A M . ZOOLOGIST, 9:887-894 (1969).
Ecological Aspects of Some Coral-Boring Gastropods and Bivalves
of the Northwestern Red Sea
GAMIL N. SOLIMAN
Department of Zoology, Faculty of Science,
University of Cairo, U.A.R.
SYNOPSIS. More than 17 molluscan species were obtained from burrows in coral
substrata at Al-Ghardaga (Hurghada on maps) on the Red Sea coast, six of which in
particular bore into living colonies. The species reported in this paper belong to the
families Mytilidae, Coralliophilidae, and Gastrochaenidae. The direction of boring in
living corals is to the outside, the borers keeping pace with the growing coral layer to
maintain their burrows open. Coral growth is generally of a higher rate than that of
borers, and burrows are accordingly mostly much larger than their inhabitants. There
is evidence in such cases that burrows form initially by growth of coral around the
settling young. Boring of Lithophaga species is mostly due to the abrasive action of the
shell which moves straight and posteroventrally without any rotation. In coralliophilids, boring is also executed mechanically by the turning movements of the shell.
Boring in dead coral is directed inwards, and burrows are nearly as large as the
borers. The latter avoid the blocking of their burrows (e.g., by a living coral
incrustation) either by great siphonal extension (Rocellaria) or the free ends of the
shell may be strengthened to maintain the capability of boring in the opposite
direction (Lithophaga laevigata). Both L. luevigata and Modiola chmamomeus bore
mainly mechanically by the rocking movements of the shell. Chemical boring is still a
possibility, particularly in the posterior narrow region of burrows of Modiola lodging
the extended pallial siphons which are deprived of any effective mechanical devices for
boring. The role of boring algae in rarefying bored coral material has also to be
included as an indirect chemical factor.
The role played by rock-boring molluscs
in the economy of coral reefs is well
known. The amount of primary break-off
of corals around Al-Ghardaga, the site of
the present work, due to molluscs was estimated by Bertram (1936) to be 20 per
cent. This percentage, however, seems less
than that which is actually taking place. In
addition to most dead rocks, a considerable number of living corals are attacked
by boring molluscs, six species of which are
considered here.
Rock-borers have been claimed to be
uncommon in living corals, these being
protected against boring by the coral polyps. Otter (1937, p. 346) observing that
only Lithophaga hanleyana bored comThe author wishes to express his thanks to
Professor H. A. F. Gohar, past-director of the
Oceanographic Institute at Al-Ghardag?, Red Sea,
for suggesting the subject of this paper and for his
continuous advice and encouragement during the
work.
Author's present address: Faculty of Science,
University of Baghdad, Iraq.
monly in Porites at Low Isles, Great Barrier Reef, Australia, arrived at the conclusion that attack of living corals could only
be possible in dead areas. Conditions at
Al-Ghardaga, on the other hand, are
rather different. Nearly all specimens of
living species of Montipora, Favia, Cyphastraea, and Stylophora collected lodgeboring molluscs in varying degrees of infestation; and Echinopora and Goniastraea
are at least partly attacked. As a general
rule, boring molluscs are confined either to
dead or living corals, and furthermore to
a particular coral species.
The molluscs studied in this paper belong to the two lamellibranch families,
Mytilidae and Gastrochaenidae, and to one
gastropod family, Coralliophilidae. Mytilid representatives are common in living
and dead corals; those of the Gastrochaenidae occur exclusively in dead rocks, while
coralliophilids are restricted to living coral
colonies.
887
888
GAMIL N. SOLIMAN
MOLLUSCS BORING IN LIVING CORALS
Boring by such molluscs is largely controlled by the growth of the coral. Constantly in danger of having their burrows
blocked, borers have to keep pace with the
growing coral layer. The activity of boring is thus directed to the outside, that is,
posteriorly. Consequently in burrows with
a definite calcareous lining, this lining is
reduced or totally absent in the posterior
region, but gradually thickens toward the
bottom of the burrow. The aperture of the
burrow is sharply defined by the fringing
coral polyps which grow closely around the
protruded pallial siphons. In some cases
(as in L. hanleyana) the siphonal region
of the burrow is provided with a thick
calcareous crust which may protrude beyond the edge of the burrow, probably to
prevent encroachment of the coral polyps
across or into the aperture.
The rate of growth of the coral always
exceeds that of the borer, though in varying degree, and burrows (in contrast to
those in dead rocks) are thus longer than
their inhabitants. The statement of Otter
(1937) that the rate of growth of the borer
is proportional to the speed of boring
could thus only be applied to the case of
boring in dead rocks where the difference
in size of the burrow and its dweller is
slight. This is not the case in living coral
colonies where the speed of boring is hugely correlated with the growth of the coral
itself. It frequently happens, as indicated
by the many cases of small molluscs inhabiting burrows that are double or more
their own size, that the speed of boring
exceeds the growth rate of the borer.
