Little neighbours in Hamburg: free-living aquatic flatworms (Platyhelminthes)AuthorDiez, Yander L.0000-0001-8741-4799Museum of Nature Hamburg – Zoology, Leibniz Institute for the Analysis of Biodiversity Change (LIB), Martin-Luther-King-Platz 3, D- 20146, Hamburg, Germany & Research Group Zoology: Biodiversity and Toxicology, Centre for Environmental Sciences, Hasselt University, Universitaire Campus Gebouw D, B- 3590 Diepenbeek, BelgiumAuthorSchmidt-Rhaesa, Andreas0000-0003-4102-9371Museum of Nature Hamburg – Zoology, Leibniz Institute for the Analysis of Biodiversity Change (LIB), Martin-Luther-King-Platz 3, D- 20146, Hamburg, GermanytextEvolutionary Systematics20242024-12-1982279310journal article10.3897/evolsyst.8.1394684D0ADC1E-13E8-404E-A10A-E28C371EBC96Polycelis tenuis
Ijima, 1884
Fig. 14Known distribution.
Species recorded from
The Netherlands
(
Young 1972
),
Finland
,
United Kingdom
and
Ireland
(
Luther 1961
), Gernany (
Schwank 1981
;
Martin and Brunke 2012
), Macedonia (
Kenk 1978
),
Romania
(
Felicia 2018
), and
Russia
(
Luther 1961
).
Material.Six specimens
studied alive, preserved in absolute ethanol for future molecular analyses; collected in Kirchwerder-Fünfhausen, submerged vegetation and litter in an irrigation channel,
0.1–0.2 m
deep.
Remarks.
Mature specimens measuring
0.5–1.2 mm
, dark coloured (Fig.
14 A, B
). Marginal eyes (Fig.
14 C, D
: e) distributed over the anterior third of the body. The pharynx (Fig.
14 B, C
) is located over the midbody and the mouth (Fig.
14 C, E
: m) opens anterior to the male copulatory organ. The male copulatory organ (Fig.
14 B, E
: mco) is 940–1100 µm long (n = 1; varying according to the relaxing stage) and 620 µm at widest. The male organ is spiny over its distal 300–480 µm, and forms a penial papilla (Fig.
14 E
:
pp
). The spines (Fig.
14 F
) are 17–18 µm long (n = 10). Two adenodactyls (Fig.
14 E
: ad) are located posterior to the male bulb and open into the common atrium, oriented forward, and exhibiting a glandular lumen.
Polycelis tenuis
.
A.
Habitus of swimming adult specimen;
B.
Squeezed adult specimen;
C, D.
Squeezed juvenile;
E.
Atrial organs;
F.
Spines of the penial papilla. Scale bars: 200 μm (
D
); 600 μm (
E
); 100 μm (
F
).
Three species of
Polycelis
have been documented in
Germany
, and they are widespread across Europe:
P. felina
,
P. nigra
, and
P. tenuis
(
Volk 1903
;
Ronneberger 1975
;
Schwank 1981
;
Müller and Faubel 1993
). Among these, only
P. nigra
has been reported in
Hamburg
(
Volk 1903
). Species within the genus
Polycelis
are primarily distinguished by the structure of the male bulb and adenodactyls.
Polycelis tenuis
shares with
P. felina
the presence of two adenodactyls, structures absent in
P. nigra
. However,
P. felina
is easily identifiable by the presence of two tentacles in its anterior body region. The penial papilla of
P. tenuis
is armed with spines along its distal half, whereas
P. nigra
exhibits two to three spine rows distally, and
P. felina
lacks any spines in this region (
Hansen-Melander et al. 1954
;
Luther 1961
;
Harrath et al. 2012
).
Volk (1903)
did not provide detailed morphological information about the specimens of
P. nigra
recorded in
Hamburg
. Given the necessity of studying the morphology of atrial organs for accurate identification of these triclads, this record requires confirmation.