Unionicola (Anodontinatax) intermedia (Koenike 1882)

SYNONOMY—

Unionicola (Anodontinatax) intermedia (Koenike 1882) in Vidrine 1986a, 1996, Edwards and Vidrine 2013 and Gerecke et al. 2016

Unionicola (Pentatax) intermedia (Koenike 1882) in Cook 1974, Hevers 1978 and Viets 1987

DIAGNOSIS: Dorsum with 4 small plates; female genital field with 2 pairs of acetabular plates that are similar to those of Pentatax and Vietsatax; male genital field similar to Pentatax; pedipalps similar to Pentatax; coxal plates grouped into 4 groups; reticulate structure in secondary sclerotization; male fourth walking leg sexually dimorphic with groups of serrated setae on the distal genu and mesial tibia; all walking legs with deeply bifid tarsal claws with dorsal prong longer than ventral prong as in the subgenera Unionicola and Vietsatax. Female leg II in Vietsatax is considerably longer than leg III—the difference in length in U. intermedia is not as obvious.

NOTES: Unionicola intermedia appears to be a European mussel mite (Tuzovskij and Semenchenko 2015). The females are residents, while the males are transients; thus, many collections contain one sex or the other depending on the site being sampled. The males have a set number of (pectinated) setae aggregated into clusters on the fourth and fifth segments of the fourth walking leg: Ge (IV-L-4) 18-20 setae, Ti (IV-L-5) 17-19 setae. The American species, U. wolcotti (Piersig 1900) has 6-8 and 9-11.

Vidrine (1996) discussed the American species that closely resembles this species. “Unionicola wolcotti is a sibling species with U. intermedia (Marshall 1933b, Mitchell 1957b, and Vidrine 1986a). The latter species commonly parasitizes species of Anodonta in Europe and Asia (Mitchell and Pitchford 1953, Mitchell 1957b, and Hevers 1978c). The females of both species are apparently indistinguishable, while the males are separated based upon the structure of the fourth walking legs (Mitchell 1957b and Vidrine 1986a). Unionicola intermedia is adequately figured in Hevers (1978c), but the female dorsum is figured in Vidrine (1986a), based upon specimens from Holland and Poland. Hevers (1978c) also provided

rather extensive series of measurements of German specimens of this latter species.”

Four species in this subgenus have been described from Asian mussels in the genus Sinanodonta. The species include U. ischyropalpus Wen, Wu and Zhu 2008, U. papillata Wen et al. 1998, U, penicillatus Wen et al. 2005 and U. rezvoi Sokolow 1931.

How do we separate the Asian species? Females of U. rezvoi have 3 pairs of dorsal plates (the posterior pair very small) compared to 2 pairs in U. intermedia, none in U. papillata, 2 pairs in U. penicillatus, and none reported in U. ischryopalpus. Unionicola papillata and U. penicillatus are known from females only; both have distinctive pedipalps, especially the tibia (P-4), where the former has an enlarged, ventral paillous protrusion and the latter has a very swollen segment. Unionicola ischropalpus can also be distinguished by its medially rounded posterior coxal plate group, although they are somewhat rounded in U. rezvoi. Females of U. penicillatus are identical to U. intermedia, but the tarsus of the pedipalp appears to be shorter relative to the latter. Males of U. penicillatus have pectinate setae on leg IV: Ge (IV-L-4) 8-10 setae, Ti (IV-L-5) 22-24 setae—compared to: U. intermedia with Ge (IV-L-4) 18-20 setae, Ti (IV-L-5) 17-19 setae and U. rezvoi with: Ge (IV-L-4) 13-14 setae, Ti (IV-L-5) 17-18 setae, but the setae in U. rezvoi are not as tightly aggregated as the other species (based on drawing in Tuzovskij and Semenchenko 2015).

Here is a table of the recorded body lengths, which may help in sorting the species:

*Body lengths in micrometers (µm):

U. intermedia—females 770-940 (TS), 520-1160 (G); males 750-800 (TS), 630-690 (G).

U. rezvoi—females 1615-1700 (TS); males 750-800 (TS). U. penicillatus—female 1289; male 827 (W1). U. papillata—female 855 (W2). U. ischryopalpus—female 721-830 (W3).

*Sources: TS= Tuzovskij and Semenchenko 2015, G= Gerecke et al. 2016, W1= Wen et al. 2005, W2= Wen et al. 1998 and W3= Wen, Wu and Zhu 2008.

Based on the measurements, U. rezvoi and U. penicillatus females are noticeably larger and form a size cluster, while the other 3 species form another cluster. The males form a single cluster.

It should be obvious that these species form a radiation around a general body form with distinctive modification in the fourth walking legs of the males. The females are difficult to separate, and the males require the counting of setae.

HOSTS—This species is usually found in mussels of the genera Anodonta and Pseudanodonta.

Figure 1: female dorsal plates from Tuzovskij and Semenchenko 2015:

Jouni Taskinen provided specimens from Finland Anodonta for my study and photography. The following plates are taken from these specimens.

Plate 1: female venter:

Plate 2: female venter:

Plate 3: male venter:

Plate 4: male venter:

Plate 5: pedipalp:

Plate 6: genu, tibia and tarsus of pedipalp:

Plate 7: male leg I:

Plate 8: male leg IV:

Plate 9: female leg I:

Plate 10: female leg IV (2 views):

Plate 11: female legs:

Plate 12: female genital field (2 views):

Plate 13: male genital field:

Plate 14: tarsal claw of leg IV and close view of genu, tibia and tarsus:

Plate 15: Top left: female venter. Top right: female venter. Middle left: female pedipalp. Middle right: leg I. Bottom left: tarsus of pedipalp. Bottom right: tarsal claw of leg I.

Plate 16: Top left: female dorsal plates visible deep to the coxal plates. Top right: female leg IV. Bottom left: female genital field. Middle right: tarsal claws of leg IV. Bottom right: close view of tarsal claw of leg IV.