For the parasitological survey, 23 cichlid species were screened for metazoan ecto- and endoparasites (Supplementary Table 1). Sampling was conducted at Toby's place on the Zambian shoreline of Lake Tanganyika. While most fish species were obtained in August 2012, S. diagramma and H. microlepis were captured in August 2011 and July 2013, respectively. One species, A. burtoni, was obtained in July 2013 at Kapata, which is about 20 km southward. About 7 to 18 individuals per species (usually ten) were caught by chasing fish into standing nets (Supplementary Table 1). After capture the fish were kept in tanks of 0.8 m x 0.8 m x 1.2 m depth or 0.8 m x 0.8 m x 2 m depth. Before usage, tanks were cleaned, dried and filled with lake water.
All fish were dissected in the field within four days post-capture. The day of dissection (0, 1, 2 or 3 days after capture) was recorded in order to keep track of changes in parasitological parameters while the fish were kept in the tanks. Individual fish were killed with an overdose of MS222. The parasitological survey consisted of three parts. First, the outer surface and the mouth cavity of the fish were inspected for ectoparasitic monogeneans and crustaceans (copepods, branchiurans, isopods), bivalves, and any kind of helminthic cysts. Second, the four gill-branches on the left were dissected and stored on 100% analytical ethanol (EtOH), and later in the lab screened for ectoparasitic monogeneans, crustaceans (copepods and branchiurans), bivalves, and any kind of helminthic cysts. Third, fish were screened for intestinal monogeneans, digeneans, acanthocephalans, nematodes, and any kind of helminthic cysts. To do so, stomach, intestines, gall and urinary bladder were dissected and inspected in a petridish with lake water. Finally, the sex of the fish was determined by visual inspection of the genital papilla and gonad development.
The parasitological survey was performed with a stereomicroscope and by multiple observers. Observers were recorded in order to keep track of observer bias. A single observer screened the outer surface and the mouth cavity of the fish. The number of observers varied between years for gills and intestines (gills: two observers in each year; intestines: three, four and one observer(s) in 2011, 2012 and 2013, respectively). All parasites were counted and identified to genus or class level and preserved as follows. Monogeneans were isolated using dissection needles and were either mounted on slides in ammonium picrate glycerine for further morphological characterization, or stored on 100 % EtOH. Acanthocephalans and nematodes were stored on 80 % EtOH, while intestinal monogeneans, branchiurans, copepods, any kind of helminthic cysts, bivalves and unknown groups were stored on 100 % EtOH.
