lagocephalus

<footnote n="1">¹ Centre de Formation et de Recherche sur les Environnements Méditerranéens, UMR 5110, Université de Perpignan Via Domitia, Perpignan, France</footnote>

<footnote n="2">² CNRS, Centre de Formation et de Recherche sur les Environnements Méditerranéens, UMR 5110, Perpignan, France</footnote>

<footnote n="3">³ ENTROPIE, IRD, Université de La Réunion, CNRS, Université de Perpignan Via Domitia, Laboratoire d'Excellence CORAIL, Perpignan, France</footnote>

<footnote n="4">⁴ Hydro Reunion, La Reunion, France.</footnote>

<footnote n="5">⁵ UMR 7208 BOREA (Biologie des Organismes et Ecosystèmes Aquatiques), Sorbonne Université, MNHN, CNRS, UMPC, Université Caen, Univ Antilles Guadeloupe, IRD - Station Marine de Dinard - CRESCO, Dinard, France</footnote>

<footnote n="6">⁶ Centre IRD de Nouméa, ENTROPIE, IRD, Université de La Réunion, CNRS, Laboratoire d'Excellence CORAIL, Nouméa, France</footnote>

<footnote n="7">⁷ ECOBIOP, INRA, UPPA, Aquapôle INRA, Saint Pée sur Nivelle, France</footnote>

<footnote>* Corresponding author : Raphaël Lagarde, email address : raphael.lagarde@univ-perp.fr</footnote>
Age at first maturity (AFM) is one of the four major life-history traits, along with mortality rate, fecundity and size at first maturity, describing fish species life-history strategies (He and Stewart, 2001). The way AFM varies indicates how species react to natural and anthropogenic disturbances (Winemiller and Rose, 1992). Fish species with an opportunistic strategy (sensu Winemiller and Rose, 1992) are characterized by a low AFM and a continuous reproductive effort. These opportunistic species have strong (re)colonizing capacities and are expected to be less sensitive to frequent and stochastic disturbances (Winemiller and Rose, 1992). However, within a given species, AFM can vary among individuals as it depends on other life history traits. The study of AFM variability among fish population (phenotypic plasticity) has been the focus of many studies due to its implication on ecological and evolutionary process (Beverton, 1992) and fisheries management (Olsen et al., 2004). Most of these studies however described the AFM plasticity at large spatial (i.e., regional) and temporal (i.e., pluri-annual) scales (Silva et al., 2006). However, Goto (1989) described plasticity of AFM for a Cottidae between different localities along a 17 km watershed. Describing the variability in AFM relates to different aspects of an individual's life history may be valuable to better understanding a range of strategies fish species can develop.
Tropical fish species represent an interesting model to study intra-seasonal variability of AFM due to their potentially extended reproductive season. The fish fauna inhabiting the rivers of small tropical islands is mainly composed of species migrating between the freshwater and sea (McDowall, 1997). Most of these species are amphidromous (Keith, 2003): they reproduce in freshwater, their larvae drift to the sea where they grow for several months before returning to freshwater as post-larvae or young juveniles that grow and become adults in freshwater (McDowall, 1988). Amphidromous fish successively inhabit two different environments where seasonal variations of different parameters, especially temperature, impact their biology. For example, Teichert et al. (2012, 2016a) demonstrated that larval growth of two amphidromous gobies, *Sicyopterus lagocephalus* (Pallas, 1770) and *Cotylopus acutipinnis* (Guichenot, 1863), was positively correlated to sea water temperature. After they returned to freshwater, seasonal variation of the size at first maturity was observed for both species: individuals matured at a smaller size during the summer months (Teichert et al., 2014a, 2016b).

In Reunion Island, *S. lagocephalus* and *C. acutipinnis* dominate the freshwater fish assemblages. These two species grow and reproduce in sympatry from the estuary to >600 m above sea level (Teichert et al., 2013b, 2014b). *Sicyopterus lagocephalus* is a cosmopolitan species that recently colonized numerous islands from the eastern Pacific Ocean to the western Indian Ocean (Keith et al., 2005a). In contrast, *C. acutipinnis* is endemic to the Mascarenes Archipelago (Keith et al., 2005b). Females of both species are multiple spawners and have similar fecundity of 13,000 to 15,000 eggs per gram of female (Teichert et al., 2014a, 2016b). Females lay eggs under cobles and boulders located in the lotic areas of the streams (Teichert et al., 2013b) and the males guard the nest until the larvae hatch (Teichert et al., 2013a). *Sicyopterus lagocephalus* and *C. acutipinnis* initiate their ovarian growth when water temperature increase above 18 and 19°C respectively (Teichert et al., 2014a, 2016b). The reproductive season is limited to the few months when water temperature is above these
78 thresholds in middle and upstream reaches when it can be year-round in downstream reaches.

79 The life span of these species in Reunion Island is unknown but it can be estimated to reach
80 several years based on the observations of seven years old *Sicyopterus japonicus* individuals
81 observed in the Ota river, Japan, by Iida et al. (2013). This assumption is also confirmed by
82 Bell (2009) who kept a single *Sicydium punctatum* individual 16.5 years in captivity. The AFM
83 of *S. lagocephalus* and *C. acutipinnis*, and its variation in relation to other parameters of an
84 individual's life history, is unknown.

