Evidence For Sublethal Predation and Regeneration
Among Living and Fossil Ascophoran Bryozoans
Evidence for partial predation on ascophoran bryozoans was hitherto mainly found in borings of the
frontal shield. However, during this and many other studies, borings are only observed rarely. Indeed, many
predators (e.g. nudibranch gastropods) are known to gain access to the internal organs via the operculum
while leaving no traces of frontal wall damage. This type of predation may, nevertheless, be evidenced by
the presence of intramural buds underneath undamaged zooecia, indicated by the presence of one or more
orificial rims within the primary one, and implies that the damage occurred during lifetime of the colony. This
skeletal signature was observed to occur in Late Cretaceous Acanthostega, as well as in Miocene to Recent
Lepraliomorpha, and in Recent Hippothoomorpha. Its infrequent presence may suggest that ascophorans
are not important target species for many predators, that not all taxa are able to secrete intramural buds,
and/or that only certain types of feeding mechanisms trigger this type of regeneration. Information on
feeding habits of modern predators on ascophorans, and reactions of different ascophoran taxa to various
types of predation, are needed to verify exactly when and why intramural buds are formed in preyed zooecia.
The evolutionary history of predator-prey relationships is
one of the most important aspects in the history of life (e.g.
Vermeij 1987). Yet interactions are notoriously difficult to
prove in the fossil record (e.g. Bishop 1975) and even inter-
pretations based on direct evidence of predation, such as drill
holes and repair scars, are often an intricate matter (Leighton
2002). Bryozoans are potentially well-suited for the analysis of
types and relative frequencies of predation because (1) physical
defenses (such as spines, frontal walls, avicularia, and ooecia)
are usually calcified and well-preserved in the fossil record
and, (2) apart from large predators (e.g. fish, echinoids) that
accidentally or deliberately graze whole colonies, predation
by smaller, zooid-level predators (e.g. nudibranch gastropods,
pycnogonids) is generally sub-lethal, as the entire colony is not
consumed; repair of damaged auto- or heterozooids by the sur-
viving ramets thus provides evidence for the occurrence of the
damage during colony lifetime. This paper is on zooid-level or
partial predators, which feed on single or few zooids at a time
and which are the same size or slightly larger than their prey.
Whereas feeding mechanisms and potential prey of
certain groups of predators are relatively well studied (for
nudibranchs see Nybakken and McDonald 1981, Todd 1981,
Todd and Havenhand 1989, Cattaneo-Vietti and Balduzzi
1991), studies on predation from the bryozoan’s point of
view are rare (e.g. Ryland 1976). The first comprehensive
work on modern predators is only now being published
(Lidgard 2008a; see also Lidgard 2008b). The sparse fossil
history of predation on bryozoans was recently reviewed by
McKinney et al. (2003). Whereas certain taxa (e.g. naticids
and muricids) leave characteristic skeletal marks (see Taylor
1982, and references therein), the data compiled by McKin-
ney et al. (2003) also show that the same (or no) damage to
the skeleton may be induced by different predators during
predation. Lidgard (2008a) gives some 180 species of Recent
partial predators from a range of higher taxa, including
nematodes, annelids, as well as several groups of arthropods
and gastropods. Because little is known about how different
types of predation are reflected in the bryozoan skeleton,
care must be taken before a specific predator can be identi-
fied as having caused damage to zooids in fossil bryozoans.
Upon damage of a zooid, anascan and ascophoran bryo-
zoans are able to produce a new zooid within the original
one, termed an intramural bud (Levinsen 1907; Buchner
1918; Taylor 1988; Wilson and Taylor 2006). In anascans, the
newly formed zooecium is evidenced by a second mural rim
located immediately inside the primary rim (e.g. McKinney et
al. 2003, figs. 2D, E), whereas ascophorans may form a new
skeleton, including a new intramural orifice rim, inside the
damaged primary zooecium (e.g. Wilson and Taylor 2006, figs.
2A, B). However, previous studies of predation on, and re-
parative buds of, ascophoran bryozoans have only considered
skeletons that were damaged, although many taxa are known
Institut für Erdwissenschaften, Universität Graz, Heinrichstr. 26, 8010 Graz, Austria.
