Hunting behavior in the carnivore marine snail, Nassarius nitidus

Hunting behavior in the
carnivore marine snail,
Nassarius nitidus
Andrea Kilströmer
Degree project for Bachelor of Science in
Biology
15 hec
Department of Marine Ecology
University of Gothenburg
Contribution number
540
Supervisor:
Prof. Kerstin Johannesson
Hunting behavior in the carnivore marine snail,
Nassarius nitidus
Andrea Kilströmer
Department of Marine Ecology-Tjärnö, University of Gothenburg
ABSTRACT Little is known about the mechanisms involved in food localization within the
species of Nassarius nitidus. In the present paper I shall perform experimental tests of
hypotheses based on chemical cues and group behavior. Studies presented on sibling species
have confirmed the involvement of olfactory signals in the water; hitherto this has not been
studied in N. nitidus.
Two main experiments were conducted in which the existence of a food-seeking
response, activated by the presence of an extract in the nearby surroundings, was verified.
Confirmation was attained when food-extract penetrated the water surface and the individuals
affected (97 %) quickly unburied themselves and descended down the food gradient with the
intention of stumbling across the food source. An alternative option might also have been the
occurrence of a rheotaxis response triggered by the mere exposure of a food stimulus; ergo
move against the current when detection of the odor has been confirmed and regardless of a
food gradient. However, this theory was later discarded; since the current itself proved to
have no- or even a discouraging effect on the whelks’ food-seeking response (70 % moved
downstream; 30 % moved upstream). As it happened the key turned out to be the
establishment of the food gradient in the water, which guided the specimens (44 out of 45) in
the right direction.
INTRODUCTION Chemical compounds are
widely recognized as being involved in the
interactions in marine communities,
although the mechanisms by which they
mediate are still unidentified (Kohn 1961).
In 1971, Gurin and Carr conducted an
experiment in which they discovered a
protein, derived from oyster fluid, with the
capacity of stimulating Nassarius obsoletus
in its search for food. This was one of the
earliest discoveries in which a food-seeking
response, in a marine animal, was allocated
entirely due to the presence of a chemical
entity. Thus the implication of chemical
cues in food localization is yet to be
discovered and more research is required on
the subject.
The present paper was undertaken in
order to address the issue of chemical
stimuli and acquire an insight in the hunting
behavior of N. nitidus. The following
questions were included: (I) How does the
marine snail N. nitidus locate food? (II) Do
the whelks exhibit a rheotaxis response?
And if so, can it be triggered by either; (i) a
food gradient or, (ii) by the mere exposure
of a current when first activated by food
extract from above?
Earlier work has described a positive
food-seeking response in Nassariid species,
due to the establishment of a gradient of
chemical stimuli (Crisp 1978; Morton and
Yuen 2000; Bachelet et al. 2004). The
authors discovered the existence of a
behavior guiding the whelks to move
upstream in the direction of the food source.
The whelks moved with determination
against the current and pinpointed the
location of the food item. Similar studies on
the species of our interest however, are
scarce and not much has been investigated
or presented.
Two additional questions was also
undertaken thus to standardize the methods;
(III) could a time-lag in the food-seeking
response be expected between fresh and
decaying carrion? (IV) How long time is to
be expected between newly saturated snails
and their next search for food?
1
Taxonomy
The
taxonomic
identifications
and
distributions of the group nassariids
(Gastropoda, Prosobranchia) has for a long
time been tainted with confusion and
uncertainty. Sibling species has been
confounded or even wrongly considered the
same species. The nassariid used in the
present paper, Nassarius nitidus (Jeffreys
1867), was for a long time confused with its
sibling species Nassarius reticulatus
(Linnaeus 1758). This was due to their
similarity in both appearance and
distribution.
Rolán and Luque (1994) were the first
to demonstrate that the two nassariids in
fact were different species; with both
morphological and behavioral differences.
Their findings also provided evidence for a
differentiated preference in habitat; where
the more sheltered bay was preferred by N.
nitidus, and a more exposed open shore was
preferred by N. reticulatus. Later Sanjuan et
al (1997) conducted a genetic data analyze
and
investigated
the
biological
characteristics for the two species. Their
results were corresponding with the
previously presented data, and showed a
strong indication towards two valid taxa at
the specific level for N. nitidus and N.
reticulatus.
