Turbellarian taxonomic database

Pseudohaplogonaria opisthandropora

Mamkaev 1971 (citation)

Convoluta opisthandropora sp. n. (Fig. 6)

Locality. This species was found in July-August 1966 in the sublittoral of the Possjet Bay and its outskirts
(at eastern shores of the Bay of Expedition: to the north from the Cape Shelekh and at Churhado spit from
the side of the Cape Shelekh, 3-4 m depth, sand; at Sivuchja Bay near the Cape “False Islet”, 3-7 m depth,
at the Furughelm Island, in the western bay, 2-5 m depth, sand, or a mixture of sand, gravel, and crushed
stone; at shores to the east from the “Little Cruiser [Crejserok] Bay, 3-5 m depth, sand). Apparently, the
same form was found in summer 1963 near the Putjatin Island (dredging in the south-western bay); however,
the bursa was not observed; possibly, it has not yet formed in encountered specimens. 

Description. The body is elongated, gradually tapering to the posterior end, usually somewhat flattened
6A, B). The body shape is easily changeable. The mouth is in the end of 2/5 – the beginning of 3/5 of body
length. The ventral side of the preoral area is concave. The length of mature specimens is 0.8-1.2 mm.
Epidermal glands with brown well-shaped secretion in form of flexible rods are strewn along the whole body
surface (Fig. 6A, D). The length of the glands is from 9 to 18 µm (more often about 15 µm). The
non-pigmented frontal glands are also present. A certain number of the frontal glands is united into the
frontal organ,
opening with a terminal pore; the remaining glands open independently, forming about a dozen of short
longitudinal rows. The length of cilia, covering the body, is 7-8 µm. The diameter of the statocyst reaches
17 µm; the diameter of the cup-shaped statolith is 11-12 µm. A relatively thick and dense layer of the
peripheral parenchyma is sharply separated from the endocitium. Numerous granules of the latter easily change
positions that speaks of its attenuated, washy state. The lateral testes are located in the anterior half of
the body. A pair 
of relatively short bundles of spermatozoa extends from the testes, which then unite into a
single relatively long bundle, reaching 1/3 of the body length (Fig. 6, B). The sperm are of the filiform
type, having a length of 130-140 µm. The anterior end of the sperm is pointed; the posterior half is
gradually tapered to the posterior end (Fig. 6, C). In active spermatozoa, placed in water, the anterior
quarter is
stiff, immotile, while the remainder part undulates. The unpaired bundle of sperm extends to the tubiform
ciliated antrum, opening at the posterior end of the body with a terminal pore (Fig. 6, B). The spermatozoa
in this bundle are tightly packed together and oriented backward into the antrum with their straight
parts. As a result, such a bundle of sperm resembles a stylet composed of spicules (of a kind found, for
instance, in Actinoposthia). The length of the antrum is 30-45 µm (it changes, in particular, upon the
stretching and contraction of the body). A crown of glands, encircling the bundle of sperm, pass into the
proximal end of the antrum. Beside that,  numerous glands with the shaped secretion in form of the finest
needles with the length of 9-10 µm, pass into the antrum (essentially, in its proximal half). The ovary,
starting in the middle area of the body with two lateral germinal zones, becomes then unpaired and extends
to the last quarter of the body. The seminal bursa is located behind the ovary; its distinctive cuticular
nozzle is oriented forward (in mature specimens – toward the egg, which is positioned in the end of the
(Fig. 6, E). In specimens having length of 1-1.2 mm, the length of the nozzle constituted 21-30 µm (in
0.8 mm in length – 17 µm). The vagina and female gonopore are seemingly absent. 
The systematic position. The described species is unusual for an acoel in its combination of a tubiform
ciliated antrum, opening at the posterior end of the body, and a bursa, provided with a cuticular nozzle.
The caudal ciliated antrum is typical for the fam. Haploposthiidae from Abursalia (together with the presence
the ventral mouth, and, of course, with the absence of a bursa), whereas the bursa with a cuticular nozzle
is a character of the fam. Convolutidae (Bursalia; Mamkaev, 1967). While making an assessment of the
position of the given species one should decide at the outset, whether its bursa and antrum are consistent
with the corresponding organs of haploposthiids and convolutids. In my opinion, the bursa of C.