Other than green algae, the outer portions of the boring shells are devoid of
animal and plant incrustations; the living
coral seems to protect borers against infestations to which those in dead corals are
commonly exposed. Green boring algae
usually attack solid corals but are nearly
absent from branching corals into whose
tissues they gain access only through the
mouth of burrows.
The initial formation of burrows in liv-
ing corals is likely to take place passively
by the growth of coral polyps around the
settling young borers. A minute home is
gradually formed around the animal,
opening distally to the exterior where the
siphon or siphons protrude. Actual boring
begins, as the animal keeps pace with the
growing coral layer, by initiation of byssal
or pedal attachment, and, in lithophagas,
by cementing calcareous granules to the
surface of the posterior region of the shell
which then takes part in the process.
LITHOPHAGAS
Three species of the well known mytilid,
Lithophaga, were collected among this
group of molluscs, namely, L. cumingiana
Dunker, boring in Stylophora flabellata; L.
hanleyana D., in Cyphastraea microphIhalma, C. chalcidicurn, and Montipora
lanuginosa (Goniastraea in a few cases);
and L. lima Jousseaume, in M. lanuginosa.
Cyphastraea and Montipora are characterized by having a rather thin encrusting
corallum which is heavily attacked by boring algae in the form of a well defined
green layer a few millimeters thick, just
beneath the surface and spreading all over.
The shell valves of all three species are
commonly completely encrusted with calcareous deposits varying in amount, consistency, and distribution on the different
parts of the shell. In L. cumingiana and L.
hanleyana one can differentiate three
regions of deposits (Fig. 1). The first
region (Dep. R. 1) is mainly dorsal, extending laterally and anteriorly where it
forms a cap over the anterior extremity of
the shell. Deposits on this region are pastelike and incoherent, being formed of calcareous particles entangled in mucus
secreted by the pallial glands. The same
type of deposit covers the corresponding
wall of the burrows.
The second region (Dep. R. 2) is lateral
and extends ventrally and posteriorly to
join the third. Deposits here are thin, adhesive, and evenly (but frequently unequally) distributed on both valves. In
some specimens the shell in this region is
CORAL-BORING GASTROPODS AND BIVALVES
Dep.R.1
5 mm.
FIG. 1. Mytilids boring in living corals. Lateral
view of shell of (A) Lithopliaga cumingiana; (B)
L. hanleyana; and (C) L. lima, showing distribution of calcareous deposits. Dep. R. 1, 2, and 3,
regions of deposits; Dep. R. ar. and Dep. r. pr.,
anterior and posterior regions of deposits.
eroded, and the periostracum is almost entirely removed in some parts.
The third region (Dep. R. 3) constitutes
the posterior portion of the shell valves.
Deposits on each valve are in the form of
an adhesive crust which looks as if it were
a part of the shell itself, being thicker
towards the free edge (up to 1.5 mm)
where it is toothed. In L. cumingiana the
particles of deposit follow the same course
as the striations. In L. hanleyana the crust
has the form of chevron-like ridges and is
overlapped anteriorly by the pasty deposits
of the first region. Coarse calcareous granules fill the grooves of this region with
which green algae are often associated.
In L. lima, calcareous deposits are disposed mainly in two sections which in
adult specimens overlap slightly (Fig. 1).
Deposits of the anterior region (Dep. R.
ar.) are thin, smooth, and adhesive, whereas those of the posterior region (Dep. R.
889
pr.) have an exceedingly rough appearance
and nearly mask the periostracum completely. They are composed of posteriorly
directed spiny or pillar-like particles more
pronounced towards the posteroventral
edge and further fuse into irregularly
toothed ridges. The dorsal part of the posterior region is commonly covered with
fine calcareous debris, where accumulation
of pasty deposits may be noticed in some
old shells.
In very young shells of these species the
regions of deposits are not well differentiated from each other, except that posteriorly the shell is more granular and rough.
There are successive stages in which the
surface of the posterior region changes
gradually from dispersed granules to the
distinctive structure characteristic of adults
of each species.
The burrows inhabited by these species
are easily distinguished by their dumbbellshaped aperture at the surface of the coral.
This is especially so in moderate-sized and
large specimens. In the young (length of
shell less then 4 mm), the aperture is nearly rounded and the dumbbell shape becomes acquired as the diameter of the opening exceeds about 1.5 mm. The distinctive
shape of the orifice corresponds to the
shape of the two protruding pallial siphons which control the growth of coral
polyps around their edges. Accordingly the
inhalent aperture is normally wider (as is
the inhalent siphon) than the exhalent.
