This work aimed to assess the suitability of a microalgal blend as a dietary ingredient for feeding juveniles of marine carnivorous and herbivorous teleost, as is the case of Sparus aurata and Mugil cephalus, respectively, and to isolate microorganisms from different media and characterize them on the base of their enzymatic activities and their antagonism against important fish pathogens. Thirty juveniles of each species (70 ± 3.2 g S. aurata mean weight and 47 ± 2.8 g M. cephalus mean weight) were distributed in four tanks (15 individuals each) corresponding to four independent dietary treatments (control and microalgae diets designed for each species). Fish were fed their corresponding diets ad libitum for 108 days. At the end of the trial, fish were weighed, and plasma, liver, perivisceral fat, and the entire intestines were obtained for the evaluation of growth performance and metabolic assessment. Furthermore, 117 bacterial strains were isolated in different culture media from the gastrointestinal tract of S. aurata fed the microalgae blend and further characterized for their potential use as probiotics in aquaculture. S. aurata fed the microalgae-supplemented diet (25% dietary inclusion) showed a significant increase in weight gain, specific growth rate, feed efficiency, hepatosomatic, and intestine length indices. However, growth performance and somatic indices in M. cephalus were not affected by the experimental diets. Plasma samples from S. aurata fed the microalgal diet revealed higher levels of glucose and triglycerides and a decrease in cortisol levels. No significant differences were found in any biochemical parameters among the experimental diets in M. cephalus. In conclusion, both species demonstrated a favorable adaptation to the nutritional formulation employed in this study, and bacterial strains UMA-169 and UMA-216 (both identified as Bacillus pumilus) could be considered for use in aquaculture as they might benefit host health by improving digestion and absorption of different energy sources and by minimizing the colonization of pathogenic species.

Environmental salinity is a critical factor influencing the physiological and metabolic processes of teleosts. Despite its importance, the molecular mechanisms underlying these responses, particularly those involving specific signaling pathways and gene expression regulation, remain poorly understood. To elucidate the role of lipid metabolism in osmotic regulation, the present study investigated the effects of varying salinity levels (2, 20, 40, and 60 ppt) on growth performance and metabolic status, including the biosynthesis of LC-FAs and VLC-FAs, respectively, in neural tissues (brain and eyes), of the euryhaline fish Fundulus heteroclitus over a 62-day period. The findings revealed multiple physiological adaptations to salinity variation, encompassing both molecular and metabolic responses. Salinity had a significant impact on growth performance, with fish exposed to the highest salinity level (60 ppt) exhibiting reduced growth. At this salinity, plasma levels of lipid-related metabolites, i.e., triglycerides and cholesterol, were decreased, whereas both osmolality and cortisol levels increased. Hepatic glucose and lactate levels increased with rising salinity, while glucose and triglyceride concentrations in muscle tissue declined. Additionally, intestinal lipase activity was significantly higher at 60 ppt. Although no significant differences were observed in the total UFAs content of both tissues, in the brain, significant differences were detected in the levels of 16:1n-7, 18:1n-9, 18:2n-6, 20:3n-3, 20:4n-6, and 20:5n-3, whereas in the eye, differences were observed only for 16:1n-7 and 20:5n-3. Gene expression analysis revealed that salinity exerts a regulatory effect on the expression of fads2b and elovl4a in the eye, with up-regulation observed at 60 ppt. In contrast, no significant changes in the expression of fads or elovl genes were detected in the brain. These findings highlight the contribution of non-osmoregulatory organs, such as the brain and eyes, in the osmotic adaptation of teleosts. Collectively, the results suggest that lipid metabolism plays a key regulatory role in the adaptation of F. heteroclitus to salinity fluctuations.

Rising seawater temperatures associated with climate change are expected to increasingly challenge the sustainability of bivalve aquaculture, particularly in estuarine environments where thermal variability is naturally high. The grooved carpet clam (Ruditapes decussatus), a species of high ecological and economic value in southern Europe, is strongly influenced by weather conditions. This study aimed to investigate the effects of a chronic thermal challenge (21 days at 28 °C) on the growth performance, intermediary metabolism, and fatty acid composition of R. decussatus raised with and without substrate. Clams were acclimated to either control (18 °C) or high-temperature (28 °C) conditions, and biometric, biochemical parameters (glucose, glycogen, lactate, triglycerides, and cholesterol) and fatty acid profiles were analyzed. Our results denote that exposure to elevated temperature significantly reduced total weight, tissue biomass, meat yield, and condition index in both clams reared with and without substrate. Thermal stress induced marked metabolic alterations, characterized by increased lactate accumulation and depletion of triglyceride reserves, indicating a shift toward anaerobic metabolism and enhanced lipid mobilization. However, glycogen and cholesterol levels remained largely unchanged. Fatty acid analysis revealed a strong temperature-driven remodeling of lipid composition, characterized by significant reductions in total lipids and unsaturated fatty acids, which highlighted changes in key fatty acids, such as arachidonic acid (ARA; 20:4n-6) and eicosapentaenoic acid (EPA; 20:5n-3). In contrast, docosahexaenoic acid (DHA; 22:6n-3) levels remained unchanged under high-temperature conditions. Principal component analysis confirmed temperature as the main factor structuring fatty acid profiles, while substrate exerted only minor effects. Overall, these findings demonstrate that sustained exposure to sublethal high temperature profoundly affects growth performance, metabolic balance, and lipid homeostasis in R. decussatus, overriding the possible physiological benefits associated with substrate presence. The results highlight the vulnerability of this species to future warming scenarios and underscore the importance of incorporating thermal stress considerations into sustainable clam aquaculture management strategies in estuarine environments.