85 In this context, the present study first confirmed the daily periodicity of otolith increment
86 formation in freshwater in both species. It then aimed to describe the AFM of *S. lagocephalus*
87 and *C. acutipinnis* based on histological observation of ovarian samples and interpretation of
88 age from daily otoliths increments. The final objective was to determine how the duration of
89 the larval and the juvenile stages influenced the AFM of each species.

## 90 Materials and methods

91 *Validation of daily increment deposition in sagittal otoliths*

92 Post-larvae of *S. lagocephalus* (N = 240) and *C. acutipinnis* (N = 140) were collected
93 immediately after their return to freshwater, in December 2012 and March 2014 respectively,
94 at the mouth of Saint Etienne River (Fig. 1) using a portable electro-shocker (Hans Grassel
95 IG 200-2). Fish were immediately transported to the laboratory in two aerated 10 l buckets filled
96 with water from the river. Individuals of *S. lagocephalus* (N = 120 per tank) were reared in two
97 1000 l tanks from December 2012 to February 2013 when individuals of *C. acutipinnis* (N = 70
98 per tank) were reared in two 200 l tanks from March 2014 to April 2014. They were fed with
99 periphyton-covered cobbles that were collected in the same river. The cobbles were renewed
100 every three days to provide sufficient food. *Sicyopterus lagocephalus* were reared for 70 days
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whereas *C. acutipinnis* were reared for 40 days because of the increasing difficulty to feed
individuals as they grew larger. The water temperature during the rearing period was kept at
25±1°C, a temperature regularly observed in Reunionese rivers (Teichert et al., 2014a), and the
lighting conditions were 12 h light and 12 h dark.

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On arrival at the laboratory, all individuals were immersed in a 30 l aerated tank containing a
100 mg.l⁻¹ solution of alizarin Red-S (ARS) for 12 h. This immersion produced the initial
otolith mark (M₀) before fish rearing. A second mark (M₁) was produced using the same
protocol after 14 and 12 days of rearing, and the third mark (M₂) after 28 and 29 days of rearing
for *S. lagocephalus* and *C. acutipinnis* respectively. A fourth mark (M₃) was only produced for
*S. lagocephalus* after 62 days of rearing. At the end of their rearing period, fish were all
euthanatised with an overdose of eugenol and preserved in 95% ethanol.

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Among the reared individuals, 28 *S. lagocephalus* and 33 *C. acutipinnis* were randomly
selected and their sagittal otoliths were extracted under a binocular microscope. Two techniques
can be used for aging fish based on otoliths microstructure: interpreting the microstructure of
the whole otolith or after first sectioning the otoliths. Interpreting the microstructure of the
whole otolith is much faster, and has been proven accurate for larvae and small juveniles
including Sicydiinae gobies (Teichert et al. 2012, 2016a). However, this technique may become
inaccurate when fish become older such that interpreting the structure of a sectioned otolith is
necessary (Sponaugle, 2009). The two different techniques were used to verify that the daily
periodicity of increment deposition in otoliths was validated. For interpreting the structure of
the whole otolith, 23 sagittae of *S. lagocephalus* and 27 sagittae of *C. acutipinnis* (left or right
sagittae were selected randomly) were placed in a drop of immersion oil, examined at 400X
through a transmitted light microscope (Olympus CX41) and photographed with a microscope
camera (Olympus C-5060). The location of the ARS marks was determined by examining and
photographing the otoliths under UV light with the same microscope fitted with a fluorescent

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attachment (Olympus U-RFLT50). Estimation of the number of increments between each mark was performed with IMAGE J 1.47 software (Rasband, 2015). To interpret the structure of sectioned otoliths, an additional 5 sagittae for *S. lagocephalus* and 7 for *C. acutipinnis* were embedded in Crystal-Bond and ground using grinding discs (Struers, France) of decreasing grades (800 and 1200 grains.cm⁻²) to expose the core. Sectioned otoliths were examined, photographed and analyzed as described previously.

The validation of the daily periodicity of increments deposition in sagittal otoliths was performed using linear regression between the estimated number of increments and the known number of rearing days. If the slope and intercept of the regression did not differ significantly from 1 and 0 respectively, the daily deposition of increments was validated (Panfili et al., 2002).

