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Organisms show an extraordinary variation in their life history traits, both at inter- and intraspecific level, exhibiting phenotypic variations among populations inhabiting different habitat. The variability in life history traits has been related to environmental conditions, as a result of selective processes or phenotypic response to them. In addition to the huge diversity in life history traits, several trade-offs among them have been recognized. The variability in life history traits and in their trade-offs related to environmental conditions represents a key aspect in the study of evolutionary processes. A wide variation in life history traits is documented among teleost fishes, both among and within species, reflecting the effects of evolutionary forces acting on them over time and across environmental conditions. Several life history traits, such as growth rate, age at sexual maturity, fecundity, reproductive investment, egg size, hatching size, are strongly influenced by temperature and food availability. Notothenioid fish represent a unique example of fish adaptive radiation in marine environment. They dominate the waters surrounding the Antarctic continent both by species number, with over 120 species (47%), and biomass (90-95%). Notothenioids distribution is limited exclusively to the Antarctic and sub-Antarctic regions (South America, New Zealand and South East Australia). Variability in their life history traits at inter- and intra-specific levels has been described, with a latitudinal trends in some reproductive traits. In this framework, despite the availability of recent phylogenies of notothenioids, a comparative analysis aimed at studying habitat dependent evolution of reproductive strategies has not yet been performed. Given the particular characteristics of their habitats and the uniqueness of notothenioid fishes, they can be considered an excellent taxon model to investigate the evolution of life history traits in relation to environmental factors. The aim of this PhD project was to investigate the evolution of life history traits of notothenioid species in relation to environmental variables (such as sea water temperature, primary productivity and sea ice cover), controlling for phylogenetic relationships. The applied methodology (Independent Contrast Method) represents a key aspect in this study since its robustness can prevent the attribution of correlations between life history traits and environmental variables to evolutionary processes, being instead a consequence of phylogenetic relationships among considered species. Using life history traits estimated from collected species and reported in data available in literature, the study focused the attention on gametogenesis and, gonadal investment, such as fecundity and egg size. Such a comparative study of habitat dependent variation of life history traits is interesting for evolutionary biology, given the extreme adaptations shown by Antarctic fishes to their peculiar environment, as well as for conservation biology, because the knowledge of reproductive traits and of their sensitiveness to environmental changes is recognized as a crucial information for the sustainable management of exploited species and their resilience capacity to overcome current climate change. In the present research, life history traits of 17 species, belonging the eight notothenioid families were investigated (Papers I, II, III, IV, V, VI), providing original information on their biology and strengthening the reliability of the available literature. Intraspecific variability in life history traits was examined in three species, including in the analyses specimens sampled in different areas (Papers I, III and IV). Unpublished life history traits data of 15 species were included in the interspecific comparison (Paper VI). Species included in this study were collected during two cruises carried out onboard the German R/V Polarstern (ANT-XXVIII/4, 2012 and ANT-XXIX/9, 2014) off the Antarctic Peninsula and in the Weddell Sea. Samples, belonging to four notothenioid species, were provided by partner scientists during past Antarctic expeditions (2009, 2010 and 2011 during the austral summer) in different areas (i.e. South Orkney, South Georgia, Burdwood Bank and Western Antarctic Peninsula). Comparing the results obtained in studies dealing with individual species, the histological analyses generally indicated some similarities among notothenioids. In all the analysed species, females presented a group synchronous ovarian type, in which two clearly distinct groups of oocytes were visible. The more advanced stage of maturity in maturing or mature females were oocytes at late stage of vitellogenesis, constituting the batch of the current season, while the other group included oocytes at the cortical alveoli or previtellogenic stages, representing the next year batch. The occurrence of late vitellogenic oocytes together with others at the cortical alveoli stage supports a prolonged oogenesis, likely lasting more than one year. Testes were of the unrestricted spermatogonial testicular type. In mature males, testes were completely filled with sperm with few cysts of early-stages of spermatogenesis, indicating that sperm maturation completes before the beginning of the breeding season. Long lasting gametogenesis is likely related to the low temperature and to the high reproductive investment. The study on individual species provided new data on absolute and relative fecundity, egg size, gonadosomatic index (GSI) and age estimates and therefore information useful for the comparative analyses (Paper VI). By means of age estimates through otolith reading, growth rate, age/length relationship, maximum age and age at sexual maturity were assessed (Papers II, III, IV, V). As expected for cold water fish species, notothenioids exhibit slow growth rate, late sexual maturity (at about 50-87% of maximum size) and long life span (up to 24 years). The intraspecific comparison highlighted that the observed variation in life history traits could be addressed to local conditions (i.e. local prevailing currents linked to cold water masses and melting ice) and/or be a consequence of the species reproductive habits, including egg type, presence/absence of parental care, adult distribution and mobility. In Chaenocephalus aceratus, a benthic species with demersal eggs and parental care, fecundity was higher in the warmer study area, that is not influenced by cold water masses coming from Weddell Sea, respect to the colder area. In the Antarctic silverfish (Pleuragramma antarctica), the only species lying cryopelagic eggs (i.e. eggs laid to the lower surface of platelet pack ice), and in Notothenia rossii, in which individuals shift from benthic