A.M. ZOOLOGIST,
9:279-282
(1969).
Thermal Tolerance of the Bivalve Mollusc, Modiolus modiolus L.
KENNETH R. H. READ
Biological Science Center, Boston University, Boston, Massachusetts 02215
and
New England Aquarium, Central Wharf, Boston, Massachusetts 02110
SYNOPSIS. A critically heat-stressed population of Modiolus modiolus has been studied
in a tidal pool. Over a period of 5.5 hours the temperature in the shallows rose from
19°C to 32.5°C. Some of the animals were killed outright; others succumbed over the
next few days. Most of the animals, although distressed at the time of exposure, seeined
to have recovered by the following week. The animals most adversely affected were
those whose shells protruded above the surface of the pool; these individuals gaped
and became desiccated. The reactions of the species are consistent with previous
physiological experiments. High temperature plays a role in limiting the vertical distribution of Modiolus modiolus.
The tolerance of the mytilid mollusc,
Modiolus modiolus, to high temperature
has been studied in the intact organism in
laboratory experiments by Henderson
(1929), Read (1964), and by Read and
Cumming (1967). Read and dimming
concluded that Modiolus modiolus is probably limited in its geographic distribution
where sea temperatures rise to the neighborhood of 23°C or higher for periods of
the order of a month.
This paper reports the limiting of Modiolus modiolus in its vertical distribution in
tidal pools by short-term exposures to
high temperatures over the tidal cycle. It is
one of the few accounts of the exact time/
temperature relationships involved in the
mortality of invertebrates in the natural
environment.
CKNKKAL DESCRIPTION
The tidal pool in which the observations
were conducted is on the north bank of
the outlet of The Salt Pond, Blue Hill,
Maine on the north side of Maine Route
175. The pool is about 7 m across and
about 40 cm deep at the deepest point; it
is about 1.7 m below high water mark as
indicated by the brown line on the rocks.
F thank Mr. and Mrs. Peter Helburn for their
hospitality at their cottage on The Salt Pond, Blue
Hill, Maine.
279
The mean distance between tide marks in
the area is about 3 m.
Modiolus modiolus lives in the pool at
the time of writing (July 1968); the population extends from the bottom of the pool
to its surface where the valves of a few
individuals protrude into the air. The animals range to 8 cm in length. The gastropod mollusc, Littorina littorea, abounds in
the pool and a few green sea urchins,
Strongylocentrotus droehbachiensis, also
live there. At the time of observation, large
numbers of blue mussels, Mytihts edulis,
1-2 cm long, crowded the margins of the
pool.
Observations on the pool started on July
18, 1968 at 11 AM Eastern Daylight Saving Time (clocks one hour ahead of sun
time). The sky was cloudless all day, and
the wind was slight due to the sheltered
location. At 1:30 PM the air-temperature
was 26.0°C. The following day was foggy
and a strong cool wind was blowing. At
10:20 AM the tide left the pool indicating
that the previous day, i.e., July 18, it
would have left the pool at about 9:30
AM. The temperature of the water as the
tide left the pool was 19.0°C. For the purposes of this paper it has been assumed
that the tide actually did leave the pool at
9:30 AM on July 18 and that the temperature of the water at that time was 19.0°C.
Thermometers used in this work were
280
KENNETH R. H. READ
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FIG. 1. Temperature in the shallows of the tidal
pool (depth 1-5 cm) as a [unction of time. Between
11:00 and 11:50 AM several readings of temperature, but not their respective times, were taken;
the ranges of time and temperatures only are
recorded.
calibrated against an instrument certified
by the National Bureau of Standards.