The technique for interpreting the structure of the otolith (i.e., whole or sectioned otolith) was included as a qualitative factor in the regression, and its significance was tested with a test of deviance reduction using the *F* statistic.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Location of the Saint Etienne and Marsouins Rivers, Réunion Island, Western Indian Ocean.</figcaption></figure>

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Fish were collected from a mid-stream reach of Marsouins River (2.2 km from the river mouth; Fig. 1) every month between January and December 2011. The water temperature was continuously recorded every 20 mn with Gemini Data Loggers Tinytag Aquatic 2 (precision 0.5 °C). A minimum of 50 point-abundance samples (Persat and Copp 1990) were performed by electrofishing (portable electro-shocker, Hans Grassel IG 200-2). Females <54 mm total length (L<sub>T</sub>) and <39 mm, which are the maximum sizes at first maturity observed over a year for *S. lagocephalus* and *C. acutipinnis* respectively (Teichert et al., 2014a, 2016b), were collected. In the field, females were identified based on the form of their urogenital papilla. In the laboratory, each female was classified as immature or mature based on histological observations of its ovarian tissues performed following the methods described in Teichert et al. (2014a). Females were considered as immature when all oocytes in the histological section were at a primary growth stage, with no evidence of past reproduction (e.g. presence of atresia or post-ovulatory follicle, thick ovarian wall). Conversely, the histological sections showing oocytes at different development stages (e.g. cortical alveoli, vitellogenic or hydrated oocyte) or showing evidence of previous spawning events were assigned to mature females (Teichert et al. 2014a, 2016b).

Paired sagittal otoliths were extracted from each female, and one sagitta (randomly left or right) was embedded in Crystal-Bond and ground using grinding discs (Struers, France) of decreasing grades (800 and 1200 grains.cm⁻²) to expose the otolith core. Sagittae were examined, photographed and analysed following the protocol described above. In Sicydiinae, the transition between sea and freshwater is characterised by a metamorphosis check mark (i.e. a structural discontinuity on the otolith; (Shen and Tzeng, 2002)). For otoliths of fish captured in freshwater, the pelagic larval duration (PLD) can be estimated by counting the number of increments between the core and the check mark. Similarly, the freshwater duration (FD)
corresponds to the number of increments between the check mark and the edge of the otolith.
FD subtracted from the date of capture allows estimation of the date of return to freshwater and
the sum of PLD and FD (i.e., the age of the individual) subtracted from the date of capture
allows estimation of hatch date.

Estimation of PLD and FD were made by an operator who did not know the sampling date or
fish length. A subsample of 60 sagittae for each species was also examined blindly by a second
operator (without knowing the results of the first operator, the sampling date, or the fish length).
The PLD and FD estimated by each operator were compared using orthogonal regressions. If
the slope and intercept of the regression between the estimations made by the two operators
were not significantly different from 1 and 0 respectively, it was assumed that estimations of
PLD and FD were consistent between the two operators.

## Statistical analysis

The age at first maturity was estimated using logistic general linear models (GLM). Maturity
was considered as a binary variable (immature/mature), the age of the females being the
explanatory variable. Age at first maturity was considered to be the age when 50% of the
females were mature (A50).

To investigate if the date of return to freshwater influenced when females became mature, the
time in freshwater until 50% of the females were mature (FD50) was estimated for three different
periods. These four-month periods were defined according to the variation in mean water
temperature observed at the sampling site during the 30 days before sampling (Table 1). The
three periods also correspond to the maximum reproductive season, the minimum reproductive
season and the beginning of the reproductive season based on observations made on ovaries
and drifting larvae (Teichert et al., 2014a, 2016b; Lagarde et al., 2017). Maturity of females,
considered as a binary variable (immature/mature), was explained using logistic GLM with FD,
the period of return in freshwater and the interaction between FD and the period of return in freshwater as explanatory variables. Similar analyses were performed to investigate if the date of return to freshwater influenced the size at which 50% of the females were mature ($L_{T50}$).

Finally, the effect of individual PLD on $FD_{50}$ was described using logistic GLM with FD and the interaction between FD and PLD as explanatory variables. For all GLMs the significance of explanatory variables was tested with a test of deviance reduction using the $Chi^2$ statistic. All statistical analyses were performed with the open source R v. 3.3.1 software (R Development Core Team, 2016)