to bentho-pelagic habits achieving the adult phase, no differences emerged between populations, with the exception of some differences in oocyte size and GSI, likely related to temporal differences in sampling periods. The results of the interspecific comparative analyses highlighted some correlation between environmental factors and life history traits, revealing potential evolutionary forces. Differently from the other biological traits considered in this study, the maximum size did not correlate with any environmental variable. These results support the observation that the Bergmann’s rule, describing the occurrence of a positive relationship between maximum size and latitude, with larger specimens found in colder environments (higher latitudes), does not apply at high latitudes. In notothenioids egg size was negatively related to primary production. Primary production, in turn, is negatively related to latitude, and general theoretical models and relevant studies indicate a positive relationship between egg size and latitude. Food availability and temperature have been claimed as factors driving the general increasing trend in egg size with latitude. Primary production can be considered a proxy of food availability, therefore in notothenioids food availability appears to be the main factor influencing egg size. A positive relationship between egg and larval size is common, and larger larvae are expected to show enhanced competitive abilities, improved capacity to feed on a wide size range of prey items and enhanced starvation resistance, and ultimately higher survival probabilities in extreme conditions. Egg size is also positively related to the maximum parental body size. Generally, large fishes, as for instance tunas or the Ocean sunfish Mola mola, are not more likely to have large eggs than small ones, nonetheless fish size may indeed constrain, rather than determine, the range of possible egg sizes, as suggested also by the results on notothenioids. Large notothenioids could therefore have a wider range of tactics in the partition of reproductive effort between fecundity and egg size. While Low Antarctic species produce small and large eggs, High Antarctic species present only large eggs. Considering the trade-off between egg size and number, a positive relationship between relative fecundity (i.e. the number of eggs in relation to body weight) and primary production was expected but it was not found. Conversely a positive relationship was observed between relative fecundity and mean water temperature, although it remains an unsolved issue which need to be further investigated. Energetic investment in female reproductive effort, i.e. maximum female GSI, showed a positive relationship with sea ice cover. Seasonal pack ice melting, occurring in summer at intermediate Antarctic latitudes, triggers phytoplankton blooms, resulting in an increased primary production with a cascading effect on the pelagic food web. At higher latitudes, where maximum values of GSI have been detected, the sea ice cover is permanent through the year and the primary production is likely to remain low, being also influenced by the long and dark Antarctic winter. These extreme conditions may therefore trigger a higher investment in reproduction, including the investment in eggs (size and/or number) represented by the female GSI. The insights provided by this study shed light on the major factors that appear to drive the evolutionary processes occurring in the Antarctic environment. The comparative method proved to be a robust tool in investigating adaptive response to different environmental conditions. Despite notothenioids demonstrated to be an excellent model group to study evolutionary process, further investigations, extending to other taxa and species geographical distributions, are necessary to trace more general and comprehensive patterns in the evolution of life history traits. In any case, shared reproductive features such as low fecundity, large egg size, high reproductive investment in gonads and, in some cases, in parental care, low growth rate and late sexual maturity, depict notothenioids as a taxon highly vulnerable to climate change and fishery re-opening scenarios.

This thesis attempts to clarify the evolution of secondary sensory cells in Tunicata (Chordata), the Vertebrata sister group, to study these cells in species belonging to different classes using a morphological approach, and to gain insights into their development using a molecular approach based on important developmental genes. In chapter 1, we analysed the mouth of three species of Thaliacea, representative of the three orders, to verify the presence of secondary mechanoreceptors comparable with the ones in the coronal organ described in the Ascidiacea class. We showed that oral secondary sensory cells are present with the exception of one species. We hypothesised that the presence of these cells in the oral region is related to the animal feeding mechanism. In chapter 2, we extended the morphological study to the class Appendicularia. These species are known to possess an oral sensory organ, the circumoral ring, composed of secondary sensory cells. We described these cells in detail, thus completing the data collection of the main tunicate groups. Moreover, based on these results and data from the literature, we performed a cladistics analysis using the cephalochordate amphioxus and vertebrates as outgroups. Our research showed that all tunicate secondary sensory cells may have evolved from a monociliated prototype, which successively differentiated into the present variety of secondary sensory cells. Chapter 3 describes the development of the oral secondary sensory structures in the model Ciona intestinalis (Ascidiacea). In this animal, we found that secondary sensory cells after the metamorphosis could be identified as cells with short cilia and microvilli, which later become pluriciliated. Moreover, we have provided evidence that secondary sensory cells can proliferate during development. Finally, chapter 4 shows that some genes involved in neural and sensory cell differentiation in vertebrates (Notch, Delta, Hairy/Hes, Atoh, Musashi) and other molecules implicated in neurotransmission (TRP channels and glutamate) are expressed in the secondary sensory cells of C. intestinalis. Taken together, the results presented in this thesis confirm that secondary sensory cells are a common feature of tunicates and have an important role as mechanoreceptors, likely playing a role in the feeding process. Moreover, our data suggest that important genes involved in the differentiation and function of these cells might have been present in the common ancestor of tunicates and vertebrates. Thus, these cells represent a plesiomorphic feature in these two groups.