The presence of fine sand in the muddy sediments affects habitat selection and accelerates the growth rate of Limnodrilus hoffmeisteri and Limnodrilus claparedianus (Oligochaeta)

Assessment of food conditions for animals is usually based on the quality and quantity of available food, whereas less attention is paid to other factors affecting the processing of ingested food. One of the commonly used mechanisms involving non-digestible objects from the environment is lithophagy (using swallowed stones or sand grains as gastroliths). Therefore, the aim of this laboratory study was to investigate the impact of sand present in muddy bottom sediments on ubiquitous freshwater oligochaetes: Limnodrilus hoffmeisteri and Limnodrilus claparedianus. They belong to the most common tubificid species and, as detritivores, play crucial role in the functioning of water ecosystems. We showed that the presence of sand in the environment has positive effects on oligochaete habitat preferences and growth. They selected the mixture of mud and 60–160 µm sand over pure mud and mud with larger sand fractions. Their growth rate was highest in the sediments with an admixture of this sand fraction, and simultaneously the largest number of sand grains of this size was found in their digestive tracts. In our opinion, the oligochaetes used the sand grains to grind the ingested food, which should accelerate digestion and nutrient assimilation, and stimulate their growth rate.