<table><caption>**Table 1.** Sampling dates, corresponding mean daily water temperature (and range) during the 30 days before sampling (T30, °C), number of fish sampled (N) and range of total length ($L_T$, mm) for each species. The three periods of return to freshwater which were used for grouping the fish in the analyses are specified.</caption><thead><tr><th rowspan="2">Sampling date</th><th rowspan="2">T<sub>30</sub> (range)</th><th rowspan="2">Period of return to freshwater</th><th colspan="2"><i>S. lagocephalus</i></th><th colspan="2"><i>C. acutipinnis</i></th></tr><tr><th>N</th><th>Range of L<sub>T</sub></th><th>N</th><th>Range of L<sub>T</sub></th></tr></thead><tbody><tr><td>18 January 2011</td><td>21.6 (20.2-22.5)</td><td>Maximum temperature –</td><td>10</td><td>32-52</td><td>14</td><td>26-39</td></tr><tr><td>16 February 2011</td><td>21.6 (20.9-22.6)</td><td>maximum of</td><td>18</td><td>33-50</td><td>19</td><td>25-39</td></tr><tr><td>18 March 2011</td><td>22.0 (21.3-22.9)</td><td>reproductive</td><td>12</td><td>36-52</td><td>31</td><td>25-39</td></tr><tr><td>20 April 2011</td><td>21.2 (20.6-22)</td><td>season</td><td>16</td><td>35-54</td><td>29</td><td>26-39</td></tr><tr><td colspan="2"><strong>Total period #1</strong></td><td></td><td><strong>56</strong></td><td><strong>32-54</strong></td><td><strong>93</strong></td><td><strong>25-39</strong></td></tr><tr><td>18 May 2011</td><td>20.1 (19.2-21.2)</td><td>Decreasing temperature –</td><td>21</td><td>34-54</td><td>24</td><td>26-38</td></tr><tr><td>15 June 2011</td><td>18.9 (18.3-19.4)</td><td>minimum</td><td>21</td><td>29-54</td><td>16</td><td>28-39</td></tr><tr><td>13 July 2011</td><td>18.4 (16.9-19.3)</td><td>reproductive</td><td>15</td><td>32-53</td><td>11</td><td>23-39</td></tr><tr><td>17 August 2011</td><td>18.1 (17.6-18.8)</td><td>season</td><td>19</td><td>31-54</td><td>10</td><td>26-39</td></tr><tr><td colspan="2"><strong>Total period #2</strong></td><td></td><td><strong>76</strong></td><td><strong>29-54</strong></td><td><strong>61</strong></td><td><strong>23-39</strong></td></tr><tr><td>14 September 2011</td><td>18.5 (17.7-19.2)</td><td>Increasing temperature –</td><td>19</td><td>31-53</td><td>15</td><td>27-39</td></tr><tr><td>12 October 2011</td><td>18.7 (18.1-19.3)</td><td>beginning of the</td><td>15</td><td>33-52</td><td>17</td><td>27-37</td></tr><tr><td>16 November 2011</td><td>20.2 (19.2-21.7)</td><td>reproductive</td><td>21</td><td>32-53</td><td>22</td><td>22-39</td></tr><tr><td>14 December 2011</td><td>21.2 (19.9-21.9)</td><td>season</td><td>18</td><td>33-52</td><td>15</td><td>24-38</td></tr><tr><td colspan="2"><strong>Total period #3</strong></td><td></td><td><strong>73</strong></td><td><strong>31-53</strong></td><td><strong>69</strong></td><td><strong>22-39</strong></td></tr><tr><td colspan="2"><strong>Total</strong></td><td></td><td><strong>205</strong></td><td><strong>29-54</strong></td><td><strong>223</strong></td><td><strong>22-39</strong></td></tr></tbody></table>
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# Results

209

## Validation of daily increments deposition in sagittal otoliths

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For all sagittae (N = 61), the location of the metamorphosis check mark superimposed on the initial ARS mark M0. The technique for interpreting the structure of the otoliths (i.e. whole or sectioned) did not influence the estimated FD ($F_{1,58} = 0.25$; $P = 0.62$ for *S. lagocephalus* and $F_{1,60} = 0.58$; $P = 0.45$ for *C. acutipinnis*). For *S. lagocephalus* the mean estimated number of increments corresponded to the number of rearing days between M₀ and M₁ and between M₀ and M₂ (Table 2). However, the mean estimated number of increments was slightly lower than the number of rearing days between M₀ and M₃. The intercept of the regression between the estimated number of increments and the number of rearing days (from M₀ to M₃) was not significantly different from 0 ($t_{58} = -1.3$, $P = 0.11$) whereas the slope was significantly different from 1 ($t_{58} = 4.2$, $P < 0.001$). When only considering the rearing period between M₀ and M₂, the intercept and slope of this regression were not significantly different from 0 ($t_{32} = -0.4$, $P = 0.69$) and 1 ($t_{32} = 0.1$, $P = 0.91$) respectively. For *C. acutipinnis*, the mean estimated number of increments corresponded to the number of rearing days between M₀ and M₁ and between M₀ and M₂ (Table 2). The linear regression between the estimated number of increment and the number of rearing days confirmed these results with the intercept and the slope not significantly different from 0 ($t_{60} = 1.4$, $P = 0.16$) and 1 ($t_{60} = 1.0$, $P = 0.31$) respectively.
<table><caption>**Table 2.** Mean number (#) of rearing days, of estimated increments (± standard deviation, SD) and summary of the linear regressions performed between M₀ and M₂ and M₀ and M₃ for *S. lagocephalus* and between M₀ and M₂ for *C. acutipinnis*.</caption><thead><tr><th rowspan="2">Mark</th><th colspan="3">S. lagocephalus</th><th colspan="3">C. acutipinnis</th></tr><tr><th>Mean # rearing days</th><th>Mean # of increments ± SD</th><th>Linear regression</th><th>Mean # rearing days</th><th>Mean # of increments ± SD</th><th>Linear regression</th></tr></thead><tbody><tr><td>M₁</td><td>14</td><td>13.9 ± 0.3</td><td>-</td><td>14</td><td>13.9 ± 0.3</td><td>-</td></tr><tr><td>M₂</td><td>28</td><td>27.8 ± 1.0</td><td>intercept = -0.24<br>slope = 1.00<br>adjusted R² = 0.98</td><td>14</td><td>13.9 ± 0.3</td><td>intercept = 0.89<br>slope = 0.97<br>adjusted R² = 0.95</td></tr><tr><td>M₃</td><td>62</td><td>60 ± 1.7</td><td>intercept = -0.61<br>slope = 1.04<br>adjusted R² = 0.99</td><td>-</td><td>-</td><td>-</td></tr></tbody></table>