OBSERVATIONS
Changes in temperature recorded in the
pool during July 18 are summarized in
Figure 1. Over the 5.5-hr period during
which the pool was left by the tide the
temperature in the shallows rose from the
putative value of 19°C at 9:30 AM to
about 32.5°C at 3 PM. Between 11:00 and
11:50 AM temperatures of the water
ranged from 26.3°C to 28.8°C in the shallow margins of the pool where depths
varied from 1 to 5 cm. Where the temperature had risen to 27.3°C and above some of
the animals gaped excessively and failed
to respond quickly when squeezed between
thumb and forefinger. Animals in the
worst condition were those whose valves
protruded into the air. At a depth of 20
cm the temperature was 22°C and the animals were in good condition.
Between 1:30 and 1:52 PM watertemperatures had risen to 29.5°C-31.9°C in
the shallows. The animals all appeared in
various stages of distress with the ones at
the highest temperatures most affected. At
this time some of the animals whose shells
poked into the air gaped extremely and
were drying out.
Between 2:15 and 2:55 PM watertemperatures in the shallows had risen to
30.2°C-32.5°C. Animals at these temperatures showed very poor responses when
squeezed. At 2:30 PM the profile of temperature in the center of the pool was as
shown in Figure 2. Temperatures ranged
from 28.7°C at the surface to 27.7°C at the
bottom. Under this regime most of the animals seemed in good condition and when
touched on the edge of the mantle reacted
quickly. A few here and there, however,
especially among those near the surface,
did gape to some extent. The green sea
urchins had all collected at the bottom of
the pool but seemed unaffected. Unlike the
urchins, Litlorina littorea did not migrate
to the bottom and moved actively around
the entire pool.
At 3:07 PM the flood tide reached the
pool and the temperature fell abruptly. By
3:10 PM the temperature in the shallows
had dropped to 24°C and by 3:15 PM to
21 °C. Animals which did not respond at
32.5°C began to react to being squeezed as
the temperature fell. Here and there, however, were individuals that failed to respond.
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FIG. 2. Temperature as a function of depth in the
center of the pool at 2:30 PM.
THERMAL TOLERANCE OF
By 7:15 PM the temperature in the pool
which was still flooded was 17.2°C.
The next day was cool and foggy. At
10:20 AM when the tide left the pool the
water-temperature was 19.0°C. At this time
at least 15 dead and decomposing animals
were counted in the shallows. Over half of
these were at the surface and had probably been subjected to desiccation the previous day. Many of the animals in the shallows gaped somewhat and had rather slow
reflexes when touched on the edge of the
mantle.
July 20 was a sunny day with an appreciable northwest wind. At 4:40 PM, just
before the tide flooded the pool, the temperature was 27.7°C in the shallows and
26.5°C at the bottom of the pool. At this
time many more dead and decomposing
animals were observed in the shallows but
none in the depths of the pool.
On July 23 many dead animals were
observed in one patch in the shallows but
the majority seemed to have survived.
DISCUSSION
Modiolus modiolus has been observed
under conditions of critical heat-stress.
When the temperature rose from an assumed value of 19°C to 30.2°C-32.5°C in
about 5.5 hr, limited mortality and injury
occurred in some of the exposed animals.
It is likely that more prolonged exposure,
or exposure for the same time to a higher
temperature, would have exacted a greater
toll. Animals whose shells protruded
above the surface were worst affected. With
a few exceptions, animals whose temperature never rose above 27.7°C during the
5.5-hr period of exposure appeared to be
unaffected. The stressed animals had one
day's respite, but the next day the temperature rose to 27.7°C in the shallows; this
could have been the cause of considerable
additional mortality. It might be that animals weakened by the first day's challenge
succumbed when subjected to an additional, though lesser, stress applied two days
later.
Modiolus modiolus
281
It is likely that short-term, intense exposures to high temperature limit the vertical distribution of Modiolus modiolus in
tidal pools. The severity of the exposures
is related to tidal and meteorological conditions. To be maximally heated a tidal
pool in the temperate zone must be exposed in the middle part of a summer's clay
when both wind and cloud cover are low.
