# Independent impacts of age and telomere length on oyster sperm motility

Andréaz Dupoué, Arnaud Huvet, Carole Di Poi

► To cite this version:

Andréaz Dupoué, Arnaud Huvet, Carole Di Poi. Independent impacts of age and telomere length on oyster sperm motility. Royal Society Open Science, 2026, 13 (1), pp.251845. 10.1098/rsos.251845. hal-05523170

HAL Id: hal-05523170

https://hal.science/hal-05523170v1

Submitted on 23 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
<figure><img src="image_2.png" /></figure>

Research

<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

Check for
updates

**Cite this article:** Dupoué A, Huvet A, Di Poi C. 2026 Independent impacts of age and telomere length on oyster sperm motility. *R. Soc. Open Sci.* **13**: 251845.

https://doi.org/10.1098/rsos.251845

Received: 24 September 2025

Accepted: 4 December 2025

**Subject Category:**
Organismal and evolutionary biology

**Subject Areas:**
molecular biology, ecology

**Keywords:**
ageing, gamete, reproduction, shellfishes, spermatozoa, telomeres

**Author for correspondence:**
Andréaz Dupoué
e-mail: andreaz.dupoue@ifremer.fr

# Independent impacts of age and telomere length on oyster sperm motility

Andréaz Dupoué<fnref n="1" />, Arnaud Huvet<fnref n="1" /> and Carole Di Poi<fnref n="1" />,<fnref n="2" />

<figure><img src="image_5.png" /></figure>

Aging often goes with the deterioration of gamete quality. However, alteration of male sperm quality alongside age is not univocal between species, and this relationship remains largely elusive in bivalve molluscs. Here, we questioned whether ageing attenuates sperm motility and if telomere length (TL), a marker of individual quality, can further mark sperm performance in the Pacific oysters (*Crassostrea gigas*). We assessed covariation between sperm motility, age and TL in males (n = 45) from 2 to 9 years old. We replicated the measures at four reproductive sessions, corresponding to two types of maturation conditions (controlled in the lab or natural in the field). We found negative correlations between male age and sperm velocity, and slower sperm in natural conditions compared to controlled ones. TL did not shorten with age, but it was positively associated with sperm velocity while maturing under controlled conditions. Our results underscore that sperm swimming performance decreases with ageing, a relationship identified for the first time in a bivalve species. Further research is needed to understand environmental effects and the predictive aspects of telomere dynamics in covarying with sperm quality.

## 1. Introduction

When animals grow old, reproductive performance usually decreases [1]. This so-called reproductive senescence is illustrated by progressive deterioration of reproductive investment in old breeders and negative impacts of parental ages on offspring. For instance, offspring may suffer deleterious impacts from advanced parental ages, such as lower growth rate, immune breakdown and, overall, shorter longevity [2]. This phenomenon has been documented in a large number of taxa [3], but the mechanisms remain largely unclear. Among cellular

<footnote n="1">¹LEMAR, UMR 6539, Ifremer, CNRS, UBO, IRD, Plouzané, France</footnote>

<footnote n="2">²SECOPOL, UMR 241, Ifremer, UPF, IRD, ILM, Vaira'o, French Polynesia</footnote>
alterations carrying deleterious effects across generations are the deterioration of the gametes in ageing individuals, which includes a drop in global gamete count and meiotic dysfunctions due to molecular modifications (e.g. epigenetic changes, DNA fragmentation, aneuploidy) [4,5]. However, in males, the senescence of sperm is not univocal between animal species [6]. This lack of inter-species consistency thereby requires investigations on several non-model animals.

In bivalve molluscs, the impacts of age on sperm quality remain surprisingly elusive. To date, sperm motility of shellfishes has been essentially explored regarding the influence of external factors (e.g. seawater temperature, pH and pollutant) [7–9] and the resistance of sperm to cryoconservation for aquaculture purposes [10]. Conversely, intrinsic characteristics related to individual reproductive quality remain misunderstood. During spermatogenesis, many molecular processes occur, including telomere elongation, which resets the innate telomeric capital passed to the offspring [11,12]. Telomeres are nucleoproteins with repeated DNA motifs (5'-TTAGGG-3')*n*, that cap linear chromosomes and protect nuclear genomes [13]. Telomere length (TL) decreases after each cell division, so without repairing activity of telomerase, shortening TL represents one hallmark of ageing [14]. Because telomere shortening has also been associated with many physiological stresses and to deleterious intergenerational effects, TL is supposedly an ageing mediator of semen quality and risk of infertility [11]. Yet, this general pattern may not apply to adult bivalves, which do not necessarily experience marked telomere shortening with age [15], including species that live for centuries [16].

