Praeaphanostoma

P. diatomovorum sp. nov. joins P. schillingi (HooGE =&=TYLER 2015) as the second Praeaphanostoma without a penis, a morphology that is incongruous with the proposed diagnostic character of Isodiametridae: a tubular isodiametric penis with inner circular muscles fibres and outer non-anastomosing longitudinal fibres (Hooge & Tyler 2005). Hooge & Tyler (2015) originally described P. schillingi as a species of Haplogonaria DöRJES 1968 because of this discrepancy and assumed the molecular data was due to error. Atherton & Jondelius (2021) later reassigned the species to Praeaphanostoma based on results from their phylogenetic analyses as well as the morphological similarities between P. schillingi and P. rubrum DöRJES =1968. With P. diatomovorum sp. nov., our results strengthen the inclusion of P. schilingi in the genus and suggest that an isodiametric penis should not necessarily be a diagnostic feature of the genus or family.

Unfortunately, without the definitive presence of a penis, the generic diagnosis of Praeaphanostoma does overlap with the generic diagnosis of Haplogonaria . Since no sequence data is available for any species of Haplogonaria, current phylogenetic analyses cannot clarify the situation. Representation of Haplogonaria in a comprehensive phylogenetic analysis must be prioritized in the future to establish the relationships between species of Haplogonaria and Praeaphanostoma and to help uncover apomorphic characters for the two genera.

P. diatomovorum sp. nov. can be easily distinguished from P. schillingi by the smaller body size and by the lack of bright red body pigmentation. Further, P. diatomovorum sp. nov. can be distinguished from the majority of species of Haplogonaria by its general body shape and size as well as the moderate size of the frontal organ. Finally, it differs from H. minima (WESTbLaD 1946) and H. psammalia FaubEL 1974 by the amount of rhabdoid glands (sparse vs numerous), the size of the bursa (~65 µm in length vs 100–150 µm) and the lack of a bursal cap; and from H. arenaria (ax 1959) by its less well-developed testes, the presence of a short vagina and the absence of vacuoles.

Praeaphanostoma neglectum sp. nov. possesses many typical features of the genus: a walled seminal bursa without appendages, a muscular seminal vesicle with an embedded penis and a small frontal organ. However, it is distinguished from the other species by the combination of an unpaired ovary, an absence of vacuoles, and a body length less than 1 mm.

With the inclusion of the two new species presented here, there are a total of 18 nominal species of Praeaphanostoma described globally. A key to the genus is presented below.

Key to Praeaphanostoma

1. Ovary unpaired....................................................................................... 2

- Ovaries paired....................................................................................... 11

2. Vacuoles present...................................................................................... 3

- Vacuoles absent...................................................................................... 6

3. Large vacuoles restricted to posterior half of animal; posterior tactile bristle present; gonopore common........................................................................................... P. chaetocaudatum DöRJES 1968

- Vacuoles numerous along entire body; tactile bristle absent; gonopores separate.................................... 4

4. Seminal vesicle located near mid-body; posterior tail long, tapering, heavily vacuolated; male and female gonopores well separated............................................................................ P. brevifrons DöRJES 1968

- Seminal vesicle located closer to posterior end; male and female gonopores close together........................... 5

5. With numerous gland cells surrounding the distal portion of the seminal vesicle; penis lumen without glandular materials............................................................................. P. vitreum EHLERS =&= DöRJES 1979

- Gland cells surrounding the seminal vesicle few or absent; dense nuclei and glandular material present in penis lumen............................................................................. P. wadsworthi HooGE =&=TYLER 2003

6. Body without pigmentation............................................................................. 7

- Body pigmentation present.............................................................................. 9

7. Body long, 3.5 mm; penis curved, ~200 µm long........................................... P. longum DöRJES 1968

- Body length less than 1 mm; penis small or absent........................................................... 8

8. Rhabdoid glands sparsely present; penis absent........................................... P. diatomovorum sp. nov.

- Rhabdoid glands abundant in rows; penis distinct, 12 µm in length.............................. P. neglectum sp. nov.

9. Body brown-olive in color............................................... P. thalasophilum EHLERS =&= DöRJES 1979

- Body pigmentation red................................................................................ 10

10. Rhabdoid glands sparse; genital atrium absent; muscular penis present......................... P. rubrum DöRJES 1968

- Rhabdoid glands numerous; genital atrium present; penis absent..................... P. schillingi (HooGE =&=TYLER 2015)

11. Gonopores separate.................................................................................. 12

- Gonopore common................................................................................... 13

12. Seminal bursa with muscular walls; vaginal sphincter well-developed; rhabdoid glands arranged in longitudinal rows.................................................................................... P. parvum RIEGER =&= oTT 1971

- Seminal bursa musculature and vaginal sphincter weak; few rhabdoid glands scattered.......... P. sizilianum (RIEDL 1954)

13. Vacuoles numerous along entirety of the body.............................................................. 14

- Large vacuoles absent or restricted to posterior body half..................................................... 15

14. Vagina lined with microvilli and granular gland cells.............................. P. foramivora HooGE =&=TYLER 2008

- Vagina without microvilli and granular gland cells............................. P. musculosum EHLERS =&= DöRJES 1979

15. Vacuoles absent; vagina without sphincter....................................... P. gusana HooGE =&=EPPINGER 2005

- Posterior vacuoles and vaginal sphincter present............................................................ 16

16. Pigmented eyespots present......................................................... P. ornatus (kozLoFF 2000)

- Eyespots and body pigmentation absent................................................................... 17

17. Genital atrium inconspicuous; penis tubular without lobes......................... P. baltalimaniaformis (DöRJES 1968)

- Genital atrium large; penis with paired distal, finger-like lobes.......... P. hyalinorhabdoida (kåNNEbY=&= JoNDELIuS =2013)