1: dark, non-costate, apical whorls slightly convex or almost flat.
2: costate whorls convex (usually more so than on non-costate specimens).
3: apex often lilac.
4: lips form a continuous peristome, usually marked entirely or partially brown.
5: distinct thin parietal lip.
6: short columellar lip reflected onto body whorl, so no umbilcal groove visible.
7: falciform (sickle-shape-mark, a.k.a. ‘comma’) crossing white labial varix.
Key identification features
Rissoa parva Numbers equate with similar species.
* Brown falciform mark (comma) across labial varix on adults
06 Rissoa parva. Shell height 3.2 mm. Lleyn Peninsula, Wales. March 2015. & 08 Rissoa parva. Shell height 2.8 mm. Menai Strait, Wales. September 2011. . Diagnostic when present, but absent from shells less than 2mm high 11 Rissoa parva. Shell height 2.6 mm. Menai Strait, Wales. September 2011. and from some larger shells.
1. Protoconch various colours, including lilac or purplish, but not
confined to single tiny apical embryonic whorl 27 Pusillina inconspicua (B 1.8mm, C 2mm, D 2mm) with Rissoa parva (A 2.5mm) & P. sarsii (E 2.3mm). All specimens have five whorls (including minute, apical, embryonic whorl). B & E leg. coll. J.M.Light. .
2. Cephalic tentacles have single medial opaque white line
08 Rissoa parva. Shell height 2.8 mm. Menai Strait, Wales. September 2011. ; sometimes discontinuous
20 Rissoa parva. Shell height 3.2 mm. Lleyn Peninsula, Wales. March 2015. . When magnified, narrow translucent slit may be
detected in line 05 Rissoa parva. Shell height 3.9 mm. Weymouth. Early April 2012. ; occasional specimens,
mainly juveniles, lack white line.
3. If present, 5-6 widely-spaced, often strongly-developed, costae
(axial ribs) on adapical view of body-whorl 14 Rissoa parva. Shell height 4.4 mm. Weymouth. Early April 2012. .
4. Max. height 5mm.
5.Long, whitish penis, tapering to a filament at tip
21 Rissoa parva. Shell height 3.2 mm. Lleyn Peninsula, Wales. March 2015. .
6. On equal-sized specimens, foot narrower than on P. inconspicua 28 Pusillina inconspicua on right (height 2.6mm), compared with Rissoa parva on left (2.5mm) and centre (3.1mm) .
7. Opaque white posterior pedal gland extends almost to edge of sole 28 Pusillina inconspicua on right (height 2.6mm), compared with Rissoa parva on left (2.5mm) and centre (3.1mm) .
8. Head/snout translucent whitish; sometimes has narrow blackish mark and/or a few opaque white marks 16 Rissoa parva. Shell height 2.6 mm. Menai Strait, Wales. September 2011.
9. Short columellar lip reflected onto body whorl, so no umbilical groove visible 01 Rissoa parva. Costate (ribbed) form. Shell height 3 mm. Lleyn Peninsula, Wales. March 2015. .
10. Periphery of body whorl rounded on adults, angulated on young (4-5 whorls) 04 Rissoa parva. Shell heights 3 mm & 1.8 mm. Menai Strait, Wales. September 2011. .
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Pusillina inconspicua
1. Tiny purple apical spot confined to single embryonic whorl, not on the two other (larval) whorls of the protoconch. 08 Pusillina inconspicua. Heights 1.3 mm, 1.6 mm & 1.6 mm. Brighton, England. November 1997. Leg. coll. J.M.Light. . Diagnostic when present, but absent from some.
2. Cephalic tentacles translucent white with two parallel irregular rows of substantial, opaque-white, hyphen-like marks in each tentacle 17 Pusillina inconspicua. Height 2.1mm. Lleyn Peninsula, Wales. March 2015. ; in lateral view one row may conceal other 10 Pusillina inconspicua. Height 2.1mm. Lleyn Peninsula, Wales. March 2015. .
3. If present, 9-15 closely-spaced, fine costae (axial ribs) on adapical view of body-whorl 30 Pusillina inconspicua A-E, comparison of costae on body-whorl, visible in apertural view, with P. sarsii F-H. Leg. coll. J.M.Light. All at same scale. . This feature differentiates from P. sarsii, but many populations have few or no costate individuals.