Since tidal pools are exposed during midday for only a relatively small part of the
summer months, and since meteorological
conditions, even in summer, are very variable in Maine, the combination of tidal
and meteorological events necessary for
maximal heating probably occurs infrequently. This would explain how Modiolus modiolus is able to grow as large as 8
cm at the surface of the tidal pool studied.
These conclusions are consistent with
observations of Hodgkin (1956) on West
Australian reefs. He reports "that mortality among intertidal organisms near the
top of their normal vertical range is an
annual event." He concludes that "this annual mortality (due to high temperature)
is a potent factor in maintaining the characteristic shore zonation."
Henderson (1929) heated Modiolus
modiolus in the laboratory at a rate of
l°C/5-min interval and reported that the
"average lethal temperature" was 36.3°C.
If it is assumed that 5 minutes of exposure
to 36.3°C is lethal, then the data of Battle
(1929) on the heat-death of skate myoneural and muscle preparations would indicate that the exposure necessary to
achieve lethality at a temperature a little
more than 5°C lower, i.e., 31 °C, would be
about 2 hr. This is consistent with the
observations recorded in this paper.
It is interesting to compare the results
for Modiolus modiolus with those of Glynn
(1968) on echinoids in Puerto Rico.
These experienced heavy mortality during
extreme midday low water stands, though
because of the tropical location the lethal
temperatures were a few degrees higher. As
for Modiolus modiolus, desiccation was an
important cause of death. The inability of
282
KENNETH R. H. READ
Modiolus modiolus to retain water in its
mantle cavity, due to byssal gape, contributes heavily to the difficulty the species
encounters in maintaining itself out of
water. Glynn raises the possibility that the
effects of the sun's rays in shallow water
cannot be discounted as a cause of death.
The effects of parasitism on thermal
tolerance are also of probable importance
(Vernberg and. Vernberg, 1963).
Observations of Modiolus modiolus from
Massachusetts to Blue Hill, Maine indicate that the species never seems to grow as
large in tidal pools as under subtidal conditions; those found intertidally in Massachusetts in the Chondrus zone, where they
are but briefly exposed, are also small.
Within a kilometer of the tide pool described in this paper, but at a depth of
15-20 m, there lives a population of Modiolus modiolus whose length reaches 12 cm
and more. In this zone, summer temperatures never rise much above 15°C.
Whether Modiolus modiolus in the tidal
pools or intertidal zone are killed by summer heat, winter cold, or ice, or by other
physiological stress before they can reach
full size, or whether their growth is merely
stunted must await further investigation.
REFERENCES
Battle, H. I. 1929. Temperature coefficients for the
rate of death of the muscle in Raja erinacea
(Mitchill) at high temperatures. Contrib. Canad.
Bid. Fish. 4:501-526.
Glynn, P. W. 1968. Mass mortalities of echinoids
and other reef flat organisms coincident with
midday low water exposures in Puerto Rico.
Marine Biol. 1:226-243.
Henderson, J. T. 1929. Lethal temperatures of
Lamellibranchiata. Contrib. Canad. Biol. Fish.
4:397-412.
Hodgkin, E. P. 1969. Catastrophic destruction of
the littoral fauna and flora near Fremantle. January 1959. West. Aust. Nat. 7:6-11.
Read, K. R. H. 1964. Ecology and environmental
physiology of some Puerto Rican bivalve molluscs
and a comparison with boreal forms. Carib.
J. Sci. 4:459-465.
Read, K. R. H., and K. B. dimming. 1967. Thermal tolerance of the bivalve molluscs Modiolus
modiolus L., Mytilus edulis L. and Brachidontes
demissus Dillwyn. Comp. Biochem. Physiol. 22:
149-155.
Vernberg, W. B., and F. J. Vernberg. 1963. Influence of parasitism on thermal resistance of the
mud-flat snail, Nassarius obsoleta Say. Expll.
Parasitol. 14:330-3J'J.