Is the Gut Content of Milnesium (Eutardigrada) Related to Buccal Tube Size?

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The Linnean Society of London Milnesium, with its relatively short and wide buccal tube, is considered carnivorous. Species in this genus exhibit differences in buccal tube length, standard buccal tube width, and the buccal tube length/width ratio. To determine whether buccal tube size is correlated with the type of prey, ˜4000 specimens of various Milnesium species were examined. Among those with identifiable gut contents, ˜97% contained tardigrades, rotifers, or both, whereas only ˜3% contained nematodes or amoebas. In total, 189 females with guts containing only tardigrades, only rotifers, or both were analysed with general linear model multivariate analysis. Milnesium specimens containing only tardigrade remnants were larger and had longer and wider buccal tubes than those containing only rotifers. Those with only tardigrades in the gut also had a significantly lower buccal tube length to width ratio than those with only rotifers in the gut. Specimens with a mixed diet (rotifers and tardigrades) had intermediate values of buccal tube width and length, and these differed significantly from individuals containing rotifers or tardigrades only. Variation in buccal tube dimensions was linked with interspecific and age (size)-related factors, and differences in buccal tube dimensions associated with gut content remained significant, even when the analysis was controlled for the interspecific variation. No evidence of cannibalism was observed.