A.M. ZOOLOGIST, 9:753-758 (1969).
Effect of the Nature of the Substratum on the Force Required to
Detach a Common Littoral Alga
H. BARNES
The Dunstaffnage Marine Research Laboratory, Scotland
AND
J. A. TOPINKA
Aflelj)hi University Institute of Marine Science, Oakdale, New York
SYNOPSIS. It has been shown that the force required to detach the holdfasts of Fucus
vesiculosus depends upon the nature of the substratum; adhesion is less to barnacle
shells. The lower adhesion appears to be due to a partial dissolution of the shell by
acidic material, probably exndates. Growth of germlings on various substrata indicates
that thalli and primary rhizoids develop better on calcareous substrata. Some ecological consequences of the phenomena are examined.
During the course of a study of the
physical factors affecting the behavior of
plants and animals in the littoral zone, it
seemed evident from field observations
that it was easier to detach Fucus from
barnacle shells than from rock. The difference is clearly of some ecological interest
and the problem was therefore pursued in
greater detail.
THE STRENGTH OF ATTACHMENT TO
SHELL AND ROCK
10
Experiments on the force required to
detach algae from various substrata will be
described elsewhere; briefly, with the
plant still attached to the substratum, the
base of the stipe immediately above the
holdfast was fastened to a spring balance
which in turn was held on a moveable
arm. The latter was raised and, pulling the
plant at right angles to the substratum,
the force required to detach the alga (or
break the stipe) was observed directly by
reading the balance scale. Plants of Fucus
vesiculosus of various sizes were used; for
the present purpose the force (g) required
to detach the plant has been plotted
We are very grateful to Dr. A. C. Churchill for
advice and much help with the experiments on the
culture of Fucus germlings.
10
Wtignt of plant in g
FIG. 1. The force required (g) to detach Fucus
vesiculosus plants of various sizes (g wet weight)
when pulled vertically from natural rock arid barnacle shells.
against the wet weight determined after
lightly drying the whole plant.
It is evident from Figure 1 that at any
given size the force required to detach the
plant is greater when the latter is attached
to rock than when it is attached to a barnacle shell. Such a lower efficiency of attachment may depend either upon physical factors such as the micro-structure of
the substratum, or upon chemical or biological factors. The form of the attachment,
namely numerous small rhizoids arising
from a basal holdfast, and their capacity
thoroughly to exploit the micro-roughness
753
754
H. BARNES AND J. A. TOPINKA
10
20
Weight of plant in g
FTC. 2. Thickness (in arbitrary units) of calcareous
layer detached with holdfast of Fucus vcsiculosus
plants of various sizes (g wet weight).
of the substratum, together with the similarity in roughness between rock and
shells, suggest that the difference is not
primarily dependent on physical factors.
Visually, the under surface of a holdfast
pulled—or even very carefully removed—
from shells, always has a white 'crust' still
attached, and microscopic examination of
hand-cut sections clearly showed that a 'layer' of shell is removed with the holdfast.
Careful teasing out under water of such a
section shows that much of the calcareous
matter, particularly in the upper part, is
spongy and penetrated throughout by rhizoids. Staining with Nile Blue confirms
these observations, the stained rhizoids
showing up clearly as the shell material is
dissected away. This penetration of the
rhizoids may be further demonstrated by
placing a section under a cover slip and
flooding with a 5% solution of EDTA at
pH 7.0. As the complexing reagent diffuses
under the cover glass the calcareous matter is dissolved and the interpenetrating
rhizoids are clearly revealed (Fig. 3).
An attempt was made to measure the
thickness of the calcareous layer adherent
to the holdfast on tearing the latter away
(Fig. 2). It is evident from this figure that
the thickness is independent of the size of
the plant so that penetration is not a function of size. Presumably the strength of
attachment, which increases with weight
(size), is largely due to an increase in the
number of rhizoids with increasing size of
the holdfast and not by a greater penetration of individual rhizoids.
