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RESEARCH COMMUNICATIONS
CURRENT SCIENCE, VOL. 93, NO. 6, 25 SEPTEMBER 2007
826
*For correspondence. (e-mail: rjtnd@rediffmail.com)
Snail-pollination in
Volvulopsis
nummularium
Khoisnam Sarma
1
, Rajesh Tandon
1,
*,
K. R. Shivanna
2
and H. Y. Mohan Ram
3
1
Department of Botany, University of Delhi, Delhi 110 007, India
2
Ashoka Trust for Research in Ecology and the Environment,
659 ‘A’ Main Road, Hebbal, Bangalore 560 024, India
3
No. 194,
SFS Flats, Mukherjee Nagar, Delhi
110 009, India
Pollination is a crucial ecological process that aids sexual
reproduction in flowering plants. Although a variety
of animals are known to bring about pollen transfer,
pollination by snails (malacophily) has remained a
rare and obscure phenomenon. Here we conclusively
demonstrate the incidence of malacophily in
Volvulopsis
nummularium
(family Convolvulaceae, commonly known
as the morning glory family), a prostrate rainy-season
weed, which is also visited by honey bees. Flowers open
in the morning and last only for half a day.
Apis cerana
indica
and Graceful Awlsnail (
Lamellaxis gracile
) are
the pollinators. Snails are exclusive pollinators on rainy
days, when bees are not active. Contrary to the belief
that snails are destructive, we found that they do not
affect the natural fecundity of
V. nummularium
. Manual-
pollinations indicated that the plants were facultative
autogamous. Pollination in
V. nummularium
by snails
and honey bees represents an interesting guild, which
is of adaptive significance in achieving high reproduc-
tive success without resorting to obligate selfing.
Keywords:
Honey bees, malacophily, reproduction,
snails,
Volvulopsis nummularium
.
P
OLLINATION
is a significant process and one of the pre-
requisites for ensuring fruit- and seed-set in all sexually
reproducing seed plants
1–3
. A great majority of plants are
pollinated by animals
4
and only a few involve abiotic
agencies
1,5–7
. Among the zoophilous species, nearly 80%
are pollinated by insects. The remaining involve a variety
of birds, reptiles and mammals
8–11
. Pollination by snails
and slugs (malacophily) is a rare and infrequent pheno-
menon; so far it has been reported in seven species:
Rohdea
japonica
,
Philodendron pinnatifidum
,
Colocasia odora
,
Calla palustris
,
Lemna minor
,
Chrysosplenium alterni-
folium
and
Phragmipedium caudatum
12–14
. Prostrate habit
of the plant and floral arrangement in which the stigma
and anthers do not extend much beyond the corolla, are
believed to be conducive to malacophily
12
. However,
some investigators doubt the possibility of snails or slugs
being successful pollinators, and consider malacophily to
be ‘notorious and obscure’
15
or even ‘ridiculous’
14
.
Snails are usually active at night and also during the
day in the rainy season. In an unexpected field observa-
tion, we found mass floral foraging by the terrestrial gar-
den snail – the Graceful Awlsnail (
Lamellaxis gra
cile
) on
a common garden weed,
Volvulopsis nummularium
, in
which flowers open in the morning and close by noon.
This is a small (~15 mm in length) terrestrial snail com-
monly found throughout India and other tropical and sub-
tropical parts of the world, except Australia
16
. We made a
detailed investigation on the pollination ecology and
breeding system of
V. nummularium
to ascertain whether
(i) snails have a definite role in pollination, (ii) their ex-
clusive foraging could induce fruit and seed-set and (iii)
phytophagy by snails is destructive to the natural fecun-
dity of this plant.
V. nummularium
(L.) Roberty (syn.
Evolvulus nummu-
larium
L., family Convolvulaceae), a native of tropical
America, is a weed with prostrate and creeping habit that
has naturalized in moist places in several parts of India
17
.
