A M . ZOOLOCIST, 9:765-774 (1969).
Marine Borers in Calcareous Terrigenous Rocks of the Pacific Coast
JOHN E. WARME
Department of Geology, Rice University, Houston, Texas 77001
AND
NEIL F. MARSHALL
Scripps Institution of Oceanography, University of California, La Jolla, 92037
SYNOPSIS. Biological attack of carbonate-cemented sandstones and mudstones (Cretaceous and Eocene ages) and semi-consolidated calcareous muds (Holocene age) were
studied near La Jolla, southern California. Rocks were collected from the intertidal
zone to a depth of 40 m using SCUBA. The deeper samples came from Scripps and La
Jolla submarine canyons.
In order to learn of the borers and their preferred substrata, we studied the borers
in situ as well as in aquaria, examined their borings with x-ray radiography, and
analyzed the bored rocks by petrographic techniques.
In preferred substrata a sequence of boring activities resulted in over half of the
initial rock volume being excavated to depths of 10 cm. The major initial borers were
bivalves (pholads, Lithophaga) , which made straight smooth-walled holes up to 10 cm
deep and at all angles to the rock surfaces. When abandoned, these holes were
modified by other borers, and interconnected by smooth or roughhewn sinuous
passageways 1 to 15 mm in diameter. The passageways were concentrated along
bedding planes and in selected layers of rock, meeting at common points to form
galleries. Many crustaceans and worms occupied the passageways, and several kinds of
saapings and/or etchings characterized the walls. Species accomplishing this have not
yet been determined. A variety of borings 1 to 5 mm in diameter have single or
multiple openings into the larger passageways.
Rates of biological erosion of the rock were obtained from the depth of penetration
of the borers and estimated length of life spans in years. Yearly attrition is estimated
to range from 2 to 10 mm in some localities.
Physical and chemical submarine weathering of these rocks alone seldom penetrates
more than one cm from the exposed surface. Because biological penetration is much
greater, it is an important factor in shaping the seabed. Borers attack fine-grained
rocks more intensely than those that are coarse-grained, perhaps accounting for the
dearth of mudstones on the seabed near La Jolla, except for the steep and more
rapidly eroding walls of the submarine canyons. Most rocks presently exposed on the
relatively flat upper continental shelf (10 to 30 m) appear to be remnants of highly
indurated sandstone strata, less susceptible to biological attack.
Marine invertebrates boring into submerged rocks off the coast of California
accelerate submarine erosion. A variety of
species attack a wide spectrum of rock
types. This "bioerosion" (Neumann, 1966)
is responsible for important physical characteristics of rocks exposed on the sea floor,
and contributes to topographic changes
that may be significant with time. Some
man-made materials placed in the sea are
also likely substrata for attack.
We wish to thank all those who have aided this
T h e
fuJ a n n Q t a t e d
bibliographv of
project, including, among others, R. Dill, A.
o f l
Flechsig, N. Fotheringham, R. Crigg, J. Roselius, Clapp and Kenk (1963) covers most paand T. Scanland. G. E. Fryer prepared the photopers on marine borers prior to 1955. Subsegraphs. Special thanks are due E. W. Fager and F. quellt papers treating rock borers of the
P. Shepard, both of whom have supported our n o r t h e a s t e r n P a c i f i c O c e a n include Turner
work in many ways. Part of this research was
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accomplished under Office of Naval Research Con(19D4.
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(l&w). Hodgkin
tract NONR-2216 (23).
(1962), and Evans (1967, 1968). These
765
766
JOHN E. WARME AND NEIL F. MARSHALL
papers are concerned largely with molluscs. The considerable excavation of rocks
by other groups of borers has not yet been
adequately described. Many of the problems regarding rock borers stated in the
literature over the past 200 years still remain unsolved. Important questions involve both the borers and the rocks which
they bore and should be considered with
reference to each taxonomic group of
borers, preferably at the specific level: (1)
Are borings accomplished primarily by
chemical or by mechanical means, and is a
calcareous substratum necessary? (2) What
chemical compositions, grain sizes, packing arrangements, and hardnesses of rocks
are preferentially bored? (3) Which taxa
initiate boring, and which ones modify or
merely occupy abandoned voids in the
rocks? (4) At what rate is boring accomplished, and what is the relative importance of bioerosion compared with physical degradation of submarine outcrops?
(5) Is bioerosion as important on the continental shelf and in deeper habitats as it
appears to be in some intertidal and shallow-water regions?
