C The Malacologwal Society of London 1996
J Moll Stud. (1996), 62,121-124
RESEARCH NOTES
Foraging and homing behaviour of the limpet, Patella vulgata: a geographical
comparison
D.R. Gray1 and E. Naylor
School of Ocean Sciences, Marine Science Laboratories, University ofWales-Bangor, Menai Bridge,
Gwynedd LL59 5EY
As yet few generalizations have been formulated,
concerning the tuning, distance and direction of foraging behaviour of Patella vulgata (Linnaeus, 1758).
This limpet has previously been reported to be
active during daytime high tides and during emersion at nighttime.'1" Various authors have investigated local factors affecting foraging activity patterns
in P vulgata*' but little attempt appears to have
been made to make geographical comparisons of
populations of this species as proposed in a previous
study.2 This aimed to investigate the foraging patterns and homing behaviour of the limpet Patella
vulgata at two widely separated geographical sites
within the British Isles.
The two localities chosen, in North Wales and in
Cornwall, permitted comparisons to be made, using
identical observing techniques of limpets in similar
positions on the shore, at two latitudes and in localities showing marked differences in the timing during
the day of high spring tides.
Limpets were observed at Menai Bndge
(53°13'15"N, 4°09'50*W), on the Isle of Anglesey,
and at Porthoustock (50°03'10*N, 5"03'30rW) on the
Lizard peninsula. The sites were sheltered and in
each locality limpets were studied on a vertical wall
constructed of granite blocks. Twenty limpets of similar size at the mid-shore level were observed at each
site (Menai Bridge mean limpet shell length = 43.2 ±
s.d. 8-33 mm; Porthoustock = 42.6 ± s.d. 8.6 mm, ttest for size differences gives p = 0.84).
At Menai Bridge, the area used for study was
located directly beneath the Menai Bridge pier. The
wall faced in an East-south-easterly direction, its
flora dominated by the macroalga Ascophyllum
nodosum Le. Jol. and its fauna by P. vulgata.
Semibalanus balanoides (L.) was present, covering
less than 30% of the study area. The tidal range at
Menai Bridge is 6.7 m on mean spring tides, and 3.6
m on mean neap tides,' with highest spring' tides
phased around 0200 and 1400 h.
At Porthoustock the study site was part of the
'West of England Pier*, an abandoned quarry with a
sheltered West-south-westerly facing wall. Macroalgae were absent on the wall and the fauna was dominated by Semtbalanus balanoides and P vulgata.
Barnacle coverage of the wall surface was estimated
'Proem addresc Department oi Zootofy *nd Enxomalofj, Rhodes
University, Grahuatown, 6140. Sooth V n a
to be greater than 90% The tidal range at Porthoustock is 4.7 m on mean spring tides and 2.3 m on
mean neap tides, with highest spring tides phased
around 0700 and 1900 h.
At each site, the 20 limpets were marked with
small plastic numbers on 'Dymo tape' attached to
the shell with ' Araldite' glue.7 This was most successfully achieved after cleaning the shell using a fine
wire brush and sandpaper, and blotting off of all
excess water To determine the position of any
limpet at any given time, permanent reference points
were embedded into the substratum at each site.
These consisted of 7 bolts fixed into the substratum
50 on apart in a line which ran across the upper limit
of the limpet population. Each bolt was labelled with
an identification letter (A-G) and successive readings of limpet position were taken by measuring the
distance of a limpet from the two reference points
which were closest to its home site. This is a modification of the method of triangulation used by other
authors."-73
Observations of activity of the 20 limpets were
made directly, over three periods for each site during
8th, 10th and 12th June, 1992, at Menai Bridge and
7th, 9th and 11th July, 1992, at Porthoustock, at
intervals of 1 h when the limpets were uncovered by
the tide and by diving every 2 hours when the
limpets were covered. In both localities the observations were made over 3 days of minimal neap tides.