Present address: Oberösterreichische Landesmuseen, Geowissenschaftliche Sammlungen,
Welserstr. 20, 4060 Linz-Leonding, Austria. <firstname.lastname@example.org>
to feed on zooids without leaving a trace on the skeleton. The
goal of this paper is to demonstrate that one skeletal feature
that also occurs in damaged and repaired zooecia, namely the
inner orificial rim within that of the damaged zooecium (Wil-
son and Taylor 2006, 567, fig. 2A), may indeed indicate this
type of sublethal predation in otherwise complete zooecia.
A range of cleaned and well-preserved fossil and Recent
ascophoran bryozoans were examined by scanning electron
microscopy (SEM). Recent material comprised specimens
from various eastern Atlantic locations (Arctic, UK, Spain,
Portugal, Ghana, Sierra Leone) as well as from the Mediter-
ranean Sea. Fossil material included specimens from the Late
Miocene (Tortonian) of the easternmost Atlantic (Guadalqui-
vir Basin, southwestern Spain; see Berning 2006), from the
Pliocene Coralline Crag of southeastern England, and from
the mid-Pliocene (Piacenzian) of the north-central Mediter-
ranean Sea (Castell’Arquato, northern Italy; see Pizzaferri and
Berning 2007). The specimens are housed in the Natural His-
tory Museum London (NHM), the Croatian Natural History
Museum Zagreb (CNHM), the Senckenberg Research Institute
and Natural History Museum in Frankfurt (SMF), and in the
Geological Museum “G. Cortesi” in Castell’Arquato (Italy,
Table 1. Extant and fossil species in which intramural orificial rims in undamaged zooecia were observed. Information on age, location and depth of occur-
rence (if available) is provided.
Species Age Location Depth of occurrence (m)
Haplopoma graniferum Recent Birlurbuy Bay, Ireland
Recent Kermadec Ridge, New Zealand 55–310
Escharina johnstoni Recent Guernsey, UK
Recent Gulf of Aden, Yemen 76
Recent Adriatic Sea, southern Croatia 30–40
Recent Madeira 75–90
Recent Sierra Leone 80
Hippaliosina clavula Pliocene Mediterranean Sea, northern Italy
Pliocene Mediterranean Sea, northern Italy
Miocene Victoria, Australia
MG). In addition, a number of publications showing SEM
illustrations of modern and fossil bryozoans were screened
for skeletal marks left behind by partial predators.
Despite the wide range of localities, material and taxa
analyzed, damage to the frontal wall indicating partial predation
was extremely low. Unequivocal evidence for predation was
hardly present, e.g., symmetrical drill holes were never observed.
Instead, damages in the frontal wall usually showed irregular
fractures, which make it difficult or impossible to determine
whether these damages were predator-induced or the result of
accidental breakage. Still rare, yet found somewhat more often
than unequivocal damage of the frontal wall, were superficially
undamaged zooecia containing a secondary, intramural orifi-
cial rim in the primary orifice. This feature was observed to
occur in both Recent [Haplopoma graniferum (Johnston 1847),
Escharina johnstoni (Quelch 1884), Herentia majae Berning et
al. 2008, Therenia peristomata Berning et al. 2008, Buffonellaria
sp.] and fossil species [Cleidochasmidra canakkalense Ünsal and
d’Hondt 1979, Hippaliosina clavula (Manzoni 1869)] (Fig. 1
and Table 1). In both H. graniferum and T. peristomata, newly
formed intramural orificial rims were also observed to occur
in ovicellate zooecia (Figs. 1.1 and 1.2, and 1.5, respectively).
Sources: 1. From Gordon 1984; 2. Amui and Kaselowsky 2006; 3. Berning et al., 2008; 4. Berning and Kuklinski 2008; 5. Pizzaferri and Berning 2007; 6. Cook
and Bock 1996; 7. Gordon and Voigt 1996.
Figure 1. Intramural buds underneath undamaged zooecia, evidenced by the presence of one or more orificial rims inside the primary one. 1, Haplopoma grani-
ferum (Johnston), Recent, Ireland (NHM 1822.214.171.124); intramural buds in an autozooecium (upper arrow) and in an ovicellate zooecium (lower arrow); scale bar
100 µm. 2, Close-up of orifice of the ovicellate zooecium in Figure 1.1; scale bar 50 µm. 3, Herentia majae Berning et al., Recent, Adriatic Sea (CNHM Inv.br.
