Investigating the Gill-Oxygen Limitation Theory (GOLT) in the context of gill parasite diversification

Abstract

The Gill-Oxygen Limitation Theory (GOLT) claims that the two-dimensional surface area of gills in Water-Breathing Ectotherms (WBE) is the limiting factor in the supply of oxygen to support the growth of their three-dimensional bodies. This limitation would lead to a shift in a range of physiological processes, most notably a slowing down of growth. Given how GOLT could affect all aspects of growth in WBE, we hypothesise that its effects would be stronger on larger-bodied than on smaller-sized species and that it would affect freshwater taxa more than marine species. Indeed, freshwater WBE, especially those occurring in the tropics, often have to cope with large fluctuations in oxygen concentrations. Hence, we propose African lates perches, Lates spp., the largest tropical freshwater fishes that are entirely dependent on their gills for gas exchange, as a model to test the predictions of the GOLT. We will also study their gill parasites, and investigate how they affect gill function. Several species and populations of African Lates spp. are infected by a single species of gill-infecting monogenean parasite: Dolicirroplectanum lacustre. This finding is surprising as, normally, parasite species diversity exceeds that of their host. Our research will apply the GOLT to understand this lack of diversification. We hypothesize that strong evolutionary pressures on the gill environment contribute to the limited parasite diversification. By measuring physiological gill stress and assessing both host and parasite stress, we aim to investigate the impact of gill function limitations on parasite and host growth. Studies suggest that monogenean parasites may impair gill function, with larger fish showing greater investment in combating these parasites. We will also examine the extent of gill damage caused by parasitic infections and use this to model how parasite infections affect fish growth. Our study will involve histopathological analyses of gill samples from infected and non-infected fish across different sizes and species. We will assess gill damage, infection levels, and stress responses, correlating these with environmental proxies such as oxygen and salinity. The results of this study will help solidify the quantitative basis of GOLT and expand its applicability to inter-specific interactions. By elucidating the relationship between gill stress, host-parasite dynamics, and environmental fluctuations, our research will contribute to a deeper understanding of the evolutionary pressures shaping parasite diversification in variable environments.