RESEARCH NOTES
116
J. Moll. Stud. (1997), 63, 116-120
© The Malacological Society of London 1997
Pre-copulatory behaviour of the freshwater hermaphrodite snail Biomphalaria
glabrata (Say, 1818) (Gastropoda: Pulmonata)
Jacqueline A. Trigwell1, G.B.J. Dussart1 and M. Vianey-Liaud2
'Ecology Research Croup, Canterbury Christ Church College, North Holmes Road, Canterbury, Kent,
CT11QU, England
Laboraloire de Ginttique et Environnement, Institute des Sciences de /'Evolution, (UMR-CNRS 5554),
University de Montpellier II, Place Eugine Bataillon, 34095 Montpellier, France
2
Biomphalaria glabrata is an hermaphrodite snail
which when isolated will reproduce by self-fertilization u but, when paired, preferentially reproduces by
outcrossing2. These reproductive modes, together
with the existence of pigmented and albino strains of
Biomphalaria glabrata, make it relatively easy to
study the genetics of this medically important snail.
The copulatory activity and reproductive biology
of many species of freshwater pulmonate molluscs
has been reported in the literature, but in describing
copulation most authors have recounted only the
physical aspects of the act and few have attempted to
divide the behaviour into different phases.
Runham3 divided courtship and mating into four
areas, (1) pre-courtship, (2) courtship, (3) copulation
and (4) postcopulation. He suggested that precourtship was the active searching for a mate and
that courtship began when the male actor mounts
the female actor's shell. He suggested courtship
continues as the snail moves around its partner's
shell and ends when the male actor is in a suitable
position for the act of copulation to take place.
During experiments which necessitated observing
pairs of Biomphalaria glabrata from initial pairing to
post-copulation, the snails were seen to exhibit a
specific mode of behaviour which happens only in
the period immediately before a copulation. The
objective of the present work was to describe and
quantify the pre-copulatory behaviour of B. glabrata.
In order to ensure that the observations were not
influenced by the possibility of learned behaviour,
sexually mature virgin snails that had reached
maturity within fourteen days of each other were
used for all experimental work. To obtain these,
snails were isolated soon after hatching and
remained this way until required for experimental
work. They were deemed to be sexually mature after
observation of the developing embryos in their first
self-fertilized egg masses, the only outward sign that
sexual maturity has arrived. During the course of
experimental work more than 600 pairs of snails
were observed, from the moment of pairing to post
copulation but for the purpose of this paper the
activity of 32 pairs was recorded.
In B. glabrata the time from pairing to the onset of
the act of copulation can vary from as little as ten
minutes to as long as one and a half hours but when
snails were paired in small boxes containing 65 ml
pond water, the following consistent pattern of
behaviour was observed. When sexually mature
virgin snails are paired for the first time, if a copulation is going to take place, one snail is more active
than the other and it approaches and then follows
the partner snail (Fig. 1). As the active snail moves
closer and finally makes contact with the second
snail (Fig. 2), the latter usually moves away. However the active snail maintains contact and mounts
the shell of the second snail (Fig. 3). It moves over
and around the whorl of its partner's shell, from the
umbilicus to the aperture (Figs 4, 5). Once it has
aligned itself on the left side of its partner's body,
near to that snail's female genital pore (Fig. 6), the
active snail stops moving. According to the position
it is in, it may then drop its body downwards,
through 180°, thus facilitating easy entry of the penis
into the female genital pore of its partner (Fig. 7). It
is now obvious that the active snail intends to take
the male role because its everted penis can be seen
(Fig. 8). As the penis is everted and during the
period of intromission, the male-acting snail lays its
tentacles towards the right, away from its penis.
When the male actor is orientated in the copulatory
position, its penis probes for the female genital pore
of the other snail (Fig. 9). As the penis probes
further, the female actor partially withdraws into its
shell and, as its foot is withdrawn, it folds and
wrinkles (Fig. 10). Eventually the female actor
relaxes its body, the penis of the male actor enters its
female genital pore and full copulation takes place
(Fig. 11). The sequence of probing by the penis and
intromission may take place several times before the
male actorfinallystops. When the copulation is over,
a successively reciprocal copulation4 might take
place. This is initiated by the snail acting as the
female in the first copulation everting its penis and
assuming the male role in the second or reciprocal
copulation. Alternatively the copulation might end
with the two snails simply moving apart (Fig. 12).
