# Comparative Tissue Responses of Marine Mollusks on Seasonal Changes in the Northern Adriatic Sea
## 4 Discussion  
Mollusks respond differently to stressors compared to other marine organisms. They comprise species ranging from sensitive to very resilient to stress and are, thus, frequently used to establish the ecological quality of benthic communities [23,24]. The most important factor for stress-related parameters for both species in this work was found to be the season-induced temperature change. In response to temperature changes, mollusks produce reactive oxygen species (ROS) and activate antioxidant enzymes [25]. The lowest temperature (9.1 \({}^{\circ}\)C) was registered in winter at both sampling depths, which contributed to a decline in enzymatic antioxidant defense in scallop tissues, having lower SOD and GPx activity and higher concentrations of TBARS and TAS in this season. The scallop shell growth ceases at such low temperatures and resumes as water temperature increases [6]. Long-term temperatures between 8.5 and 9.0 \({}^{\circ}\)C are the lethal limit for ormers, and at low temperatures, they are limited in their ability to absorb food and grow [3,26]. In ormers, low temperatures in winter contributed to lower TAS, TBARS, SOD and GPx concentrations. The significant difference of winter TAS and TBARS levels between ormers and scallops in this work could be correlated with their variations in reproductive cycles, as well as in antioxidant defense system, since in scallop tissues, they seem inversely related to the accumulation of lipid peroxidation products. Therefore, a decrease in scallop antioxidant capacity in winter was accompanied by the MDA/TBARS increase, as previously established [27]. TAS is a quantitative representation of the total contribution from a wide range of antioxidant molecules and provides an assessment of an organism's ability to resist oxidative stress [28]. The lower TAS in ormers in winter points to the dynamic equilibrium between different procokands and antioxidants in tissues, with the prevailingly low SOD and GPx. The antioxidants SOD and GPx convert superoxide anion radicals to hydrogen peroxide and catalyze the decomposition of superoxide radicals into less toxic molecules, respectively [29]. Not only antioxidant capacity, but also tissue Mg, Ca and GLU showed decreased levels in ormers in winter, which did not correlate with scallop tissues. In fall, in ormer tissues a number of parameters had their yearly peaks, such as Mg, Ca, SOD, GPx, TRIG, CHOL and GLU. Additionally, Mg, Ca, TRIG, CHOL and GLU were elevated in scallop tissues. Tissue Mg and Ca are active in ionic homeostasis; they participate in prostaglandin release from tissues and act in reproduction control [30, 31, 32]. They approximate the environmental ion concentrations and varying salinity of water [32]. Furthermore, tissue Mg has a regulatory role in oxidative processes and is considered an essential cofactor of glutathione (GSH) synthesis. Its decrease leads to a reduction in GSH levels and intensifies the production of ROS [33]. Calcium regulates adenosine diphosphate binding and actin affinity in mollusks. In the absence of Ca, tension generation by the striated adductor muscle and actin-dependent motility are repressed. An increase of Ca above micromolar levels significantly elevates these activities [34]. Figure 4: Seasonal gametogenic cycles of _Pecten jacobaeus_ (**A**) and _Haliotiş tuberculata_ (**B**). Histograms showing the relative abundance of different gonadal maturity stage: 2—early active stage; 3—late active stage; 4—ripe stage; 5—partial spawn stage. In preparation for reproduction, energy demands arise. Northern Adriatic scallops can spawn in spring or fall. Gonads are mature from May until July, while a second maturation peak can occur from November until February; however, most are spring-spawned [6]. Ormers, on the other hand, spawn for a protracted period, if not synchronously, but may also spawn twice a year. During spawning, they may cease feeding [14]. Tissue concentrations of TRIG, CHOL and GLU act as main energy sources for metabolic demands [35; 36; 37].