###### Abstract  
The possibility of simultaneous production of halophyte and euryhaline fish creates huge interest in both commercial aquaponics systems and in research. The aim of the present study was to investigate the effect of three different salinities (8, 14, and 20 ppt) on the growth performance and survival rate of sea bream (_Sparus aurata_) and rock sapphire (_Criithmum martitimum_) in an experimental brackish aquaponic system. Furthermore, induction of heat shock proteins (Hsps) and phosphorylation of mitogen-activated protein kinases (MAPKs) were assessed through the sodium dodecyl sulfate-polyacrylamideamide gel electrophoresis (SDS-PAGE) and immunoblot analysis. A total number of 234 sea bream individuals were divided into nine autonomous aquaponic systems. The experiment lasted 45 days. In total, 54 individuals of rock sapphire were used and were distributed into groups of six individuals per hydroponic tank using the raft method. Water quality showed stable fluctuation throughout the experiment, strongly supporting fish and plant growth performance and survival in both treatments. The results show that better growth performance for both sea bream and rock sapphire (height increase) was evident in salinity 8 ppt compared to salinities 14 ppt and 20 ppt. Minimal or mild histopathological alterations were detected in gills, midgut, and liver for all three salinity groups. Exposure to different salinities modified Hsp60 and MAPKs expression in a tissue- and time-specific manner. During exposure to 8 ppt, constant Hsp60 levels and phosphorylation of MAPKs at 15 days may indicate a prominent protective role in the gills. The results show that sea bream and rock sapphire can be used in brackish aquaponics systems with satisfactory growth performances, thus allowing for a range of commercial applications generating interest in their production. growth performance; rock samphire; gilthead sea bream; MAPKs; HSBs stress 20233385 20233385 20233385 20233385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 2023385 202385 202385 2023385  
gradually increasing extreme environmental events, such as tides and tsunamis often cause large and rapid fluctuations in habitat salinity [3]. Salinity constitutes a crucial abiotic factor for fish that influences both their distribution and activity [4]. Aquaponics systems represent an innovative application that can contribute to the reduction in salinity levels and ever-increasing scarcity of resources due to the continuous growth of human population. Furthermore, the reckless use of freshwater with limited availability compared to the increased availability of brackish water and saltwater triggered the interest of researchers to exploit saline water in aquaponics systems [5; 6]. Previous studies report that aquaponics systems are identified to date as ecological and sustainable food production methods that combine aquaculture and hydroponics in a recirculating system [6; 7; 8]. In such applications, aquatic organisms (fish, invertebrates) co-cultivated with plants (vegetables, herbs, or algae) may create a symbiotic environment [5; 7; 9; 10; 11]. Animals, plants, and microorganisms coexist in such multicultural systems offering environmentally friendly solutions and resource use efficiency. In aquaponic systems, the nutrient-rich water acts as a natural fertilizer for plants, while the latter act as water purifiers helping aquatic organisms grow more efficiently. Fish excrete their waste, and bacteria convert it into available nutrients for plant growth [6; 7; 12; 13; 14; 15]. Ammonia (released as a metabolic product through fish gill mediators) is involved in the process that enables the simultaneous growth of plants, bacteria, and fish. Ammonia-oxidizing bacteria (AOB) (_Nitrosomonas_ spp.) oxidize toxic ammonia to nitrite ions and consequently to nitrate ions through the nitrite-oxidizing bacteria (NOB) (_Nitrobacter_ spp.) [16].