Maria Papadaki

The aim of this study was to evaluate the effects of a gonadotropin-releasing hormone analogue (GnRHa), administered through injections or with sustained-release implants, on spermiation and sperm quality in thicklip grey mullet, Chelon labrosus. Male broodstock transferred from extensive lagoons were randomly allocated to three treatment groups: (a) weekly saline injections (Saline-INJ), (b) weekly GnRHa injections (10 μg kg⁻¹ bw; GnRHa-INJ), and (c) a single GnRHa EVAc implant (50 μg kg⁻¹ bw; GnRHa-IMP). Males were maintained with females under the same treatment protocols and allowed to spawn. Sperm condition, density, motility, duration of motility, and androgen levels were evaluated weekly at five sampling points (Days 0, 7, 14, 21, and 28). All males exhibited complete spermiation during the first two weeks (Days 0 and 7), followed by a gradual decline throughout the rest of the study (Days 14 and 21). This pattern of decline was not as evident in the groups treated with GnRHa, especially in the GnRHa-INJ group. Sperm density, motility, and motility duration varied strongly over time; however, no significant differences were observed among treatments. GnRHa treatment prolonged the spermiation period and delayed its seasonal decline without significantly altering sperm quality. These results may enable more frequent sperm collection and greater sperm availability, as inferred from the spermiation condition (SCI) and qualitative observations.

Thick-lipped grey mullet (Chelon labrosus) has important characteristics that make it a promising candidate species for diversifying Mediterranean aquaculture. However, spontaneous spawning in captivity has not been documented, mainly due to failure of females to spawn, highlighting the need for further research on reproduction control. This study evaluated the efficacy of GnRHa administration, using repeated intramuscular injections or slow-release Ethylene–Vinyl Acetate (EVAc) implants combined with a dopamine antagonist (metoclopramide, Met), in terms of spawning performance and egg quality. Three groups were established: (a) saline injection (0.9% NaCl; Saline-INJ), (b) GnRHa [Des-Gly¹⁰, D-Ala⁶-ProNEth⁹-mGnRHa] injection at 10 μg kg⁻¹ BW (GnRHa-INJ), and (c) EVAc implant containing GnRHa at 50 μg kg⁻¹ bw (GnRHa-IMP). Over four weeks, Saline-INJ and GnRHa-INJ females received weekly saline or GnRHa, respectively. GnRHa-INJ and GnRHa-IMP females also received weekly Met (15 mg kg⁻¹ bw). GnRHa induced 11 spawns (1,768,680 eggs), nearly triple the Saline-INJ group (4 spawns, 394,400 eggs). Daily relative fecundity (DRF) and fertilization success were highest in GnRHa-INJ (56,982 eggs kg⁻¹ day⁻¹; 59.7%), followed by GnRHa-IMP (20,375; 18.8%) and Saline-INJ (13,061; 9.1%). Multiple injections showed a trend toward higher spawning performance and egg quality compared to implants, although variability was high and further replication is needed. Nevertheless, optimizing both GnRHa delivery methods could further enhance their effectiveness while maintaining operational benefits for aquaculture.

Environmental effects and, particularly, temperature changes have been demonstrated to influence the activity, function, and well-being of teleosts. Temperature may change seasonally in the wild, and in captivity under aquaculture operations. Moreover, climate change is expected to shift temperature profiles worldwide. MicroRNAs (miRNA) are important temperature-sensitive gene-expression regulators acting at the post-transcriptional level. They are known to be key regulators in development, reproduction, and immune responses. Therefore, early larval development of the European sea bass (Dicentrarchus labrax), one of the most extensively cultured species in Mediterranean aquaculture, was investigated at early rearing temperatures, i.e., 15, 17.5, and 20 °C, in regard to the impact of temperatures on miRNAs through sncRNA high-throughput sequencing but also at the phenotypic level in terms of growth, sex, vision, and skeletal deformities. Expression profiling revealed stage- and temperature-specific miRNA expression targeting genes with roles in reproduction and immune response mainly at the flexion and all-fins stages. Similar stage- and temperature-specific results were also observed concerning the number of rod cells and lower jaw elongation. The present work presents for the first time highly promising results on the influence of early rearing temperature at the post-transcriptional level during European sea bass development, with a putative impact on reproduction and immune response, as well as regarding teleost vision and larval development.

There is growing interest about marine bivalve aquaculture globally, not only for the market value of the goods produced, but also for the socio-economic and environmental services that this activity can provide. Arca noae is an endemic Mediterranean bivalve of commercial value, whose previously undescribed population in Elounda Bay we studied in terms of its structure and reproduction, while constructing a timeseries of the basic environmental parameters of the bay, thus, gaining fundamental knowledge for the potential future exploitation of the species in the area. We found a variable spatial distribution of arks in the study area, with local high peaks in the population density, consisting of smaller size individuals, in comparison to other areas. Because of protandry of the species, human pressure on this population could have a strong negative effect, by targeting the limited numbers of large females in the study area. The reproduction pattern was similar to the reports from other Mediterranean locations. The abiotic conditions in Elounda Bay differed from those in the adjacent coastal zone, confirming that the Bay is a unique semi-enclosed marine area in the island of Crete.

