03 Rissoa parva

Shell-form intermediate between costate and non-costate.

1: protoconch of three whorls; as always, lacks any sculpture.

2-2: three whorls after protoconch with strong costae; produced in summer when optimum growth conditions (Wigham, 1975b).

3: glossy and translucent body-whorl lacking costae; produced in winter/early spring when stress of cold and wave-exposure prevented development of costae. (Wigham, 1975b).

Shell height 3.8 mm. Weymouth. Early April 2012.

 

Rissoa parva (da Costa, 1778)

 

Full SPECIES DESCRIPTION BELOW

Sets of OTHER SPECIES: www.flickr.com/photos/56388191@N08/collections/

 

Synonyms: Turbo parvus (da Costa, 1778); Turboella parva (da Costa, 1778); Rissoa interrupta ( J. Adams, 1800)

Meaning of name: Rissoa = named for G.A. Risso.

parva = (Latin) small

 

Vernacular: foldet tangsnegl (Danish); klein drijfhorentje (Dutch);

 

GLOSSARY below.

 

Shell description

Polymorphic; two main forms, costate 1Rp flic.kr/p/uxFKZK and interrupta (non-costate) 2Rp flic.kr/p/tT8koN , with continuum of intermediate forms 3Rp flic.kr/p/uxy617 .

Usual features of all forms

Up to 5mm high and 3mm broad. Profile generally, moderately tall ; body whorl usually 63-70% shell height, with apical angle 34-44º but varies 25-50º; young profile lower. Whorls well defined by fine suture. Periphery of body whorl rounded on adults, angulated on young (4-5 whorls) 4Rp flic.kr/p/uMPnUQ . Sculpture: apart from costae on some, surface appears smooth, but microscopic spiral lines sometimes detectable, strongest pair of lines may form a cord a short distance below periphery 5Rp flic.kr/p/uxy5iW (only very slight indications of spiral striae found on any shells examined by author). Larval protoconch occupies 2.75-4 apical whorls, always lacks costae; dirty- whitish, brownish, greyish 6Rp flic.kr/p/tTi1dH or tinted lilac. Ground colour, apart from protoconch, dingy-white to horn-colour to chocolate brown, with orange-brown costally-orientated lines varying in number and extent, sometimes absent. White labial varix on mature adults, with groove (not always visible) between varix and edge of aperture 7Rp flic.kr/p/uxy4yQ ; dark brown streak nearest aperture bends sharply in diagnostic falciform (sickle-blade-shape) mark across the varix 6Rp flic.kr/p/tTi1dH & 8Rp flic.kr/p/uxy4h7 . Juveniles lack varix; and falciform mark usually absent on shells below 2mm high 11Rp flic.kr/p/uQoM66 ; on some larger shells falciform mark never develops 10Rp flic.kr/p/tT8hTC . Aperture varies oval to D-shape ; usually 35-47% of shell-height; adapical angle about 90º; lips form a continuous peristome, usually marked brown 1Rp flic.kr/p/uxFKZK & 2Rp flic.kr/p/tT8koN . Curved outer (palatal) lip, thickened by varix on mature specimens, thin and sometimes transparent on juveniles 11Rp flic.kr/p/uQoM66 ; slightly everted at base, angulated where meets columellar lip on young shells , rounded on older ones 12Rp flic.kr/p/uxy3jf . Short columellar lip reflected onto body whorl, so no umbilical groove visible; distinct thin parietal lip 1Rp flic.kr/p/uxFKZK . Internally, aperture white with brown exterior marks visible when shell translucent. Operculum horn-colour, translucent showing dark underlying opercular lobe 13Rp flic.kr/p/uQoLzr , excentric spiral with nucleus near columellar lip when withdrawn 14Rp flic.kr/p/uxy9Hq .

