Hibernation perturbs the number of hemocytes and causes hematological turnover: basal traits to understand season-dependent physiological variations in Helix pomatia (Gastropoda: Helicidae)

* Correspondence: suljevic.damir@gmail.com A great success in the evolution of animal behavior and physiology is the ability to survive seasonally unfavorable conditions, especially when it comes to variable and extremely low temperatures (Roots, 2006). Environmental variations lead animals to a state of hibernation, which causes depression of metabolic and physiological processes, as well as a slow development and reproduction so that the animal can survive (Dugbartey and Henning, 2013). Life activities of the gastropods depend on environmental factors throughout the whole year. Shells have great importance because they reduce water loss during summer days. The formation of the calcareous epiphragm, with which they bind to substrates, is an additional protection mechanism of evaporation (Elmslie, 1998). Snail hibernation includes shell closing by the secretion of the epiphragm, while energy is gained from stored food reserves during this process (Holtz and Von Brand, 1940). In experimental conditions, aestivation can last for years, which suggests that the metabolic rate depends on both exogenous and endogenous factors (Nowakowska, 2011). Respiration is reduced during hibernation and about 10% of the fluid is lost, which affects heart rate, oxygen consumption, and energy requirements (Nicolai et al., 2011). Metabolic changes during hibernation affect the value of circulating hemocytes. The phagocytic activity of hemocytes and lectins, as a defense mechanism, takes place thanks to the presence of calcium ions in the snail hemolymph. Consequently, the changes in the values of calcium ions affect the hemocyte number (Saleddin and Wilbur, 1984). Many studies confirmed the importance of hemocytes in immune responses, antibody recognition, phagocytosis, encapsulation, and cytotoxic reactions (Yoshino et al., 2001). Two types of hemocytes, granulocytes and hyalinocytes, have been identified in the hemolymph of Biomphalaria (Ratcliffe, 1985). Seasonal factors and different experimental tests affect the total number, type, and behavior of hemocytes (Oliver and Fisher, 1995). The role of hemocytes today is most widely considered as an immunological response to infections. Seta et al. (1996) reported that the phagocytic potential of hyalinocytes is questionable and that only granulocytes are crucial in the immune response. Vinaud et al. (2008), Delgado et al. (2001), and Matricon-Gondran and Letocart (1999) identified four types of hemocytes based on their size (small, medium, large, and gigantic cells) in the hemolymph of Biomphalaria glabrata. According to morphological criteria, there are two types, round cells and spread cells (Sminia, 1981), while in the study of Adamowicz and Bolaczek (2003), they refer to these cells as type I and type II. Wojtaszek et al. Abstract: The total number of hemocytes and their morphological characteristics were analyzed in Helix pomatia Linnaeus, 1758, by light microscopy during different phases of hibernation. Three types of hemocytes were identified: hyalinocytes, agranulocytes, and granulocytes. Total number of hemocytes decreased significantly during hibernation. Most intensive changes in cell size, morphology, and appearance of special structures in granulocytes happened during the hibernation period itself. Termination of hibernation coincides with very important granulocyte transformation and the appearance of specific gigantic short-lived cells. All of the changes in cell number and morphology are characteristics of different hibernation phases and have a major role in adaptive hypothermia. Granulocytes are hemocytes of great importance in immune response while specific functions of hyalinocytes and agranulocytes remain largely unexplored.