Dietary preferences and diet effects on life-history traits of tardigrades

Tardigrades may be divided into the following feeding groups: herbivores, carnivores and omnivores. However, little is known about their specific feeding preferences. Here, we used a number of laboratory experiments with 18 potential food sources, representing a wide variety of organisms, to test feeding preferences, survival and fecundity of three tardigrade species, representing different feeding modes. We also tested for differences in preferences between juveniles and adults, and differences in survival between two age groups: one that started the experiment as juveniles and the other as adults. In our experiments, Milnesium inceptum is confirmed to be a carnivore, being able to reproduce only on animal prey. We also show that Hypsibius exemplaris is a herbivore, feeding on cyanobacteria, algae and fungi. Paramacrobiotus fairbanksi, on the other hand, is demonstrated to be an omnivore, feeding on cyanobacteria, algae, fungi and animals. In some cases, juveniles preferred different types of food than adults. Reproduction was strongly affected by food type. Finally, we demonstrate that tardigrades may ingest food types that they are not able to digest. Thus, gut content analysis may be misleading as a method of studying tardigrade feeding habits.