Since the history is surrounded with
uncertainty, only a few papers have been
presented in which N. nitidus is the
employed name. Therefore, some sources
used in the present paper refer to papers on
N. reticulatus instead.
Ecology
The netted dog whelk N. nitidus is a marine
scavenging snail, frequently found in
shallow waters with sandy bottoms along
the Swedish west coast (Tallmark 1980). As
seen in many other marine snails, N. nitidus
spends its first few days as plankton drifting
in the pelagic before settling in the sediment
at a depth of 20 meters. In the deep water
the larva undergoes metamorphosis and
transforms into a fully developed whelk.
The whelk lingers in the deep water,
partly buried in the sediment with only its
siphon protruding, until it has reached
sexual maturity. Sexual maturity occurs
when the snail is about four years old and
has reached an average length of 15 mm. At
this stage in life it begins for the first time to
go on seasonal migrations. Moving from the
deep water early in the spring with water
temperatures around 7°C, and reaching the
shallow waters in June. During the summer
months it then remains stationary within the
shallow waters. The return towards deeper
water begins in September, to avoid the risk
of freezing during the colder month, and is
fully completed in December.
Moving over surfaces most snails and slugs
leave behind a silvery mucus trail, which
eases locomotion (Hosokawa et al. 2009).
Besides locomotion the trail is also thought
to be a sort of communication between
conspecifics, enabling them to congregate
and find a suitable partner by trail following
(Stafford and Davies 2005; Johannesson et
al. 2008). A pre-existing trail also reduces
the energy costs associated with the
production of mucus, since it now can
benefit from locomoting over the old one
(Davies and Black well 2007).
As mentioned above, N. nitidus is a
scavenger and rely on the unpredicted
supply of carrion presented in the nearby
surroundings. The abundant blue mussel
Mytilus edulis is a favored sustenance which
it can devour when found damaged (Morton
2000). Injured individuals of this species
cause the snails to peer out of the sediment
and proceed towards the mussel. They then
quickly start to feed by extending their
proboscis and take big chunks of the item.
The proboscis originates from the early
larval stage and by which it can quickly
process a meal (Page 2005). The amount
consumed can reach up to an astonishing
50% of their bodyweight per day (Morton
1990).
Feeding snails contribute to an
increase of compounds in the water mass
2
(personal observation), which in turn can
lure larger predators to the area; such as
hermit crabs (Pagurus), shore crabs
(Carcinus maenas), etc. For this reason they
need to eat fast, or they will lose the
opportunity to feed or even end up being the
prey along with the blue mussel.
Eggs from the common sand goby,
Pomatoschistus minutus, has also been seen
eaten by N. nitidus (Jarvi et al.
unpublished); since the breeding season of
the sand goby peaks at the same time as the
snail begins their inshore migration, it
provides an appreciated meal. This results
in a “predator-prey relationship” between
the two species; the goby trying to defend
its nest and avoid a decrease in clutch size,
and contrary the snail trying to acquire food
by sneaking past the guarding male.
MATERIAL AND METHODS The research
was carried out at the Sven Lovén Center
for Marine sciences at Tjärnö on the
Swedish west coast during April to May of
2010. Experiments were designed to
investigate the feeding behavior of N.
nitidus.
Similar-sized adults (shell height 16.3 -23.6
mm) of N. nitidus was collected from the
direct surroundings of the Sven Lovén
Center, using either a sink net baited with
flesh from the fish Molva molva or by hand
after a crushed M. edulis had been
introduced in the water. All whelks were
then maintained indoors in aquaria with
running seawater and without sediment.
During the experiments, water temperature
and salinity ranges were 5.4 to 15.8 °C and
23.1 to 31.2 ‰, respectively. The whelks
were held under two different feeding
conditions based on their purpose, due to
the differences in the experiments;
(I) in the series the whelks were
initially feed until satiated, and then kept
starved during the foregoing trails
(II) the whelks were maintained in
aquaria without food for two weeks prior to
the experiments.