Costate form

Moderately strong shell, old specimens sometimes thickened. Strong, slightly-curved, steep-sided, narrow-topped, white costae cross all of spire-whorls (excepting apical larval protoconch, about 1.5mm height) but only cross adapical two thirds of body-whorl, terminating sub-peripherally (at cord of two spiral striae according to most sources, but not detected on most shells examined by author) 5Rp flic.kr/p/uxy5iW . Typically 8 costae on body-whorl, 12 on penultimate whorl and 12 on the antepenultimate whorl 14Rp flic.kr/p/uxy9Hq . Costate whorls distinctly convex, apical non-costate whorls slightly convex or almost flat 1Rp flic.kr/p/uxFKZK . Microscopic spiral lines sometimes detectable in gaps between costae. Shell-surface usually opaque and matt on and between costae. Brown lines, if any, in grooves between white costae 9Rp flic.kr/p/tT8i85 but often absent 5Rp flic.kr/p/uxy5iW or obscured by debris/growths 15Rp flic.kr/p/tT8gh1 ; frequently only visible brown lines are falciform mark crossing labial varix and spiral band on base of body-whorl 14Rp flic.kr/p/uxy9Hq .

Non-costate, interrupta form

Shell thinner and more fragile than costate form; surface usually fairly glossy and translucent 4Rp flic.kr/p/uMPnUQ & 16Rp flic.kr/p/uxy8G7 . Usually, whorls less convex than on costate form; giving narrower ,sometimes almost straight-sided, profile 4Rp flic.kr/p/uMPnUQ . Orange-brown lines usually, but not always, extend across whole of penultimate and earlier whorls 12Rp flic.kr/p/uxy3jf . Lines vary on body-whorl; they can traverse almost whole whorl from suture to basal spiral band without a break 9Rp flic.kr/p/tT8i85 or with a break at the periphery before continuing off-set from first half 4Rp flic.kr/p/uMPnUQ or with a break and gap at the periphery, and recommence on same course 12Rp flic.kr/p/uxy3jf ; or lines may only traverse adapical part of the body-whorl 8Rp flic.kr/p/uxy4h7 sometimes lines are wavy and variation can be found on the same specimen 2Rp flic.kr/p/tT8koN ; on some shells marks indistinct 10Rp flic.kr/p/tT8hTC .

Intermediate forms

Many gradations between the two forms above of shape, size, solidity, sculpture, glossiness and colour can be found. After smooth protoconch, whorls may be entirely costate or entirely non-costate, or may start as one and change to the other on later whorls 3Rp flic.kr/p/uxy617 & 29Rp flic.kr/p/uQ8TJv .

 

Body description

Ground colour of flesh whitish, translucent to almost transparent. Snout deeply bifid 11Rp flic.kr/p/uQoM66 with longitudinal slit mouth in terminal cleft 17Rp flic.kr/p/uQoKev ; sometimes has medial narrow blackish mark and/or a few opaque white marks 11Rp flic.kr/p/uQoM66 . When pale-yellow salivary glands and bright-yellow buccal mass retracted, snout translucent whitish and head yellow 18Rp flic.kr/p/tT8fro ; when gland and buccal mass pushed forwards, snout yellow and head translucent whitish 19Rp flic.kr/p/uQoJHv . Cephalic tentacles translucent white with single medial opaque white line that may be continuous 8Rp flic.kr/p/uxy4h7 or discontinuous 20Rp flic.kr/p/tThWja . Under high magnification narrow translucent slit may be detected in line 5Rp flic.kr/p/uxy5iW ; occasional specimens, mainly juveniles, lack white line. Slight swelling at base of tentacle 19Rp flic.kr/p/uQoJHv bears black eye with dorsal opaque-yellow patch. Body behind tentacles translucent greyish-white, with yellowish areas 11Rp flic.kr/p/uQoM66 . Dorsal surface of foot translucent greyish-white with broad, dark, transverse band to anterior of mid-point 21Rp flic.kr/p/tT8evA . Anterior quarter of foot has opaque-white, heart-shaped, anterior pedal mucous gland 11Rp flic.kr/p/uQoM66 . Posterior half of foot has opaque-white posterior pedal gland, visible laterally as a white peripheral border 21Rp flic.kr/p/tT8evA . Constriction in foot shows as crease in side when foot contracted 22Rp flic.kr/p/tThVGt ; facilitates transverse folding of narrow foot 5Rp flic.kr/p/uxy5iW ; and medial dividing line on sole allows longitudinal folding. When extended, anterior of sole axe-shaped; bilaminate edge has outlet within for heart-shaped anterior pedal mucous-gland 20Rp flic.kr/p/tThWja . Most of posterior two-thirds of sole occupied by opaque-white posterior pedal mucous-gland with central pore 17Rp flic.kr/p/uQoKev . Dark transverse band on side of foot extends a short way onto sole. Operculum rests on large purple-black/ burnt-umber opercular lobe that extends prominetly like wings 17Rp flic.kr/p/uQoKev and 6Rp flic.kr/p/tTi1dH . Long, translucent-white, dorso-ventrally-flattened metapodial tentacle extends from opercular disc to posterior of foot 13Rp flic.kr/p/uQoLzr or beyond 6Rp flic.kr/p/tTi1dH ; often held in an arch 19Rp flic.kr/p/uQoJHv . Mantle dull-yellow 20Rp flic.kr/p/tThWja to greyish-white 17Rp flic.kr/p/uQoKev with translucent white pallial tentacle at adapical angle of shell-aperture 6Rp flic.kr/p/tTi1dH . Grey-white ctenidium with about 20 short stout filaments, and blackish osphradium, within mantle cavity 23Rp flic.kr/p/tThVtn . Whitish penis, with tapered tip 21Rp flic.kr/p/tT8evA , attached behind right tentacle on males; very long so may be doubled back into mantle cavity 24Rp flic.kr/p/uxy6CC .