In this study, we aimed at deciphering the effects of age, environment and TL on sperm swimming motility in the Pacific oyster (*Crassostrea gigas*). This species reproduces once a year, with gonad maturation and gametogenesis starting in mid-spring during phytoplankton blooms and seawater warm-ups [17]. During this period, males cohabit with females, and once the temperature reaches a threshold of 18–19°C for several days in mid-summer, the individuals release their gametes in the water column [17–19]. The Pacific oyster is a sequential hermaphrodite, with a tendency towards protandrous sex determination [20]. Meaning that young individuals are more likely to produce sperm, whereas gamete synthesis will progressively unbalance to female oogenesis in older oysters. Therefore, chronological age may influence male semen quality and reproductive success. In support of this, recent investigations in *C. gigas* confirmed that sire age strongly affects offspring development, including negative impacts on telomere dynamics early in life [15]. In addition, oyster sperm quality varies seasonally, with reduced motile sperm in late autumn [21]. We therefore hypothesize that male oysters may undergo sperm senescence, and that both maturation environment and TL could influence sperm performance. We tested this hypothesis by comparing sperm motility, a marker of fertilizing ability [22–24], in males aged 2–9 years and maturing either under controlled conditions or in the field. In addition, we measured TL in both a somatic tissue (gills) and in germinal cells (sperm) to assess intra-individual TL correlation and to determine whether TL represents a reliable predictor of sperm motility across tissue types.

## 2. Material and methods

### 2.1. Reproductive males and maturing conditions

All adult males used in this study were captured at the juvenile life stage ('oyster spats') when it is possible to know their age (6-month-old). Spats were obtained at the same locality each year on the French coastline (Aix Island, France: 46.0121° N; 1.1609° W) from 2014 to 2021. Although this single geographic origin could have led to a specialized phenotypic response based on genetic background, wild populations of *C. gigas* exhibit poor genetic diversity and structure in Southern Europe because most populations are also derived from multiple introductions of oysters spats obtained from this same origin around Arcachon and Marennes d'Oléron [25,26]. Individuals sampled here should thereby be representative of mixed genotypes from the south of Europe. Each year, collected oysters were regrouped by cohort (one cohort equals one year); they were transferred to an intertidal tideway (Aber Benoit, France: 48.5751° N; 4.6051° W) and maintained there for years in mesh bags (100 × 50 × 5 cm) to reach sexual maturity and grow old. This field site remains too cold for natural oyster spawning and settlement, and it is therefore used for aquaculture purposes and maximizing oyster growth and energy storage [27]. Indeed, averaged seawater temperature remains below 17°C, even in summer, due to strong currents and high tidal range of the English Channel [28]. As a result, natural spawning is greatly prevented throughout the year, since spawning typically requires several consecutive days above 18–19°C [17–19]. In our study, oysters matured either in controlled conditions (in the laboratory)
<table><caption>**Table 1.** Summarized male oysters' traits between the four reproductive sessions, gathered in two types of maturing conditions. Table reports means ± s.d. of oyster age, shell length, gill TL and sperm TL and the absence of difference in oyster traits between reproductive sessions.</caption><thead><tr><th rowspan="3">oyster traits</th><th colspan="2">controlled maturation</th><th colspan="2">field maturation</th><th rowspan="3">statistical effects of<br>reproductive<br>session</th></tr><tr><th>session 1: March<br>2023</th><th>session 2: March<br>2024</th><th>session 3: early<br>Sept 2023</th><th>session 4: late<br>Sept 2023</th></tr><tr><th>(n = 12)</th><th>(n = 11)</th><th>(n = 13)</th><th>(n = 9)</th></tr></thead><tbody><tr><td>age (years)</td><td>4.25 ± 2.11</td><td>3.96 ± 2.04</td><td>4.36 ± 1.26</td><td>4.43 ± 1.97</td><td>F<sub>3,41</sub> = 0.1, p = 0.941</td></tr><tr><td>shell length (cm)</td><td>10.48 ± 2.89</td><td>10.54 ± 1.88</td><td>10.53 ± 1.45</td><td>12.09 ± 1.50</td><td>F<sub>3,41</sub> = 1.4, p = 0.247</td></tr><tr><td>gill TL (z-score)</td><td>-0.49 ± 0.65</td><td>-0.15 ± 1.02</td><td>0.05 ± 0.93</td><td>-0.05 ± 1.00</td><td>F<sub>3,41</sub> = 0.8, p = 0.491</td></tr><tr><td>sperm TL (z-score)</td><td>0.59 ± 0.90</td><td>-0.08 ± 0.91</td><td>0.26 ± 0.81</td><td>0.11 ± 1.64</td><td>F<sub>3,41</sub> = 0.8, p = 0.500</td></tr></tbody></table>

or in field conditions as described below. Importantly, individuals exhibited comparable traits of interest (age, shell length, gill TL and sperm TL) across maturation conditions and reproductive sessions (table 1).

