Unionicola (Unionicola) ypsilophora (Bonz 1783)

Plates 188-190 in Vidrine (1996a)

Synonomy—

• Unionicola (Parasitatax) ypsilophora (Bonz 1783), Vidrine 1986c, 1992b

• Unionicola formosa-ypsilophora complex, Baker 1982

• Unionicola (Unionicola) ypsilophora in Harvey 1998, Edwards and Vidrine 2013, Tuzovskij and Semenchenko 2015 and Gerecke et al. 2016.

Type locality and host-- Europe. The host was one of the Anodonta of Europe. Etymology—‘Ypsil’ means "Y-shaped," and ‘ophora’ means "to bear," thus ypsilophora reflects the obvious Y-shaped, white, excretory "bladder" borne on the black dorsum of these mites. Diagnosis-- Character states of the subgenus; male dorsum with an elongate plate; female dorsum with 2, small, widely separated plates; male posterior coxal group with well sclerotized and distinct posterior borders. Males (5 specimens)-- Length including capitulum 1150 (1000-1250) (all measurements are in micrometers (µm)); length of posterior coxal group 500 (480-540); dorsal lengths of pedipalp segments: Ti 185 (170-200); Ta 65 (60-70); dorsal lengths of leg segments: leg I: TFe 188 (170-200); Ge 255 (230-290); Ti 240 (210-270); Ta 178 (160-200); leg IV: TFe 263 (230-300); Ge 418 (360-480); Ti 553 (500-640); Ta 448 (410-500). Notes-- Specimens were measured from: Pyganodon cataracta (Say) from Sydney River southwest of junction of Highways 4 and 5, Cape Breton county, Nova Scotia, Canada, collected 4 May 1962 by A. M. Rick; P. cataracta from Lake George at southwest end, New Brunswick, Canada, collected on 30 June 1962 by A. M. Rick; A. implicata Say from 100 acre pond, Rhode Island, collected on 10 September 1978; Obovaria olivaria (Rafinesque) (an incidental occurrence) from Mississippi River ca. 8 km southeast of Guttenberg, Clayton County, Iowa, collected 1977 by S. L. H. Fuller; and Pyganodon grandis (Say)) from Mississippi River at Lansing, Allamakee County, Iowa, collected 1977 by S. L. H. Fuller. I have a fairly

long series of these specimens from Pyganodon cataracta from Swartzwood Lake, New Jersey. Unionicola ypsilophora is the type species of the genus (Harvey 1998). It is considered by many as the classical mite infesting freshwater mussels. It is one of 8 nearly identical species that are difficult if not impossible to morphologically separate without extreme methods. To make things much more interesting, I am convinced that there are probably 30 species involved and not only the 8 here named. Two things are used to separate the species: the sclerotization of the posterior coxal plates of the males and the sclerotization features of the dorsal plates of the females. There are a couple of notable exceptions: U. foili and U. formosa can only be separated using DNA analyses, but they occur in different genera of hosts, and U. thienemanni and U. primoryensis can be separated by the number of pairs of acetabula in the genital fields of both sexes. A ninth species is included with these mites, U. difurcatus—an African mite that is not readily a member of this group in that it appears in a number of character states to be intermediate between the subgenera Unionicola and Prasadatax. I have not seen the specimens, and the images leave much to be desired; rather than exclude it or treat it elsewhere, I am punting and retaining it in the subgenus Unionicola. How do I separate the 8 species using morphological crtieria:

• U. ypsilophora—distinct posterior borders on male posterior coxal group and female with 2, small, widely separated, dorsal plates.

• U. formosa—indistinct posterior borders on male posterior coxal group and female with 2, small, widely separated, dorsal plates.

• U. foili—indistinct posterior borders on male posterior coxal group and female with 2, small, widely separated, dorsal plates.

• U. uchidai--distinct posterior borders on male somewhat elongated, posterior coxal group and female with a horse-shoe-like, single, dorsal plate.

• U. thienemanni—indistinct posterior borders on male posterior coxal group and female with a single large dorsal plate not extending to the level of the posterior end of the posterior coxal plates, and both sexes with 20 or more pairs of acetabula in the genital field.