This characteristic shape is lost, becoming
oval, when the surface around the aperture
becomes overgrown by coralline algae or
weeds.
Burrows have nearly the shape of the
shell at the bottom, become slightly wider
in the middle, and narrow gradually
toward the outside, thus changing from an
almost circular cross section to an oval
one. Like their shells, the burrows of
L. cumingiana and L. hanleyana are
covered internally with pasty deposits of
varying thickness on their anterior and
dorsal aspects (Fig. 2, A and B). In the
case especially of L. hanleyana there are
dense bottom deposits (more than 2 cm
890
GAMIL N. SOLIMAN
Br.Ln.
1cm.
FIG. 2. Lithophagous burrows. (A) of L. cumingiana; (B) of L. hanleyana; (C) of L. hanleyana,
showing bottom deposits; and (D) of L. lima.
Alg., boring algae; Br. burrow; Br. Dep., loose
calcareous deposits; Br. Lin., calcareous lining; Dep.
Bt., bottom deposits; S. Ln., calcareous lining of
siphonal region.
thick in some cases) irrespective of the size
of the borer itself (Fig. 2, C). Such deposits show a tendency to stratify but the
layers are ill defined and not compact.
Thus, although the length of the burrow
often much exceeds that of the shell, most
of the extra space is occupied by the accumulations at the bottom.
Beneath the calcareous paste, burrows
have a definite adhesive calcareous lining
in the form of a compact, hard, smooth
stratum best observed in L. hanleyana and
L. lima (Fig. 2, B and D). It is thicker in
those parts of the burrow with thick paste
and seems to originate from the latter.
There are stages in which burrow deposits
are in the form of slightly hardened paste;
in others, they are stiff and condensed into
distinct hard layers. The outer region of
the burrow is devoid of any lining and is
always rough, abraded, and pitted, particularly in L. lima. In a few cases the burrow
possesses a mid-ventral longitudinal ridge.
Having reviewed the main characters of
the shells and their burrows, I shall next
discuss the boring mechanism. The existence of calcareous deposits of definite
structure and distribution on shells and in
burrows, the accumulation of thick deposits at the bottom of burrows in certain
instances, the hard calcareous incrustation
on the posterior region of the shell, the
frequently eroded periostracum, the constant attachment by byssus threads, and the
smooth interior of burrows are evidences
that boring of Lithophaga in living corals
is primarily effected by mechanical abrasion by the valves. A smooth depression could be produced artificially by gently rubbing the spiny posterior part of the
shell valve of L. lima against the surface of
Montipora under water. However, one
cannot exclude the possibility of a chemical factor, whether intrinsic or extrinsic.
Boring algae are known to play a role in
the rarefaction of coral material which is
thus rendered less resistant to penetration.
The constant association of dense filamentous algae with the dorsal deposits of the
shells and burrows of L. cumingiana often
results in the formation of shallow depressions in the dorsal wall of the burrow filled
with calcareous paste. Yonge (1955), although assuming the secretion of an acid
mucus by L. plumula, adds that "the shape
of the shell is such as to suggest that, with
adequate byssal attachment, boring can be
purely mechanical." Hass (1943) suggested that chemical boring was the only
means of attacking rock, but could not
explain how L. plumula bored successfully
in non-calcareous argillaceous shale both
at La Jolla and Pacific Grove on the
California Coast.
The direction of boring, as can be
judged from the distribution and thickness
of the lining of burrows, is posterior and
partly ventral. Considering that the burrow in cross section has unequal axes like
those of the animal, that the byssus threads
are confined to the midventral region of
the burrow, that the aperture of the burrow is dumbbell-shaped, and that the bur-
CORAL-BORING GASTROPODS AND BIVALVES
row in some species possesses a ventral
ridge, it appears that only a rocking movement can possibly take place during boring.
The paste-like deposits on shells and in
burrows result from accumulation of part
of the calcareous fragments ground off
mainly by the posterior part of the shell,
and held in mucus secreted from the exposed anterior and posterior parts of the
mantle. The greater thickness of bottom
accumulations in burrows of L. hanleyana
is mainly due to the higher rate of growth
of the coral bored (Cyphastraea and Montipora). By their position, structure, and
mode of development, the coarse-ridged
posterior deposits on shells seem to originate from coral particles abraded from the
outer region of the burrow, rather than
from siphonal secretion. They are rendered harder, probably by being mixed
with glue-like mucus secreted from the
pallial glands, and cemented to the surface
of the valve in a definite orientation.
