Integrative taxonomy of the genus Onchidium Buchannan, 1800 (Mollusca, Gastropoda, Pulmonata, Onchidiidae)
Author
Dayrat, Benoit
Author
Goulding, Tricia C.
Author
Apte, Deepak
Author
Bhave, Vishal
Author
Joseph Comendador,
Author
Qua, ng, Ngo Xuan
Author
Tan, Siong Kiat
Author
Tan, Shau Hwai
text
ZooKeys
2016
636
1
40
http://dx.doi.org/10.3897/zookeys.636.8879
journal article
http://dx.doi.org/10.3897/zookeys.636.8879
1313-2970-636-1
55CD34169B9040DDA34A2026640E1E83
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.
Figure 14. Internal anatomy, holotype,
Onchidium reevesii
(NHMUK 20160036). A Digestive system, dorsal view, scale bar 7.5 mm B Reproductive system, anterior, penial accessory gland, scale bar 3 mm C Reproductive system, anterior, penial sheath, scale bar 5 mm. Abbreviations: agf, accessory gland flagellum; dd, deferent duct; ddg, dorsal lobe of digestive gland; i, intestine; pdg, posterior lobe of the digestive gland; ps, penial sheath; rg, rectal gland; rm, retractor muscle; st2, stomach chamber 2.
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
Labbe
, 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.