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## Estimation of age at first maturity

Between 18 January 2011 and 14 December 2011, the mean water temperature during the 30 days before the sampling ranged from 18.1 to 22.0 °C. A total of 205 *S. lagocephalus* (29-54 mm *L*<sub>T</sub>) and 223 *C. acutipinnis* (22-39 mm *L*<sub>T</sub>) were captured (Table 1). For both species, the slope and intercept of the orthogonal regressions between the estimations of PLD and FD made by the two operators were not significantly different from 1 and 0 respectively. The intercepts ± 95% confidence interval (CI) ranged from -3.6 ± 10.4 to 3.1 ± 5.5 and the slopes from 0.98 ± 0.05 to 1.03 ± 0.05. Moreover, the Pearson correlation coefficients of these regressions (R²) ranged from 0.95 to 0.98 highlighting a good estimation consistency between the two operators. It was concluded that the values of PLD and FD estimated by the two operators were not significantly different, and a single operator analysed the 308 remaining otoliths.

The age of the females ranged from 124-392 days for *S. lagocephalus* and from 123-316 days for *C. acutipinnis* (Fig. 2). The age at first maturity (A₅₀) ± standard error (SE) estimated by the logistic GLMs was 276 ± 8 days (df = 203, Chi² test p < 0.001) for *S. lagocephalus* (Fig. 2a) and 219 days ± 2 days (df = 221, Chi² test p < 0.001) for *C. acutipinnis* (Fig. 2b).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Logistic GLM estimating the probability of maturity regarding to the age for females of *a) Sicyopterus lagocephalus* and *b) Cotylopus acutipinnis*. The grey dashed lines represent the age when 50% of the females were mature (A50), Nm and Ni the number of mature and immature females respectively. The gray shaded area represents the range of the predictions ± standard error.</figcaption></figure>

The FD of females ranged from 8-196 days for *S. lagocephalus* and from 16-216 days for *C. acutipinnis*. The period of return to freshwater significantly influenced the time in freshwater for females of each species to become mature (Table 3). The lowest FD50 (± SE) was observed
for females of *S. lagocephalus* returning between January and April (FD<sub>50</sub> = 70 ± 5 days, Fig. 3a). For *C. acutipinnis*, the lowest FD<sub>50</sub> was observed for females returning between September and December (FD<sub>50</sub> = 90 ± 7 days, Fig. 3b). The highest values of FD<sub>50</sub> (130 ± 8 days for *S. lagocephalus* and 130 ± 6 days for *C. acutipinnis*) were observed for females of both species returning between May and August. *Sicyopterus lagocephalus* females returning between September and December had a FD<sub>50</sub> of 90 ± 10 days, but *C. acutipinnis* females returning between January and April had a FD<sub>50</sub> of 120 ± 9 days. Similar patterns were observed for the size at first maturity (*L*<sub>T50</sub>) for both species (Supplementary materials S1 and S2). However, the variability of *L*<sub>T50</sub> was more limited compared to FD<sub>50</sub> with a *L*<sub>T50</sub> (± SE) ranging between 45 ± 2 mm and 52 ± 2 mm for *S. lagocephalus* and between 30 ± 1 mm and 35 ± 2 mm for *C. acutipinnis*.

**Table 3.** Summary of deviance reduction on logistic GLM estimating the probability of maturity with individual freshwater duration (FD), period of return to freshwater (F_return) and the interaction of FD and F_return as explanatory variables. The significance of explanatory variables was tested with a test of deviance reduction using the Chi² statistic.

<table><thead><tr><th>Species</th><th>Variable</th><th>Deviance</th><th>d.f</th><th>P-value</th></tr></thead><tbody><tr><td rowspan="4">S. lagocephalus</td><td>Null</td><td>246</td><td>204</td><td>-</td></tr><tr><td>FD</td><td>178</td><td>203</td><td>&lt;0.001</td></tr><tr><td>F_return</td><td>130</td><td>201</td><td>&lt;0.001</td></tr><tr><td>FD x F_return</td><td>129</td><td>199</td><td>0.7</td></tr><tr><td rowspan="4">C. acutipinnis</td><td>Null</td><td>297</td><td>222</td><td>-</td></tr><tr><td>FD</td><td>157</td><td>221</td><td>&lt;0.001</td></tr><tr><td>F_return</td><td>124</td><td>219</td><td>0.001</td></tr><tr><td>FD x F_return</td><td>121</td><td>217</td><td>0.2</td></tr></tbody></table>

274

275
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Logistic GLM estimating the probability of maturity regarding to the freshwater duration (FD) and the period of return to freshwater (i.e. September to December, January to April and May to August) for females of **a) Sicyopterus lagocephalus** and **b) Cotylopus acutipinnis**. The grey dashed lines represent the freshwater duration when 50% of the females were mature (FD<sub>50</sub>) for each period, N<sub>m</sub> and N<sub>i</sub> the number of mature and immature females for each period respectively. The gray shaded area represents the range of the predictions ± standard error.</figcaption></figure>