The purpose of this paper is to discuss
the borers, their borings, and the rocks
which they attack on the coast of southern
California. Most of the rocks we studied
were in the category of clastic terrigenous,
i.e., sandstones, mudstones (shales), and
conglomerates, all with some carbonate
content. Of particular interest were the
deeply bored and honeycombed mudstones
exposed on the walls of submarine canyons. These rocks harbor many taxa which
appear particularly well adapted to life in
their borings or in the abandoned borings
of other organisms. The excavations in
these rocks are clearly biogenic. It is not
clear, however, which organisms are responsible for producing some kinds of tunnels, galleries, and other voids, as well as
scratch or etch marks lining the excavations. We have attempted to document the
stages of bioerosion which lead to disintegration of the rocks, and to discuss the importance of this activity for the processes
of erosion and deposition on the sea floor.
ROCKS INVESTIGATED AND METHODS
EMPLOYED
Rocks that we analyzed came from natural marine outcrops. They were collected
from the intertidal zone to depths of 40 m
using SCUBA; the deeper samples were
recovered in and near Scripps and La Jolla
submarine canyons, off La Jolla, California (Fig. 1).
Samples weighed from 1 to 40 kg, and
were stored in aquaria with circulating
fresh sea water. Some samples were kept in
aquaria for several weeks in order to observe the borers and their behavior; others
were broken apart to recover the borers.
All macroscopic specimens were collected
from each rock sample and preserved.
The geometric and spatial relationships
of the borings within the rocks were studied
using x-ray radiography (Figs. 2, 3, 6). In
some cases radiographs were prepared in
stereo pairs in order to study the spatial
relationships of the borings within the
rocks in three dimensions.
Several different rock types are present
near La Jolla with which to test the preference of different taxa for various substrata. Detailed petrologic analyses were
made of these rocks because boring animals must contend with such variable
properties of the substratum as chemical
composition, texture (grain sizes), structure (bedding), compaction, and hardness.
Rocks studied include those exposed on
the sea cliffs north of Scripps Institution
of Oceanography and offshore in Scripps
Submarine Canyon (Fig. 1). These are Eocene sandstones, shales, and conglomerates
(Limbaugh and Shepard, 1957, p. 633).
Cretaceous sandstones and mudstones are
exposed south of Scripps on the cliffs and
offshore from Point La Jolla (R. F. Dill,
personal communication). Holocene semiconsolidated muds, sands, and conglomerates lie at the headwall and shoreward rim
of La Jolla Submarine Canyon (Dill.
1964; Shepard and Dill, 1966, p. 56).
Carbonate Contents
Almost all of the sedimentary rocks col-
BORERS IN CALCAREOUS TERRIGENOUS ROCKS
767
FIG. 1. Map showing shoreline and bathymetry
offshore near Scripps Institution of Oceanography,
where samples of biologically attacked rocks were
collected. Note La Jolla and Scripps submarine
canyons.
lected showed some reaction to dilute hydrochloric acid. Eleven samples of varied
lithologies gave carbonate percentages of 5
to 50% as measured by the induction furnace method (combustion, collection, and
measurement of carbon as CO2). Almost
all of the carbon measured was in the form
of carbonate.
Petrographic analyses on thin sections of
these rocks revealed that the Cretaceous
and Eocene sandstones are composed of
sand-sized grains of quartz and several other minerals, with a microcrystalline carbonate cement filling the interstices. The
sandstones were 34 to 38% carbonate, largely occurring as interstitial cement. Uniform spheres may be arranged in various
geometries that yield 26 to 49% porosity (Graton and Fraser, 1935), and the
sandstones analyzed were largely or entirely cemented with carbonate.
Rocks of Eocene age from Scripps Sub-
marine Canyon are composed mostly of silt
and/or clay-sized particles (mudstones)
and contained 5 to 50% carbonate. The
most calcareous specimens thus approach
limestones, and are generally fine grained
and dense. The original source of the carbonate in these rocks is not apparent; it
was recrystallized, or precipitated after mobilization, during diagenesis and lithification.
