RISSOA (RISSOSTOMIA) MEMBRANACEA (J. ADAMS, 1797) (GASTROPODA: PROSOBRANCHIA: RISSOIDAE) IN PUCK BAY (SOUTHERN BALTIC SEA)

In the paper the author describes the morphology and anatomy of the Rissostomia species inhabiting Puck Bay. A detailed description of the shell and its variability (including the colour spots and ribs occurring very rarely) is given. Then the author describes the head pigmentation, variable and quite uncharacteristic. The concise descriptions of the soft parts morphology, ctenidium, radula and male reproductive organs are given. The variability of the giant penis is described, as well as the structure of the female reproductive organs. Then the author compares the Polish Rissostomia with the drawings and descriptions of R. membranacea given in the literature, as well as with the specimens of the species from the localities outside the Baltic Sea. He establishes that the Polish Rissostomia belong to Rissoa membranacea which is an extremely polymorphic species. key woRDs: shell variability, pigmentation, ctenidium, radula, reproductive organs, penis http://dx.doi.org/10.12657/folmal.002.003 Folia Malacol. 2: 53–61 Folia Malacologica 2/1988 was originally published as No. 1112 of Scientific Bulletins of University of Mining and Metallurgy, Cracow. This digitalised version was prepared by the Association of Polish Malacologists and first published on-line on December 30th, 2016. ZESZYTY NAUKOWE AKADEMII G6RNICZO-HUTNICZEJ IM. ST ANISlAWA STASZICA Nr 1112 Folia Molacologica z. 2 Krak6w 1988 ANDRZE.J F.ALNIOWSKI RISSOA (RISSOSTOMIA) MEMBRANAOEA (~. ADAMS, 1797) (GASTROPODA: PROSOBRANCHIA: RISSOIDAE) IN PUCK BA V (SOUTHERN BALTIC SEA) Ab s t r a c rt : In the paP,er the author describes the morphology and anatomy of the Rissos.tomia species iohabi ting Puck Bay. A detailed des.cription of the shell and ~ts variability {Including tne colour spots and ribs ocC.tJrring very r.arely) is @l·ven. Then the a~,tthor ~e.scribes the head pigmentation, var1able and quite uncharacteristic. The concise descriptions of the soft parts morphology, ctenidium, radula and male reproductive organs are given. Jha v•riability of the giant pe~is ~s described, aq well as the structure of the female reproductive organs. Then t!'le authot compares the Polfsh Rissostomia with the drawings and descriptions of R. membraoacea given in .the literature, as well as with the specimens Bf the species from the localities outside the Baltic Sea. He establishes that the Polish Rissostomia belong to Rissoa membrana~ea which is an extremely polymorphlc species.


INTRODUCTION
Falniowski, Dyduoh and Smagowicz ( 1977} in a paper on the molluscs of Puck Bay (a part of the Bay of Gdansk, the Southern Baltic Sea, Poland) ~ecord$d the occurrence of a ~ species being new for the Polish fauna.1he ~peci' es belongs to the ,s41Jgen.usRissostomia Sar.s, l87B (the systematics and nomenclature of the genus and subgenus level aftar Coan 1964).This Rissostomia was described by • smago~icz (1977) as Rissostomia brullO §ttric.ea.The presented paper is devoted to a detailed description of the Polish Rissostomia and, moreover, ~t is an attempt at tne determination of its systematic position.

Republic of Poland
This digitalised version of Folia Malacologica is funded by the Ministry of Science and Higher Education, Republic of Poland under the agreement no . 646/P~DUN/2016 allocated to the activities of disseminating science and by the courtesy of the Rector of the AGH University of Science and Technology, Cracow

SHELL DESCRIPTION
The shells of the Polish Rissostomia ) are sharply conical, elongately ovate or conically•ovate.The spire is high or very high, usually narrow, very variable.The proportion: shell breadth : shell height is variable within a wide range, but in general the shells are nearly always rather slender or slender.I must point out that the drawings present the whole range of the shell variability, and then the majority of the phenotypes figured are not common.or even actually rare.The real majority of th~ snails have the shells characterized by rngh, slender, conical and sharp spires.The apex rather sharp, the corrosion of the youngest whorls extremely rare: I have noticed it in two specimens only .
The mouth prominent, oval, usually broad or very broad.Its upper part without an angle or, exceptionally, with a very slightly marked angle.Very often there is a narrow distance between the peristome and the body whorl.It is noteworthy that this arrangement of the mouth in relation to the body whorl, the breadth and outline of the aperture and the always continuous peristome are the hest diagnostical features for Rissoa (Rissostomia) membranacea (J.Adams,l797) given in the literature.
The umbilicus is absent only exceptionally: usu~lly present, often as a groove running along th& whole zone of contact of the mouth with the body whorl.
• The shell walls rather thick, but sometimes relatively very thick, always thicker than in the other Polish Rissoacea.

