I only very rarely find Chaetonotus arquatus in the mud layer of the Simmelried. I have not yet been able to find the species in my other sampling sites.

The quite strong, curved spines of Chaetonotus arquatus are widely separated from each other, which can be recognized even at small magnifications (s. fig. 1 a-d). Chaetonotus arquatus also has two elongated spines at the posterior end, which protrude over the toes with furcae (s. fig. 9).

The dorsal spines have no secondary tip and arise from almost square-shaped scales, which do not overlap and are clearly separated from each other (s. fig. 4). The spines are clearly thickened at the base (s. fig. 6). The dorsal scales in the middle of the body of the specimens in my population had clearly rounded corners and were also larger than Voigt drew them (s. fig. 6 and drawing above). The scales on the head have a clear distal incision, while they are more semi-circular in the neck region (s. fig. 5). However, they all have simple spines as in the middle of the body. At the posterior end, however, the scales are more oval and waisted, but with only a keel. I could not recognize a naked area in the anal region, as described by Schwank (1992).

The scales in the middle of the body are cup-shaped, as drawn by Voigt (s. drawing above). This shape can be recognized if the focal plane is on the margin of the body (s. fig. 7). Due to the cup shape, the surface of the body appears “humpy”, making it difficult to recognize the shape of the scales in unsquashed specimens. Unfortunately, I have not yet been able to examine the ventral scales.

More images and information on Chaetonotus arquatus: Dr. Michael Müller-Mikroskopie Forum-Chaetonotus arquatus