KINDS AND SEQUENCE OF BORING IN SCRIPPS
SUBMARINE CANYON
The most completely attacked rocks that
we studied were well-bedded, Eocene, calcareous mudstones from the steep to vertical walls of Scripps Submarine Canyon
(Fig. 1). These strata are hard, wellcemented rocks. Almost all strata exposed
at the shoreward rim of the canyon (15 m
depth) exhibited some boring, but are
768
JOHN E. WARME AND N E I L F. MARSHALL
FIG. 2. ABOVE. Fragment of a 30-kg specimen of
Eocene miidstone collecled from Scripps Submarine
Canyon, depth 16 m. Specimen is 3 cm thick,
maximum length 14 cm. View is bottom o£ bedding
plane surface, 3 cm below surface exposed to
the sea. Circular borings were made by Lithophaga
plumula. One bivalve is visible in broken calcareous sheath; other borings are unlined and
lined with sheaths. Irregular tunnels have been
excavated along bedding plane, BELOW. X-ray radiograph of same rock. Lithophaga are visible in
lined, partially-lined, and unlined borings. Ten
pairs of Nettastomella roslrata valves are within
the rock, occupying pear-shaped borings. Note internal passageways.
BORERS IN CALCAREOUS TERRIGENOUS ROCKS
subject to intermittent covering with sand
or are in the path of sandfalls down the
sides of the canyon. Rocks in the axis of the
canyon are intermittently covered with
sand and other debris. The better-exposed
rocks on the sides of the canyon, however,
were riddled with borings (Fig. 2).
Common species in Scripps Submarine
Canyon that contribute to bioerosion by
nestling on or attaching to rock surfaces
include (in approximate order of abundance) attached molluscs (Chama, Crucibulum, Aides, Hinvites), and many species
of vagrant and tube-building polychaetes,
tunicates, sponges, bryozoans, corals, and
algae. Animals boring deeper into the
rocks include the bivalve,
Lithophaga,
several species of pholads, and sipunculids.
In addition, a network of sinuous tunnels
and interconnected galleries (Fig. 3) is
commonly developed and inhabited by
many species of polychaetes, as well as arthropods (shrimp, amphipods, crabs), and
some fishes. It is not yet clear which of
these accomplished the tunneling and
which were merely residents of abandoned
excavations in the rock.
The rocks in Scripps Submarine Canyon
exhibited several stages of bioerosion that
are also shown to a lesser degree in other
localities. The boring takes place through
the surface and into the interior of the
rocks, and eventually accounts for their
destruction. The following stages outline
the series of events that lead to disintegration of the rocks:
(1) Rock surfaces are colonized by
epilauna and borers.
(2) Vagrant and sessile epifauna pit,
groove, and otherwise erode the surface, as
well as protect it in places by incrustation. Sipunculids and other forms enlarge
crevices and abandoned burrows, and perhaps initiate new borings.
(3) Boring bivalves penetrate deeply,
especially Lithophaga plumula (up to 10
cm), which bores at all angles to the surface. The small pholad, Ncttastomella rostrala, is common in shallow burrows (upper 3 cm; see Fig. 2).
769
(4) As borings become abundant they
intersect at depth.
(5) Adult Lithophaga secrete carbonate
linings up to 5 mm thick on the walls of
their borings. These linings are exhibited
more commonly in larger specimens, but
are formed by small individuals if their
borings are intersected by the borings of
other Lilhophaga or those of other species.
(6) Skeletons of epifaunal molluscs,
corals, worm tubes, and bryozoans become
infested with borers that appear to be restricted to these rather pure carbonate substrates (e.g., Gliona, some cirratulid polychaetes); these skeletons are undermined
along with some of the underlying rock.
(7) Mortality of boring bivalves yields
empty borings that are then occupied by
nestling bivalves and many other organisms which modify the borings to varying
degrees.
(8) All borings tend to become interconnected, especially along bedding planes
or layers that were preferentially bored.
Interconnections are in the form of
straight or sinuous tunnels with diameters
of about 1 mm up to 15 mm (Fig. 3).
Smaller tunnels are usually smooth-walled
and are occupied by polychaetes. Larger
tunnels appear roughhewn (Fig. 4) or
smooth (Fig. 5) and are occupied by a
variety of species.
(9) Systems of major interconnected
and branching passageways thus develop,
commonly opening to the surface through
old pholad or Lithophaga burrows. Leading from these are many smaller borings
with single openings (of sipunculids, Nettastomella), or multiple openings (polychaetes). These animals feed within the
passageways rather than at the rock surface.
(10) Development continues until much
of the rock is removed; it becomes physically weakened and may start to fragment. Many inhabitants of the passageways (i.e., polychaetes) seal themselves off
from their neighbors by depositing chitinous or feltlike plugs. Chaetopterus, or a
related genus, occupies many of the larger
passageways, effectively closing them by
770
JOHN E. WARME AND NEIL F.
