Tropidoatractus spinosus was first described as Metopus spinosus by Kahl in 1927. In 2018, Rottovera et al. redescribed the species and transferred it to the genus Tropidoatractus based on morphological characters.

In my localities Simmelried and Bündtlisried I find Tropidoatractus spinosus rarely, but regularly. The species is easily recognized by the slightly twisted, S-shaped body and the typical caudal  spine, which is shifted to one side of the body. In my population I could find some differences to the descriptions of Tropidoatractus spinosus I have. On all drawings (apart from Rotterova et al., s drawings above) of Tropidoatractus spinosus the caudal spine is drawn on the left side of the body. In my specimens, however, it was localized exclusively on the right body margin (s. figs. 1 a-c and 2 a-c). My specimens were up to 100 µm long (s. fig. 2 a-c) and thus much longer than Kahl indicates (50–80 µm) but in the range indicated by Rottovera et al. (60–115 µm). Very striking in all specimens I examined was that the macronucleus was completely filled with at least two species of bacteria (s. fig. 3 a-b), whereas the cytoplasm was free from bacteria. According to Rotterova et al. these bacteria are presumably parasitic comparable to Holospora in the micronucleus of Paramecium. However, in all specimens of Tropidoatractus spinosus I observed, the macronucleus was infected with these bacteria. This makes a parasitic nature of the bacteria very unlikely, because an infection rate of 100% does not occur even with Holospora. The two types of bacteria are not homogeneously distributed in the macronucleus. In the margin of the macronucleus thin rods with a length of about 4 µm are localized (s. fig. 3a), while in the center there are mainly thicker rods with a length of about 5 µm (s. fig. 3b).