The histological process of gonadal differentiation, together with the endocrine changes of sex steroid hormones and some of their precursors, was studied in hatchery-produced greater amberjack Seriola dumerili from 101 until 408 days post-hatching (dph), with samplings conducted every 50 days. Histological processing showed that sex differentiation began at 101 dph with the formation of the ovarian cavity in females, while the presumptive males did not yet contain any germ cells in their gonad. At 150 dph, we observed the first germ cells in the developing testes. Sex differentiation in almost all sampled individuals was complete at 408 dph. No size dimorphism was observed between the sexes, and the sex ratio was 1:1, suggesting that there was no influence of early rearing in captivity on sex differentiation. Plasma concentrations of adrenosterone (Ad), androstenedione (Δ4), 11-ketotestosterone (11ΚΤ), testosterone (Τ), estradiol (Ε₂), progesterone (P4) and 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) were measured in males and females with the use of liquid chromatography tandem mass spectrometry (LC–MS/MS) to examine their role in the sex differentiation process. From the seven hormones, the only one that exhibited differences between the sexes was 11-KT and the plasma 11-KT concentration was found to be a useful indication of greater amberjack sex. Variations were observed in the mean values of Ad, Δ4, 11-KT, T, P4 and 17,20βP over time in one or both sexes, indicating their involvement in the sex differentiation process.

The effect of fasting on spawning performance, maternal, and egg nutrient composition, and on embryo/larval development was monitored in gilthead seabream (Sparus aurata). Two broodstocks were fasted during two consecutive years, for a period of 43 and 54 days within the spawning season, in a preliminary (year 1, 5-year-old breeders) and the main study (year 2, 6-year-old breeders), respectively. Mean daily fecundity showed a declining trend during fasting in the main study only, while fertilization success was high in both years and it was not affected by fasting, as was hatching and 5-day larval survival. There was a loss of 23.5% of maternal body mass due to fasting, and a reduction in gonadosomatic and hepatosomatic indexes, as well as crude protein in maternal muscle and gonads, but not in the liver. After fasting, muscle Σω-6 PUFA and C18:3ω-3 were reduced while C20:4ω-6, 20:5ω-3/20:4ω-6, and C22:6ω-3/20:4ω-6 increased; in the liver, significant reductions were observed in C16:0, C18:3 ω-3, 20:5ω-3/C22:6ω-3 and increases in C18:0, C20:5ω-3, Σω-6 PUFA, and 20:5ω-3/20:4ω-6; in gonads, C15:0, ΣMUFA, 20:5ω-3/C22:6ω-3, 20:5ω-3/20:4ω-6 were increased, while C18:1ω-9 and C20:5ω-3 decreased. Contrary to maternal tissues, the energy density and proximate composition of the eggs did not change due to fasting. The study suggests that fasting of gilthead seabream breeders for 6–8 weeks during the spawning period does not affect spawning performance, egg proximate composition, or embryo and early larval development since maternal nutrient reserves are mobilized to maintain optimal egg nutrient composition.

Sturgeon sperm maturation occurs outside the testes during the transit of testicular spermatozoa (TS) through the kidneys and the Wolffian ducts. A method of in vitro TS maturation in sterlet Acipenser ruthenus was used to investigate the effects of temperature and hormonal stimulation of spermiation on the ability of TS to complete this process. Spermatozoa motility parameters after in vitro maturation of testicular sperm, concentrations of sex steroid hormones and testis morphology were studied in three groups of sterlet: (1) after overwintering in ponds (OW), (2) adapted to spawning temperature (ST), and (3) adapted to spawning temperature with hormonal induction of spermiation (ST-HI). Blood plasma concentrations of testosterone, 11-ketotestosterone and 17,20β-dihydroxy-pregnenolone increased significantly after hormonal induction of spermiation (group ST-HI). In all groups, TS were not motile. After in vitro sperm maturation, motility was up to 60% only in group ST-HI. The data suggest that the ability of TS to be matured in vitro was not related to the environmental temperature, while hormonal stimulation of spermiation during the spawning season was an absolute requirement for optimal in vitro maturation.

The histological process of gonadal differentiation together with the endocrine changes of glucocorticoids and sex steroids was studied in hatchery produced greater amberjack (Seriola dumerili) from 101 until 408 days post-hatching. In the samplings, which were conducted every 50 days, no size dimorphism was observed between the sexes and sex ratio was 1:1, suggesting that there was no influence of early rearing in captivity on sex differentiation. Plasma concentrations of cortisol, cortisone, adrenosterone (Ad), androstenedione (Δ4), 11-ketotestosterone (11ΚΤ), testosterone (Τ), estradiol (Ε₂), progesterone (P4) and 17,20β-dihydroxyprogesterone (DHP) were measured in males and females with the use of liquid chromatography tandem mass spectrometry (LC-MS/MS) to examine their role in the sex differentiation process. From the nine hormones, the only one that exhibited differences between the sexes was 11-KT. However, variations were observed in the mean values of cortisol, Ad, Δ4, 11-KT, T, P4 and DHP over time in one or both sexes, indicating their involvement in the sex differentiation process. Moreover, the 11-KT/E₂ ratio was found to be a useful diagnostic biomarker for greater amberjack sex, from as early as 150 dph.