Half of the oysters (n = 23 males) were sampled from Aber Benoît in winter (February 2023 and February 2024) and brought to the laboratory to mature under controlled conditions. Controlled conditions consisted of exposing oysters in 3001 seawater tank for five weeks under food-enriched regimen and stable physicochemical environment (see all detailed zootechnical conditions in [27]). Here, sperms were sampled shortly after maturation, so sperm retention lasted for less than two weeks. During this controlled maturation, oysters were exposed to a UV-treated 1 µm filtered natural seawater temperature of 17°C and a balanced mix of phytoplankton of *Tisochrysis lutea* and *Chaetoceros sp.* provided *ad libitum* at a daily ratio close to 12% dry weight algae/dry weight oyster.

The second half of the oysters (n = 22 males) was left in the field and sampled in early and late September (table 1). In this habitat, natural maturation is supported by the spring phytoplankton bloom, but gametes can never be released because seawater temperature never reaches the spawning threshold. As a consequence, field maturation involves natural environmental variability and a relatively prolonged sperm retention period (>2 months), since spermatogenesis starts in May–June and ends with partial or complete absence of spawning [29]. Here, sperm was collected the day after bringing oysters back from the field.

Oysters were sacrificed the day of reproduction; sex was visually determined under binoculars (presence of oocytes or sperms; operational sex ratio approx. 1 male : 10 females) and male sperm was collected by gonad stripping as described before [30]. From 400 oysters aged 2–9 years (8 cohorts), we separated males from females and isolated sperm from n = 45 males. We rinsed oyster milt with sterilized seawater and filtered sperm on a sieve (diameter: 60 µm) to remove any gonad tissue waste. We collected two aliquots (2 × 1 ml) of sperm in seawater, one for sperm motility analysis and one that was flash frozen in liquid nitrogen and kept at –70°C until TL assays. Concomitantly, we dissected 10–15 mg of somatic tissue (gills) that were immediately flash frozen and stored at –70°C until telomere assays.

## 2.2. Sperm performance assays

We used a microscope-camera (Olympus BX51, Japan; ×20 magnification, dark field; Camera Qicam Fast 1394) system to measure sperm swimming performance as described previously [23]. We assessed sperm motility at room temperature (20 ± 1°C) after diluting 450 µl of sperm solution to adjust the concentration in 500–700 µl of sterilized seawater containing 100 µl of pluronic acid at 10 g l⁻¹. Within an hour post-activation in seawater, aliquots (14 µl) of the sperm solution were placed in FastRead cells (Fischer ScientificVR, USA) for video acquisition. This timing ensures reliable assay since sperm remains performant for over 20 h post-activation [23]. We recorded an average of 5 videos per male (5 s each at 60 frames per second) as recommended for this species [7]. Videos were processed with freely available OpenCASA plugin developed for ImageJ software [31]. We considered four characteristics of sperm trajectories, including straight line velocity (VSL, in µm s⁻¹), average path velocity (VAP, in µm s⁻¹), curvilinear velocity (VCL, in µm s⁻¹) and linearity coefficient (LIN) [31].
<figure><img src="image_6.png" /><figcaption>**Figure 1.** Inter-individual relationship between TL measured in germline (sperm) and somatic tissue (gills). Regardless of maturing conditions (orange: controlled, green: field), sperm TL correlates positively with gill TL ($r^2 = 0.47, p < 0.001$). The dashed line ($y = x$) illustrates that TLs are longer in sperm than in gills for 72% of individuals ($n = 45$).</figcaption></figure>

## 2.3. Telomere length assays

We measured TL from gills (10–15 mg) and sperm (100 µl; approx. $10^6–10^7$ spermatozoa per male). We extracted DNA following a protocol optimized for this species. Gills and sperm were lysed at 45°C for 1 h in 500 µl of lysis buffer and 15 µl of proteinase K from *Tritirachium album* (20 mg ml$^{-1}$; SigmaAldrich). We treated samples with 3 µl of RNase (Promega) for 15 min at 37°C followed by 200 µl of protein precipitation (Promega). We ultracentrifuged samples at 26 900g in order to isolate DNA from supernatant and to precipitate DNA with isopropanol. We pelleted DNA at 1370g and washed it with 70% ethanol. Ethanol was removed and DNA was resuspended in 50 µl of TE buffer. Following extraction, we verified DNA integrity with electrophoresis (1% agarose gel) and checked concentration and purity (A260/A280: 1.84 ± 0.01; A260/A230: 2.19 ± 0.11) on a NanoDrop 2000 (Thermo Scientific).