Limpets were recorded as 'at home' when they
were on their home scar and as 'active** when away
from their home scar. At night, observations were
made using red light only, since in previous studies
white light was observed to cause limpets to clamp
down and stop foraging.u At the end of the study
period, all limpets were removed from the rock, their
identity verified and their lengths measured.
A 'foraging angle' was obtained for each excursion
by drawing a line through the home scar and the furthest point reached by the limpet during that excursion and measuring the angle in a clock-wise
direction from the vertical. The mean vector (r) of
foraging directions of the 2 populations irrespective
of the maximum distance travelled were calculated.'
Figure 1 shows the percentage of limpets active
during each observation period over each day of
study at Menai Bridge and Porthoustock. From these
data it is clear that foraging took place at both locali-
122
RESEARCH NOTES
(d)
• Dart
0
2
4
6
0
8 10 12 14 16 18 20 22 24
Time in hours (B.S.T.)
2
4
6
2
4
8 8 10 12 14 16 18 20 22 24
Time in hours (B.S.T.)
2
4
6 8 10 12 14 18 18 20 22 24
Time in hours (B.S.T.)
(e)
8 10 12 14 16 18 20 22 24
Time in hours (B.S.T.)
<D
IS
2
o
d
o
0
2
4
6
8 10 12 14 16 18 20 22 24
Time in hours (B.S.T.)
80
Ughl
Dark
70
60
Dark
I
50
40
30
d
20
S
i
i
m
10
0
2
12
ilk
4
6
1. A, ,,
8 10 12 14 16 18 20 22 24
Time in hours (B.S.T.)
RESEARCH NOTES
123
Figure 1. Showing the percentage of limpets active against the time ui hours for Menai Bridge: a) 8th, b) 10th
& c) 12th June 1992 and for Porthoustock: d) 7th, e) 9th & f) 1 lth July 1992 Arrows indicate time of low tide.
(a)
270°
270°
180°
Downshore
180°
Figure 2, Foraging directions shown by Patella vulgata at a) Menai Bndge and b) Porthoustock. Results
obtained over the 3 days have been pooled. Each cross represents one excursion, n—number of excursions
observed and plotted, r—an estimate of the non-uniformity of the circular distributions given as mean vector
lengths by the Rayleigh test
ties during low tides which occurred at night or
around dusk and dawn, but not during daytime low
tides and not at high tides. In most cases, foraging
commenced soon after emersion and the limpets
were back at their home site by the time they were
covered with water on the next flood tide During
the 3 separate days of observation of 20 limpets at
Menai Bridge and 2 days of 20 limpets at Porthoustock there were 60% occurrences when a limpet was
observed to move away from its home scar during a
24 h period. Of these only one failed to return to its
home scar during the 24 h period of observation. On
the first day of observation at Porthoustock (7th July)
when both low tide times occurred during daylight
hours only 6 of the 20 limpets were observed to move
Limpets at Porthoustock travelled significantly
greater distances on each foraging excursion (x =
0 39 m ± s.d. 0.47) than those at Menai Bridge (x =
0.18 m ± s-d. 0.16) (Mann-Whitney, p < 0.0019).
Variability between the foraging distances travelled
each day at each site was tested using the KruskalWallis test. There was found to be no significant difference between the distances travelled by limpets at
Menai Bridge on the 3 different days (x = 10.60 >
011). However, the daily distances travelled by the
limpets at Porthoustock were found to differ significantly from each other (x2 = 5.99 < 8.88), attributable
to the differences between the 7th and Uth July (Z =
238; p < 0.05, Dunn multiple comparison technique)
On 7th July both low tides fell during daylight hours
and total foraging activity was suppressed.
The foraging directions of all limpets observed to
forage at the two sues, together with the calculated
mean vectors, are plotted in Fig. 2. At Menai Bridge,
the mean direction of foraging was 107.7° (Figure
2a) and that at Porthoustock 207° (Figure 2b) The
foraging directions of each locality was significantly
different from random and were predominantly
downshore.