 

Key identification features

Rissoa parva

1) Usual maximum height 5 mm.

2) Shell varies thin and unribbed 04Rp flic.kr/p/uMPnUQ to thick and ribbed 05Rp flic.kr/p/uxy5iW & 06Rp flic.kr/p/tTi1dH .

3) Periphery of body whorl rounded on adults, angulated on young (4-5 whorls) 04Rp flic.kr/p/uMPnUQ . No distinct subsutural collar.

4) Brown comma (falciform mark) across labial varix on adults 06Rp flic.kr/p/tTi1dH & 08Rp flic.kr/p/uxy4h7 ; diagnostic, but absent from shells less than 2 mm high 11Rp flic.kr/p/uQoM66 and from some larger shells. Lip not flared.

5) Head/snout translucent whitish; sometimes has narrow blackish mark and/or a few opaque white marks 11Rp flic.kr/p/uQoM66 , but not dorsally dark for whole width between tentacle bases on head, snout and anterior of body.

6) Cephalic tentacles have single medial opaque white line 08Rp flic.kr/p/uxy4h7 ; sometimes discontinuous 20Rp flic.kr/p/tThWja . When magnified, narrow translucent slit may be detected in line 05Rp flic.kr/p/uxy5iW ; occasional specimens, mainly juveniles, lack white line.

7) Yellow eye patches 19Rp flic.kr/p/uQoJHv .

8) Foot is translucent white with a broad, dark, transverse band anterior of the mid-point 21Rp flic.kr/p/tT8evA .

9) Opercular lobe entirely pigmented black or brown 17Rp flic.kr/p/uQoKev .

10) Confusion with R. membranacea is especially likely when on Zostera .

 

Similar species .

Rissoa membranacea (J. Adams, 1800) from north-east Atlantic.

1) Maximum height usually 9 mm in Britain, 13.4 mm recorded in northern Norway.

2) Shell varies thin and unribbed to thick and ribbed 25Rp flic.kr/p/2p6TZdc .

3) Tall, or moderately tall, spire with subsutural collar 25Rp flic.kr/p/2p6TZdc.

4) No comma on labial varix. Entire lip everts into a flared aperture on mature adults 25Rp flic.kr/p/2p6TZdc .

5) Flesh of head and body whitish, translucent, extensively covered by various shades of grey to brown-black pigment dorsally with opaque white spots 26Rp flic.kr/p/2p6SY6C .