We ran qPCR from 5 ng of DNA by measuring real-time fluorescence on a Bio-Rad CFX96 Touch (Bio-Rad, USA) to amplify telomere regions using Sybr Green Supermix (BioRad, California, USA) and universal telomere primers (Tel1 and Tel2), and corrected by the amplification of a reference gene (glyceraldehyde-3-phosphate dehydrogenase—GAPDH) as described previously [15]. Samples were measured in duplicates with high repeatability (telomeric DNA: $r = 0.98, p < 0.001$; GAPDH: $r = 0.99, p < 0.001$) and we used a DNA positive control (pool of 30 samples) on each plate to smooth interplate variation and a non-template control to verify the absence of contamination. Relative TL was determined using gene efficiency ($E_{Telo} = 99.3\%$ and $E_{GAPDH} = 105.4\%$) obtained from the serial dilution of the positive control and Cq differential for each gene as follows:

$$TL = \frac{1.99^{Cq_{Telo(POOL)}} - Cq_{Telo(Sample)}}{2.05^{Cq_{GAPDH(POOL)}} - Cq_{GAPDH(Sample)}}$$

On each plate, we measured three samples three times to assay intraplate repeatability of TL measures ($r = 0.88, p < 0.001$) and we used four individuals on all plates to determine interplate repeatability of TL ($r = 0.92, p < 0.001$). Final individual TL was z-scored within each plate to ease model convergence in statistical analyses and removed TL variation between plates (function *scale_by*, package *standardize*).
<figure><img src="image_7.png" /><figcaption>**Figure 2.** Multivariate profiles of sperm performance measures in the two maturing conditions (orange: controlled, green: field and 95% confidence ellipses). We used the coordinates along the main axis of this PCA as an integrative score of sperm velocity (VSL: straight line velocity, VAP: average path velocity, VCL: curvilinear velocity, see [31] for details). The second axis is related to the linearity of sperm trajectory (LIN) and did not covary either with age or TL.</figcaption></figure>

## 2.4. Statistical analyses

We used linear mixed models using RStudio software [32] and *lmer* functions (package: lme4, lmerTest) using maximum likelihood to analyse data. Models were ranked by AIC with the *dredge* function (package MuMln) and selected for ΔAIC < 2 and parsimonious degree of freedom. We also estimated the statistical power of fixed effects from selected models (package simr) using the *powerSim* function based on 1000 simulations.

First, we explored the variables shaping variation in TL. We tested the effects of oyster age (in years), adjusted oyster length (residuals of relationship between length and age), tissue (gills or sperm), maturing conditions (controlled or field) and first-order interactions. We used a linear model to assay the inter-individual correlation between sperm and gill TL, and extract residuals of this relationship as an index of TL elongation in sperm (see below).

We then tested the variables explaining the sperm performance. We used a principal component analysis (PCA; package ade4 and factoextra) to check the co-variation between VSL, VAP, VCL and LIN. We ran three model comparisons specific to each TL measurement (gills, sperm and residual sperm gain).

# 3. Results

## 3.1. Factors influencing variation in TL

TL differed between tissues, with sperm TL being longer than gill TL ($\beta$ = 0.4 ± 0.1, *p* = 0.001, mean statistical power: 0.91; *figure 1*). TL did not correlate either with age, nor with maturing conditions and any interaction terms (all *p* > 0.073).
<figure><img src="image_8.png" /><figcaption>**Figure 3.** Additive effects of male traits (n = 45) and environmental maturing conditions (orange: controlled, green: field) on sperm velocity (means ± s.e. of male PC1 scores inverted to ease interpretation). (A) Male age and field conditions negatively impacted sperm swimming performance. (B) Gill TL predicted sperm velocity only when males matured under controlled conditions (solid line) and (C) Sperm elongation of TL does not explain variation in sperm speed (dashed lines).</figcaption></figure>

## 3.2. Factors influencing variation in sperm performance

PCA regrouped three indexes of velocity while the linearity coefficient appeared orthogonal to sperm speed (figure 2). We considered the coordinate along the main axis to extract an integrative score of sperm velocity (VSL, VAP and VCL), and we used the opposite value (x · -1) to get a positive measure of velocity and ease interpretations (figure 2).