The PLD of the females ranged from 86-289 days for *S. lagocephalus* and from 66-171 days for *C. acutipinnis* (Fig. 4). For both species, the FD<sub>50</sub> decreased significantly with PLD (Table 4, Fig. 4). This result indicates that the longer a fish stayed in the ocean as larvae, the faster they matured once returned to freshwater. For example, *S. lagocephalus* with a PLD of 100 days had a FD<sub>50</sub> of 150 days but when their PLD increased to 250 days, their FD<sub>50</sub> decreased to 70 days. Similarly, *C. acutipinnis* with a PLD of 70 days had a FD<sub>50</sub> of 140 days, but when their PLD increased to 160 days, their FD<sub>50</sub> decreased to 100 days. For *S. lagocephalus*, FD<sub>50</sub> decrease by 53% when PLD increased by 150%, these values are respectively 29% and 129% for *C. acutipinnis*.
<table><caption>**Table 4.** Summary of deviance reduction on logistic GLM estimating the probability of maturity with the freshwater duration (FD) and the interaction between FD and pelagic larval duration (PLD) as explanatory variables. The significance of explanatory variables was tested with a test of deviance reduction using the Chi² statistic.</caption><thead><tr><th>Species</th><th>Variable</th><th>Deviance</th><th>d.f</th><th>P-value</th></tr></thead><tbody><tr><td rowspan="3"><i>S. lagocephalus</i></td><td>Null</td><td>246</td><td>204</td><td>-</td></tr><tr><td>FD</td><td>178</td><td>203</td><td>&lt;0.001</td></tr><tr><td>FD x PLD</td><td>151</td><td>202</td><td>&lt;0.001</td></tr><tr><td rowspan="3"><i>C. acutipinnis</i></td><td>Null</td><td>297</td><td>222</td><td>-</td></tr><tr><td>FD</td><td>162</td><td>221</td><td>&lt;0.001</td></tr><tr><td>FD x PLD</td><td>150</td><td>220</td><td>&lt;0.001</td></tr></tbody></table>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Freshwater duration when 50% of the females were mature (FD<sub>50</sub>, black lines) on which are superimposed the pelagic larval duration (PLD) and freshwater duration (FD) of matures (black dots) and immatures (open dots) females of **a) Sicyopterus lagocephalus** and **b) Cotylopus acutipinnis**. The FD<sub>50</sub> was estimated with a logistic GLM describing the probability of maturity regarding to the FD and the interaction between the FD and the PLD.</figcaption></figure>

Discussion

*Otolith daily increments in freshwater*

This study demonstrated the daily periodicity of increment deposition in otoliths of *S. lagocephalus* and *C. acutipinnis* after they returned to freshwater. This observation is consistent with two other studies that validated the daily periodicity of increment deposition during the larval period of *Sicyopterus* species (Iida et al. 2010; Hoareau et al., 2007). For
S. lagocephalus, a slightly lower number of increments compared to the known number of days reared in aquaria was observed for individuals kept more than 30 days in captivity. This discrepancy may be explained by the decrease in the fish growth rate that made increments more difficult to detect. The reduced growth rate of individuals kept in aquaria during similar experiments has been explained by the difficulties to feed the fish in the laboratory (Siegfried and Weinstein, 1989) and the stress caused by handling or by ARS marking (Iida et al., 2010).

In the natural environment, it is likely that the growth rate is more elevated than in captivity as more food is probably available and the stress caused by handling and ARS marking inexistent.

As a consequence, the daily increments are easier to detect over a longer period in freshwater.

## Influence of individual life history on age at first maturity

The time spent by females of *S. lagocephalus* and *C. acutipinnis* in freshwater before they become mature varied depending on the period when they returned to freshwater. At first maturity, *S. lagocephalus* females had a PLD equal to or greater than their FD₅₀: PLD ranged from 130-200 days when FD₅₀ ranged from 70-135 (Fig. 5a). Oppositely, for *C. acutipinnis* females, the relative length of their PLD and FD varied depending on the period they returned to freshwater. Females returning earlier in the year had a PLD shorter than FD₅₀ but the PLD of those returning later became longer, and their FD₅₀ became shorter (Fig. 5b). As a result, for each period of return, the ratio between PLD and FD₅₀ was more elevated for *S. lagocephalus*, ranging from 1.0-2.7, than for *C. acutipinnis* (0.7-1.2). Despite the low number of mature females *S. lagocephalus* which returned to freshwater from May to August, the great differences of FD₅₀ observed between each period are unlikely due to a sampling bias. These differences can be explained by the environmental conditions, particularly water temperature, that they encountered in freshwater. Indeed, *S. lagocephalus* and *C. acutipinnis* initiate their ovarian growth when water temperature increase above 18 and 19°C respectively (Teichert et al., 2014a, 2016b). Fish returning at the beginning of or during the reproductive season (i.e.,
336

between September and April) had enough time to initiate ovarian growth during the current
spawning season (Fig. 5). Fish returning between May and August did not have enough time to
initiate ovarian growth before the water temperature decreased below the 18-19°C threshold.