MARSHALL
•V S
FIG. 3. ABOVE. Fragment from same rock as specimen in Figure 2. Thickness 1.5 cm; max. length
20.5 cm. Upper and lower surfaces are bedding
planes. Rock exhibits extensive passageways (1 cm
diam) meeting at common points. Smaller passage-
ways are also present. Note roughhewn passage
walls (see Fig. 4) . BELOW. X-ray radiograph of
same rock as above. Small club-shaped borings at
top center of photo were probably made by
sipunculids.
BORERS IN CALCAREOUS TERRIGENOUS ROCKS
h,.
771
nivores (e-g-, vagrant crabs, polychaetes)
also feed there.
RATES OF BIOEROSION
FIG. 4. Close-up of F i g u r e 3 ( a p p r o x . 7 X 9 cm)
sliowing r o u g h h e w n scratch or etch m a r k s o n passage walls.
secretion of networks of leathery tubes.
Terebellid worms occupy abandoned borings and pack them with fine-grained sediment rich in organic debris and mica
flakes.
(11) Skeletons of the epifauna are torn
loose, fall off, or are excavated together
with the attached bedrock, exposing fresh
rock surfaces to continued colonization.
These borers probably do not derive
nourishment from the rocks they bore.
Some species in permanent borings feed
deep within the networks of tunnels, both
by suspension feeding (e.g., Nettastomclla,
bryozoans, sponges) and by deposit feeding (sipunculids?). It is possible that car-
We have not yet made precise measurements of the rates of biological erosion in
the rocks investigated. We have estimated
them, however, by assuming longevity values for the important boring species, and
by knowing the depths of their borings.
Comfort (1957) states that documented
molluscan life spans seldom exceed 5 to 10
years.
The most direct inference regarding
rates of erosion of the mudstones in
Scripps Submarine Canyon comes from
Lithophagn, whose borings normally open
flush with the rock surface (Yonge, 1955).
In the adult stage many Lithophaga borings become lined with a calcareous sheath
secreted by the animal. We collected examples in which these sheaths projected
well beyond the substrate surface, some
being almost entirely exposed. In one example several sheaths were cemented together and largely free from the rock matrix (Fig. 6). Living specimens occupied
some of these sheaths. Assuming a life span
of 10 or even 20 years, attrition of the
substrate took place around the sheaths at
a rate of several mm or perhaps even a cm
per year.
Another indication that bioerosion is
rapid in the mudstones of Scripps Submarine Canyon is that many sipunculid and
Neltaslomella borings leading off passageways deep within the rock are occupied by
living specimens. A single sipunculid boring may be occupied for several generations of worms, but the high frequency of
occupied Nettastomella sites in the interior of the rocks suggests to us that the
systems of passageways were excavated
within one or a few life spans of this species, in a total time of a few years or
decades.
PHYSICAL VS. BIOLOGICAL EROSION
FIC 5. Example o£ smooth passage walls, exposed
in rock broken along bedding plane. Diameter of
passage ca. 1 cm.
Although the actual rate of bioerosion
can only be estimated at present, the rela-
772
JOHN E. WARME AND NEIL F. MARSHALL
FIG. 6. X-ray radiograph of Lithophaga in calcareous sheaths that have been largely exposed by
erosion of surrounding rock. Specimen on upper
right was alive at time of collection. Note variety of
other borings and passageways. Maximum length
of lock ca. 14 cm.
tive importance of biological processes
which contribute to submarine erosion of
hard calcareous rock appears to be greater
than that of physical and chemical processes. Many rocks which we collected
offshore from La Jolla and studied by thinsection petrography exhibited weathering
to depths ranging from a few mm to a cm
(Fig. 7). Invertebrates boring into the
same rocks commonly penetrated to depths
of 10 cm, and in some cases 50 cm or
more. Where these rock strata are subaerially exposed on the cliffs and mesa in La
Jolla, weathering proceeds to depths of decimeters or meters within a few years after
exposure. Our observations indicate that
weathering of rocks can be significantly
different in subaerial regions compared to
the submarine environment, and that
borers play the dominant role in erosion of
some submerged rocks. They not only
effectively penetrate the rocks, but also
bare new rock surfaces to physical and
chemical deterioration.
biological activity culminates in rapid destruction of substrates that are preferred by
the borers and which are favorably situated on the ocean floor. The rocks that are
more readily attacked are generally calcareous, fine-grained, and relatively soft.