4. Max. height 3mm, usually less. When shells with equal numbers of whorls are compared, P. inconspicua shorter than R. parva and P. sarsii 27 Pusillina inconspicua (B 1.8mm, C 2mm, D 2mm) with Rissoa parva (A 2.5mm) & P. sarsii (E 2.3mm). All specimens have five whorls (including minute, apical, embryonic whorl). B & E leg. coll. J.M.Light. .
5 Long, whitish penis, with blunt rounded tip and no filament 26 Pusillina inconspicua. Height 1.6mm. Lleyn Peninsula, Wales. March 2015. .
6. On equal-sized specimens, foot wider than on R. parva 28 Pusillina inconspicua on right (height 2.6mm), compared with Rissoa parva on left (2.5mm) and centre (3.1mm) .
7. Opaque white posterior pedal gland stops well-short of edge of sole.
8. Snout often has broad medial blackish or brown-umber band 23 Pusillina inconspicua. Height 2.4mm. Lleyn Peninsula, Wales. March 2015. that extends onto head and body 14 Pusillina inconspicua. Lleyn Peninsula, Wales. March 2015. . (This feature enabled immediate separation by IFS of all P. inconspicua from all R. parva at a Welsh site, but it may vary; Høisaeter, 2009, found dark markings distinct at one site, but faint or absent at two other sites.)
9. Columellar lip only slightly reflected, usually leaving an umbilical groove between it and body-whorl 01 Pusillina inconspicua. Adult profile variation. B-D leg. coll. J.M.Light. & 03 Pusillina inconspicua. Illustrations from J. Alder’s 1844 original description of the species. Type locality: deep water off Cullercoats, Northumberland. .
10. Periphery of body-whorl well-rounded on both adults and juveniles 02 Pusillina inconspicua. Heights 1.3 mm, 1.6 mm & 1.6 mm. Brighton, England. November 1997. Leg. coll. J.M.Light. .
Pusillina sarsii (Lovén, 1846) Ps 27 Rissoa parva album. COMPARISON images of Pusillina sarsii. Heights 3.2mm, 3.1mm, 2.3mm. Donegal, Ireland. June 1989. Leg. J. Light.
WoRMS recognises P. sarsii as a valid species distinct from P. inconspicua, but several authors express varying degrees of doubt and of difficulty differentiating the two species at some sites. “– – neither the shape of the shell nor the sculpture have much diagnostic value.” (Høisaeter, 2009). “One possibility – – is that P. sarsi is a series of local populations of P. inconspicua, adapted to brackish water.” (Warén, 1996). Live specimens not observed by IFS, so soft-part details dependant on incomplete published descriptions. Not recorded from North Sea Caithness-Kent, NBN interactive U.K. map data.nbn.org.uk/imt/?mode=SPECIES&species=NHMSYS00210…
1. Protoconch (3 apical whorls) coloured as rest of shell, sometimes dark bronze or faint lilac tinge, but not a purple spot confined to single tiny apical embryonic whorl.
2. Cephalic tentacles usually spotted with yellow or opaque-white (Jeffreys, 1867, as Rissoa albella).
3.If present, 6-8 widely-spaced, rounded, shallow costae (axial ribs) on adapical view of body-whorl 30 Pusillina inconspicua A-E, comparison of costae on body-whorl, visible in apertural view, with P. sarsii F-H. Leg. coll. J.M.Light. All at same scale. but many populations have few or no costate individuals.
4. Max. height 4 mm. When shells with equal numbers of whorls are compared, P. sarsii taller than P. inconspicua 27 Pusillina inconspicua (B 1.8mm, C 2mm, D 2mm) with Rissoa parva (A 2.5mm) & P. sarsii (E 2.3mm). All specimens have five whorls (including minute, apical, embryonic whorl). B & E leg. coll. J.M.Light. .
5. – 7. no information found.
8. Brown and white speckles on the head (Fretter & Graham,1978).
9. Columellar lip only slightly reflected, usually leaving an umbilical groove between it and body-whorl.
10. Periphery of body-whorl rounded on both adults and young, not angulated.