6) Cephalic tentacles translucent white with crowded, randomly arranged, opaque, white spots 26Rp flic.kr/p/2p6SY6C .

7) Variably sized, opaque, pure-white, dorsal patch behind each eye 26Rp flic.kr/p/2p6SY6C .

8) Foot is translucent white with some opaque white spots and, dorsally, pale grey and/or pale-brown shading 26Rp flic.kr/p/2p6SY6C .

9) Opercular lobe pale apart from some dark pigment proximally.

10) ELWS, mainly on Zostera in north-east Atlantic.

 

Pusillina inconspicua (Alder, 1844)

1) Maximum size varies locally 1.3 to 3 mm.

2) Shell varies thin and unribbed to slightly thicker with slight ribs 27Rp flic.kr/p/2p6LWXg .

3) Profile varies from squat to fairly tall (spire 23% to 40% of shell-height). Whorls smoothly rounded without angulation 27Rp flic.kr/p/2p6LWXg .

4) Tiny purple apical spot confined to single embryonic whorl; diagnostic when present, but absent from some. Labial varix absent or weakly developed; no brown comma 27Rp flic.kr/p/2p6LWXg .

5) Flesh translucent whitish with some opaque white and yellow patches. Snout often has broad medial blackish or brown-umber band which fills the gap between the tentacles and extends onto the head and body 28Rp flic.kr/p/2p6SXUW .

6) Cephalic tentacles translucent white with two parallel rows of substantial, opaque-white, hyphen-like marks in each tentacle 28Rp flic.kr/p/2p6SXUW .

7) Opaque eye patches vary yellow to whitish.

8) Foot is translucent white with a broad, dark, transverse band anterior of the mid-point.

9) Opercular lobe blackish or brownish at anterior only, projecting posterior of lobe is whitish

10) ELWS, on seaweeds.

 

Habits and ecology

Often abundant, up to 100 000 /m² at LWST on rocky and stony shores; extend from MTL to 15m sublittorally. Lives at full marine salinity down to 20 p.p.t. and survives short periods at 15 p.p.t. On finely branching red algae that filter and retain suspended sediment (and R. parva settling veligers), such as Lomentaria, Plumaria, Ceramium, Callithamnion, Corallina, Plocamium and Cryptopleura; on branching hydroids, in Laminaria holdfasts, and, occasionally, (adults only) under stones. Numbers of R. parva positively correlate with amount of sediment retained; broad algal fronds rarely occupied as do not retain much sediment and do not afford firm grip for adhesive mucous lines or foot suited to grasp filaments. Most numerous in summer e.g. at one site 76 000/m² in summer, 5 000/m² in winter (Fretter & Graham, 1978) . Shell sculpture apparently responds to small changes in environment. Costate-form is a larger % of adult population (those over 1.9mm high) sublittorally and on sheltered shores e.g. on two shores near Plymouth in September 1972, 93.7% at Wembury (sheltered) and 2.3% at Polhawn (exposed). Costate form more dominant in late summer than in late winter on any particular shore e.g. Wembury 93.7% of adults costate in September 1972, fell to 16.7% in February 1973 (Wigham, 1975a). Intermediate individuals with costate early post-protoconch whorls followed by non-costate body-whorl 3Rp flic.kr/p/uxy617 probably settled in summer when growth conditions optimum, so costae formed; and then over-wintered when stress of cold and wave-exposure (perhaps including dislodgement of sediment-food from algae) prevented development of costae . Those with early non-costate post-protoconch whorls followed by costate whorls probably settled in late winter/spring and lived on into following summer 29Rp flic.kr/p/uQ8TJv . Shells sometimes heavily calcified with stony alga, Titanoderma (synonym Lithophyllum) 5Rp flic.kr/p/uxy5iW & 15Rp flic.kr/p/tT8gh1 .