31); intramural bud in an undamaged autozooecium; scale bar 50 µm. 4, Close-up of the orifice in Figure 1.3; scale bar 50 µm. 5, Therenia peristomata Berning
et al., Recent, Madeira (NHM 19126.96.36.1993); intramural buds in an undamaged autozooecium (left arrow) and ovicellate zooecium (right arrow), as well as in
a damaged zooecium (lower right); scale bar 100 µm. 6, Close-up of autozooecium orifice indicated in Figure 1.5; note the immersed distal shelf in the orifice
supporting the operculum. Scale bar 50 µm. 7, Buffonellaria sp., Recent, Sierra Leone (SMF 3015); autozooecia with primary orifice rim (lower right), with one
additional orificial rim (top) and with two orificial rims (lower left and bottom); scale bar 100 µm. 8, Close-up of uppermost orifice in Figure 1.7; scale bar 50
µm. 9, Cleidochasmidra canakkalense Ünsal and d’Hondt, Pliocene, N Italy (MG 0922); intramural buds in two early astogenetic autozooecia (arrows); note the
intact peristome especially in zooecium at lower left; scale bar 100 µm. 10, Close-up of orifice of the zooecium at upper right in Figure 1.9; scale bar 50 µm. 11,
Hippaliosina clavula (Manzoni), Pliocene, N Italy (MG 0727); intramural bud in an autozooecium (arrow); scale bar 100 µm. 12, Close-up of orifice indicated in
Figure 1.11; scale bar 50 µm.
In the literature, similarly few records of intramural orifi-
cal rims, or any other predator-induced damages, were found.
For instance, the primary orifice in Turbicellepora torquata
Hayward 1978 from the Bay of Naples, figured by López
de la Cuadra and García-Gómez (2001, fig. 3F) is actually a
tertiary one, as a second and third intramural orifice rim are
located within the first one. Because the entire zooecium is
not figured, however, it is not possible to determine whether
or not the frontal wall was damaged. Nevertheless, such ma-
tryoshka doll-like zooecia were also observed in Buffonellaria
sp. (see Berning and Kuklinski 2008), in which the frontal
shield was intact (Fig. 1.7). Other records of Recent bryozoans
comprise an Arthropoma species from the Kermadec Ridge,
which Gordon (1984: pl. 30, fig. A, zooecium at upper right)
figured as A. cecilii (Audouin 1826), and a species from the
Gulf of Aden, which Amui and Kaselowsky (2006, fig. 22,
the figured orifice is that of the undamaged zooecium at
upper right in fig. 23) identified as Escharina pesanseris (now
considered to be Bryopesanser sp., see Tilbrook 2006). Other
fossil specimens are Characodoma excubans (Waters 1881) from
the Australian Miocene, figured by Cook and Bock (1996,
figs. 2 [middle zooecium], 8, 11), as well as Castanopora labiata
(Levinsen 1907) from the Late Cretaceous of Denmark, fig-
ured by Gordon and Voigt (1996, fig. 1B) (see Table 1).
Considering the diversity and ubiquity of partial preda-
tors in shallow shelf environments that include bryozoans as
part of their diet or that even specialize on a certain species
(e.g. McBeth 1971; Ryland 1976; Gordon and Rudman 2006;
Lidgard 2008a, 2008b), the occurrence of systematic skeletal
damage in ascophoran zooecia is astonishingly seldom reported.
For instance, in Tilbrook’s (2006) tome on bryozoans from the
Solomon Islands, not a single figured zooid shows an unequivo-
cal boring in the frontal wall. Only two zooecia of Bryopesanser
capitaneus Tilbrook 2006 have large, irregularly fractured holes
of unknown origin that underneath show the frontal walls of
intramural buds (Tilbrook 2006, pl. 56, fig. E, upper center and
right). Thus, whether ascophorans are target species of partial
predators or mere incidental prey, the great majority of preda-
tors seem to gain access to their food without leaving a trace
on the bryozoans’ frontal wall. Although the frequencies of
borings observed in modern and fossil mollusks and echinoids
are generally high (20-80%), predation intensity is taxon-specific
and may be very low (0-5%; see, e.g., Nebelsick and Kowalewski
1999; Hoffmeister and Kowalewski 2001; Kelley and Hansen
2003). In Paleozoic and Recent brachiopods predation traces are
generally found in 0-2% of the shells (Kowalewski et al. 2005).
The low frequencies of borings and/or intramural buds ob-
served in ascophoran bryozoans may therefore not be unusual.