It seems hardly likely that the delicate
rhizoids mechanically penetrate the intact
shell and indeed it is usually considered
that penetration of calcareous materials by
plants and lower invertebrates is probably
the result of chemical action. When
fronds of Fucus, still attached to the original substratum, were maintained in close
contact for two weeks with a piece of polished marble in sea water, the surface of
the marble was lightly etched. It is evident
from the foregoing that the disintegration
of the shell to produce a pulpy mass
through which the rhizoids are able to
penetrate may be due to the direct dissolution of calcium carbonate by exuded acidic
material or by the multiplication of bacteria metabolizing the exudates and
causing a demineralization of the shell: it
may even be that some calcicole species of
bacteria are particularly involved in the
second of these possibilities. The excretion
of organic material by unicellular algae
has now been investigated in some detail,
particularly by Fogg and his colleagues
(for a review, see Fogg, 1966); it is evident
that a large proportion of the products of
photosynthesis may be liberated as extracellular products. Among such products
is glycollic acid. That littoral algae are
also capable of organic exudation has been
demonstrated by Sieburth (1969); exudation is a function of photosynthetic activity
and of emersion. In the light some 30 mg
C/100 g/h were lost from F. vesiculosus
and some 50-380 mg C/100 g when
emersed weed was returned to sea water.
Carbohydrate material was the main component of the exudates from littoral algae.
Under natural conditions the exudate may
be consumed by planktonic bacteria, or by
the microflora present on the littoral alga,
the relative importance of the two depending on the conditions and particular alga
involved. In the present instance bacterial
metabolism may, therefore, be implicated
in the breakdown of carbohydrate to give
acidic substances which in turn attack the
calcium carbonate of the shell.
DETACHMENT OF ALGAE FROM VARIOUS SUBSTRATA
IDO
Fig. 3. Photomicrographs o£ the gradual dissolution
o£ 'spongy' calcium carbonate when a hand
section of detached holdfast is flooded with 5%
EDTA; time difference between first and fourth
photographs, 10 min; note gradual exposure o£
rhizoids.
GROWTH OF GERMUNGS ON VARIOUS
SUBSTRATA
fragments), scrubbed, washed thoroughly
in distilled water, and then allowed to dry.
Pieces of each substratum were placed in
small glass dishes, 40 ml of filtered sea
water were added, and the whole was autoclaved. Germlings were obtained by taking
ripe plants of Fucus vesiculosus, cutting
open the receptacles, and allowing eggs and
sperm to be released into sea water; to
ensure at least initial viability, the fertilized eggs were first allowed to develop until a single rhizoid had appeared. Batches
of such germlings were washed several
times with filtered sea water, and then
pipetted into a dish containing sterile
substratum and sea water. The sea water
was changed every 4-6 days, but after the
In view of the very different nature of a
calcareous shell and many natural substrata such as rocks, and remembering the
calcicole character of some algae, it seems
possible that the rate of development ot
Fucus germlings may differ according to
the substratum on which they are growing.
The rate of growth of Fucus germlings on
pieces of Balanus eburneus and Mercenaria mercenaria shells, on broken chips of
marble and rock, and on a glass surface
(glass dish) have now been followed.
The substrata to be tested were cleaned
with detergent or dilute sodium hydroxide
(with nitric acid in the case of the rock
756
H. BARNES AND J. A. TOPINKA
TABLE 1. Development of germlings of Fucus vesieulosus on various substrata: constant conditions, 15"C,
12-hr Ught/lS-hr dark; mean values; measurements (in mm) after 46 days; closed system in Petri dishes.