Respiration inhalent current into mantle cavity at left of head has water quality tested by black osphradium before reaching grey-white ctenidium with about 20 stout filaments 23Rp flic.kr/p/tThVtn ; exhalent current passes from mantle cavity at right of head. Locomotion: very active clambering among fine algae and hydroids. Anterior pedal mucous gland discharges within bilaminate anterior edge of sole 20Rp flic.kr/p/tThWja which spreads mucus to lubricate creeping. Medial division of sole allows independent movement on either side, and longitudinal folding 5Rp flic.kr/p/uxy5iW to grip algal strands 7Rp flic.kr/p/uxy4yQ and mucous threads; turning facilitated by transverse folding at constriction in anterior part of foot 30Rp flic.kr/p/tT8dyf . Posterior pedal gland 17Rp flic.kr/p/uQoKev secretes adhesive mucus that is shaped by medial groove on sole into strong threads that harden on contact with sea water and used to anchor the snail and act as climbing lines in its movement around algae or between substrate and pool surface below which it often crawls. Feeds by grazing microphytes (diatoms etc) and algal fragments from surface of algae and hydroids with its radula. Yellow salivary gland on either side of radula tube 18Rp flic.kr/p/tT8fro secretes mucus to lubricate action of radula and cement food particles together. Breeds all year, with summer maximum; up to six generations p.a.. Sexually mature when labial varix developed. Males about 40% of population, smaller than females; long penis develops 24Rp flic.kr/p/uxy6CC when shell reaches 4.5 whorls; fertilization internal. Lens shape spawn capsules, about 0.7mm diameter, containing 6-50 white eggs, each 0.1mm diameter, laid at night on weed or shells of other R. parva. Hatch in three weeks at 12-13ºC into colourless veliger larvae that develop border of orange-red marks on large bilobed velum by 3-whorled shell stage; right velar lobe often larger than left (illustration, full description and differentiation from Pusillina inconspicua pp. 18-20 in Fretter & Pilkington, 1970). After fairly lengthy time in plankton 6Rp flic.kr/p/tTi1dH , settle on fine weed, and metamorphose when shell 2.75- 4 whorls, diameter 0.25 – 0.48mm. Those settling in summer grow at 1mm height per month and live for 3-5months. Autumn settlers over-winter, growing at 0.5mm/month, to breed in spring and die at age 8-9months.

 

Distribution and status

Northern Norway to Canary Islands, Mediterranean and Black Sea. Not Baltic. GBIF map www.gbif.org/species/5192339 . All round Britain and Ireland; scarce or absent in N.E. Irish Sea and coast of East Anglia, U.K.map NBN species.nbnatlas.org/species/NBNSYS0000177721

 

Links and references

Forbes, E. & Hanley S. 1849-53. A history of the British mollusca and their shells. vol. 3 (1853), London, van Voorst. Free pdf at archive.org/stream/historyofbritish03forbe#page/98/mode/2up

Use slide at base of page to select pp. 98 - 103 .

 

Fretter, V. and Graham, A. 1962. British prosobranch molluscs. London, Ray Society.

 

Fretter, V. and Graham, A. 1978. The prosobranch molluscs of Britain and Denmark. Part 4– Marine Rissoacea. J. Moll. Stud. Suppl. 6, 153-241

 

Fretter, V. and Pilkington, M.C. 1970. Prosobranchia. Veliger larvae of Taenioglossa and Stenoglossa. Conseil international pour l'exploration de la mer. Fiches d'identification. Zooplankton, 129-132. Free PDF (enter title in “find a file”) at www.ices.dk/sites/pub/Publication%20Reports/Forms/Marine....

 

Graham, A. 1988. Prosobranch and pyramidellid gastropods. London.

 

Jeffreys, J.G. 1862-69. British conchology. vol. 4 (1867). London, van Voorst. Free pdf at archive.org/stream/britishconcholog04jeffr#page/22/mode/2up . Use slide at base of page to select pp. 23-26.

 

Rasmussen, E. 1973. Systematics and ecology of the Isefjord marine fauna (Denmark). Ophelia, 11, 1-495.

 

Warén, A. 1996. Ecology and systematics of the north European species of Rissoa and Pusillina (Prosobranchia: Rissoidae). J. mar. biol. Ass. U.K. 76, 1013-1059.