Present results concur with those of previous studies, )JJ ' 0JI suggesting that over a wide range of localities Patella vulgata forages away from its home scar
predominantly during nocturnal low tides. It is
unlikely that differences in tidal tuning in different
geographical localities account for occurrence of
limpet foraging at high tide times on some shores as
have previously been reported.112 Indeed, recordings
of spontaneous foraging activity by P vulgata under
constant conditions in the laboratory demonstrate an
endogenous predominantly circatidal rhythm, with
peaks at times of expected low tide,13 suggesting that
this is the pattern which is particularly strongly
selected for.
On the evidence presently available, therefore, it
seems most likely that local rather than wide geographical factors determine regional variations in
limpet foraging patterns. For example, high tide foraging by Patella vulgata on Isle of Man shores has
been shown to occur only in limpets on gently shelving shores.' Limpets on vertical surfaces were
observed to forage at low tide, attributable to a
greater nsk of dislodgement by waves when the tide
is in, particularly on barnacle covered shores. Differing patterns of foraging activity by P vulgata have
also been related to the neaps/spnngs cycle and to
position on the shore in relation to the quantity of
124
RESEARCH NOTES
3
available food organisms. In addition it has been
shown that though P vulgata exhibits a primarily circaudal rhythm of foraging in constant conditions in
the laboratory,13 alternate peaks are suppressed by
light during daytime low tides when electronic
recordings of limpet foraging are made on the shore.1
Though the tuning of foraging activity was similar
in the Porthoustock and Menai Bridge localities
studied, the duration of low tide foraging was significantly greater in the former location (0.39 ± 0.47 m)
than in the latter (0.18 ± 0.16 m). Mean distances
travelled by P. vulgata at Porthoustock are the same
as those reported for Isle of Man limpets regarded as
particularly mobile 2 The longer distances travelled
by Isle of Man limpets were attributed to movement
whilst under water. However, our limpets moved
when exposed to air at low tide so some other explanation should perhaps be sought for varying foraging
excursions. A possible explanation for the differences observed in the present study is the greater
barnacle cover at the Porthoustock site (>90%) compared with the Mcnai Bridge site (<30%) Longer
searches for food would be required if barnacles hinder limpet foraging1413 or consume settling algal
spores by filter feeding."
Finally, our observations addressed the question
as to the direction of foraging excursions of limpets
in the two localities. Over the period of study in each
locality foraging excursions were non-random, with a
mean vector downshore in each case. Patella vulgata
has been shown to move consistently upshore from
dense barnacle zones to bare rock in some localities"
and several other patellids show vertical movements
in relation to tidal rise and fall." However, in this
study the dominant foraging directions were not the
same in each locality and it is possible that preferred
directions may change with time. Our findings,
therefore, suggest that in future studies of foraging
and trail following in P. vulgata greater attention
should be paid to the possibility of a learning component of such behaviour, for example in relation to
the return to optimal feeding patches.
We are grateful to Dr P. Delia Santina for his
methodological advice; Alan Gray, Andrea Gray
and Paul Wing for assisting D.R.G during the periods of observation and Alan Hodgson for critically
reading the first draft of this manuscript.
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J MolL Stud. (1996), 62,124-126
Flash-freezing using cold fluids: field and laboratory methods for
preventing retraction of snails during fixation
Peter A. Bowler,1 Timothy P. Johnson,2 and William J. Mautz3
'Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92717-2525
'Department of Applied Sciences, University of Glamorgan, Pontypndd, Mid-Glamorgan CF3 1DL,
Wales. 'Department of Community and Environmental Medicine, University of California, Irvine, CA 92717
'These Euapta [holothurians] were a nervous lot We
tried to relax them with Epsom salts so that we
might kill them with their donate heads extended,
but the salts, no matter how carefully administered,
caused the heads to retract, and soon afterwards
they threw their stomachs out into the water . . We
intoxicated them with pure oxygen and then tried
the salts, but with the same result Finally, by admin-