FWGNA > Freshwater Gastropods of Tennessee > Dichotomous Key
Tennessee photobar
Dichotomous Key
Eastern Tennessee River Drainages

Disclaimers: (1) This key isn't literally dichotomous. (2) It is intended for use only with the 40 species and subspecies of freshwater gastropods confirmed for the drainages of the Tennessee River above the Alabama line, including southwestern Virginia, western North Carolina, and north Georgia, plus 1 species unconfirmed (U). The management will not be responsible for any loss, damage, or injury resulting from its application elsewhere.

1a) Operculum present. . . Subclass Prosobranchia (2)
1b) Operculum absent . . . Subclass Pulmonata (28)

2a) Operculum concentric [photo] . . . (3)
2c) Operculum paucispiral [photo] . . . (4)

3a) Aperture greater than 50% of shell height . . . Viviparus subpurpureus
3b) Aperture 50% of shell height or less . . .  Campeloma decisum

4a) Adults over 10 mm shell length . . . Family Pleuroceridae (5)
4b) Adults under 10 mm shell length . . . (19)

5a) Shell with prominent siphonal canal, often bearing spines . . . Io fluvialis
5b) No siphonal canal . . . (6)

6a) Shell height less than 1.7 times width . . . (7)
6b) Shell height greater than 1.7 times width . . . (9)

7a) Adult shell height typically exceeding 15 mm . . . Leptoxis crassa
7b) Shell height rarely exceeding 15 mm . . . (8)

8a) Shell never carinate, tentacles constricted at eye,
    body bluish gray . . . Leptoxis praerosa 
8b) Shell often carinate, tentacles taper gradually,
    body dark with orange or yellow hue . . . Leptoxis carinata 

9a) Shells typically pustulate . . . Lithasia verrucosa
9b) Shells costate, at least on early whorls . . . (10)
9c) Shells never costate or pustulate . . . (11) 

10a) Spiral cords present . . . Pleurocera catenaria catenaria
10b) Spiral cords absent . . . Pleurocera laqueata

11a) Shell height greater than 3 times shell width . . . Pleurocera troostiana 
11b) Shell height less than 3 times shell width . . . (12)

12a) Adult shells never carinate . . . (13)
12b) Adult shells may bear a single strong carination . . . (15)

13a) Body of animal typically orange or brown, with black flecks.  Inhabits 
    upper tributaries of the Hiwassee . . . Pleurocera modesta
13b) Body primarily black.  Widespread . . . (14)

14a) Shell broader, apex length better fitting the regression A = 0.157B + 2.46,
    where B is body whorl length . . . Pleurocera simplex
14b) Shell more slender, apex length better fitting the regression A = 0.556B - 0.09,
    where B is body whorl length . . . Pleurocera gabbiana

15a) Inhabits softwater rivers and streams, shell usually at least slightly
    carinate throughout, apex typically eroded . . . Pleurocera proxima
15b) Inhabits hardwater rivers and streams, apex not typically eroded . . . (16)

16a) Shell acutely-spired, always bearing a single, strong sub-peripheral 
    carination . . . (18)
16b) Shell variably carinate - bearing peripheral carination, subperipheral carination, 
    or no carination at all . . . (17)

17a) Populations inhabiting larger rivers and bearing squat, robust shells . . . 
    Pleurocera clavaeformis unciale
17b) Populations inhabiting creeks and small rivers, bearing more slender shells . . . 
    Pleurocera clavaeformis clavaeformis

18a) Robustly-shelled, inhabiting the main Tennessee River and major
     tributaries . . . Pleurocera canaliculata canaliculata 
18b) Shell slender, inhabiting creeks and smaller rivers . . .
    Pleurocera canaliculata pyrenellum

19a) Primarily terrestrial in life habit, found above the water 
    level . . . Family Pomatiopsidae (20)
19b) Entirely aquatic in life habit . . . Family Hydrobiidae (21)

20a) Whorls rounded, shell sutures deeply impressed, apex
    acute . . . Pomatiopsis cincinnatiensis
20b) Whorls more ovoid in outline, sutures not deeply impressed, shell more
    fusiform . . . Pomatiopsis lapidaria

21a) Penis with a single duct . . . (22)
21b) Penis with two ducts . . . (27)

22a) Penis simple, unlobed . . . (23)
22b) Penis with glandular terminal lobe . . . (25)
22c) Penis with two accessory lobes . . . (26)

23a) Shell umbilicate . . . Clappia umbilicata
23b) Shell not umbilicate . . .  (24)

24a) Sturdy, heavily-shelled inhabitant of rocks in the Hiwassee R . . . Somatogyrus virginicus
24b) Sturdy, heavily-shelled inhabitant of rocks in the Powell R . . . Somatogyrus parvulus
24c) Slender, lightly-shelled cave dweller . . . Holsingeria unthanksensis

25a) Lightly-shelled, umbilicus open, endemic to springs in Sequatchie
    County, TN . . . Marstonia ogmorhaphe
25b) More heavily-shelled, umbilicus closed, main Tennessee River and larger 
    tributaries . . . Marstonia arga

26a) Body pigmented, proximal penial lobe containing tubular 
    gland . . . Fontigens nickliniana
26b) Body unpigmented, proximal penial lobe containing bulbous 
    gland . . . Fontigens orolibas (U)

27a) Operculum entirely paucispiral . . . . Amnicola limosa
26b) Operculum initially multispiral, becoming paucispiral . . . Lyogyrus granum

28a) Shell dextral . . . Family Lymnaeidae (29)
28b) Shell sinistral, not planispiral . . . Family Physidae (30)
28c) Shell sinistral, planispiral . . . Family Planorbidae (32)
28d) Shell patelliform . . . Family Ancylidae (35)

29a) Aperture greater than 50% shell height . . . Lymnaea columella
29b) Aperture less than 50% shell height . . . Lymnaea humilis

30a) One-part penial sheath . . . Physa acuta
30b) Two-part penial sheath . . . (31)

31a) Whorls convex . . . Physa gyrina
31b) Whorls not notably convex . . . Physa pomilia

32a) Adult greater than 10 mm shell diameter . . . (33)
32b) Adult less than 10 mm shell diameter . . . (34)

33a) Spire appears deeply indented when viewed from either aspect . . . Helisoma anceps
33b) Spire appears flattened when viewed from one aspect . . . Helisoma trivolvis

34a) Shell bearing rounded periphery . . . Gyraulus parvus
34b) Shell with weak carination, off mid-whorl . . . Menetus dilatatus

35a) Apex entirely smooth . . . Laevapex fuscus
35b) Apex bearing fine radial striae . . . (36)

36a) Apex distinctly to the right of the midline . . . Ferrissia fragilis
36b) Apex approximately in the midline . . . Ferrissia rivularis   [photo]