 

Wigham, G.D. 1975a. The biology and ecology of Rissoa parva (da Costa). [Gastropoda: Prosobranchia]. J. mar. biol. Ass. U.K. 55: 45-67.

 

Wigham, G.D. 1975b. Environmental influences upon the expression of shell form in Rissoa parva (da Costa). [Gastropoda: Prosobranchia]. J. mar. biol. Ass. U.K. 55: 425-438.

Abstract at journals.cambridge.org/action/displayAbstract?fromPage=on...

 

Current taxonomy: World Register of Marine Species (WoRMS) www.marinespecies.org/aphia.php?p=taxdetails&id=141365

 

Glossary

‰ = (salinity) parts salt per thousand parts water.

abapertural/adapertural = away from/towards the aperture.

abapical/adapical = away from/towards the apex of the shell.

aperture = mouth of gastropod shell; outlet for head and foot.

bifid = divided into two parts by a cleft.

body whorl = most recent and largest whorl of a gastropod shell, which accommodates the withdrawn body, apart from viscera in the spire.

 

buccal mass = anterior of digestive system including a radula, odontophore and muscles.

 

cephalic = (adj.) of or on the head.

columella = solid or hollow axial “little column” around which gastropod shell spirals; hidden inside shell, except on final whorl next to lower part of inner lip of aperture.

 

columellar = (adj.) of or near central axis of spiral gastropod.

columellar lip = lower (abapical) part of inner lip of aperture.

costa = (pl. costae) rib on a whorl of a gastropod shell across direction of coiling and spiral striae. costal = (adj.) of, or arranged like, costae.

 

costate = bearing costae.

diatom = microscopic aquatic alga with siliceous cell-walls.

ctenidium = comb-like molluscan gill; usually an axis with a row of filaments either side.

 

ELWS = extreme low water spring tide (usually near March and September equinoxes).

 

falciform = sickle blade shape.

H. = (height of gastropod shells) distance from apex of spire to base of aperture.

 

labial varix = strong or broad costa (rib) along edge of outer lip of aperture.

 

mantle = (a.k.a. pallium) sheet of tissue which secretes the shell and forms a cavity for the gill.

 

metapodium = hind part of the foot.

metapodial = of or on the metapodium.

MTL= mid tide level

mucus = (noun) viscous, slippery substance secreted by various glands on molluscs.

 

mucous = (adj.) pertaining to mucus.

odontophore = approximately ellipsoid structure containing a pair of cartilaginous bolsters which support the radula. Protruded and retracted like a tongue to operate radula.

 

opercular = (adj.) of the operculum.

opercular disc = part of foot on which the operculum rests.

opercular lobe = extension of opercular disc beyond edge of operculum.

 

operculum = plate of horny conchiolin used to close shell aperture.

 

osphradium = organ for testing inhalant water for particles and/or chemicals.

 

palatal lip = outer lip of gastropod aperture.

pallial tentacle = tentacular extension of the edge of the pallium (mantle).

 

pallium = (a.k.a. mantle) sheet of tissue which secretes the shell and forms a cavity for the gill.

 

parietal lip = upper part of inner side of gastropod aperture, often lacking clear lip structure with just a glaze on side of whorl adapically of columellar lip.

 

pedal = (adj.) of the foot.

periphery = perimeter of the body whorl of a gastropod at its widest.

 

peristome = entire rim of aperture.

plankton = animals and plants that drift in pelagic zone (main body of water).

 

protoconch = apical whorls produced during embryonic and larval stages of gastropod; often different in form from other whorls forming teleoconch.

 

radula = chitinous ribbon of teeth; extended on odontophore to rasp food.

 

stria = (pl. striae) very narrow spiral groove.

suture = groove or line where whorls of gastropod shell adjoin.

teleoconch = entire gastropod shell, apart from apical protoconch.

veliger = shelled larva of marine gastropod or bivalve mollusc which swims by beating cilia of a velum (bilobed flap).

 

 

Done