Chelon labrosus represents a promising species for current aquaculture to lean towards a more sustainable model. The control of reproduction in captivity is essential to develop the industrial production of new species. This work adds to the knowledge of the reproductive aspects of males and females of this species. We aim to (1) develop a methodology for sex identification of reproductive and prereproductive specimens of the studied species based on the plasma levels of steroid hormones—estradiol (E2) and 11-ketotestosterone (11-KT); (2) determine the maturity evolution of the females throughout the spawning season by gonadal biopsy; and (3) describe sperm quality throughout the breeding season (volume, motility, density). The results show that an 11-KT/E2 ratio > 0.4 is 100% effective in distinguishing males from females, provided that individuals present a size greater than that of first sexual maturity. A double trend was observed in the development of oocytes, with an increase in the initial diameter until the middle of the spawning season, followed by a divergence of the group, with some females continuing the maturation process and others going into regression. Sperm data show stable quality at the beginning and in the middle of the spawning season and a significant loss of quality at the end of the reproductive period. The present study also shows that, between the beginning and the middle of the reproductive season, males of this species can recover sperm volume removed by stripping.

RESUMEN | Los ácidos grasos de cadena muy larga (˃ C24) (VLC-FA) juegan un papel crítico durante el desarrollo temprano de los vertebrados, ya que estos compuestos se acumulan en los tejidos neurales que se forman rápidamente, asegurando su función normal. La funcionalidad de los VLC-FA ha despertado el interés científico, centrándose en el estudio de las proteínas elongasas 4 de ácidos grasos de cadena muy larga (Elovl4), que son responsables de su biosíntesis a partir de ácidos grasos más cortos (precursores). Para comprender mejor el metabolismo y los potenciales requisitos a nivel tisular de los VLC-FA en teleósteos marinos, el presente estudio tuvo como objetivo determinar el patrón de expresión tisular de los genes que codifican las isoformas de Elovl4, es decir, elovl4a y elovl4b, en diferentes ventanas del desarrollo (larvas y adultos) de dorada (Sparus aurata) y lenguado senegalés (Solea senegalensis). Los resultados indicaron que en las larvas de S. aurata, elovl4a se expresa ampliamente en la región cerebral, mientras que elovl4b se expresa intensamente en los ojos. Curiosamente, en las larvas de S. senegalensis se observó un patrón de expresión tisular opuesto, siendo elovl4a y elovl4b fuertemente expresado en ojos y región cerebral, respectivamente. En adultos de ambas especies, aunque se detectaron transcritos de elovl4 en la mayoría de los tejidos analizados, los mayores valores de expresión de elovl4a y elovl4b se observaron en el cerebro y en los ojos, respectivamente. Es importante destacar que el diferente patrón de expresión tisular observado para ambos genes elovl4 asociados a la etapa pre y posmetamórfica de S. senegalensis podría ser indicativo de las necesidades particulares de VLC-FA vinculadas a la funcionalidad de los tejidos neurales en cada etapa de desarrollo. Estos hallazgos pueden contribuir a una mejor comprensión del metabolismo de VLC-FA específico en función de la especie de teleósteo marino. ABSTRACT | Very long-chain (˃ C24) fatty acids (VLC-FA) play critical roles during early development of vertebrates, since these compounds are accumulated in the rapidly forming neural tissues, ensuring their normal function. The functionality of VLC-FA has aroused scientific interest, focusing on the study of elongases protein 4 of very long chain fatty acid (Elovl4), which are responsible for their biosynthesis from shorter fatty acids (precursors). For a better understanding of the metabolism and the potential tissue-specific requirements of VLC-FA in marine teleosts, the present study aimed to determine the tissue-expression pattern of the genes that encode for Elovl4 isoforms, i.e. elovl4a and elovl4b, in different windows of development (larval and adult stages) of Gilthead seabream Sparus aurata and Senegalese sole Solea senegalensis. The results indicated that in S. aurata larvae, elovl4a is widely expressed in the head, while elovl4b is strongly focused in the eyes. Interestingly, in S. senegalensis larvae an opposite tissue-expression pattern was observed for both elovl4 isoforms. In adults of both fish, although elovl4 transcripts were detected in most tissues analyzed, elovl4a and elovl4b genes were strongly expressed in brain and eyes, respectively. Importantly, the differential tissue-expression pattern of both elovl4 isoforms associated to the pre- and post-metamorphic stage of S. senegalensis could be indicative of the VLC-FA particular needs linked to neural tissues functionality in each development stage. These findings can contribute to a better understanding of the species-specific VLC-FA metabolism in marine teleosts.