Balanus
oalanoides
Shell
Mean total length of thallus
Mean number of rlrizoids
/germling
Mean total rhizoid length
/germling
Mean length of a rhizoid
Mean no. tlialli/germling
Mercenaria
mercenaria
Dish
Shell
Dish
Eock
Marble
Marble Dish
Rock
Dish
Dish
alone
2.1
2.1
—
—
1.3
1.8
1.5
31.0
4.0
35.9
1.1
32.2
19.3
23.0
14.8
26.0
1.1
9.8
166.3
176.3
171.2
—
59.0
78.3
33.2
86.7
25.6
5.16
3.47
5.68
1.50
initial autoclaving, the cultures, held at
15°C under a 12-hr/12-hr light-dark regime, were not maintained under sterile
conditions. After 46 days the germlings
were washed, carefully teased off each substratum, and transferred to 5% formalin for
preservation prior to examination; it is
believed that few rhizoids were lost or badly damaged during detachment.
The rate of development of a germling
(i.e., the whole product of the fertilized
egg) was estimated by measuring the total
length both of the individual thalli and the
rhizoids, and counting the number of the
latter per germling (Table 1). A comparison can be made (with the exception of
the clam shells) between growth on the
added substratum itself and on the dish in
which it was contained and also between
each of these and growth on the dish containing no added substratum. The results
were subjected to an analysis of variance,
and in what follows the P = 0.05 level is
accepted as significant; the mean values
are given in Table I.
After 46 days, growth of the thallus is
best on the clam shells when compared
with all other substrata tested; barnacle
shells were better than marble or glass but
not rock, and the latter better than glass;
but growth on marble was similar to that
on glass. This difference is not entirely due
to the greater number of separate thalli
per germling since the mean length of
each main thallus was also greatest on the
clam shells; indeed the number of thalli
per germling and total length of thalli
4.77
2.21
—
—
3.06
1.08
3.40
1.33
2.24
1.47
3.33
1.67
2.61
1.0
almost parallel one another. The effect of
the substratum is also seen on the production of rhizoids as well as on development
of the thallus. Contact with a calcareous
substratum clearly enhances rhizoidal development. Barnacle shells and clam shells
showed no difference in the number or
length of the rhizoids; but the production
of rhizoids on marble and rock was much
lower as regards both number and length,
the values being even less than on the glass
of the container; fewer rhizoids, but of
similar length to those on marble and
rock, were produced on the controls grown
in dishes containing no substratum. When,
however, the rhizoidal development on
the barnacle shells is compared with that
on the dish in which they were held it
is clear that an increase in growth and
number was also found for the germlings
on the glass of that container: the effect
was 'transmitted' to the culture medium.
A similar effect is evident in the dishes
containing rock and marble when compared with the control dish. In view of the
thorough cleaning of the shells it seems
unlikely that the effect—either directly or
indirectly through the advent of bacteria—was due to the presence of organic matter; it seems likely—again, either
directly or indirectly (possibly through
Ca-+ ions or pH) to be due to the calcareous material itself and/or to the physical
nature of the surface; it may be due to
leaching out of trace elements. It seems
reasonable to assume that, although this
increased rhizoidal growth was 'trans-
DETACHMENT OF ALGAE FROM VARIOUS SUBSTRATA
mitted' to the medium under the static
conditions of the experiment and so influenced the growth rate of germlings attached to the glass of a dish containing
shells, under natural conditions with water
movement there would be no effect on
adjacent germlings growing on rock. It is
not certain how far any of these effects
would be found in the secondary rhizoids
which eventually come to make up the
attachment of the holdfast.
Apart from the differences in primary
rhizoid development referred to above, a
microscopic examination of germlings
grown under the various conditions showed
that although fewer primary rhizoids were
developed on rock, this substratum with its
greater micro-roughness had a more obvious
and thicker growth of secondary rhizoids
forming a mat—the young holdfast. This
suggests that such growth—less evident in
the other cultures and particularly on the
smooth glass surface—may be stimulated by
the available area of contact.
ECOLOGICAL CONSEQUENCES
Burrows and Lodge (1950) have described marked variations in the Fucus
population on a semi-exposed shore made
up of limestone ledges in the Isle of Man.