We have observed a simultaneously reciprocal copulation taking place in approximately five percent of
pairings (Fig. 13). If there is no intention on the part
of either snail to engage in a copulation, they either
remain relatively still in the container, or glide
slowly around and when one snail encounters the
other, it passes over or around it.
Individuals of Biomphalaria glabrata were paired
and observed for a total of 120 mins. The times from
(1) pairing to mounting (between 9 and 93 minutes,
RESEARCH NOTES
117
Female.
actor
Contact
Figures 1-12. Stages in the copulation of Biomphalaria glabrata. A camera lucida was used to draw these
diagrams from photographs taken whilst the snails were in a transparent box. The snails were exhibiting
typical behaviour and some of thefiguresshow the snails crawling up the side of the box (e.g. Figs 7-11).
Figure 13. Biomphalaria glabrata during a simultaneous reciprocal copulation.
118
RESEARCH NOTES
9
Penis
probing
Mouth
Foot
12
Disengagement
Penis
Penis
Penis of
male actor
RESEARCH NOTES
x = 41.92, SD = 26.89), (2) mounting to eversion
of the penis (between 1 and 29 minutes, i = 5.65,
SD = 6.46) and (3) eversion of the penis to the end
of intromission Le, the end of the copulation, (from
5 minutes to in excess of 87 minutes, X = 48,24
SD = 16.82) were recorded. Forty-five percent (14)
of the pairings resulted in a successively reciprocal
copulation taking place, whilst the others simply
moved apart once the copulation was over. In fifteen
percent of pairings the snails did not exhibit precopulatory activity, and no subsequent copulation
took place.
Although female actors have been seen to register
their reluctance to take part in a copulation either by
moving away whilst the male actor has been mounting their shell or by forceful shell swinging when the
male actor is mounted, they nearly always seem
to acquiesce once the male actor's penis starts to
probe for the female genital pore. Occasionally a
copulation is interrupted at this point and the two
snails separate but when the copulation is resumed,
starting again with the pre-copulatory behaviour, the
two snails take the same sexual roles.
Experimental work has shown that when B.
glabrata are paired, and if a copulation is to ensue,
there is always an active snail and a passive snail. We
know that the more active snail takes the male role
and because of its heightened activity it is probable,
though not certain, that the male actor makes the
decision that a copulation will eventuate. Conversely
it is possible that the decision to copulate is made by
the female; by choosing not to take the male role the
female actor might be forcing its partner into involuntarily adopting the male role. However, given that
the time between pairing and mounting can be as
short as nine minutes, we think the latter is unlikely.
It thus appears that (1) there is a consistent and
specific pattern of pre-copulatory behaviour, (2) the
most active snail is always the male actor, (3) the
male actor appears to make the decision that a copulation will take place and (4) the female actor can
decide to terminate the copulation, but this rarely
happens. Once one snail of a pair has made the
decision to copulate, it is possible to predict which
one will take the male role soon after the onset of
pre-copulatory behaviour. Once a copulation has
begun, the female either remains still, or continues
to move slowly around with the male actor firmly
attached to its shell. The folding, wrinkling and
partial withdrawal of the foot seen in Fig. 14 may be
a mechanism to avoid penetration. The female actor
in a Lymnaea stagnalis5 pairing can show similar
withdrawal behaviour and Barraud suggests that this
movement is an attempt to disrupt the current penetration. Rudolph4 found that when Stagnicola elodes
were copulating, the female actor suddenly drew
part of the right side of its body into its shell. He
speculated that this happened as the male actor
inserted its penis into the female actor's gonopore.
Either or both of these reasons could apply to B.
glabrata, though as the withdrawal of the foot occurs
as the penis is probing for the female genital pore,
we suggest that it may be an avoidance mechanism.
119
During some episodes of pairing, after one snail
has mounted the shell of the other and prior to intromission, both snails evert their penes. In correctly
aligned snails this phenomenon may be a pre-cursor
to a simultaneously reciprocal copulation.4 There
may be only one position that facilitates this type of
copulation; Le. the two snails embrace by clasping
the other's shell just above the aperture, with the
head facing towards the opposing aperture (Fig. 13).
These observations of simultaneously reciprocal
copulations can be contrasted with the observations
of Rudolph4 who states that in Stagnicola elodes, a
simultaneously reciprocal copulation is impossible
because of the position of their gonopores.
It would seem that there is a specific pattern of
pre-copulatory behaviour and that in this pattern,
sexual rules are fixed at an early stage. The
knowledge that B. glabrata exhibits such a behaviour
pattern can be used in a range of experimental work.