SHELL SURFACE VARIABILITY
The surface with no spiral sculpture, matt, ro~h.opaque.Periostracum thick and porous, silky, light brown or brownish.Sometimes, with the periostracum especially thick the shell light grey.The ~i sible inside the aperture endostracum nearly always light orange; a narrow belt along the lip -light.The colour of the periostracum, as well as of the endostracum, very characteristic of the species.

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A majority of the shells of the Polish Rissostomia are unicolour.In few specimens only I have found the occurrence of longitudinal dark brownish spots (Figs 1,17; 2,7; 2.12; 2,18 a~d 3,1), They are situated on the surface of no more than two last whorls and associated with ribs, but sometimes are present also on the surface of smooth shells.The spots from oelicate to well marked; sometimes very distinct close to the suture but down delicate (Fig. 2,7).In one specimen I have observed very well marked and broad spots on the surface of the completely smooth shell (Fig. 2,18).The spots occurred almost entirely on the females' shells.
Among several thousand of specimens of Rissostomia from Puck Bay only several dozens of females have ribbed shells ( Figs 2,7; 2,12 and  3,1 -3,5).The ribs are always weak and low or even vestigial.On the penultimate whorl the ribs are marked more weakly or similarly as on the body whorl~ The ribs present on 1 1/2 -three whorls, usually on about two.The maximum number of ribs per whorl: 11 -16, usually: 15.The last rib situated 0 -3/ 4 whorl from the mouth, usually about 1/2.

NOTES ON THE SHELL VARIABILITY
The ~hara~ter of the variability described above and presented in the drawings is typical for Rissoidae.It is marked in the shape of the ~ell, its pro~or~ions.(abig range, although a majority of specimens Present •o•nly a nar~ow .se.ction of this range}'>, the shape of the spire, the ~onv.~xity of the whorls and the depth of ttre suture (the same remark), the breadth of the mouth and its shape, the shell dimensions, the thicknees of the.walls of the shell, the colour of the periostracum •and, • sometim~s.~~hepigmented spot~ and ribs . .

SOFT PARTS MORPHOLOGY ANO COLOURATION
The anterior lobe of foot.broad and flatly rounded, usually bears a band of dark pigment arranged in two joined together sectors of arcs.The body sometimes creamy, but more often from light brownish, by light yellowbrownish , to dark brown.The visceral hump sometimes darkly ~ig mented, but often completely lacking pigment.The eyes situated on proportionally very big knobs.

HEAD PIGMENTATION
The pigmentation of the head (Figs 4,1 -4,5) is fairly variable and not very characteristic, and completely different from that pictured for R. membranacea by Rehfeldt (1968), or by Meyer and MBbius (1872) and Graham {1971).The tentacles yellowish or yellow, always without pigment.In many specimens there is no pigmentation of any other soft part as well.In the others pigment may occur at the snout, posteriorly to the snout, and •rarely at the bases of the tentacles, in the form of rather narrow transverse Qelts (Fig. 4,1).
At the area situated posteriorly to the snout pigment never occurs in the form of geminate spots, but only may form a continuous transverse belt being variable in its breadth and intensity.The pigmentation may be in the form of from hardly visible dots to a completely black belt, sometimes lightened along its borders (Fig. 4,5).The belt often connects with the pigmented area of the snout (Figs 4,1 -4,3).
The pigmentation of the snout (Figs 4,1 -4,4) is also variable in intensity.Sometimes brightened in the middle and more intensive to the sides {Fig.4,1).Sometimes the snout pigmentation in the form of geminate spots situated at the lateral sides of the snout (Fig. 4,4).The snout pigmentation need not the snout, and vice-versa.accompany that of the area posterior to

•
The Rissostomia from the Polish coast of the ~altic Sea has no prostate.The function of prostate is fulfilled by a strongly developed glandular epithelium inside the massive penis (Fig. 3,12).This confirms the data of Johansson (1939) on the R. membranacea female reproductive organs.This function is just the reason for propo~tio~ally the giant dimensions of the penis: shell heigHt (mm) 3 •• 3.6 3.7 3.7 3.8 3.8 3.9 4.0 4.tl 4.3 4•:•3 4.9 penis length (mm) 1.8 1.6 3.3 2.0 1.8 2.1 2.9 1.9 2.5 Q.6 2.3 3.3 57 The variability of the dimensions is evidently high.However, I"have not found any markedly small penis being typical of the snails more seriously inf~cted with larval trematodes.A weak cor re la t.ion bll,tween the length of the penis and the height of the shell (~ = 0.572) is striking.For example, I ha~e found a 3.3 mm long with the 4.9 mm high shell as well as in another penis in a specimen one whose shell was only 3.7 mm high.Besides the dimensions the penis variability is weakly marked (Figs 3,11 and 3,19).
The penis is simple, without any outgrowths.In the medial part usually thicker than at the basis, terminally narrow and sharp, just dagger-shaped while not co~tracted.The penis is usually arched.The vas deferens well visible along nearly the whole penis, only inside its distal Rart less easy to distinguish.At the proximal part it runs in zigzag -forming three loops, while at the medial part it runs straightly.Inside the medial part of the penis the vas deferens very strongly Pigmented from light to medium brown, and very strongly thickened.
In one specimen I noticed an accessory duct running inside along more than a half of the penis, right to the vas deferens (Fig,3:19).The function of the duct is unknown, and nobody has observed it so far.The penes of the Rissostomia .from Puck Bay are identical with those of the R. membranacea specimens from the Netherlands.