Sperm velocity decreases with male age ($\beta$ = -0.34 ± 0.09, *p* < 0.001, mean statistical power: 0.97; figure 3A), and was slower when maturing in the field than under controlled conditions ($\beta$ = -1.31 ± 0.33, *p* < 0.001; figure 3A). The influence of gill TL on sperm motility depended on maturing conditions (interaction term: $\beta$ = -1.0 ± 0.4, *p* = 0.011, mean statistical power: 0.78), since gill TL positively correlated with sperm motility in controlled maturing conditions ($\beta$ = 0.62 ± 0.26, *p* = 0.023; figure 3B), but not in field conditions ($\beta$ = -0.3 ± 0.2, *p* = 0.175; figure 3B). Of note, the explicative power of sperm TL was similar but slightly lower than gills (marginal r² = 0.33 versus 0.35) and residual sperm TL gain (residuals from the relationship between sperm and gill TL; figure 1) did not explain variation in sperm performance (figure 3C).

## 4. Discussion

In this study, we explored whether age, TL and/or maturing conditions can shape sperm motility. Despite a limited sample size and an uneven distribution of oyster ages, statistical powers remain in good standards (>0.78) to interpret results. First and foremost, results confirm that ageing oysters experience a deterioration of sperm motility. Shellfish experience similar hallmarks of ageing to vertebrates [33]. For instance, older Pacific oysters experience many physiological changes, including DNA damage responses, fatty acid oxidation and mitochondrial dysfunctions [15], which are likely to mechanistically alter semen quality [34]. Supporting this first evidence of sperm senescence in a non-model bivalve species, we posit reproductive mode, here protandrous hermaphrodism, may selectively contribute to favour greater sperm motility in first breeding years before switching to oogenesis. It is particularly relevant for aquatic species with external fertilization, whereby sperm velocity in the water column is determinant to ensure individual reproductive success. Indeed, external fertilizers, relative to species with internal fertilization, may experience greater variability in reproductive success and greater sensitivity to environmental stressors, leading to profound evolutionary consequences on gamete biology [35]. Our sampling was cross-sectional, so decreased sperm motility in old males might reflect the selective disappearance of young ones that invested more in spermatogenesis earlier in life and died (e.g. live fast–die young strategy) [36]. This alternative hypothesis calls for a longitudinal survey of sperm motility on the same individuals over time, preferably in tagged oysters, including record holders for longevity (>20 years old).

This study also shows that telomeres elongate in sperm as compared to a somatic tissue, in this case oyster gills. It is noteworthy that telomere dynamics may be tissue-specific, but in other bivalves, gill TL usually correlate with other somatic tissues such as mantle, muscle or digestive glands [16,37].
Telomere elongation in gonads during spermatogenesis occurs in many species, including humans, and contributes to reset viable TL over generations [11,12] and likely results from the enhanced activity of telomerase in germline tissues [15]. In addition to the age effect, gill or sperm TL correlated with sperm motility in controlled conditions, which may confirm the general outcome that TL is predictive of animal quality and intergenerational effects in bivalve [15]. This is also concomitant with other studies across taxa, especially in humans, where longer sperm TL correlates with better sperm quality or reproductive outcomes [11]. However, findings appear inconsistent across species, methods and contexts. For example, in the guppy (*Poecilia reticulata*) and in the Australian-painted dragon lizard (*Ctenophorus pictus*), the relationship can even reverse, reflecting potential trade-offs between reproductive effort and telomere maintenance [38,39]. Such context dependence is further supported by our study where sperm motility is altered by the interaction between TL and maturing conditions. Indeed, prolonged sperm retention in the field environment obviously altered both sperm motility and the relationship between velocity and TL.

Understanding the main differences between experimental and field conditioning at the Aber Benoît field site would enable us to identify new avenues of research to further investigate this environmental effect. In temperate regions, Pacific oysters exhibit a seasonal reproductive cycle related to environmental parameters as such: initiation of gametogenesis in winter when water temperature is low; active phase of gametogenesis in spring when water temperature increases; maturity and spawning in summer, when temperature is above 19°C [17,19]. This general pattern is clearly modulated by the availability of food in the environment, gamete production being higher in eutrophic environments as it is directly linked to energy from food [17,19]. Ultimately, the main constraint of the Aber Benoît coastal area is that field oysters exhibit an incomplete reproductive cycle [29] and prolonged sperm retention [29], with partial or no spawning at all given that water temperature never reaches the thermal threshold [17,19]. Gonad re-absorption, an energy-saving process of gametic degeneration, constitutes a profound molecular and cellular disturbance of gametes [30]. Here, the gametes collected in September at Aber Benoît were late in the season when gonad resorption had already begun [29], which probably altered sperm quality and its proxies, as observed in oysters [21] and fishes [40]. Sampling males at different stages of sperm retention will help disentangle the effects of reproductive state on sperm quality and on correlations with oyster TL. Given that we did not measure genetic distances between individuals, we cannot exclude whether particular genotypes also explained variation in TL, sperm motility or both. This assumption remains unlikely due to genetic homogeneity of wild populations in southern Europe [25,26]. Further research in diverse field conditions is now required to confirm that TL does also predict sperm motility regardless of environmental imprinting.