It is therefore necessary to distinguish between
pre-copulatory behaviour (which indicates a snail's
intention to copulate), a copulation, an insemination
and a fertilization.
Pre-copulation—a specific mode of behaviour that
precedes a copulation.
i
CopulationInsemination
I
*•
Fertilization
-penetration of the female genital
pore by the penis of the partner, with
or without the transference of sperm.
the introduction of sperm into
the female genital tract during
copulation.
the union of sperm and ovum
(resulting in the production of
embryos).
For some experiments, it is sufficient to know that
there is the intention to copulate; for others it is
necessary to know that a copulation has taken place,
although the outcome is irrelevant; some experiments require the knowledge that an insemination
has taken place even though no fertilization has
occurred; others necessitate a copulation which
results in a fertilization.
The ability to recognise consistently a snail's
intention to copulate as a male could mean a considerable saving of time in certain experiments. For
example, in an experiment designed to determine
whether B. glabrata prefers to copulate with a
partner of the same size, larger or smaller, it is useful
to be able to recognise that mounting, orientation
and eversion of the penis indicates a snail's intention
to copulate as a male and that, if left undisturbed, it
will copulate with the compliance of its partner.
For this type of experiment, once the intention to
copulate has been established, the snails could be
parted before the actual copulation has taken place;
it is not necessary to wait until the female actor has
produced eggs to confirm a copulation.
Although the act of copulation has been investigated in a range of freshwater gastropod species,
there has been little specific analysis of their precopulatory activity. In the basommatophoran species
120
RESEARCH NOTES
so far studied, the male actor appears to initiate a
copulation by following and mounting the shell
of the female actor. In Stagnicola elodes*, Bulmus
globosus6-1, Lymnaea stagnalis5^9, Lymnaea truncatula'° and Physa gyrina", copulatory activity appears
to be initiated by the male actor crawling onto the
shell of the female actor. DeWitt" reported that
copulating Physa gyrina go through a mating
sequence of mounting, positioning, preputium
eversion, intromission and dismounting, and that
mate rejection includes shell swinging and jerking,
biting (head/preputium contact) and moving away so
that the male actor cannot reach the female actor's
gonopore. As soon as the male actor has everted and
inserted its penis, Lymnaea Iruncatula10 stops moving
for the duration of the copulation. All of these
behaviours, except the biting practised by Physa
gyrina", are similar to those exhibited by B. glabrata.
We suggest that Runham's3 pre-courtship and
courtship form the pre-copulatory behaviour of B.
glabrata, which begins when the potential male actor
first searches for and then pursues a mate, and ends
when the now obvious male actor starts to evert its
penis in order to copulate.
In Lymnaea stagnalis8, the period of isolation prior
to a copulation determines whether a snail is the
sperm donor or receiver, and the longest-isolated
snail takes the male role. A male actor then paired
the following day with an isolated virgin snail takes
the female role. However, similar experiments in this
laboratory to test the masculinity and receptivity of
B. glabrata have so far been inconclusive and are
continuing. We are also conducting experiments to
determine the copulatory roles chosen by snails
during successive copulations with (a) the same
partner and (b) different partners. These experiments indicate that when paired with the same
partner, certain pairs of snails seem to show a preference to copulate in a particular sexual role. By contrast, when populations of Bulinus cernicus were
examined by Rollinson et al.13, snails from some
populations were found to preferentially take the
female role during copulatory activity whilst those
from other populations preferred to take the male
role. This is unlikely to be relevant to our observations since all the snails came from a single colony.
Also, the major point at issue here is that there
appears to be a male actor pattern of behaviour and
a female actor pattern of behaviour. We contend
that a male actor originating from a 'male-acting'
population, such as those identified for Bulinus
cernicus", or a male actor from a balanced population with equal numbers of male and female actors
will, in either case, show this behaviour pattern
during copulation.
Although we have been unable to determine how
a snail chooses a particular copulatory role, we
believe that the decision to copulate is made by the
male actor, and that the male actor initiates the
copulation by exhibiting pre-copulatory behaviour.
Recognising this pre-copulatory behaviour in Biomphalaria glabrata can be useful in both understanding the fundamental biology of this important
schistosome vector species, and in allowing
economies of effort in behavioural and genetic
experiments.
We are indebted to Heike Reise for many helpful
comments that resulted in considerable improvements in our paper.
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