FEMALE REPRODUCTIVE ORG~NS
The female reproductive organs (Figs 4,9) carrespond with the scheme presented by Jonansson (1939) for R. membranacea.The upper accessory gland of the oviduct {albuminoid) is relatively big.The oviduct between the upper and the lower accessory gland of the oviduct (nidamental) is thickened.The upper blind sac of the oviduct, which can be identified as receptaculum seminis, is connected directly with the oviduct.The receptaculum small and tube-shaped.
The lower blind sac of the oviduct, which can be identified as bursa copulatrix, is somewhat bigger than the receptaculum; narrow close to the outlet and broad along its upper part (Figs 4,7).In its normal position, when the• organs are not artificially arranned to show their structure, the bursa lies flatly along the oviduct (Figs 4,9).After joining the outlet of the nidamental gland, the oviduct runs into a short, thin-walled vagina whose outlet is situated at a papilla on the wall of the m.antle cavity.
The variability of the female reproductive organs of the Polish~ sostomia is slight.The organs of the Dutch specimens of R. membranacea are practically identical with those of the specimens of Rissostomia from Puck Bay.

DISCUSSION OF THE SYSTEMATIC POSITION OF THE POLISH RlSSOSTOMIA
Johansson (1939) gives a photograph of R. membranacea shell, being very similar to the Polish Rissostomia.Also some of the specimens drawn by Cuchcin (1975) from the Black Sea are evidently similar to the specimens from Puck Bay.The same can be said about "R.membranacea from B" from Danish waters (Rehfeldt 1968) and, to a lesser degree, about the specimen figured by Jaeckel (1976).The drawings of McMillan (1968), Graham (1971), Nordsieck (1972) andPalazzi (1978) differ from the Polish "R.brunosericea", although the descriptions accompanying them pay attention to the features, which are in majority present in the Polish specimens.An exception could be the monograph of ' Nordsieck {1972), however, this monograph is plenty of mistakes, inexactitudes and vaguenesses, and is a source of big amount of chaos occurring nowa-Qays in the taxonomy of Rissoidae .
The variability of.R. membranacea is as wide as to force many authors to regard the species as a complex one (among others Schwartz 1864, M6rch 1871, Meyer and M6bius 1872, Muus 1967, Rehfeldt 1968and Nordsieck 1972).However, all the attempts at the division of R. membranacea into several separate species made so far seem doubtful in respect of their bases.
Accordingly, the numerous data show that this is one species, strongly genotypically polymorphic and showing a great ecophenotypical plasticity.For example, Rehfeldt (1968) found the correlation between the occurrence of morphs with different shell proportions, whorls' number, shape of the spire and dimensions, and some environmental factors like salinity or water movements.Mars (1956) and Gostan (1966) noticed the occurrence of the morphs with smooth shells parallelly to the morphs with ribbed shells.At the ( most strongly freshened stations they found the smooth morph only, so the scarceness of the ribbed morph in Puck Bay confirms their observations.The environmental stress of another character than enlowered salinity also stops the production of ribs, similarly as in other Rissoa.Cuchctn (1975) observed that in laboratory conditions the offspring of the ribbed form was always smooth.
Among numerous described forms of R. membranacea, f. baltica Nilsson, which is very simil ~r or even identical with the form from Puck Bay, is worth of attention; f. baltica is known from the Baltic Sea: from Straights to Rugia Island, occurring at a salinity not less than 7%., so the salinity in Puck Bay is equal to the lower limit of the salinity range known for this form.Rehfeldt (1968) described the types A and B within Danish R. membranacea.Besides the shell shape the types differ in their pigmentation and life history as well.The types were distinguished on the basis of the value of the diameter of the first half of the whorl of the emorional shell.In the type A -with a pelagic larve -the diameter is 85 ~m.while in the type B -without a pelagic larve, the development inside the capsule to the stage of a creeping gastropod -120 ~m.In the specimens from Puck Bay the diameter was 75 -100 -110 ~m, and the variability within this range was continuous.This neither confirms the occurrence of one of the types described from Danish waters in Puck Bay, nor lets one deduce the mode of development of the Polish R. membrana- i, szerzej, pigmentacj~ glowy, podkre~lajQC znacznQ zmienno~c tej pigmentacji i brak cech charakterystycznych.Podano zakresy zmienno~ci liczby lamelli skrzela u samic oraz u samc6w, podkre-~lono brak cech charakterystycznych tarki.Wskazano na brak prostaty w m~skich narzQdach rozrodczych i wystQpowanie proporcjonalnie ogromnego prQcia, kt6rego zmienno~c om6wiono.Podobnie opisano ienskie narzQdy rozrodcze, charakteryzujQce si~ malQ zmienno~ciQ.Kr6tko om6wiono protokonch.