They are constantly renewed and strengthened by additional deposition of coral material.
891
2 mm.
FIG. 3. Coralliophilids. (A) Magilopsis
lamarckii;
(B) Leptoconchus globosus; (C) L. cumingii; (D)
Egg capsule of L. cumingii; (E) L. cumingii during
egglaying.
ridges much developed on the siphonal
region as well as across the last suture. The
periostracum is conspicuously thin, especially over the body whorl. The shell is
usually covered with a complete green
algal crust mingled with coral fragments.
It is so thick, particularly in small specimens, that it conceals the shell entirely.
Paste-like calcareous deposits are lacking
on the shells.
A creeping foot is well developed and
Boring is not a constant process. The
posterior edges of the shell may be rather acts as a pivot affixing the snail strongly to
far from the aperture, the interval being the wall of the burrow. The pallial siphon
occupied by the highly extensible pallial is markedly extensive, sometimes attaining
siphons. Moreover, stratification in the a length equal to that of the shell. No
bottom deposits, as well as in the compact radula or jaws are present.
The aperture of the burrow is rounded
lining of the burrow, may be taken as evidence that boring is executed in successive and minute, sometimes drawn into a narrow siphonal canal of varying length, and
periods of activity and of rest.
mostly partially lined with a calcareous
secretion of the pallial siphon (Fig. 4, A).
COR ALLIOPHI LI DS
Burrows themselves are unlined, frequentOf these poorly known gastropods, three ly possessing soft muddy, not pasty, acspecies are abundant, Leptoconchus cum- cumulations on the bottom.
ingii (Deshayes), boring in the living
Small burrows are goblet-shaped in L.
tops of Favia stelligera and Goniopora; L. cumingii (Fig. 4, A), rather elongated in
globosus (D.), boring in Goniastraea and M. lamarckii (Fig. 4, C), and compressed
Echinopora; and Magilopsis lamarckii in L. globosus (Fig. 4, B). Elongated bur(D.), in Cyphastraea (Fig. 3). A detailed rows, irrespective of the size of the shell,
morphological and anatomical account of are common in the first two species, and
these gastropods was given by Gohar and infrequent in the latter because of the thin
Soliman (19636).
corallum of the corals bored. The long
The shell, although thin, is markedly burrows (up to 50 mm) are mostly divided
rough, with sharp, uneven, longitudinal internally by one (Fig. 4, D) or more
892
GAMIL N .
flourishing Cyphastraea. The larvae remained unaffected for several days. The
same result was obtained when experimenting with larvae of M. lamarckii on Cyphastraea, but the coral polyps in this case,
were badly contracted.
MOLLUSCS BORING IN DEAD CORALS
1ctn.
1cm.
1cm.
FIG. 4. Coralliophilid burrows. (A), (D), and
(E) , of L. cumingii; (B) of L. globosus; (C) and
(F) of M. lamarchii.
ridges defining the spaces successively occupied by the gastropod; the outermost ridge
defines that last occupied. Ridges may be
regular and complete: spiral, annular, or
longitudinal; or irregular (Fig. 4, E, F).
Especially in L. cumingii, burrows are so
aggregated that they often intercommunicate. Minute burrows (3 mm long) are
merely fine tubes without definite borders
piercing the coral skeleton.
The characters of the shell, foot, and
burrows indicate that boring is likely to
take place by mechanical abrading by the
shell which is capable of incomplete rotation. Histological examination of the fleshy
parts protruding out of the shell revealed no acid glands. The shell apparently makes its way through the coral in steps,
producing cross ridges. Enlargement of
the burrow may also result in the formation of perpendicular ridges. In a few
instances the passage of the borer through
the coral is so gradual that the ridges are
ill defined and the walls rather even. The
shell siphon from time to time takes part
in lengthening the burrow as well as
widening the siphonal region.
In an attempt to test the assumption of
Otter (1937) that as a defensive device
coral polyps consume all veligers coming
across them, I set some veligers of L.
cumingii free directly on the surface of a
Excluding the factor of coral growth
affects boring into dead corals in two ways:
(1) the direction of boring is inward; and
(2) boring activity is reduced; thus, burrows are normally of moderate length and
proportional to that of the shell. However,
the danger of blocking of burrows by corals living nearby is still possible. Thus, in
order to maintain connection with the exterior, some borers are capable of excessive
siphonal elongation (Rocellariu), while
others can reverse the direction of boring
through the action of the posterior portions of their valves which are reinforced
with hard deposits (Lithophaga laevigata).
