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Biological invasions of non-native species are one of the most serious factors threatening global biodiversity. Their negative impacts are particularly evident in the case of freshwater ecosystems and native crayfish populations that live there. Generally, the higher dominance of non-native crayfish species over the native species is relatively well known, but the interactions between non-native crayfish species are significantly less investigated. Although their distribution is increasingly more important and it is obvious that non-native crayfish species will dominate in European waters. However, it is not yet clear which of these crayfish will be more successful. The aim of this diploma thesis was to create a literature review on the topic of interspecific interactions in crayfish with emphasis on native and non-native crayfish species occurring in Europe. The experimental part of the thesis was focused on the comparison of direct interactions and competition for shelter between the widespread spiny-cheek crayfish Faxonius limosus (Rafinesque, 1817) and the newly increasingly occurring marbled crayfish Procambarus virginalis Lyko, 2017. These species are becoming increasingly common in the European wild, but about their mutual direct interactions and competition is largely unknown. During agonistic interactions the marbled crayfish won in significantly more fights than the spiny-cheek crayfish in the absence as well as presence of the shelter. The marbled crayfish established a significantly greater dominance over the females of the spiny-cheek crayfish in the absence and in the presence of shelter, but not over the males. It is also important to say that the presence of the shelter reduces the number of fights and their total duration. There was no significant difference in dominance established by occupation of shelter between the marbled crayfish and the spiny-cheek crayfish. Still, thanks to its aggressiveness, marbled crayfish is able to compete with other non-native species or displace them from freshwater ecosystems.

A wide variety of factors are predicted to influence the intensity and outcome of agonistic interactions in animals, including the resource holding potential of the opponents and the nature and value of the resource over which the individuals are competing. Field crickets (Orthoptera: Gryllidae) have been used extensively as model organisms with which to study animal contests, but relatively few studies have examined the effect of mandible size or structure, or the level of contact with females on the intensity and outcome of agonistic interactions. To do so was the aim of the present thesis, using Gryllus bimaculatus as the study species. The first finding of this study was that there is a significant degree of sexual dimorphism for anterior components of the anatomy in G. bimaculatus. The mandibles, head and pronotum of male crickets were all relatively larger than those of females. This indicates that these traits may be acted upon by intrasexual selection. In many animal species that show sexual dimorphism, a trade-off in development sees enhanced weapon growth at the expense of testes size, but no such relationship was seen in this species. A comparison of the mandible structure of males that either won or lost at flaring and or wrestling showed that a relatively wider mandible span was a significant predictor of success during mandible displays. It was also found that specific components of tooth structure, namely the length of the incisor and length to distal tip, were significantly associated with victory at the jaw flaring stage. This is the first time that mandible shape has been shown to affect fight outcome in the Gryllidae, and also the first confirmed identification of a visual cue component of fighting behaviour. Despite the effectiveness of their weapons in fighting, body mass is a primary predictor of victory in combat between G. bimaculatus males, with the greater the degree of asymmetry in weight the more likely the heavier fighter will win. However, a study of fighting behaviour between asymmetrically matched opponents found that even males who were out-weighed by 40% were still likely to escalate the fight to grappling. Furthermore, males who were able to fend off their larger opponent in their first clash were significantly more likely to win their overall encounter. This hyper-aggressive response may therefore represent an adaptive mechanism to extreme odds and is worthy of further study. Female contact is known to be a significant promoter of male aggression and fighting enthusiasm, and mate guarding aggression is well documented in G. bimaculatus. A recreation of two contradictory studies, including one which concluded that mating makes males lose fights, highlighted that female contact after spermatophore transfer can overcome the loser effect and cause a male to re-engage with a previously dominant opponent. Fighting behaviour in this species is therefore highly flexible and factors affecting the outcome of contests are complex. There is much scope for further studies on this topic.

FundaÃÃo Cearense de Apoio ao Desenvolvimento Cientifico e TecnolÃgico / Territorial fights between males are frequently observed among insects. However, despite the existence of theoretical models directed at explaining the possible rules adopted to decide the winner, there is little empirical support for these models. The low empirical support may be explained by difficulties in identifying traits that determine the male fighting capacity (RHP- or Resource Holding Potential) and also by the lack of simultaneous tests of different models. In this study we evaluated males traits that may determine RHP in the dragonfly Diastatops obscura. Also, we evaluated the relationship between such characteristics and duration of disputes between pairs of males to test predictions derived from the models War of Attrition (GDA), Sequential Access Information (ASI) and Cumulative Access Information (ACI). Male weight was the main feature related to the winning chances. The dispute duration decreases with increasing weight and presented the opposite pattern for the loser male, rejecting the GDA as the decision rule adopted by males to settle territorial interactions. In addition the duration of the dispute was not related to the loser male weight when we considered only pairs of rivals who presented similar weight differences. This indicates that the rivals make mutual assessment of RHP, as assumed by the model ASI. Since wings are highly melanized, we suggest that males may perform rival assessments of RHP based on wings traits, as occur in odonate species. / Apesar das brigas entre os machos serem frequentes e de existirem modelos teÃricos que postulam possÃveis regras usadas para decisÃo do vencedor, pouco suporte empÃrico direcionado ao entendimento das brigas tem sido encontrado. Provavelmente esse fato ocorre devido à dificuldade em identificar quais caracterÃsticas determinam a capacidade de luta (RHP - Resource Holding Potential) dos indivÃduos e na escassez de testes simultÃneos de diferentes modelos. Neste trabalho, usamos machos da libÃlula Diastatops obscura para identificar caracterÃsticas determinantes do RHP. Posteriormente usamos informaÃÃes sobre a relaÃÃo entre tais caracterÃsticas e a duraÃÃo das disputas entre pares de machos para testar previsÃes derivadas dos modelos Guerra de Atritos (GDA), Acesso Sequencial de InformaÃÃo (ASI) e Acesso Cumulativo de InformaÃÃo (ACI). O peso representou a principal caracterÃstica determinante da chance de vitÃria em uma disputa. A duraÃÃo da disputa diminuiu com o aumento do peso do vencedor e aumentou com o aumento de peso do perdedor, refutando o modelo GDA como explicaÃÃo das regras de resoluÃÃo das brigas. Adicionalmente a duraÃÃo da disputa nÃo esteve relacionada com o peso do macho perdedor quando consideramos apenas pares de rivais que apresentaram diferenÃas semelhantes de peso. Isso indica que os rivais fazem avaliaÃÃo mÃtua do RHP, como presumido pelo modelo ASI. Uma vez que as asas sÃo fortemente melanizadas, sugerimos que eles podem realizar avaliaÃÃes da capacidade de luta ao obter informaÃÃes sobre a coloraÃÃo das asas, como ocorre em outras espÃcies de libÃlulas.