To conclude, our study confirms two important features of sperm biology, including a deterioration related to oyster age and a positive association with TL mediated by environment. Although our results highlight one potential mechanism of reproductive senescence, it is now essential to link sperm TL with additional measures of cellular quality (e.g. ATP production, mitochondrial performance, oxidative stress) together with fertilization success in mass spawning [23]. After larval development, the genotyping assignment of paternity to offspring would clarify the relative contribution of each male to the population and help to decipher how parental effects and cumulative stress and telomere dynamics can be conveyed through generations [15,41].

**Ethics.** In France, research on mollusc bivalves does not require ethical approval or permits.

**Data accessibility.** Data and codes used in this paper are available at Zenodo [42].

**Declaration of AI use.** We have not used AI-assisted technologies in creating this article.

**Authors' contributions.** A.D.: conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, visualization, writing—original draft, writing—review and editing; A.H.: conceptualization, data curation, investigation, methodology, writing—review and editing; C.D.P.: conceptualization, data curation, investigation, methodology, writing—review and editing.

All authors gave final approval for publication and agreed to be held accountable for the work performed therein.

**Conflict of interest declaration.** We declare we have no competing interests.

**Funding.** This study benefited from financial supports of the IFREMER and the ISblue project, Interdisciplinary graduate school for the blue planet (ANR-17-EURE-0015) under 'TeloRepro' project.

**Acknowledgements.** We thank Hugo Koechlin, Matthias Huber, Jacqueline Le Grand and Moussa Diagne, who were dedicated to field work, oyster maintenance and oyster sexing. We are grateful to Dominique Hervio-Heath, Pauline Merrien and Christine Dubreuil, who helped in the establishment of the protocol to extract DNA. We finally thank Tom Edward Rainbow for English revision.
References

1. Lemaitre JF, Gaillard JM. 2017 Reproductive senescence: new perspectives in the wild. *Biol. Rev.* **92**, 2182–2199. (doi:10.1111/brv.12328)

2. Monaghan P, Metcalfe NB. 2019 The deteriorating soma and the indispensable germline: gamete senescence and offspring fitness. *Proc. R. Soc. B* **286**, 20192187. (doi:10.1098/rspb.2019.2187)

3. Ivimey-Cook ER, Shorr S, Moorad JA. 2023 The distribution of the lansing effect across animal species. *Evolution* **77**, 608–615. (doi:10.1093/evolut/qpac045)

4. Secomandi L, Borghesan M, Velarde M, Demaria M. 2022 The role of cellular senescence in female reproductive aging and the potential for senotherapeutic interventions. *Hum. Reprod. Update* **28**, 172–189. (doi:10.1093/humupd/dmab038)

5. Klutstein M, Gonen N. 2023 Epigenetic aging of mammalian gametes. *Mol. Reprod. Dev.* **90**, 785–803. (doi:10.1002/mrd.23717)

6. Sanghvi K, Vega-Trejo R, Nakagawa S, Gascoigne SJL, Johnson SL, Salguero-Gómez R, Pizzari T, Sepil I. 2024 Meta-analysis shows no consistent evidence for senescence in ejaculate traits across animals. *Nat. Commun.* **15**, 558. (doi:10.1038/s41467-024-44768-4)

7. Boulais M, Demoy-Schneider M, Alavi SMH, Cosson J. 2019 Spermatozoa motility in bivalves: signaling, flagellar beating behavior, and energetics. *Theriogenology* **136**, 15–27. (doi:10.1016/j.theriogenology.2019.06.025)

8. Tallec K et al. 2021 Amino-nanopolystyrene exposures of oyster (*Crassostrea gigas*) embryos induced no apparent intergenerational effects. *Nanotoxicology* **15**, 477–493. (doi:10.1080/17435390.2021.1879963)

9. Di Poi C, Brodu N, Gazeau F, Pernet F. 2022 Life-history traits in the Pacific oyster *Crassostrea gigas* are robust to ocean acidification under two thermal regimes. *ICES J. Mar. Sci.* **79**, 2614–2629. (doi:10.1093/icesjms/fsac195)