Due to their exposed frontal membrane and largely un-
calcified frontal surface, many partial predators of anascan
bryozoans leave the calcified mural rim intact when feeding
on single or even multiple zooids. One or more intramural
buds formed within undamaged outer rims are therefore
commonly reported in Recent and fossil anascans (e.g. Poluzzi
1980: pl. 4, fig. 6; McKinney et al. 2003: figs. 2D, E; Berning
2006: figs. 10, 13, 14, 16). If the formation of spines, which
may be induced by the presence of predators in Membrani-
pora mebranacea (Linnaeus 1767), significantly slows down
the predator’s feeding rate (Harvell 1984), the completely
calcified frontal walls in ascophorans should deter predators
from boring through the skeleton (Lidgard 2003). This may
explain the general scarcity of borings.
One possibility to circumvent boring the frontal wall
was observed in a pycnogonid arthropod, which uses its
extremely long and thin proboscis to penetrate through the
frontal pores of Antarctic Cellarinella species and sucks out
the internal organs (Fry 1965, as in Cook 1985, 17). The
next weakest spot to access the ascophoran’s soft parts is the
orifice, which could be bored by a radula and/or sucked using
a nudibranch’s buccal mass, while the external cuticle may
be removed by weak radula action (e.g. Best and Winston
1984, 399). However, these types of feeding may leave little
or no mark in the skeleton. Suction feeding was observed in
dorid nudibranchs grazing colonies of Reptadeonella violacea
(Johnston 1847) and Schizomavella linearis (Hassall 1841) in
the Adriatic Sea (McKinney et al. 2003: 244). Even at high
magnification, grazed colony surfaces revealed no visible
skeletal damage (McKinney et al. 2003: figs. 1A-E). In some
highly specific and exclusive associations between dorid
nudibranchs and eurystomellid bryozoans, Okenia rosacea
(MacFarland) feeds on Integripelta bilabiata (Hincks 1884)
using a combination of suction feeding and radular abrasion
(McBeth 1971). In turn, O. hiroi (Baba), which feeds on I.
acanthus Gordon and Rudman 2006, seems to solely use the
suction technique, as damage of the spinous, gymnocystal
frontal wall was not recorded by Gordon and Rudman (2006;
they do, however, report boreholes that were possibly caused
by a marginelliform gastropod). Moreover, two ascophorans,
identified as Trematooecia magnifica (Osburn 1914) and T.
turrita (Smitt 1873) by Cook (1985), were grazed upon by a
specific nudibranch off Ghana (Cook 1985, 18-19, pl. 3, fig.
B). A single large colony of T. magnifica was seemingly able to
sustain a population of some 30 suction-feeding nudibranchs.
Yet none of the above-mentioned authors included informa-
tion on possible repair of grazed zooids or other reactions to
this type of predation by the respective bryozoans.
That the presence of one or more orificial rims inside
the primary one in an otherwise undamaged zooecium
(Fig. 1) represents an indication of sublethal predation and
subsequent zooid regeneration is supported by several lines
of evidence. Predation does indeed trigger this very type of
repair at least in certain instances and in certain ascophoran
bryozoans, as shown by the presence of intramural buds
underneath bored zooecia of Microporella hyadesi (Jullien 1888),
which also secreted new orificial rims (see Wilson and Taylor
2006: fig. 2A). Accidental damage of the orifice and internal
organs as a causal mechanism for repair seems unlikely.
Although the operculum is the weakest spot on the frontal
surface, abrasive grains or strolling organisms need to have a
sufficient size, shape and mechanical force to be able to push
in the operculum without, however, damaging the calcified
rim. As the operculum occasionally rests on an immersed
calcified shelf, which is the case in H. majae or T. peristomata
(Fig. 1.6), (see also Berning et al. 2008), greater resistance
may be provided. In performing mechanical tests on opercula
of different cheilostome species, Best and Winston (1984)
showed that, when low forces were applied to ascophoran
orifices, either the opercular hinge broke or, under increasing
loads, the orifice rim was shattered. In either case, skeletal
damage to the orifice region by the intruding object is likely
to be reflected in, e.g., broken condyles. Moreover, current-
transported grains or organisms of sufficient size should leave
marks on delicate superficial structures such as peristomes
(cf. the early astogenetic zooecium of C. canakkalense, Fig.
1.9). This was, however, not the case in most of the observed
regenerated zooecia. Another indication is that intramural
buds often occur in several adjacent zooids, implying non-
random damage via the opercula.