opisthandropora is homologous to the bursa of the convolutids. The cuticular nozzles of the given kind are
highly specific and quite elaborate, and so the similarity in this case should be considered with a good
reason as a homology (see the Remane’s second criterion of homology (Remane, 1952)). Based on this
assumption, I place this species in the genus Convoluta, for which the bursa with a single cuticular nozzle is
diagnostic character. As regards the male copulatory organ of C. opisthandropora, I inclined to consider it
homologous to the antrum of the haploposthiids; however its caudal position is most likely derived and its
considerable similarity with the antrum of lower acoels is a result of this secondary displacement. The
ciliated antrum with cirrus occurs among the species of Convoluta (e.g. in C. convoluta, see: Mamkaev,
1967), and it could be easily derived from the corresponding copulatory organs of lower acoels, which in turn
be traced back to the caudal antra of the haploposthiids (Westblad, 1949). However, one should bear in mind
that the primitive caudal position of the given organ was lost still outside the fam. Convolutidae. It
could, of course, be argued that the antrum of C. opisthandropora retained the primitive state of the
haploposthiids. But then this species should be considered not only the most primitive Convoluta, but even the
primitive member of the family, that could hardly be acknowledged. This psammobiontic (and hence, quite
species cannot be regarded as primitive Convoluta, and the given genus, in my opinion, cannot be considered
the primitive genus of the family. It seems quite likely that in the convolutids there occurred a
simultaneous multiple origin and the oligomerization of the bursas and cuticular nozzles (Dogiel, 1954;
Mamkaev, 1967),
and, if this is true, then Anaperus and Oligochaerus should be taken as the primitive genera of the family.
But they already do not have a caudal ciliated antrum. One more assumption could be made: to posit that the
ancestors of the given form have branched off from the ancestors of other convolutids, when the antrum was
still caudal, and the bursa with a single nozzle is a result of an independent (parallel) development in
this branch. However, in this case, we are compelled to derive the convolutids directly from the
with the caudal antrum (i.e. from the most primitive acoels, which seems highly unlikely). Moreover, it
seems more precarious for me to admit such a parallelism, than to accept a secondary backward displacement of
male gonopore. While we cannot adduce any reasonable argument for the first hypothesis, the second one finds
a ground in the plentiful examples of various displacements of male copulatory organs, observed in
turbellarians (especially, in lower ones). The male copulatory apparatus of the acoels is quite variable, a
diversity could sometimes be observed even inside a genus. In particular, the great diversity is found in
the genus Convoluta. And, although the variation that we encountered in C. opisthandropora differs
from the others and closely resembles the condition of some members of Haploposthia, it has rather appeared
inside the genus, than inherited from the ancestors of the haploposthia-like type. It should be noted in
this respect that in the “green convolutas” upon the passage to the life in sand there was also observed a
displacement of the male gonopore to the posterior end (see page 47). Possibly, this particular case is also
related to the life in sand. The given examples, in my opinion, further corroborate to the notion of
considerable plasticity of the acoel organization, and, in particular, of the plasticity of their male
copulatory apparatus.  

Diagnosis of Convoluta opisthandropora sp. n. Convoluta with a caudal tubiform ciliated antrum, opening with
a terminal pore. Body elongated, gradually tapering to the posterior end. Body length up to 1.2 mm.
glands with brown rod-like secretion strewn along the whole body surface. Frontal organ present. Mouth in
the end of 2/5 – beginning of 3/5 of body length. Testis paired, in the anterior body half. The ovary behind
testis, unpaired, but start with a pair of germinal zones. Bursal nozzle up to 30 µm in length. In sand at a
depth of 2-7 m. Sea of Japan. 

Fig. 6. Convoluta opisthandropora. After live specimens. A [À] – general appearance; B [Á] – morphological
reconstruction; C [Â] - spermatozoon; D [Ã] – the shaped secretion of epidermal glands; E [Ä] – bursal

[The corresponding Russian letters are given in square brackets.]

Translation by A. Petrov