Onchidium reevesii (J.E. Gray, 1850)

Dayrat, Benoit, Goulding, Tricia C., Apte, Deepak, Bhave, Vishal, Joseph Comendador,, Qua, ng, Ngo Xuan, Tan, Siong Kiat & Tan, Shau Hwai, 2016, Integrative taxonomy of the genus Onchidium Buchannan, 1800 (Mollusca, Gastropoda, Pulmonata, Onchidiidae), ZooKeys 636, pp. 1-40 : 28-31

Onchidium reevesii (J.E. Gray, 1850) Fig. 14

Onchidella reevesii J.E. Gray, 1850: 117, pl. 181, fig. 5-5a; Hoffmann 1928: 103.

Onchidium reevesii : Semper 1885: 290.

Type locality.

China. Onchidella reevesii was not described by Gray. That name, which appears on page 117, simply referred to figures 5 and 5a of the plate 181). On page 117, there is no indication of the geographic origin, and there is no indication of geographic origin on the label of the holotype either. However, on page 36, those same figures are referred to as " Onchidium -. Mr. Reeve’s drawings. China," clearly indicating that the animal illustrated on the fig. 5 (and 5a for the ventral side) of the plate 181 is from China. Hoffmann (1928) also accepted China as the type locality.

Type material.

Holotype (43/25 mm), by monotypy (NHMUK 20160036). The label says "? Holotype Onchidella reevesii Gray 1850," but there is no reason to think it is not the holotype because the specimen fits perfectly the original illustration and the label also does refer to "Gray, Figs. Moll. Anim., IV, 1850, p. 117" where the name Onchidella reevesii was first published. The holotype is in excellent condition. It was opened dorsally prior to the present study so several key features could easily be checked in the digestive system as well as the male anterior parts, which are all drawn here (Fig. 14). The posterior reproductive parts (far less critical for species identification) were not removed from the visceral cavity (because it would have required our destroying the posterior region of the holotype) and so they are not illustrated here.

Distribution

(Fig. 2). China (type locality). Based on sequences misidentified as Onchidium " struma " (a nomen nudum) by Sun et al. (2014) and re-analyzed here, Onchidium reevesii is found on most of the coastline of mainland China (except for southernmost and northernmost latitudes), from 22°30' to 34°36' of latitude north.

Internal anatomy

(Fig. 14). The intestine of the holotype is between a type II and a type III, because a transverse line can intersect the intestine six times (four times for the type II as in Onchidium typhae , and eight times for the type III as in Onchidium stuxbergi ). Although observed, the hollow spine was not extracted. At the end of the flagellum of the accessory gland, there is a flat disc which distally protects the hollow spine. The penial sheath is coiled in spirals. The retractor muscle is shorter than the penial sheath and inserts at the posterior end of the visceral cavity. There is no additional left retractor muscle. The deferent duct is highly convoluted with many loops. The penis of the holotype was not extracted for the present study.

Distinctive diagnostic features.

Externally, Onchidium reevesii differs from other Onchidium species by its ventral color, i.e., a whitish hyponotum and foot (see the dichotomous key below). Internally, the spirally coiled penial sheath of Onchidium reevesii is not diagnostic (as in Onchidium stuxbergi and Onchidium typhae ). However, Onchidium reevesii lacks the additional, anterior, left retractor muscle of the penial sheath that is unique to Onchidium stuxbergi (the only species with which Onchidium reevesii may be shown later to overlap in the southernmost part of its distribution).

Remarks.

Semper transferred Onchidella reevesii to Onchidium probably by default (i.e., as a non- Onchidella species) because he did not give any explanation and he did not examine any new material. At any rate, it just so happens that Onchidium reevesii is the correct combination, because the notum of the type specimen bears the long papillae that are typically found in Onchidium as defined here. The presence of a rectal gland and of an accessory penial gland also supports that Onchidella reevesii is an Onchidium , even though those traits are found in other genera and even though the accessory penial gland can be both present and absent within a genus. That being said, a rectal gland and an accessory gland are found in all other Onchidium species. There is a small disc at the distal end of the accessory gland of the holotype of Onchidium reevesii , a structure which we found so far only in Onchidium .

Hoffmann (1928: 69) placed Onchidium reevesii in the genus Oncis (i.e., Platevindex ) with a question mark but, in the same publication ( Hoffmann 1928: 103), accepted it as an Onchidella also. Hoffmann did not have access to new material and he does not seem to have examined the type because he did not comment on it. Britton (1984) then used the new combination Paraoncidium reevesii based on material that was sent to him from Hong Kong but without examining the type material of Onchidium reevesii . However, Britton’s identification was erroneous because he described Paraoncidium reevesii as lacking both a rectal gland and an accessory penial gland, while both glands are actually present in the holotype of Onchidium reevesii . Also, Paraoncidium Labbé, 1934 actually is a junior synonym of Onchidina Semper, 1885, and thus refers to a different clade.

Sun et al. (2014) adopted Britton’s (1984) work and their use of the name Paraoncidium reevesii is a misidentification because it refers to a species with no rectal gland and no accessory penial gland, based on our own dissections of that species. The molecular study of Sun et al. (2014) shows that there are eight onchidiid species in China. Unfortunately, their identifications are erroneous (at the specific and/or generic levels). However, by including their sequences in our comprehensive regional data set and due to our own dissections it is possible to know what those species are as well as their internal anatomy.

There are actually only two species in China with both a rectal gland and an accessory penial gland, and both species belong to the genus Onchidium . In the study by Sun et al. (2014), those two species are confused under a single name, Onchidium " struma ", which is a nomen nudum. One of those two species, Onchidium stuxbergi , is restricted to the extreme southernmost latitudes of the coast of China (up to 22°10'N) but is also distributed in Malaysia (Sabah, Peninsula), Vietnam, Brunei, and the Philippines (see above Onchidium stuxbergi ). The other species, Onchidium reevesii , seems to be endemic to China and is distributed along nearly the entire coast of China (from 22°30' to 34°36'), except for the southernmost and northernmost latitudes. It makes sense that Onchidium reevesii applies to the species that is the most widely distributed in China because its type locality ( “China”) had a much higher chance to fall within the range of Onchidium reevesii compared to that of Onchidium stuxbergi .

Onchidium reevesii and Onchidium stuxbergi , of which the distribution ranges do not seem to overlap, differ externally and internally. A few pictures of live animals of Onchidium " struma " from Shangai, northern China ( Shen et al. 2006: fig. 1), show that the foot of Onchidium reevesii is whitish (the foot of Onchidium stuxbergi is bright orange). Internally, the holotype of Onchidium reevesii is lacking the additional, left, retractor muscle that is exclusively diagnostic of Onchidium stuxbergi . The spiral coils of the penial sheath of the holotype of Onchidium reevesii are compatible with the illustration (although fuzzy) of the anterior male apparatus of specimens misidentified as Onchidium " struma " from Shangai ( Wu et al. 2007: fig. 1). However, spiral coils of the penial sheath are not diagnostic of Onchidium reevesii because they are also found in Onchidium typhae . Finally, there are fewer loops in the intestine of the holotype of Onchidium reevesii than in Onchidium stuxbergi (of type III) and, based on our data, the intestine of a species cannot be of different types.