Alternatively, partial starvation and/or senescence of
colonies may commonly occur in oligotrophic bathyal habitats
and dark caves (J.-G. Harmelin, pers. comm., 2008). Subse-
quent re-use of the existing undamaged zooecia during phases
of enhanced food supply would also result in the presence
of a secondary orifice rim in these zooecia. However, most
colonies presented here are from relatively shallow, open water
where starvation is rather unlikely to occur.
Although not as rare as bored zooecia, intramural buds
within undamaged zooecia were seldom observed, which may
suggest that either ascophoran bryozoans are not major targets
of partial predators, that not all taxa are able to secrete intramu-
ral buds, and/or that only certain types of feeding mechanisms
trigger zooid-scale repair. It seems unlikely that the loss of a
number of tentacles would initiate zooid-scale repair, whereas
only the complete loss of internal organs and external cuticle
may induce regeneration of a zooid. Another explanation for
the infrequent observation of intramural buds may be sought
in low magnifications used in many SEM photos of ascophoran
colonies. For an unequivocal determination of a secondary
orifice rim close-ups may occasionally be indispensable. For two
interdependent reasons it is furthermore regarded unlikely that
the literature survey yielded biased results because of taxono-
mists’ preferences for illustrating undamaged colony regions:
(1) in contrast to drill holes in the frontal wall, the skeleton
may not be visibly damaged by this type of partial predation
and secondary orificial rims may thus go unnoticed; (2) most
taxonomists were not aware of this inconspicuous damage and
repair of predated zooids as the existence of secondary orificial
rims was, to my knowledge, never mentioned in any taxonomic
paper. Also, the observed predation frequencies have to be
considered with some reservations, as intramural buds cannot
be produced in colonies that did not survive the attack.
The common presence of repaired avicularia, in contrast
to the relatively few repaired zooids, may be due to their ex-
posed location and their comparatively frequent encounters
with predators and other objects. For instance, distinctly
larger predatory or other organisms that are captured by the
avicularium may severely damage or remove the mandible
and attached tissue when trying to free themselves, inducing
repair of the heterozooid.
While never observed in erect growing ascophorans,
intramural orificial rims in undamaged zooecia were present
in encrusting species from the mid-to outer shelf (Table
1), which is in contrast to most nudibranch predation
records that are from shallow subtidal depths (Lidgard,
pers. comm. 2007). Whereas this may or may not be related
to sampling bias, in favor of depths that can be reached by
scuba diving, a number of other partial predators occur in
deeper habitats, such as pycnogonids (Lidgard 2008a), the
feeding mechanisms of which may as well be able to induce
the regeneration of zooids. Clearly, studies are certainly
needed that aim at showing how modern predators gain
access to their bryozoan food, if/how this is reflected in
the ascophoran skeleton, and how different groups of
ascophorans respond to different feeding mechanisms by
repairing damaged zooids.
In this study the formation of intramural buds was
observed to occur in the families Cribrilinidae, Haplo-
pomidae, Hippaliosinidae, Microporellidae, Lacernidae,
Escharinidae, Cleidochasmatidae, and Celleporidae. At
the present state of knowledge it therefore occurs in the
Acanthostega, Hippothoomorpha, and Lepraliomorpha but
not in the Umbonulomorpha. Although not easy to spot,
especially when using light microscopy, intramural orifice
rims are readily preserved in fossil bryozoans, and their
first occurrence dates back to at least the Late Cretaceous
in ascophorans with acanthostegan frontal shields (Gordon
and Voigt 1996). The appearance and frequency of this fea-
ture in ascophoran bryozoans can therefore be tested using
the fossil record.
To conclude, the paucity of predator-induced damage
and repair could be real, reflecting low occurrence of certain
types of predators in certain habitats or time intervals, or
may reflect differential regeneration ability or frequency
among various ascophoran groups. The collection of preda-
tor-prey data with a focus on ascophoran bryozoans and the
exact type of feeding of the predator is therefore needed. Fur-
thermore, in order to determine taxon-related response to
predation, future research on living ascophorans should give
(part of) the grazed colony sufficient time for regeneration
after exposure to a predator, which may occur within a few
days after damage (Bone and Keough 2005). If the cause(s)
for the formation of intramural buds in undamaged zooecia
can be ascertained, the presence of two or more intramural
orificial rims may represent another clue with which to verify
the timing frequency and evolutionary patterns of sublethal
predation of bryozoans by partial predators.
I thank S. Lidgard, J.E. Winston and M.M. Key for
helpful discussions, literature supply, and their constructive
comments on the manuscript. Most of the fossil material
was studied during a project funded by the Deutsche
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