The entrance to the burrow in dead coral is wide and ill-defined, roughly dumbbell-shaped to nearly oval, and is sharply
delimited only when coral overgrowth is
encountered, or in species capable of siphonal calcareous secretion (Rocellaria).
Since the aperture is wide, the burrow usually harbors other smaller animals, such as
chitons and worms, as well as weeds contaminated with sand, mud particles, and
other foreign bodies. The coral itself is
mostly worn out, heavily infested with boring algae, sponges, tubeworms, as well as
many other small animals which seek protection in pits and depressions.
In dead coral, burrows seem to form
initially in crevices, pits, or occasionally
within old burrows where the young molluscs attach by byssus threads. Active boring may start as soon as the anterior end of
the growing bivalve comes in contact with
the rock material.
Among the Mollusca, only bivalves are
found to bore in dead corals. Of these in
the order of their abundance at Al-Ghardaga may be mentioned various species of
CORAL-BORING GASTROPODS AND BIVALVES
Gr.dl
Rd.dl
FIG. 5. Mytilids boring in dead corals. (A) Modiola cinnamomeus; (B) Lithophaga laevigata; (C)
dorsal view o£ burrow of Modiola; (D) and (E)
lateral and ventral views of L. laevigata. Br. Ln.,
hard calcareous lining of burrows; Dep. R. 1, 2,
and 3, regions of calcareous deposits; Gr. dl.,
dorsal groove; Rd. dl., dorsal ridge; Rd. vl., ventral
ridge.
Lithophaga, Modiola, Peiricola, Rocellaria, and Clavagella. This report is concerned primarily with the two mytilids,
Lithophaga laevigata Quoy and Gaimard
and Modiola cinnamomeus Bruguiere.
Shells of these two species are markedly
stronger than those of other mytilids boring in living corals. While calcareous deposits are reduced on the shells and burrows
of Modiola the deposits attain their maximum development in L. laevigata among
all Lithophaga species studied. According
to the distribution of the deposits, three
regions can be distinguished on the shell
(Fig. 5, B). The first comprises its anterodorsal and ventrolateral aspects where deposits are in the form of a muddy paste
and most pronounced dorsally. The posterior regions of the shell valves are covered
with thick, hard deposits (3 mm thick at
the free end, and protruding posteriorly 7
mm beyond the true edge of the shell in a
specimen 44 mm long). On the remaining
region, represented by the prominent
lateral aspects of the valves, the periostracum displays variable degrees of erosion. In Modiola the periostracum is similarly worn, and partly detached in certain
parts from the anterior and anterolateral
surfaces of the shell.
Burrows generally take the shape of the
shell, but particularly in burrows of Modi-
893
ola which are not lined, one can differentiate roughly between a siphonal region
and a shell region (Fig. 5, C). A smooth,
hard, calcareous lining, however, is present
in burrows of L. laevigata and is more
developed on the dorsal and dorsolateral
aspects than elsewhere (Fig. 5, D). Median
longitudinal ridges and grooves are a
common feature of burrows of both species. There is an anterodorsal ridge in
Modiola fitting in the interumbonal
groove and continvied into a dorsal groove
receiving its high dorsal edge, and in L.
laevigata there is a less pronounced ventral
ridge projecting between the ventral margins of the valves (Fig. 5, E). The position
of these ridges and grooves allows the
movement of the shell only in an anteroposterior direction.
Although Modiola was claimed by Otter
(1937) to bore exclusively by chemical
means, evidences for mechanical boring by
the rocking movement of the shell are
numerous. These include the powerful
shell, the well developed byssal apparatus,
the calcareous fragments on shells and burrows, the erosion of periostracum, and the
presence of ridges and grooves in the wall
of the burrow. The validity of this last
feature—as evidence for mechanical boring—was reported by Yonge (1955) who
concluded that the method of boring by
local application of acid is unlikely to
leave ridges, and that the presence of such
ridges and the strong byssal attachment in
Botula are evidence that the animal bores
mechanically. The enlargement of the siphonal region of the burrow is difficult to
explain on mechanical grounds, since the
pallial siphons are not covered with wrinkled periostracum which may aid in abrasion as mentioned for Hiatella (Hunter,
1949). A chemical process is, therefore,
likely in this region of the burrow.
The previous observations apply to L.
laevigata when it bores in dead coral rock.
In case the rock is overgrown by a living
coral, the direction of boring is reversed;
the shell with its hard calcareous incrustation is capable of boring successfully posteriorly. The rate, of boring accelerates and
894
GAMIL N. SOLIMAN
the burrows become markedly elongated
and contain thick muddy accumulations in
the bottom, as in other lithophagas boring
normally in living corals.
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