• U. primoryensis—indistinct posterior borders on male posterior coxal group and female with a single large dorsal plate extending to the

level of the posterior end of the posterior coxal plates, and both sexes with less than 20 pairs of acetabula in the genital field.

• U. dimocki—distinct or indistinct posterior borders on male somewhat elongated, posterior coxal group and female with 2 dorsal plates closely placed to one another with less space between them than the width of a single plate (in some females there are at much as 7 additional plates).

• U. tetrafurcatus—indistinct posterior borders on male posterior coxal group and female with 3 dorsal plates—one larger and 2, small, widely separated, dorsal plates.

Unionicola ypsilophora is typically a Eurasian species, which may be easily confused morphologically with U. formosa and U. foili. The distinctive posterior border of the fourth coxal plate of the male permits separation of this species from U. formosa and U. foili; however, there is variation in the extent of sclerotization of these posterior borders. The females are indistinguishable. Recent studies on this species include Wen and Zhu (1999), Tuzovskij and Semenchenko (2015) and Gerecke et al. (2016). Unionicola ypsilophora is found in mussels in North America, Europe and Asia—is it a single species? My experience hints strongly that this is probably 10 species at minimum, but the process of separating them will be tedious if not impossible, unless we use DNA analyses as our guide. As in the American relatives, U. foili and U. formosa, it may have to do much with their hosts and coevolutionary events that led to the development of tight host-parasite relationships. New plates 1-2 provide images of a male and a female from Sinanodonta woodiana from China—a gift from Chungen Wen. The key diagnostic features that permit one to separate U. ypsilophora from the other 8 species are noted Hosts: In North America, these mites were found in the mussels of the genus Pyganodon. In Eurasia, the list of hosts is quite impressive, but they are famously found in species of Anodonta, Sinanodonta, Cristaria, Sinohyriopsis, Nodularia and Lanceolaria.

Figure 1. Drawings of North American U. ypsilophora (after Vidrine 1996).

Map 1. Known records of U. ypsilophora in North America (after Vidrine 1996).

Plate 188: Top left: male venter of Holland specimen. Top right: male venter of Rhode Island specimen. Middle: 2 images of male venters of specimens from the Upper Mississippi River. Bottom left: male venter of New Jersey specimen. Bottom right: female venter of New Jersey specimen (note the dorsal plates deep to the posterior coxal groups).

Plate 189: Female Unionicola ypsilophora from Pyganodon cataracta in Swartzwood Lake, New Jersey, USA: Top left: venter (note the dorsal plates deep to the posterior coxal plates). Top right: anterior venter. Middle left: dorsal plates deep to posterior coxal plates. Middle right: pedipalp. Bottom left: genital field. Bottom right: tarsus of pedipalp.

Plate 190: Male Unionicola ypsilophora from Pyganodon cataracta in Swartzwood Lake, New Jersey, USA: Top left: venter (note the single, elongate dorsal plate deep to the coxal plates). Top right: posterior venter. Middle left: pedipalp. Middle right: tarsal claw of leg IV. Bottom left: tarsal claws of legs II and III. Bottom right: composite photo of first walking leg.

New plates: Plate 1: male venter from Sinanodonta woodiana from China (Chungen Wen specimen) (note that the posterior coxal group has a very distinctive posterior border) :

Plate 2: female venter from U. ypsilophora from Sinanodonta woodiana from China (Chungen Wen specimen)(note the dorsal plates deep to the posterior coxal plates in this image—two rather narrow plates about as 5 times as long as wide and about as far apart as the total length of one of the plates):

Plate 3: female dorsal plates from U. ypsilophora from Sinanodonta woodiana in China.

Plate 4: female genital field from U. ypsilophora from Sinanodonta woodiana from China (Chungen Wen specimen).

Plate 5: Unionicola ypsilophora male from Pyganodon cataracta in the United States.

Plate 6: Unionicola ypsilophora female from Pyganodon cataracta in the United States.