A new population of Fucus which had become established on a cleared area showed
a considerable loss of plants over a period
of eight months and they found that the
most important cause of the loss was the
fact that this population was not attached
directly to the rock but to barnacles. They
stressed the interaction between Balanits
bnlavoifles, Fucus, and Patella which gave
rise to these variations. They pointed out
that while Fucus germlings present on barnacles initially receive some protection
from grazing by the limpets, subsequently,
the Fucus kills the barnacles by interfering
with their feeding activity, and the empty
shells together with the plants are easily
removed. Patella, by its grazing, will keep
such cleared rocks bare so that the settlement of Fucus in such places will await the
next settlement by barnacles.
It is evident from our observations that
757
such an interaction is not dependent upon
the barnacle being killed; any Pucus settled on barnacles is always more easily lost
from the community, but if it is lost before
the barnacle is killed then this will merely
limit the Fucus, but will not create a situation favorable to Patella: the interaction
is more restricted to that between Fucus
and Balanus and this will vary with the
forces tending to remove the adult alga,
i.e., with exposure. Increasing exposure
will, therefore, favor a barnacle zone, and
decreasing exposure, a marked Fucus-Balanus ecotone. If, because of the chemical
factors referred to above and physical factors such as any greater lodgment of the
zygotes in protected places on the cirripede
valves, the settlement and early development of Fucus is favored by Balanus, the
interaction will be more complex.
The less effective adhesion to barnacle
shells and the loss of both algae and empty
shell if the alga kills the barnacle clearly
limit the size of Fucus plants which will be
found on shells; the age and size structure
of Fucus growing on shells will be different from those found on the bare rock. A
random collection of Fucus showed that
plants growing on barnacles rarely exceeded 10 g in weight (19-20 cm length) while
on adjacent rocks the plants reached
weights of several hundred grams and a
length of almost a meter. This is in agreement with the observation of Burrows and
Lodge (1950) that on barnacles much of
the Fucus had an annual cycle whereas
second and third year plants were present
on the rocks.
Burrows and Lodge were considering
limestone rocks and, although they stressed
the killing of the barnacle, some loss of
Fucus without this may well have occurred
from barnacles but not (or less) from the
limestone rock itself. It would be interesting to know something of the relative rates
of development of the germlings and adhesion of adult Fucus to limestone and
non-limestone rocks in the field.
In addition to playing an important part
in any cyclical changes such as those described by Burrows and Lodge (1950), the
758
H. BARNES AND J. A. TOPINKA
less effective adhesion to barnacle shells
would seem to be important in the general
relation between Balanus balanoides and
fticoids as one passes from exposed to sheltered situations on British coasts. The general effect of decreasing exposure is well
known to allow fucoids gradually to displace the barnacles. Clearly, exposure will
affect each species per se and only where
conditions permit both to be present will
there be competition. With increased shelter when the forces (wave action) tending
to remove Fucus are reduced, Balanus
would be killed by the growing Fucus, and
areas would be made available for further
colonization by the algae; the long period
of settlement either in summer or winter
depending on the Fucus sp. will favor such
expansion of the algal population and
deny space to the annual settlement of B.
balanoides. On more exposed places the
Fucus may be lost before it has reached a
size sufficient to kill the barnacle. The less
effective attachment demonstrated for Fucus is equally true for Pelvetia canaliculata, which tends to occupy a position above
Balanus. Downward penetration of this
alga into the balanid zone may well be
dependent to some extent on the relative
adhesive properties.
REFERENCES
Burrows, E. M., and S. M. Lodge. 1950. A note on
the inter-relationships of Patella, Balanus and
Fucus on a semi-exposed coast. Ann. Rep. for
1949, Mar. Biol. Sta. Port Erin 62:30-34.
Fogg, G. E. 1966. The extracellular products of
algae. Oceanogr. Mar. Biol. Ann. Rev. 4:195-212.
Lewis, J. R. 1964. The ecology of rocky shores.
English Universities Press, London. 323 p.
Seiburth, J. M., and A. Jensen. 1969. Studies on
algal substances in the sea, II. The formation of
Gelbstoff (humic material) by exudates of the
Phaeophyta. J. Exp. Mar. Biol. Ecol. 3: (In
press)