Some organisms penetrate a range of
rock types. For example, Yonge (1955)
and Hodgkin (1962) demonstrated that
the substratum for Lhe bivalve, Lithophaga, must contain carbonate. However, we found in the La [olla region that
some relatively soft mudstones with less
than 5% carbonate were intensively attacked by Lilliophnga, and some dense hard
mudstones with 48% carbonate are noticeably less bored compared to adjacent softer
and less calcareous layers. We also collected Lithophaga in rocks ranging in grain
sizes from calcareous mudstones to medium-grained sandstones with calcareous cement. This animal thus demonstrates the
ability of some borers to utilize a variety of
rock types, even though they may be dependent for penetration upon the presence
of some carbonate.
It is readily apparent that natural lines
of weakness along bedding planes, cracks,
and joints also provide a purchase or access to the rock interior lor some borers
(see Fig. 3). In many underwater localities
thicker and more homogeneous strata
tend to stand in relief compared with thinly-bedded rocks of about the same com-
DISCUSSION AND CONCLUSIONS
We have studied several kinds of calcareous clastic terrigenous rocks on the
coast of southern California. Almost all of
these rocks undergo some degree of bioerosion by boring marine invertebrates, and
FIC. 7. Thin weathering halo around two borings
exposed in rock broken along bedding plane. Longest dimension of larger hole is 1.4 cm.
BORERS IN CALCAREOUS TERRIGENOUS ROCKS
position; the latter are more easily eroded
by both biological and physical processes.
However, the amount of bioerosion on any
rock specimen does not always reflect its
suitability for attack. Attack may be slow
because the rocks are not favorably exposed, such as in the path of shifting sand;
or rocks may not be in the proper habitat
for taxa which have narrow ecological restrictions other than substratum (e.g., intertidal amphineurans in the La Jolla region,
which accomplish a unique and intensive
kind of bioerosion; Emery, 1960, p. 17).
Most data in the literature on bioerosion
came from studies in intertidal or very
shallow water habitats. Our work indicates
that a rich fauna participates in boring
into a variety of rocks in deeper water on
the continental shelf. The most common
and conspicuous of the borers are the bivalves, Lithophaga plumula and Nettastomella rostrata, both of which were collected in depths ranging from the lower
intertidal zone to 40 m, and in relatively
soft fine-grained mudstones to resistant
sandstones. The abundance of these species, however, was generally much greater
in the finer grained and softer rocks. Sandstones exposed in depths of 10 to 30 m off
La Jolla were attacked by some of the
same species as were the finer grained
mudstones in Scripps Submarine Canyon,
but to a noticeably lesser extent. The mudstones are recently exposed surfaces on the
steep walls of the submarine canyon; possibly this lithology has been largely eliminated from the flatter continental shelf by
bioerosion during and since the Holocene
rise of sea level (within the last 5,000 to
15,000 years).
A major problem yet requiring investigation in these rocks is the method of excavation and significance of the networks
of tunnels present in advanced stages of
boring. Initial borings are dominated by
straight, auger-like boreholes, probably
made with rotatory motions. These enclose
the animals that made them, and are generally utilized by only one generation of
the species producing them. In more advanced stages, borings are largely well de-
773
veloped passageways occupied by mobile
species. Judging from the variation in
sizes, geometries, and characteristics of the
walls of these networks, several species of
organisms join in producing them. Smaller
polychaetes are probably responsible for
many of the smaller networks, as the
worms have been collected in situ and precisely fit their excavations. However, only a
few species have been collected that may
account for the larger passageways. These
include the crab, Pachycheles mdis, which
may also make the scratch marks on the
tunnel walls, and large terrebellid and other polychaete worms, which may account
for faint to pronounced annulations on
some passages. Similar networks are present in the softer semiconsolidated muds
at the head of La Jolla Submarine Canyon, and these in turn resemble burrows of invertebrates, particularly those of
decapod crustaceans, living in soft, unconsolidated muds and sands. There may be
many species within the arthropods, annelids, and other groups that penetrate substrata with a range of hardnesses. This has
been demonstrated for the spionid, Polydora, which may or may not bore into a
hard calcareous substrate (Boekschoten,
1966, p. 354), and for the isopod, Sphaerorna, which normally lives on intertidal
marshes and mudflats but can also erode
rocks with its mandibles (Barrows, 1919).
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JOHN E. WARME AND N E I L F. MARSHALL
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