The importance of thermal cycling for proper gametogenesis and succesful egg and sperm production in meagre (Argyrosomus regius)
Creators: name: Lancerotto, S.
name: Mello, P.H.
name: Tsoukali, P.
name: Calloni, L.
name: Kopper, S.
name: Sarracino, R.
name: Fakriadis, I.
name: Mylonas, C.C.
Publisher: name: Zenodo
Year: 2021

description: Introduction
Meagre (Argyrosomus regius) is a species with great potential for the diversification of Mediterranean aquaculture, but reproduction in captivity still remains a problem (Duncan et al., 2013). Due to biosecurity reasons (lack of pathogens), marine hatcheries use borehole seawater for their broodstock of relatively constant temperature during the year (18-20°C). Therefore, it would be cost-effective if fish could undergo reproductive maturation without the need to heat or cool the water during the year. It has been shown in gilthead seabream (Sparus aurata) that constant annual temperatures (18-20°C) had no negative effects on spawning kinetics and egg quality (Karamanlidis et al., 2017). Here, we manipulated the temperature when gametogenesis and maturation take place, and using established hormonal therapies to induce spawning, evaluated the effect on the reproductive performance of the broodstock.
Materials and Methods
Two broodstocks were maintained, under an attenuated seasonal temperature regime (SeasT, 16 to 20°C) or a constant temperature (CoT, 19.4±0.6°C), using borehole water. During the spawning season (May 2019), eight couples were placed in separate 5-m3 tanks, and using gonadotropin-releasing hormone agonist (GnRHa), induced to spawn. Females were treated once a week (n=4) with a GnRHa injection (50 μg kg-1) and males with GnRHa implants every 15 days (50 μg kg-1). Throughout the study, oocyte development and sperm quality, as well as spawning success, were monitored. Spermiation was evaluated using a subjective index as follows: S0 = no milt released, S1 = only a drop of milt released after multiple stripping attempts, S2 = milt was released easily after the first stripping attempt and S3 = copious amount of sperm release with very little pressure. Statistical analyses were done using t-test or Friedman's test (both at P˂0.05).
Results
Females exposed to a constant temperature showed a smaller mean oocyte diameter (P=0.02) (Fig. 1A), and males did not release any sperm before GnRHa treatment (Fig. 1B). Subsequent GnRHa treatments slightly improved the reproductive outputs of the CoT group. Males partially started expressing sperm, though with a spermiation index lower than their SeasT counterparts (P=0.002) (Fig. 1B). Females spawned fewer times, with a mean total relative fecundity three times lower than SeasT females (P=0.02) (Fig. 1C), and embryo survival 24h after spawning dropped to 35 % (P=0.02) (Fig. 1D).
Discussion
Constant temperature significantly compromised, but did not prevent gametogenesis in both sexes. Females had initially a lower mean oocyte diameter, being less than what isconsidered optimal for spawning induction in this species (Duncan et al., 2013, Mylonas et al., 2016). Males did not express any sperm upon gentle abdominal pressure at the beginning of the study. Fish from the SeasT group exhibited better reproductive parameters than the CoT group before any GnRHa administration. In response to GnRHa implants, CoT males produced progressively more sperm, even though the spermiation index remained low. Besides, SeasT females spawned well after hormonal treatments and exhibited a higher total relative fecundity than the CoT group. Although, the repeated hormonal therapy during the spawning season partially reduced the negative effects of constant temperature regime during gametogenesis and enabled CoT breeders to spawn, and to produce fertilized eggs both relative fecundity and embryo survival were reduced. Therefore, a seasonal thermal regime -even an attenuated one- was necessary for the proper development of the gametes, allowing for the successful spawning induction using the established GnRHa induction protocol.