# Comparative Tissue Responses of Marine Mollusks on Seasonal Changes in the Northern Adriatic Sea
## 1 Introduction  
Due to having a wide functional and taxonomic diversity, mollusks in marine environments are significant as predators, filter feeders, herbivores, scavengers and prey. By their activities, they provide and transform habitats and improve the water quality for other species. Their catch continues to increase and can be gravely affected by bottom trawling and seafood disturbance [1]. In the Northern Adriatic, Mediterranean scallop (_Pecten jacobaeus_ L) and tuberculate abalone (green ormer) (_Haliois tuberculata_ L) are particularly threatened by such activities. Mediterranean scallops are often overfished off the northwest Istrian coast, where they naturally occur in larger numbers than in the rest of the Adriaticor Mediterranean [2]. Green ormers, on the other hand, occur in exploitable quantities only in the British Channel Islands [3], but due to benthic dredging in the Northern Adriatic, their populations face a decline. Ormers are the only species in the Haliotidae family commercially harvested in Europe [3]. Neither ormers nor scallops are currently formed in the area, notwithstanding the efforts to start their cage rearing. Mediterranean scallops are bivalve mollusks, having a rapid growth in their first two years of life. Their growth is intensified by the temperature increase, although temperatures above 22 \({}^{\circ}\)C diminish their metabolic processes [4]. Scallops can reach 120-140 mm of shell length. In the Italian part of the Northern Adriatic, they attain 100 mm of length in two years [5], while on the Croatian side they need one year more to reach that length [6]. It, thus, seems that the variation of scallop growth is location-dependent. The particular location might also affect biochemical and antioxidative properties their tissue [7]. Green ormers are slow-growing gastropod mollusks, taking three years to reach 45 mm of shell length [8]. The most important variables that govern their responses to the environment are temperature and oxygen. Temperature affects major physiological processes, while aerobic metabolism provides the majority of energy for those processes, thus creating a requirement for oxygen [9]. Although ormers are facultative anaerobes, under increased metabolic demand, they may show signs of oxidative stress [10]. Adults occur in the same habitat as juveniles. That can be attributable to the lack of their migration possibilities as they rarely move more than a few hundred meters from their settlement area and usually require a firm substrate [11]. Locality is, therefore, important for both ormer and scallop growth because of food supply, exposure to wave energy and thermal conditions. The Northern Adriatic is characterized by a low average depth (30 m) and a weak bathymetric gradient. The most important oceanographic factors influencing the habitats of invertebrates in the Adriatic are seasonal salinity, the concentration of nutrients, light intensity, currents and temperature. Rivers are a significant source of nutrients in the Northern Adriatic, of which the largest is the River Po [12]. The phytoplankton distribution in the Northern Adriatic depends on the precipitation and snowmelt in the Alps, and the River Po influx of nutrients. The maximum influx in spring results from the snowmelt, while the fall maximum correlates with the yearly precipitation [13]. Seasonal changes of oceanographic factors, including temperature, algae and wave motion, greatly affect the growth and physiology of green ormers [14]. The oscillation in seawater seasonal properties might also negatively affect physiology, stress response and, ultimately, the survival of scallops in the Northern Adriatic [15; 16]. In mollusks, the cellular immune system is composed of hemocytes and their phagocytic immune response is the first line of defense, but it is also equipped with an array of other tissue defense mechanisms [17]. The tissue responses to seasonal variations in green ormers and Mediterranean scallops in the Northern Adriatic have not been reported.