During the foregoing trails the whelks were
allowed access to sediment, which also had
been
collected
from
the
nearby
surroundings of the Sven Lovén Center.
Before introducing the whelks to the
sediment, it was filtered in a 500 µm mesh.
Filtering was necessary due to the high risk
of any uncounted individuals or any larger
organisms to enter and taint the outcome of
the results. To avoid creating homogeny
sediment consisting of only fine particles,
gravel was sorted and blended in with the
rest.
Experimental design
All experiments were conducted in a larger
aquarium (100 x 41 x 6.5 cm) divided in
three long straight channels and placed
outside under natural lighting (see picture
1). A ~2.5 cm thick layer of sediment were
placed inside each channel and then filled
with seawater of an additional depth of ~2
cm. Surface seawater was allowed to run
thru the channels in a constant direction and
speed of ~1.7 cm/s.
No attempt was made to control
environmental parameters; however, the
experiments were performed during the day
under a short period of time (April-May
2010).
Picture 1: Showing the experimental setup, in which all experiments took place.
Experimentally-naïve whelks were allowed
to completely bury themselves before any
experiments were initiated. The unburying
3
response in the subsequent experiments
entailed the entire shell to emerge from the
sediment, and the exposure of the soft body.
The channels were flushed for thirty
minutes in between every replicate to ensure
no contamination. The sediment was also
disrupted in order to remove previous trails
or burrows in which chemical cues still
might remain.
Extract preparation
Two different extracts was used during the
experiments; (I) surface seawater, to
stimulate the spontaneous behavior of N.
nitidus when activated by a new fresh
release of seawater or when distracted by a
foreign object (the pipette), and (II) “food
extract”, used to reveal the response
triggered by the presence of food. This
potion was in addition divided into two
subcategories; (i) “old”, and (ii) “fresh”.
To prepare the food extract, three
freshly crushed M. edulis were blended in 3
dl pure seawater and then filtered in a 30
µm mesh.
The fresh extract was made daily to
avoid deterioration or loss of essential
compounds.
The “old” extract was stored in a
canister throughout the study along with the
same amount of seawater, stored in an exact
replica of the canister. They were then
placed in a cooling room with temperatures
close to zero.
Unburying response
The first experiment was merely to reveal
whether or not the whelks were responsive
to an olfactory signal derived from
nonconspecific carrion in an artificial
environment.
A gradient was created, using their
preferred food item M. edulis, by adding 20
ml extract close to the inflow of the
channel. This was done with the use of a
small pipette. To ensure the assembly of a
gradient upstream, extract was added three
times under a minute’s time. The unburying
response was then observed for five
minutes, or until all whelks had emerged if
that was the case. The procedure was
repeated three times for each extract, using
10 whelks in each replicate i.e. a total
number of 60 individuals.
Water flow response
To investigate if the specimens choose to
move against the water flow when triggered
by a food stimuli but without having the
presence of a food gradient in the water, a
second experiment was conducted.
Whelks were placed in a straight line
across the middle of the channel. Extract
was then sprayed directly on top of them,
using the same amount and technique as in
the previous experiments. This caused the
extract to quickly descend down the current
and leave the system and offer the preferred
“no gradient”. The response to the treatment
was observed for five minutes and a goal
bar was set to a distance of ten centimeters
upstream
and
thus
consequently
downstream. The test was then repeated
twelve times for both extracts using five
whelks in each trail i.e. a total number of
120 individuals.
Food gradient response
To analyze if the food seeking response was
activated by the assembly of a food
gradient; olfactory substances were added
repeatedly for one minute’s time in the
water mass.
The experimental set-up was designed
in the same way as in the “water flow
response” assay, apart from the distance to
which the extracts were added. To ensure
the creation of a gradient, the same
technique as in the first experiment was
practiced. The test was repeated eight times
for both extracts i.e. a total number of 80
individuals.
Series, unburying response
A series was conducted to observe whether
or not the unburying response was affected
by hunger and if they chose to seek out food
more quickly when hungry.
