RISSOA ( PUSILLINA ) INCONSPICUA ALDER , 1844 SUBSP . ALBELLA LOVEN

In the paper the author describes Polish representatives of the subgenus Pusillina (Rissoa: Rissoidae). The characteristics of the shell features and their variability (especially of the ribs) is given. Next the protoconch, ctenidium, osphradium and male reproductive organs are concisely described. A more detailed description of the head pigmentation (within which three types: A, B, and A + B are distinguished), penis variability and female reproductive organs (showing some very characteristic features) is given. Finally, the author lists the differences between the Polish Pusillina and the data in the literature on Rissoa albella, R. benzi and R. inconspicua. Basing on this, the author considers R. albella a probable subspecies of R. inconspicua, and the Polish Pusillina being identical with R. inconspicua albella. key woRDs: shell variability, protoconch, ctenidium, osphradium, head pigmentation, reproductive organs, penis http://dx.doi.org/10.12657/folmal.002.004 Folia Malacol. 2: 63–72 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. STANISlAWA STASZICA Nr 1112 Folia Malacologica z. 2


INTRODUCTION
first time recorded in Puck Bay (Polish Baltic Sea coast) the occurrence of the representatives of the genus Rissoa (Freminville) Oesmarest, 1814: one species from the subgenus Rissostomia Sars, 1878, and two species from the subgenus Pu-Sillina Monterosato, 1884 (= Turboella (Leach) Gray, 1847: the nomenclature of subgenus level is acknowledged after Coan 1964).This paper is an attempt at more exact and detailed description, as well as determination of the systematical status of the representatives of subgenus Pusillina inhabiting Puck Bay.
Rissoidae are known one of the least known groups of the European gastropods, and their systematics is full of doubts and controversies.
Unfortunately, one can suppose that the situation would not change in the nearest future, because the revision of this rich in taxons group, basing on the present level of the knowledge, seems to be simply impossible.Nevertheless, every attempt at marshalling of the data, even a contributory and provisional one, seems needed if not necessary.