337

338

339

Females returning in freshwater when water temperature was low had to pursue their somatic
growth until water warmed up at the beginning of the next austral summer. At the
biogeographical scale, the role played by freshwater temperature on age at first maturity may
explain why *S. lagocephalus* and *C. acutipinnis* reach maturity much earlier, 9 and 7 months
respectively, than *Sicyopterus japonicus* (Tanaka 1909) which reach maturity after 2 years (Iida
et al., 2013). The temperate *S. japonicus* reproduces in summer only (Iida et al., 2013, 2015)
and the juveniles return in freshwater 7-9 months after hatching (Iida et al., 2015). These
juveniles do not have enough time to mature before water temperature drops. As a consequence,
their maturation is delayed until the next summer, when environmental conditions become more
favorable.

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344

345

346

347

348
<figure><img src="image_6.png" /><figcaption>b) *C. acutipinnis*</figcaption></figure>

<figure><img src="image_7.png" /></figure>

**Figure 5.** Schematic diagram representing the life history of a) *Sicyopterus lagocephalus* and b) *Cotylopus acutipinnis* from hatching to the first reproductive event based on three periods of return to freshwater: May to August (open symbols), September to December (greys symbols) and January to April (black symbols). The period of hatching (square symbols) is estimated based on the median PLD when the period of the first reproductive event (circular symbols) is estimated based on the FD₅₀ for each periods of return to freshwater (triangular symbols). The horizontal black lines represent the estimated ranges.

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357
358

Independently of the seasonal variations previously described, females of both species that experienced a longer PLD matured earlier in freshwater (i.e., had a shorter FD₅₀). The dynamic of maturation for both species spans a continuous gradient from spending a long period at sea and maturing quickly after returning to freshwater, to spending less time at sea and waiting for a longer period of time in the river before maturing. The energetic storage accumulated by fish before maturation is usually allocated to growth and/or maturation (Martin et al., 2017).

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361

362

363

364

*Sicyopterus lagocephalus* and *C. acutipinnis* display a large range of PLD; the individuals with a longer PLD generally return to freshwater at larger size (Teichert et al., 2012, 2016a).

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366

Moreover, the Fulton condition factor tends to increase with PLD, especially for *S. lagocephalus* (R. Lagarde, unpublished data). The energetic storage of fish with a longer PLD is thus probably higher, explaining their more rapid maturation once they return to freshwater.

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368

369

370

The variation of AFM observed in the present study is most likely due to phenotypic plasticity.

371

Growth trajectory is usually described as being the main factor affecting phenotypic plasticity of AFM (Duponchelle and Panfili, 1998, He and Stewart, 2001). For amphidromous fish, growth trajectory is complex due to the influence of marine and freshwater biotic and abiotic conditions on the different phases of the growth. The estimation of the growth trajectory of an individual would particularly require the determination of its size when returning to freshwater.

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Indeed, previous studies demonstrated that the size range of individuals returning to freshwater varies between 26 and 37 mm *L*<sub>T</sub> for *S. lagocephalus* (Teichert et al., 2016a) and between 17 and 23 mm *L*<sub>T</sub> for *C. acutipinnis* (Teichert et al., 2012). This size variability at arrival to freshwater may also influence the growth trajectory of the juveniles in the rivers. This relationship between the size at return to freshwater of an individual and its growth rate as juvenile remains to be studied. Indeed, small individuals at return to freshwater may grow faster (Gagliano and McCormick, 2007), or slower (Thia et al., 2018), during their juvenile stage,

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382
<figure><img src="image_8.png" /></figure>

depending of the species they belong to, and probably the local conditions they encounter. The
estimation of fish size at arrival to freshwater could be back-calculated using the relationship
between the distance from the otoliths' nucleus to the metamorphosis check mark and the fish
length. This approach would require extensive validations and was thus beyond the scope of
this present study. However, our results highlighted that the plasticity of AFM in Sicydiinae
gobies and their complex amphidromous life cycle make them an interesting model for further
studies focused on phenotypic plasticity.

