Key to species of Rhabdochona

According to Moravec et al. (1981) the closest relative of R. zacconis Yamaguti, 1935 is probably R. canadensis Moravec & Arai, 1971; the two species “differ distinctly only in the position of the deirids (more anterior relative to length of the vestibule in R. canadensis) and the shape of the distal end and width of the left spicule”. There is possible confusion here because Moravec & Arai (1971) stated that the left spicule of R. canadensis is “conspicuously slender” but, at a width of 0.06–0.09 (see Moravec et al., 1981), it is not as slender as that of R. zacconis at 0.012–0.015. For these reasons the following key does not attempt to distinguish the two species (although separate descriptions are given). It is relevant to point out that neither Anon. (1978) nor Arai & Mudry (1983) described or illustrated the worms from Catostomus macrocheilus in British Columbia that they claimed represented R. zacconis. This species has not been previously reported from North America so it is possible that the worms represented R. canadensis rather than R. zacconis.

1 Mature eggs smooth, without filaments or gelatinous “formations”............................................. 2

- Mature eggs with gelatinous “formations”, mostly in the shape of polar caps, or filaments............................ 4

2 Left spicule longer than 0.8; length ratio of right to left spicule 1:10.2 to 1:11.9.......................... R. decaturensis

- Left spicule shorter than 0.6; length ratio of right to left spicule 1:3.1 to 1:4.2..................................... 3

3 Right spicule with truncated proximal end; tail ends in rounded tip............................... R. rotundicaudatum

- Right spicule with bulbous, convex proximal end; tail ends in sharp point............................... R. cascadilla

4 Prostom with 16 teeth; egg surfaces covered with fine protuberances and numerous short filaments.......... R. ovifilamenta

- Prostom with 10 or 14 teeth............................................................................ 5

5 Prostom with 10 teeth................................................................................. 6

- Prostom with 14 teeth................................................................................. 7

6 Prostom lacks basal teeth; deirids small, bifurcate................................................. R. catostomi

- Prostom with basal teeth; deirids medium-sized, bifurcate; eggs with gelatinous “formations” covering most of poles and lat- eral surfaces................................................................................. R. kisutchi

7 Tail tip rounded; eggs with two long single, rarely double, filaments; parasitic in Cottidae ....................... R. cotti

- Tail ends in sharp point............................................................................... 8

8 Both egg poles with numerous fine filaments........................................................ R. milleri

- Both egg poles with at least one filament, sometimes two filaments on one or both poles......... R. canadensis / R. zacconis