SHELL DESCRIPTION
The shells of Pusillina occurring in Puck Bay are presented in Figs 1,1 -1,1B and 2,1 -2,16.They are conical or conically-oval less often slightly turreted.The spire moderately elongate and rather variable.The value of proportion between the shell breadth and height very variable, and then the shells may be, in extremal cases, rather stocky or evidently slender.The apex$rather blunt, the corrosion of the oldest whorls very rare.Whorls'number: 5 1/4 -5 3/4 -6 -6 1/2.The whorls moderately convex and the suture well marked but moderately deep.The whorls convexity and suture depth hardly variable.
Whorls breadth grows regularly and moderately rapidly.The body whorl equals in height more than a half of the shell.A wide variability of the convexity of the body whorl, which can be from completely uninflated to rather markedly inflated, can be observed.On the body whorl a more or less visible keel, parallel to the suture, is observable.At the keel ribs terminate, if present.
The outline of the aperture is variable, from broadly-to elongately oval.Down close to the columella the mouth rather strongly or strongly elongated, the corner at its upper left side weak or absent, nearly always rounded.The peristome never continuous.The parietal lip narrow and short, showing the outline of a spherical triangle or a half of ellipse.No outer lip, the margins of the mouth thin.The umbilicus covered by the parietal lip, then absent; in only 2.1X of the snails studied I found the umbilicus, in the form of a narrow slit.
Shell dimensions: males: height 2.2 -2.9 -3.3 mm, exceptionally up to 3.7 mm; breadth up to 1.3 mm; females: height 2.1-2.B-3.0 mm, exceptionally up to 3.7 mm; breadth up to 2.3 mm.As one can see, the ranges for the males (Figs 2,1 -2,16) and females (Figs 1,1 -1,18) are the same, but commonly the females are somewhat bigger.The other features, like the whorl convexity or suture depth seem identical in both sexes.
The walls of the shells from Puck Bay are thin to moderately thick.The shell surface smooth, slightly glossy pr matt.A delicate spiral dotting often visible, though I have never observed any conspicuous Fig. 1. 1 -18: Rissoa inconspicua albella from Puck Bay, shells of females Fig. 2. 1 -16: Rissoa inconspicua albella from Puck Bay, shells of males spiral iculpture.The shell t'reamy" yel1owish, creamy b~ownis'h 1 ¥BP-.o-•wish, or brownish.I'n 78•.5% of the sturue.ospecim~ns I ha"ve found the occurrence of longit~dirtpl spots.T~e spqts &fe usual1y associatQd.with the ribs lying be.tw~en the latte.r,howe\let, in ~5.2% ~f the specimens the spots were present on •the shells wt th-no• r.i:bs.The spot~ yellowish, light brown or brown.The outltne Qf t~e spots ofteh with a bend or undulate.The spot~ sometim!ls m,ay be• •v•en poorly•, but liS!Ially strongly or very strongly marked.
The ribs lffiVel' occur at the oldest• who:rls, n•oW'E!ver, they appear rather early -the co.rTelati'on b•etween thfl lack of ribs and the small di-me~sjons of the Sh!!ll is eyidently weak.The first rir:,,s are smaller, disfributed less null)erously and less regularly thal'l the following ones.At tt}e surfac~ of the older wh~r!s the~e are of'ten wide ~;~aps between a 11:oup"le of• firs-t r.i.bs or.groups .o.f ribs •. fhe ribs arr:angement on the youflger owhor1s is a,I~o not vary r.egu~ar.
•l'he maximum nutllb,!Olr of riJ:75 pe•r .wl'lorl is different in the males and in th~:~ females.In the females•: 12 • -20, b~o~t in 5G% of the studied females there were 14 ribs per wl'larl, and in 20~ -16.In the males the range of variabiiity was wider: 9 -22 ribs per whorl, and the distribution of the notice~ numbers was less contagious.30.B% of males had the shells with 11 ribs per whorl, 23% -with 12, and 15.5% apiece --15 and 16 ribs per whorl.It is well to add that inside the given ran~s the variability was not continuous: not all of the values inside them were actually not~ced.
The total number of the ribbed whotl~ per the male shell was 1 1/4 --2 1/3.About 40% of the ribbed males had the shells with two ribbed whorls, and about 20% -with 1 3/4.The total number of the ribbed whorls per the female shell was 1 1/2 -2 1/2; about 40% of ~he ribbed females had 2 1/2 whorls with ribs, and about 20% apiece -1 1/2 or two.In all the ribbed females studied ribs were present to the very end of the body whorl, reaching the margins of the mouth.In the males the same pattern was observed in 22% of ribbed specimens only, while in the remaining ones the last rib was situated 1/4 -2/3 of the whorl from the margins of the apertur~: in 45% the distance from the mouth was 1/4 of the w~rl, and only in 11% as lnuch as 2/3.

SHELL VARIABILITY
The shell variability shows the typical of Rissoidae character.It " is marked in the habitus, slenderness (an especially wide variability), the inflatedness and height of the body whorl, and in the form of the keel on the body whorl (a wide variability of all these features).It can be observed as well in the outline of the aperture (including the form of the angle and the elongation), the shell dimensions and wall thickness (rather a slight variabllity).Another part of the variability is associated with the shell colouration, its spotted pigmentation and ribs.Nothing exact is known about the background of the shell variability.It seems very p~obable that we observe an example of the genotypical polymorphism.Due to a low salinity -about 7%. in Puck Bay -the gastropods are smaller than the ones occurring in higher salinities.The sexual dimorphism, as it could be seen in the drawings, is slightly expressed, with an exception of the ribs pattern .
. There is a number of papers devotev to t~e variability of ribs in rissoids, among them in the subgenus Pusillina.Rissoa parva ma Costa, 1779)is a commonly known example of the age variability of ribs (Gostan 195B, Fretter and ~raham 1962).In this species the smooth shell is characteristic of the young snail having the rudimentary reproductive .organs.The following stages of the development of the reproductive organs appear along with the formation of the first rib and an angulated outer lip, then numerous ribs and a rounded outer lip.The maturity is manifested in the presence of the labial rib.Rissoa interrupts (Adams 1 179B) in northern seas forms often the populations consisting entirely 9f unribbed specimens (Verduin 1976).Wigham (1975) observed in R. parv~ seasonal changes -not :ery regular, however -in the ratio of the ribbed to smooth.specimensnumbers.In spring ~nd summer the ri~bed gastropods dominated, while in late winter and early spring the smooth ones prevailed.In summer there occurred the intermediate specimens: between smooth and ribbed, whereas in winter the intermediates between ribbed and smooth.The Wigham's dat& seem to show that an environmental stress, like a low temperature, intensive waves action, food shortage, and probably also pollution, after reaching some level that alters ~ith an individual, stops the rib formation.Wigham (1975) regards the ribs formation as not genotypically determined, bUt Verduin (1976) doubts in this opinion.

