Changes in Solea senegalensis sperm quality throughout the year

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Abstract

Some of the Senegalese sole (Solea senegalensis) broodstock reproductive constraints are related to sperm quality. Although they present two defined spawning season (spring and autumn), males gave semen during all the year thus an exhaustive annual sperm analysis is important to determine the seasonal changes in semen quality. Sampling was performed monthly during one year, analyzing different cellular parameters to better understand sperm quality limitations obstructing sole mass production. The percentage of progressive motile cells and their linear velocity showed a decrease from March (beginning of the first spawning season) to July (when the highest temperatures were observed), followed by a slight increase in August and October (second spawning season). DNA fragmentation values showed highest values between the two spawning seasons and decreased to the end of the year. The percentage of apoptotic cells was lowest in March (beginning of the first spawning season) and the highest in November. The percentage of cells resistant to seawater exposure presented two peaks related with both spawning seasons. There was a tendency for the semen to attain a quality peak between the beginning and the middle of the first spawning season (March–May), followed by a pronounced decrease, achieving the lowest values during the months with the highest temperature. Also, the different males present in the broodstocks reach their sperm quality peak at different times, which will result in an unequal contribution for the next generation.

Introduction

In Senegalese sole (Solea senegalensis) natural spawning can be achieved in captivity with wild stocks (Imsland et al., 2003), however poor control over the reproductive cycle has obstructed the massive culture of this species. Temperature and photoperiod seem to be the key factors triggering gonadal maturation (Oliveira et al., 2008), although the effect of the manipulation of biotic factors on sperm quality has never been tested. Some attempts have been made to increase the low sperm volume and quality produced by this species using hormonal therapies such as GnRHa treatment (Agulleiro et al., 2007, Cabrita et al., 2011, Guzmán et al., 2008) or changing the feeding regime (Anguís et al., 2008).

Despite some reports in which eggs were obtained by stripping (Mañanos et al., 2008), in most cases females only spawn naturally in two periods (late spring to the beginning of summer and early autumn, when temperatures are similar), even though males spermiate throughout the year (Anguís and Cañavate, 2005, Cabrita et al., 2006, García-López et al., 2006). According to these authors sperm could be obtained at any time without manipulating fish (hormonal stimulation or manipulation of biotic factors). Cabrita et al. (2006) demonstrated that sperm motility was variable during the year, with an improvement during the spawning seasons, however an explanation for this variation is missing. In a previous work, sperm samples with low resistance of the plasma membrane to seawater hyperosmolarity were noticed during the spawning season, and this low membrane resistance was correlated with DNA integrity (Beirão et al., 2009). There is no information whether this could be due to the asynchronization of individuals, to the presence of overripe cells, or to the fact that sperm presents these characteristics all year. Thus, an exhaustive sperm analysis throughout the year is important to determine the status of spermatozoa variation in broodstocks. This knowledge may be useful for monitoring males and optimizing broodstock maintenance through manipulation of biotic parameters or hormonal therapies and to determine the relevance of using these treatments in sperm production and quality.

In the present work we evaluated sperm quality in a broodstock based on the following parameters: (1) sperm motility; (2) DNA integrity; (3) cell viability and the presence of apoptotic cells; and (4) spermatozoa plasma membrane resistance to seawater hyperosmotic conditions. The analyses were performed during one year in a broodstock kept under natural temperature, photoperiod and salinity conditions.

Section snippets

Broodstock husbandry conditions

The individuals used in the experiments were captured in the wild and maintained for more than 2 years at the Ramalhete Experimental Station (37°00′N, 7°58′W), Faro, Portugal. Fishes (1:2, female:male) were kept in 6 round fiberglass indoor tanks (3000 L), with sand substrate and compressed air provided through air stones, stock density around 5 kg m−3 (mean individual weight 1.83 ± 0.43 kg). The photoperiod, water temperature and salinity varied throughout the year according to the environmental

Sperm production and motility evaluation

In S. senegalensis semen production was highest in February and March (52.4 ± 76.4 × 106 spermatozoa and 52.2 ± 60.9 × 106 spermatozoa, respectively) (beginning of the first spawning season) as well as in October (50.5 ± 61.9 × 106 spermatozoa) (second spawning season) nevertheless there were no significant differences with the other months (Fig. 2). There was a high variability within males, all the months presenting males with more than 130 million cells and males with less than 10 million cells. The

Discussion

Despite the importance of male reproductive potential in recruitment success this question has not been studied in similar details as in females (Trippel, 2003). In S. senegalensis one of the factors responsible for the low and variable quality of the spawns seems to be low sperm quality, a fact already mentioned in previous studies (Beirão et al., 2009, Cabrita et al., 2006). In the present study we used several parameters to evaluate sperm quality, among which sperm fluency is one of the

Acknowledgements

The authors would like to thank Patricia Diogo, Vânia Santos and Emilio Mota for their help during the experimental procedures. This work was supported by a PhD fellowship (J. Beirão, SFRH/BD/31990/2006), co-founded by POPH - QREN) a research contract (E. Cabrita, RYC-2007-01650) and REPROSOL 2006.71.001.443.9 project.

References (32)

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