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

Interspecific differences in reproductive strategy

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /></figure>

<figure><img src="image_24.png" /></figure>

<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /></figure>

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /></figure>

<figure><img src="image_30.png" /></figure>

<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /></figure>

The *S. lagocephalus* spawning strategy of spending a longer period in the ocean and a shorter period of maturation in freshwater can contribute to explain its success in Reunionese rivers compared to *C. acutipinnis*. In Reunionese rivers, the abundance of *S. lagocephalus* is estimated to be twice that of *C. acutipinnis* (Teichert et al., 2014b). After returning to freshwater, juvenile Sicydiinae gobies are subjected to inter- and intra-specific competitive interactions with adults and juveniles. These interactions are exacerbated by the territorial behaviour of the males (Fitzsimons and Nishimoto, 1990) as they select their spawning habitat (Teichert et al., 2013b). The longer PLD of *S. lagocephalus* is probably associated with a higher risk of mortality during its marine stage compared to *C. acutipinnis*. However, this risk of mortality is probably more elevated during the first few days of the larval growth as observed in many other species (Houde, 1997). In this context, the difference of mortality associated with the longer PLD of *S. lagocephalus* compared to *C. acutipinnis* is probably limited. The longer PLD of *S. lagocephalus* is also associated with a larger size of post-larvae when they return to freshwater (Teichert et al., 2012) compared to *C. acutipinnis* (Teichert et al., 2016a). Because they are larger, *S. lagocephalus* juveniles are probably more competitive and are thus more likely to spawn in the first spawning habitats encountered (Teichert, 2012). The spawning habitats that are closer to the estuary are suggested to support greater eggs production and larvae
per m² as they allow the fish to have a longer reproductive season, and therefore more spawning events can occur per individual compared to upstream reaches (Artzrouni et al., 2014, Teichert et al., 2014a). Moreover, their proximity to the sea greatly reduces the drifting time of larvae after hatching, and thus the associated risk of mortality (Bell, 2009). These competitive advantages and the rapid maturation of *S. lagocephalus* can thus contribute to explain its higher abundances in Reunionese rivers. Interestingly, the shortest FD₅₀ of *S. lagocephalus* (70 days) was observed during the maximum of the reproductive season which is also the period when the risk of cyclonic floods is higher. A rapid maturation of the females during this period of the year may limit the risk of being entrained by cyclonic floods.

The successful colonisation of new environments by amphidromous species depends mainly on their capacity to disperse widely and on the competitive effectiveness of their strategies, including their reproductive strategy, when reaching new environments (McDowall, 2010). The longer PLD of *S. lagocephalus* is usually associated with high dispersal ability (Lord et al., 2010). This hypothesis was used to explain the relatively recent colonization by this species of numerous islands from eastern Pacific Ocean to western Indian Ocean (Keith et al., 2005a). The results of this study suggest that the longer PLD of *S. lagocephalus* also influences its reproductive strategy due 1) a larger size when returning to freshwater, and 2) a faster maturation compared to the endemic *C. acutipinnis*. The long PLD of *S. lagocephalus* thus appears pivotal in its recent and widespread colonisation enhancing both its dispersal capacity and reproductive success.

Acknowledgments

This study was conducted with the financial support of Electricité De France (22-30 avenue deWagram, 75382 Paris Cedex 8), Office de l'Eau Réunion (49 rue Mazagran, 97,400 Saint Denis), Région Réunion (Avenue René Cassin, BP 7190, 97719 Saint Denis cedex 9), Parc
432 national de La Réunion (112 rue Sainte Marie, 97400 St Denis) and the European Union
433 (European Social Fund). We are very thankful to all the persons who assisted us during the
434 study and especially Agathe Chateauminois. We are also very grateful to the three anonymous
435 referees for useful comments on a previous version of this paper. This study was conducted
436 under the following permit: Direction de l'Environnement de l'Aménagement et du Logement
437 de La Réunion, Permit N°11-002/DEAL/SEB.

438 Conflict of Interest

439 The authors declare no conflicts of interest.

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**Figure S1.** Logistic GLM estimating the probability of maturity regarding to the total length ($L_T$) and the period of return to freshwater (i.e. September to December, January to April and May to August) for females of **a) Sicyopterus lagocephalus** and **b) Cotylopus acutipinnis**. The grey dashed lines represent the total length when 50% of the females were mature ($L_{T50}$) for each period, $N_m$ and $N_i$ the number of mature and immature females for each period respectively. The gray shaded area represents the range of the predictions ± standard error.

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<figure><img src="image_33.png" /></figure>

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<table><caption>**Table S2.** Summary of deviance reduction on logistic GLM estimating the probability of maturity with individual total length ($L_T$), period of return to freshwater ($F_return$) and the interaction of $L_T$ and $F_return$ as explanatory variables. The significance of explanatory variables was tested with a test of deviance reduction using the $Chi^2$ statistic.</caption><thead><tr><th>Species</th><th>Variable</th><th>Deviance</th><th>d.f</th><th>P-value</th></tr></thead><tbody><tr><td rowspan="4">S. lagocephalus</td><td>Null</td><td>246</td><td>204</td><td>-</td></tr><tr><td><i>L</i><sub>T</sub></td><td>114</td><td>203</td><td>&lt;0.001</td></tr><tr><td>F_return</td><td>95</td><td>201</td><td>&lt;0.001</td></tr><tr><td><i>L</i><sub>T</sub> x F_return</td><td>90</td><td>199</td><td>0.09</td></tr><tr><td rowspan="4">C. acutipinnis</td><td>Null</td><td>297</td><td>222</td><td>-</td></tr><tr><td><i>L</i><sub>T</sub></td><td>153</td><td>221</td><td>&lt;0.001</td></tr><tr><td>F_return</td><td>96</td><td>219</td><td>0.001</td></tr><tr><td><i>L</i><sub>T</sub> x F_return</td><td>80</td><td>217</td><td>0.001</td></tr></tbody></table>

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