The whelks were feed until satiated
and then starved for a controlled number of
4
days in order to investigate their willingness
to unbury themselves. Three replicates,
using five whelks in each, were repeated
with both extracts for each day during one
week’s time i.e. a total number of 240
individuals. The unburying response was
observed in the same way as in the previous
experiments.
Food preference
A one-choice assay was conducted to
determine if a feeding preference towards
fresh or decaying carrion exists.
Five whelks were placed in a line
across the channel and treated with either
old or fresh extracts; also a control group
was tested. The extracts were added using
the same technique as in the previous
experiment and also the fixed five minutes
bar was exercised. The time it took for all
whelks to emerged, if that was the case, was
observed and noted. The experiment was
repeated five times for both extract i.e. a
total number of 50 individuals.
STATISTICAL ANALYSIS The results from
the experiments were statistically analyzed
with a Chi square 2x2 contingency table,
and the probability level was found in the
Chi square distribution table.
RESULTS
Unburying response
Control animals displayed no response
(0/30) towards the exposure of additional
seawater nor to the intruding pipette (see
figure 1). They remained buried in the
sediment with only the siphon protruding
with which they on occasion waved in the
direction of the current. On the other hand, a
significant difference was displayed when
the whelks were exposed to mussel
treatment and responded by quickly unbury
and start moving around the channel (df=1;
chi2=56.13).
Figure 1: Illustrating the number of
individuals displaying the unburying
response in N. nitidus when exposed to
treatment of either I) fresh seawater or, II)
seawater containing extracts from M.
edulis.
The displayed reactions was analyzed in a
2x2 contingency table and assumed a value
larger than the critical of 3.84, which means
that the correlations were significant at the
0.05-level. The null hypothesis could
therefore be rejected.
Water flow response
Table 1 describes, as before, how N. nitidus
is nonresponsive towards control treatment
but on the contrary show a significant
response towards the mussel treatment
(df=1; chi2=120; P<0.01)
When activated by mussel extract
from above; 70 % of the subjects moved
down the current and passed the goal bar of
10 centimeters, whereas 30 % chose to
move against the current.
Table 1: Stimulated by either clean
seawater or seawater containing extract
from M. edulis directly from overhead, the
direction of movement was investigated.
Upstream
Downstream
Clean
seawater
0
0
Extract of
M. edulis
11
26
The obtained results showed no statistical
difference
between
upstreamand
downstream movement, hence the whelks
had a randomized movement pattern. The
5
null hypothesis could therefore not be
rejected, and no confirmation was obtained
towards the rheotaxis model.
Food gradient response
Streams containing extract from M. edulis
stimulated N. nitidus to move upstream
towards higher concentrations, as seen in
table 2. 44 out of 45 responded to the
mussel treatment and displayed a significant
rheotaxis by crossing the fixed goal bar of
10 centimeters upstream. As demonstrated
earlier, no response was reported in the
control treatment.
Table 2: By the establishment of a food
gradient, consisting of extract from the
bivalve M. edulis placed in the incoming
stream, the rheotaxis response was
investigated.
Upstream
Clean
seawater
Extract of
M. edulis
Still buried
0
45
44
1
The chi2 value (86.09) was larger than the
critical value; which means that the
correlations were significant at the 0.05level; suggesting that this behavior is in
concordance with the unburying response.
Series, unburying response
The whelks ignored the presented food
initially, but became more responsive over
time after last meal (see figure 2).
On day zero and one, no whelks
emerged (0/15). All remained stationary in
the sediment with only the siphon
protruding seeming unaffected by the
induced odor in the water mass.
On day two and three respectively,
3/15 whelks unburied themselves and began
to investigate the vicinity by moving
around.
On day four, 5/15 whelks responded
positive towards the treatment and unburied
themselves immediately when they sensed
the odor in the water.
On day five, 4/15 whelks unburied
themselves when presented to the chemical
stimuli.
On day six, 6/15 whelks reacted by
unbury themselves.
On day seven, 8/15 whelks emerged
i.e. more than 50% of the total number.
Figure 2: Unburying responses of recently
(within 10 min) satiated N. nitidus and a
follow-up over the next seven days.