PROTOCONCH •
The protoconchs of Pusillina from Puck Bay are smooth, with no distinct sculpture.The diameter of the nucleus: 60 -70 -BO ~m, the diameter of the first half of the whorl: 100 -120 ~m.The dimensions are thus even smaller than those given by  for Pusillin~ species with the small protoconch, which means characterized by the P.elagic larve stage in the life history.

SOFT PARTS MORPHOLOGY AND PIGMENTATIONS
The metapodial tentacle styliform, moderately big.The head and foot translucent, either whitish or whitishyellow or light brownish.lhe visceral hump often very intensively darkly pigmented.Besides the hump only the snout and the area behind the latter are pigm:nted: tne tentacles alwats unpigmented and colourless (Figs 3,(1)(2)(3)6).The observed pattern of pigmentation varies from very delicate, sometimes even hardly visible brownish colouratioos through intensively black, dispersed dots to dark brown or completely black continuous.spots of undifferentiated intensity.Three types of the pigmenta~ion pattern: A, B, and A + B were observed.
The pigmentation pattern type A (Figs 3,1 -3,2) consists of spots located posteriorly to the snout, while the entire snout is unpigmented • At the pigmented area there are two triangular spots of pigment lying on both sides, the basis of each triangle arranged parallelly to the lateral margin of the head at its side.The size as well as the intensity of these spots are variable .. The spots are always approximately triangular in shape although they may be a little irregular; they are always arranged symmetrically or nearly symmetrically to each other.I found the pigmentation of type A in all the studied males; it was also the most common type of pigmentation in the females, characterizing 44% of the examined females.In the other females I noticed the In aboU't 22% of females "the spots of the type A occurred along with the snou;t pigmer:~tati:on of type B. This pattern being .thecombination • of b~th the patterns described above (including the whole r~nges of their variability)' caf'l be designated as A t."' B (Fig. 3, 6 ) .. Within all the types of pigmentation pattern mat~ than a half pf snails of both sexes were characterized by poot or very poor pigmentation.Moreover, in 22% of the studied females there was no pi~ment On the head .

RADUlA
The radula is typi~al of~.without a~y characteristic features.

PENIS
The uni-armed and simple penis is situated at the right side of the hea~, ~xactly along the continuation of the axis of symmetry of the rigrit tentacle, and. is directed backwards.The penis is rather small; ~~me exemplary dimensions of it are as follows: 69 shell height (mm) 2 .50 2.70 2.75 2.80 3.25 3.30 3.35 3.60 ).65 penis length , (mm) 0.50 0.50 0.55 0.30 0.45 0.75 1.05 0.90 1.25 The lenQth of the penis virtually equalled not less than 0 .50 mm; the smaller pefies were ~haracteristic of the specimens intensively in-• ,febted •ith larval trematodes.The relatively small dimensions are connected with the lack of a glandular epithelium inside the penis.This epithelium is unnecessary if there is a prostate.The penis bears no outgrowths; it is slender and rather not massive.Usually arched, it ofte~ ~erminates dagger-like.A rather thick and usually Visible vas Qeferen~ runs along the middle of the penis.Besides the dimensions the variability bf penis is very restricted.