The plants are multi-stemmed and propagate profusely
through seeds as well as by vegetative means. Profuse
growth of neighbouring individuals leads to the formation
of expanded mats with overlapping stems. The plant be-
comes conspicuous when it starts blooming with the on-
set of monsoon (Figure 1
a
). Peak flowering is reached by
mid-August and lasts for a month. White, short tubular
flowers are borne solitarily, and sometimes two in num-
ber, in the leaf axil. There is no previous report about any
specific pollinator of
V. nummularium
.
A general survey of the distribution of plants of
V.
nummularium
was carried out in Delhi region (28
°
12
′
–
28
°
53
′
N, 76
°
50
′
–77
°
23
′
E) during May–June 2006. For
the present investigation, ten sites in different undisturbed
localities were randomly chosen for detailed observations.
Phenoevents such as time of anthesis, anther dehiscence,
flower longevity, period of flowering and fruiting were
recorded. Anther dehiscence was determined by examin-
ing flowers (
n
= 30) under a stereomicroscope at different
times, from the bud stage until anthesis between 0
400 and
0800 h. Commencement of stigma r
eceptivity and its du-
ration were recorded through semi-vivo pollination studies
18
.
Ovule production in a pistil was computed by scoring the
cleared pistils (
n
= 30). Average pollen production in a
flower (
n
= 30) was estimated using a haemocytometer
19
.
Fertility of pollen grains was tested by scoring their
stainability with 1% acetocarmine
18
. Pollen viability was
assessed through fluorochromatic reaction test
20
. Pollen
viability period was determined using pollen grains sam-
pled every 60 min following anther dehiscence, till it de-
clined to zero. The nature of reserve material in the pollen
grains was identified using Sudan black for lipids and
potassium iodide for starch.
As the flowers were visited only by snails and honey
bees, detailed field observations were confined to these
foragers. The standard entomological method was em-
ployed
for collecting and preserving the honey bees
21
. To
prevent the drying of snails during sampling, they were
narcotized by asphyxiation, followed by washing thor-
oughly with distilled water to remove the mucous, passed
RESEARCH COMMUNICATIONS
CURRENT SCIENCE, VOL. 93, NO. 6, 25 SEPTEMBER 2007
827
through ascending ethanol series (20, 40, 60 and 70%)
and finally stored in 70% ethanol
16
for identification. The
samples were sent to the Zoological Survey of India,
Kolkata for identification.
The role of floral visitors in pollination and their efficiency
was ascertained on the bases of foraging behaviour,
flower-handling time, pollen load and the number of pol-
len grains deposited on the stigma after their visit. For
computing the stigmatic pollen load on the freshly opened
flowers and after the visits of the foragers, flowers
(
n
= 20, each site) were collected in dry screw-cap vials
(2 ml), and their stigmas were mounted in a drop of aur-
amine O
′
and observed under an epifluorescence micro-
scope (Nikon, AXII Optiphot). Foraging period and
flower-handling time were recorded separately for each
forager from all the study sites (a total of 90 h of observa-
tion). The average amount of pollen load on each floral
forager (snails,
n
= 300; honey bees,
n
= 200) was counted
by removing the pollen grains with a brush on separate clean
microslides under the stereomicroscope (Nikon, SMZ 800),
staining with auramine O
′
and examining them under the
epifluorescence microscope.
To ascertain whether or not the visit of snails and bees
could lead to subsequent fruit- and seed-set, the flowers
Figure 1.
a
, Close-up of
Volvulopsis nummularium
plant showing prostrate habit with open flowers.
b
,
Whole mount of a stamen showing latrorse (arrow) longitudinal line of dehiscence of the anther.
c
, Fruits
with variable number of seeds (1–4). Fruit size is proportional to seed number.
d
–
g
, Pollination mecha-
nism in
V. nummularium
.
d
, A snail approaches the flowers at anthesis.
e
, It gradually crawls towards the
anthers to forage.
f
, The shell comes in contact with the dehisced anthers and pollen grains get adhered to
the last whorl.
g
, Pollen transfer occurs when the snail visits the next flower.
h
, A snail entering a par-
tially opened flower during a rainy day. Note vertical orientation of the shell.
i
,
A honey bee foraging the
flower.