Food preference
The response time towards decaying carrion
in the one-choice assay was significantly
longer (p-value 0.003) than for fresh
carrion; 253.4 versus 44.6 s (see table 3).
Table 3: Activated by the presence of
extract from either fresh or decaying food,
the total time for all whelks to react by
unburying, or until the fixed 5 minutes bar
was exceeded if that was the case, was
scrutinized.
Fresh M. edulis
Time (s)
27
34
68
42
52
Old M. edulis
Time (s)
210
300
157
300
300
In three replicates in which old extract was
used; the time limit was exceeded and noted
as 300.
DISCUSSION Several interesting patterns
was revealed in the present study; namely
(I) the response towards a food stimuli was
to quickly unbury and seek out the food, (II)
starved individuals displayed a more eager
food-seeking behavior, (III) a preference
6
towards fresh food was found, and (IV) the
unburying response was triggered by the
presence of a food gradient in the water
mass. The unburying response, to begin
with, was performed in a quick and resolute
way, indicating that the behavior has arisen
due to the scarce distribution of food in the
ocean. The whelks need to seek out food
when the possibility is presented; or else
they won’t succeed in having a high fitness.
An individual which has starved in the
absence of available food react therefore
more swiftly since it is in a great need of
nourishment. The same is true for the
preference towards fresh food, indicating
that they are in agreement. Fresh food
supply more nutrition and a lowered risk of
competitors; since the food item has been in
the water for a shorter time and hence fewer
organisms had had time to be attracted.
When creating a food gradient, it
became clear that the whelks needed a
concentration to follow in order to find the
food source. This discovery came as a
surprise, since we first thought the whelks
should be satisfied by simply sense the odor
in the water and then strictly starts to move
against the current; instead they became
confused and unfocused. No significant
results were obtained in the experiment
using only water flow; but the rheotaxis
response was significant at the 0.05-level in
the food gradient experiment.
Morton and Yuen (2000) performed an
experiment with the sibling species,
Nassarius festivus, in which they looked at
(1) food preferences, and (2) food detection
distance. For bait they used; fish
(Lateolabrax
sp),
bivalve
(Tapes
philippinarum), soldier crab (Mictyris
longicarpus) and mud-shrimp (Upogebia
major). Their results showed that N. festivus
was able to detect a food item from a
distance of 80 cm (it should be noted that
this was the maximum length used in this
experiment which indicate that N. festivus
might be able to detect food from a greater
distance). The study also revealed that the
preferred food item was fish (Lateolabrax
sp) and bivalve (Tapes philippinarum);
which underlie our choice of experimental
food extract.
The feeding behavior of N. reticulatus
was investigated by Davenport et al. (2002)
in Scotland, UK; where they examined the
hunting behavior both with and without
substratum. Their experiments showed that
cod- and crab extract provoked the most
impressive display. It became evident when
all animals were showing activity towards
the stimulus within minutes; by waving of
the siphon, shell rocking, eversion of the
proboscis and rapid movements around the
aquaria. These results corroborate what
Mary Crisp saw in her experiments in 1978.
The activity- and unburying response
from the above mentioned studies are in
agreement with the findings in the present
paper. Where the majorities of the
specimens reacted towards the M. edulis
treatment, none on the contrary reacted
towards the control treatment. M. edulis
extract triggered the specimens to emerge
out of the sediment and quickly start to
move around the aquarium waving their
siphon in the direction of the food source.
The foreign object, saturated as the pipette,
induced no reaction nor did the introduction
of a new fresh release of sea water.
Extract was prepared in a canteen during the
length of the series and from which the
whelks was treated in order to reveal a
hypothetical behavior. In the food
preference experiment this was proved to be
a bad choice of sustenance; since it was
evident that they preferred fresh food before
old. The obtained results might therefore
have been more evident if extract had been
made daily, which is something that should
be made different if the experiment was to
be tested again. The length of the series was
also cut short due to lack of time and a
clear-cut result could therefore not be
obtained. This parameter should be more
carefully maintained in a follow-up, in order
to exhibit a more interesting result.
7
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8