FEMALE REPRODUCTIVE ORGANS
The structure of the female reproductive orpans resembles that described by Johansson (1939) for R. inconspicua.The typical of rissoids upper accessory gland of the oviduct (albuminoid gland) is moderately big.The upper blind sac of the oviduct, being functionally the recep-tacOlum seminis, passes into the lumen of the upper gland, not directly to the oviduct.This mode of organization is very characteristic (Figs 4,1 • 4,4).The lower blind sac of the oviduct, being functionally the ~ursa copulatrix, passes into the oviduct typically of all the Rissoidae.It is moderately big and spheroid-like.Also the large bean-shaped fower accessory gland of the oviduct (nidamental gland), lying along the distal part of the oviduct, is typical of all the rissoids.The Oviduct between the glands is narrow, unbroadened, what differs this Pusillina from some other Rissoa .
. The oviduct terminates forming a thin-walled vagina.Into the vagina there passes a giant, also thin-walled, "lower bursa copulatrix" (Figs 4,6) lying along the lo~er accessory gland of the oviduct.This structure does n~t occur in any other ~ whose anatomy has been studied so far (Johansson 1939, author's own observations).It is in the shape of an elongated sac narrowing towards 'the outlet into the vagina, but without a markedly distinguishable duct.The variability of the female reproductive organs is very restricted.DISCUSSION ON THE SYSTEMATIC POSITION OF POLISH PUSILLINA Falniowski, Oyduch and Smagowicz (1977) recorded from Puck Bay t~o species from the subgenus Pusillina: Turboella benzi (Aradas et Maggiore,l843) and T. sarsi (Loven,lB46).The description of the variability presented above, this of the shell as well as that of the other structures shows that the subgenus Pusillina is represented in Puck Bay rather by a single, very variable species.Moreover, this variability shows the continuous and typical of Rissoidae character and, virtually, it is observable only in the form of the shell.Nordsieck (1972) and numerous other authors regard Mediterranean Rissoa benzi, which inhabits Straight of Messina, as identical with~ albella Loven,1846 which occuls in Kattegat Straight.Verduin (1976) points out the differences between these two taxa.He emphasizes also a long distance between the ranges of their distribution as well as the absence of R. benzi anywhere outside Straight of Messina.R. benzi seems to be a narrowly specia~ized species, separate from R. albella.Additionally, the Polish specimens are markedly different from those of R. benzi from Straight of Messina.
Numerous authors (Schwartz 1863, Meyer and M8bius 1872, Nordsieck 1~72 and Jaeckel 1976) give the descriptions and dra~ings of R. inconspicua Alder, 1844 which suggest that the Polish Pusillina belong to this species .Especialli the specimens described and drawn by Meyer and M8bius (1872) are virtually identical with the Polish ones.On the other hand, in comparison with the descriptions of R. inconspicua of Ziegelmeier (1966), McMillan (1968), Graham (1971) and Ver~uin (1976) I there are three constant differences: 1. Polish swecimens ere bigger than those of R. inconspicua (the shell height up to 3.7 ~m, while in R. inconspicua up to 2.0, exceptionally 2.9 mm) .
2. The spiral sculpture in Polish Pusillina is poor or ~xtremely poor, while in R. inconspicua -often very strongly marked.
The existence of such remarkable differences of opinions in the literature explains Verduin (1976).He points out that Schwartz (1863) erroneous~y described the shell~ of R. albella as the typical of ~ conspicua.
Pusillina from Puck Bay corresponds with the descriptions and drawings of R. albella (among others McMillan 1968, Palazzi 1978), however, the sutures of the Polish Pusillina are less deep, and the habitus of their shells is rather more similar to R. inconspicua than to R. albel-~• Moreover, the number of ribs is• the Polish specimens may be higher than the one recorded for R. albella.Verduin (1976) considers~ bella a northern species; McMillan (1968) gives the distribution of the species: from English Channel to Thurso.

CONCLUSIONS
The data presented above, especially: a. the described wide variability of R. inconspicua, b. the restriction of the differences observed between R. inconspicua and R. albella, these constant and rather univocally given, practically merely to the different maximum dimensions, c. the distribution of the two taxa, d. the ranges and character of the variability of various representatives of Rissoidae, lead to considering of R. albella a subspecies or, possibly, only a form of R. inconspicua as well as to considering of the Pusillina inhabiting Puck Ba~belonging toR .inconspicua albella.Anyway, this seems just the only sol~tion of the problem of the systematic position of Polish,Pusillina as long as t~ere is no revision of all the European ~. or at least of subgenus Pusillina alone, based on solid grounds.

•
pigmel)tation patterns • of type , a•, or A + B, or complete lack of pigment -~IJ the head.The pigmentation pattern t.ype .B (figs 3,3 -3,5) is ~haracterized by .' the Lack of pij:!OJen:t pos1;eriorly tp' the snout, and by. the occurrence •of I .pigment at, botb •the lateral sides of the snout.The .intensity of pigment'atton, a~though often Oery high, i~ variable.The pigme.ntatiQn pattern in the form of broader or narrower belts, running along neatly Jh~ entire snout • excep' its very short distal section (figs 3,3 -3,4)d or in the form of rather broad spots limited tc the short proximal section (Fig. 3,5) Qf ~he snout.The pattern B I found in only 12X of females.