Over the past few decades, public attention, activities in the field of conservation and animal welfare by nongovernmental organizations (NGOs), and scientific exploration are raising awareness on... Show moreOver the past few decades, public attention, activities in the field of conservation and animal welfare by nongovernmental organizations (NGOs), and scientific exploration are raising awareness on the potential effects of sounds on marine mammals and fish species. The aim of this thesis was to explore sound-induced behavioral changes in fish using captive zebrafish as a model species. I explored behavioural parameters as potential indicators of sound-related stress, disturbance and deterrence. In four different studies, I examined various sound exposure treatments to provide insights that may be useful for future explorations for indoor and outdoor sound impact studies as well as for assessing animal welfare and productivity in captive situations. Furthermore, my findings may also raise awareness for sound levels in laboratories and the potential effect on reliability for fish as a model species for medical and pharmaceutical studies. I also explored the complexity of sound fields in indoor fish tanks by selecting a different set-up for each study, which makes behavioural analyses and direct comparisons not only relevant within each study, but also provides insight into the role of fish tank acoustics on 'natural' and experimental exposure condition. Show less
Lake Victoria cichlids show the fastest vertebrate adaptive radiation known which is why they function as a model organism to study evolution. In the past 40 years, Lake Victoria experienced severe... Show moreLake Victoria cichlids show the fastest vertebrate adaptive radiation known which is why they function as a model organism to study evolution. In the past 40 years, Lake Victoria experienced severe environmental changes including the boom of the introduced, predatory Nile perch and eutrophication. Both environmental changes resulted in a decline of haplochromine cichlid species and numbers during the 1980s. However, during the 1990s and 2000s, some haplochromine species recovered. With the use of the unique Lake Victoria cichlid of collection of the Naturalis Biodiversity Center, this thesis showed how the haplochromines have adapted their ecomorphology to the changed environment. In response to a decrease in water clarity, increased predation, larger prey and low dissolved oxygen levels, these fish adapted their eyes, body shape, upper jaw and gills in a way that would be beneficial to the survival of the fish. These adaptive responses could be the result of phenotypic plasticity or genetically based changes such as natural selection or hybridization or a combination of these three mechanisms. This research showed that the Lake Victoria cichlids are able to adapt extremely fast which is likely to have contributed to their extreme fast adaptive radiation. Show less
Mate choice by female cichlid fish from Lake Victoria plays an important role in speciation and the maintenance of species. Females are expected to select against males that are intermediate in... Show moreMate choice by female cichlid fish from Lake Victoria plays an important role in speciation and the maintenance of species. Females are expected to select against males that are intermediate in their phenotype during the process of speciation driven by sexual selection. To test this, we hybridized two species of cichlids that differ in male nuptial coloration. Male hybrids were intermediate in nuptial coloration compared to the parental species. Hybrids had a comparable fitness to the parental species and did not suffer from inviability, sterility, skewed sex ratios, reduced survival or reduced growth rates. Females preferred males of their own species over males of the other species. This implies that female mate preference can drive speciation. Females of one species were able to distinguish between males of their own species and hybrid males; females of the other species however, were not able to see this difference. When there is hybridization in nature, for instance when coloration becomes invisible by an increase in turbidity of the water, there would be asymmetrical selection against hybrids. An understanding of these processes is crucial in speciation research and for the maintenance of biodiversity. Show less
Waar mensen zijn vindt nutriënten-verrijking van het water plaats. In de zomer kunnen algen hierdoor goed groeien. Dode algen en andere resten zakken naar de bodem waarna door rottingsprocessen de... Show moreWaar mensen zijn vindt nutriënten-verrijking van het water plaats. In de zomer kunnen algen hierdoor goed groeien. Dode algen en andere resten zakken naar de bodem waarna door rottingsprocessen de zuurstofconcentratie in het water sterk kan dalen. Het water wordt hierdoor onleefbaar voor vissen en andere organismen. In het Victoriameer in Afrika is het altijd zomer en grote gebieden zijn hier de laatste decennia zuurstofarm geworden. Men kan aannemen dat dit zeer slecht is voor de visstand. Toch blijken Victoriameervissen beter te kunnen overleven dan men dacht. In ons laboratorium in Leiden zagen we drie weken oude Victoriameervisjes (cichlidensoorten) zeer snel konden wennen aan zuurstofarme omstandigheden. Zij bleken zelfs met tien keer minder zuurstof (0,8 mg per liter) net zo hard te groeien als nestgenoten die onder normale omstandigheden leefden. Volwassen vissen gaan dood of overleven dit slechts tijdelijk. Jonge visjes kunnen zo goed overleven omdat zij tot 80% grotere kieuwen krijgen en continue een veel hogere ademactiviteit hebben. De aanleg van deze superkieuwen kost zoveel ruimte dat zelfs hun kop van vorm veranderde. Het bloed dat door de kieuwen stroomt bleek meer en beter zuurstof te kunnen binden terwijl een soort meerdere nieuwe typen hemoglobine bleek te produceren. Dit is een unieke eigenschap die bij andere dieren alleen optreedt bij geboorte of bijvoorbeeld transformaties van water naar land bij amfibieën. Het waterpeil in het Victoriameer was 14.000 jaar geleden extreem laag en mogelijkerwijs heeft een voorouder van deze soort deze truc destijds onder de knie gekregen die in het zuurstofarme water van nu weer van pas komt. Show less