10. Riesco MF, Félix F, Matias D, Joaquim S, Suquet M, Cabrita E. 2019 Comparative study on cellular and molecular responses in oyster sperm revealed different susceptibilities to cryopreservation. *Aquaculture* **498**, 223–229. (doi:10.1016/j.aquaculture.2018.08.049)

11. Fernández de la Puente M, Salas-Huetos A, Valle-Hita C, Babio N, Murphy MM, Canudas S, Salas-Salvadó J. 2024 Is telomere length a biomarker of sperm quality? A systematic review and meta-analysis of observational studies. *Andrology* **12**, 277–288. (doi:10.1111/andr.13482)

12. Thilagavathi J, Venkatesh S, Dada R. 2013 Telomere length in reproduction. *Andrologia* **45**, 289–304. (doi:10.1111/and.12008)

13. Olsson M, Wapstra E, Friesen CR. 2018 Evolutionary ecology of telomeres: a review. *Ann. NY. Acad. Sci.* **1422**, 5–28. (doi:10.1111/nyas.13443)

14. López-Otín C, Blasco MA, Partridge L, Serrano M, Kroemer G. 2023 Hallmarks of aging: an expanding universe. *Cell* **186**, 243–278. (doi:10.1016/j.cell.2022.11.001)

15. Dupoué A et al. 2024 Reproductive aging weakens offspring survival and constrains the telomerase response to herpesvirus in Pacific oysters. *Sci. Adv.* **10**, eadq2311. (doi:10.1126/sciadv.adq2311)

16. Gruber H, Schaible R, Ridgway ID, Chow TT, Held C, Philipp EER. 2014 Telomere-independent ageing in the longest-lived non-colonial animal, *Arctica islandica*. *Exp. Gerontol.* **51**, 38–45. (doi:10.1016/j.exger.2013.12.014)

17. Enríquez-Díaz M, Pouvreau S, Chávez-Villalba J, Le Pennec M. 2009 Gametogenesis, reproductive investment, and spawning behavior of the Pacific giant oyster *Crassostrea gigas*: evidence of an environment-dependent strategy. *Aquacult. Int.* **17**, 491–506. (doi:10.1007/s10499-008-9219-1)

18. Bernard I, Massabuau JC, Ciret P, Sow M, Sottolichio A, Pouvreau S, Tran D. 2016 In situ spawning in a marine broadcast spawner, the Pacific oyster *Crassostrea gigas*: timing and environmental triggers. *Limnol. Oceanogr.* **61**, 635–647. (doi:10.1002/lno.10240)

19. Fabioux C, Huvet A, Le Souchu P, Le Pennec M, Pouvreau S. 2005 Temperature and photoperiod drive *Crassostrea gigas* reproductive internal clock. *Aquaculture* **250**, 458–470. (doi:10.1016/j.aquaculture.2005.02.038)

20. Broquard C, Martinez AS, Maurouard E, Lamy JB, Dégremont L. 2020 Sex determination in the oyster *Crassostrea gigas*: a large longitudinal study of population sex ratios and individual sex changes. *Aquaculture* **515**, 734555. (doi:10.1016/j.aquaculture.2019.734555)

21. Suquet M, Malo F, Quéau I, Ratiskol D, Quééré C, Le Grand J, Fauvel C. 2016 Seasonal variation of sperm quality in Pacific oyster (*Crassostrea gigas*). *Aquaculture* **464**, 638–641. (doi:10.1016/j.aquaculture.2016.07.016)

22. Boulais M, Soudant P, Le Goïc N, Quééré C, Boudry P, Suquet M. 2017 ATP content and viability of spermatozoa drive variability of fertilization success in the Pacific oyster (*Crassostrea gigas*). *Aquaculture* **479**, 114–119. (doi:10.1016/j.aquaculture.2017.05.035)

23. Boulais M, Soudant P, Le Goïc N, Quééré C, Boudry P, Suquet M. 2015 Involvement of mitochondrial activity and OXPHOS in ATP synthesis during the motility phase of spermatozoa in the Pacific Oyster, *Crassostrea gigas*. *Biol. Reprod.* **93**, 1–7. (doi:10.1095/biolreprod.115.128538)

24. Suquet M, Labbe C, Brizard R, Donval A, Le Coz JR, Quere C, Haffray P. 2010 Changes in motility, ATP content, morphology and fertilisation capacity during the movement phase of tetraploid Pacific oyster (*Crassostrea gigas*) sperm. *Theriogenology* **74**, 111–117. (doi:10.1016/j.theriogenology.2010.01.021)

25. Meistertzheim AL, Arnaud-Haond S, Boudry P, Thébault MT. 2013 Genetic structure of wild European populations of the invasive Pacific oyster *Crassostrea gigas* due to aquaculture practices. *Mar. Biol.* **160**, 453–463. (doi:10.1007/s00227-012-2102-7)

26. Huvet A, Fabioux C, McCombie H, Lapègue S, Boudry P. 2004 Natural hybridization between genetically differentiated populations of *Crassostrea gigas* and *C. angulata* highlighted by sequence variation in flanking regions of a microsatellite locus. *Mar. Ecol. Prog. Ser.* **272**, 141–152. (doi:10.3354/meps272141)

27. Petton B, Boudry P, Alunno-Bruscia M, Pernet F. 2015 Factors influencing disease-induced mortality of Pacific oysters *Crassostrea gigas*. *Aquac. Environ. Interact.* **6**, 205–222. (doi:10.3354/aei00125)

28. Garreau P. 1993 Hydrodynamics of the North Brittany coast: a synoptic study. *Oceanol. Acta* **16**, 469–477.
29. Chavez-Villalba J, Mingant C, Cochard JC, Le Pennec M. 2001 Gamétogenèse chez l'huître *Crassostrea gigas* (T.) de l'Aber Benoit (Bretagne, France), à la limite nord de son aire de reproduction. *Haliotis* **30**, 1-12.

30. Steele S, Mulcahy MF. 1999 Gametogenesis of the oyster *Crassostrea gigas* in southern Ireland. *J. Mar. Biol. Ass.* **79**, 673–686. (doi:10.1017/S0025315498000836)

31. Alquézar-Baeta C et al. 2019 OpenCASA: a new open-source and scalable tool for sperm quality analysis. *PLoS Comput. Biol.* **15**, e1006691. (doi:10.1371/journal.pcbi.1006691)

32. R Core Team. 2025 R: a language and environment for statistical computing. Vienna, Austria: R Foundation for Statistical Computing. See https://www.R-project.org.

33. Philipp EER, Abele D. 2010 Masters of longevity: lessons from long-lived bivalves: a mini-review. *Gerontology* **56**, 55–65. (doi:10.1159/000221004)

34. Boguenet M, Bouet PE, Spiers A, Reynier P, May-Panloup P. 2021 Mitochondria: their role in spermatozoa and in male infertility. *Hum. Reprod. Update* **27**, 697–719. (doi:10.1093/humupd/dmab001)

35. Monro K, Marshall DJ. 2015 The biogeography of fertilization mode in the sea. *Glob. Ecol. Biogeogr.* **24**, 1499–1509. (doi:10.1111/geb.12358)

36. Dobson FS. 2013 Live fast, die young, and win the sperm competition. *Proc. Natl Acad. Sci. USA* **110**, 17610–17611. (doi:10.1073/pnas.1317368110)

37. Godwin R, Brown I, Montgomery S, Frusher S, Green T, Ovenden J. 2012 Telomere dynamics in the Sydney rock oyster (*Saccostrea glomerata*): an investigation into the effects of age, tissue type, location and time of sampling. *Mar. Biol.* **159**, 77–86. (doi:10.1007/s00227-011-1791-7)

38. Friesen CR, Rollings N, Wilson M, Whittington CM, Shine R, Olsson M. 2020 Covariation in superoxide, sperm telomere length and sperm velocity in a polymorphic reptile. *Behav. Ecol. Sociobiol.* **74**, 74. (doi:10.1007/s00265-020-02855-8)

39. Morbiato E, Cattelan S, Pilastro A, Grapputo A. 2023 Sperm production is negatively associated with muscle and sperm telomere length in a species subjected to strong sperm competition. *Mol. Ecol.* **32**, 5812–5822. (doi:10.1111/mec.17158)

40. Gasparini C, Dosselli R, Evans JP. 2017 Sperm storage by males causes changes in sperm phenotype and influences the reproductive fitness of males and their sons. *Evol. Lett.* **1**, 16–25. (doi:10.1002/evl3.2)

41. Marasco V, Boner W, Griffiths K, Heidinger B, Monaghan P. 2019 Intergenerational effects on offspring telomere length: interactions among maternal age, stress exposure and offspring sex. *Proc. R. Soc. B* **286**, 20191845. (doi:10.1098/rspb.2019.1845)

42. Dupoue A. 2025 *Independent impacts of age and telomere length on oyster sperm motility*. Zenodo. See https://doi.org/10.5281/zenodo.17723646.