Boring crustaceans damage polystyrene ﬂoats under docks polluting marine
waters with microplastic
Timothy M. Davidson
Aquatic Bioinvasion Research and Policy Institute, Environmental Science and Management, Portland State University (ESM), P.O. Box 751, Portland, OR 97207, USA
Boring isopods damage expanded polystyrene ﬂoats under docks and, in the process, expel copious num-
bers of microplastic particles. This paper describes the impacts of boring isopods in aquaculture facilities
and docks, quantiﬁes and discusses the implications of these microplastics, and tests if an alternate foam
type prevents boring. Floats from aquaculture facilities and docks were heavily damaged by thousands of
isopods and their burrows. Multiple sites in Asia, Australia, Panama, and the USA exhibited evidence of
isopod damage. One isopod creates thousands of microplastic particles when excavating a burrow; col-
onies can expel millions of particles. Microplastics similar in size to these particles may facilitate the
spread of non-native species or be ingested by organisms causing physical or toxicological harm.
Extruded polystyrene inhibited boring, suggesting this foam may prevent damage in the ﬁeld. These
results reveal boring isopods cause widespread damage to docks and are a novel source of microplastic
Ó2012 Elsevier Ltd. All rights reserved.
Marine borers can cause substantial damage to marine struc-
tures. The most extensive and costly damage occurs in wooden
structures by teredinid bivalves (shipworms) and isopod crusta-
ceans (Cragg et al., 1999; Neily, 1927). For example, the non-native
shipworm Teredo navalis destroyed the timber pilings and supports
of docks in San Francisco Bay causing nearly 50 structures to
collapse and causing $615 million in damages (Cohen and Carlton,
1995; Miller, 1926; Neily, 1927). Crustacean borers are also very
destructive (Cookson et al., 1986; Cragg et al., 1999; Kofoid and
Miller, 1927), especially in Australia where timber replacement
costs from marine borers are around $20 million per year (in
1986 AUD dollars; Cookson et al., 1986). Moreover, borers can
attack non-wooden structures as well, such as rock sea walls
(Chilton, 1919), concrete structures (Kofoid and Miller, 1927),
and even steel support beams (Irwin, 1953).
Burrowing sphaeromatid isopods bore into numerous substrata
used in marine structures and facilities in brackish temperate and
tropical regions (Carlton, 1979; Chilton, 1919; Cragg et al., 1999;
Kofoid and Miller, 1927). Boring isopods are native to the Indo
and West Paciﬁc but are non-native in North America and perhaps
in the Caribbean (Carlton and Iverson, 1981; Carlton and
Ruckelshaus, 1997; Harrison and Holdich, 1984). These estuarine
isopods tolerate a wide range of salinities (0–43 PSU, Estevez,
1994; Riegel, 1959) and temperatures (5–42 °C; Jansen, 1971).
However, they suffer mortality after several days at the lowest
salinity (0 PSU) or temperature (5 °C; Jansen, 1971; Riegel, 1959).
In the ﬁeld, the boring isopods Sphaeroma quoianum, Sphaeroma
terebrans, and Sphaeroma peruvianum are most often found
between 5 and 31 PSU salinity (Davidson, 2008; Davidson et al.,
2008, unpublished data). The burrowing isopods S. quoianum and
S. terebrans live for 12–18 months and 10 months, respectively,
and can produce up to two cohorts before dying (Schneider 1976,
These borers are especially destructive to expanded polystyrene
ﬂoats (commonly known as Styrofoam) used in many docks.
Densely clustered colonies of these direct-developing isopods
perforate the submerged surface of the ﬂoat and appear to reduce
its functionality. While burrows are initially shallow (less than
30 mm deep, and rarely exceeding 60 mm; Davidson and de Rivera,
2012; Perry and Brusca, 1989; Talley et al., 2001), subsequent
generations and colonizers extend and build from old burrows,
creating an interconnected burrow network (as described by Talley
et al., 2001; Thiel, 1999). This extensive network substantially
reduces the density of the outer 60 mm of the ﬂoat, making the
foam noticeably weaker and more susceptible to breakage. As the
outer surface is removed, additional area of the ﬂoat becomes
vulnerable to attack. Boring sphaeromatid isopods are ﬁlter feeders
that excavate burrows for habitat (Rotramel, 1975; Si et al., 2002);
therefore, any consumption of excavated material is likely
0025-326X/$ - see front matter Ó2012 Elsevier Ltd. All rights reserved.
Present address: Smithsonian Tropical Research Institute, Apartado Postal
0843-03092, Balboa, Ancon, República de Panamá. Tel.: +507 212 8830; fax: +507
E-mail address: DavidsonT@si.edu
Marine Pollution Bulletin 64 (2012) 1821–1828
Contents lists available at SciVerse ScienceDirect
Marine Pollution Bulletin
journal homepage: www.elsevier.com/locate/marpolbul
incidental (Messana et al., 1994; Rotramel, 1975; Si et al., 2002).
While some ﬂoats are encapsulated with hard plastic shells or
sheets, many docks or facilities either do not use these materials,
or the encapsulation materials are damaged and vulnerable to iso-
pod burrowing (per. obs.).
Minute plastic particles are created through the boring process
of polystyrene ﬂoats by S. quoianum (Carlton, Chang, and Wells,
unpublished, as cited in Carlton and Ruiz, 2005), S. terebrans, and
S. peruvianum (per. obs.). Like other microplastics (deﬁned as
<5 mm in diameter, Arthur and Bamford, 2009) in the marine envi-
ronment, these particles may have detrimental effects to marine
organisms (Carpenter et al., 1972; Cole et al., 2011; Gregory,
1996; Thompson et al., 2004). Plastics persist for hundreds to thou-
sands of years in normal oceanic conditions (Barnes et al., 2009).
Also, polystyrene fragments and other minute plastics in the mar-
ine environment are readily colonized by bioﬁlm and other organ-
isms causing them to sink (Barnes, 2002; Gregory, 2009; Ye and
Andrady, 1991). Thus, these particles may interact with benthic
(Graham and Thompson, 2009; Thompson et al., 2004) and pelagic
organisms (Boerger et al., 2010; Carpenter et al., 1972; Davison and
Asch, 2011). Ingested microplastics may cause both toxicological
effects by transmitting bioaccumulating toxins (Mato et al., 2001;
Teuten et al., 2009) and possibly physical effects by occluding feed-
ing structures or inducing a false indication of satiation.
The damage caused by boring isopods to polystyrene ﬂoats un-
der ﬂoating docks can result in economic costs and contribute to
microplastic pollution. This paper reports observations of the
destructive effects of boring isopods on foam ﬂoats, quantiﬁes
the density of burrows and individuals in ﬂoats, quantiﬁes the
abundance of plastic particles created from the boring process,
and discusses the morphology and ecological implications of the
plastic particles created through the boring process. Furthermore,
results of an experiment examining how different polystyrene ﬂoat
types may prevent damage by borers are presented. Together these
observations, surveys, and experimental results reveal (a) the dam-
aging effects of non-native and native borers on the ﬂoatation in
docks, (b) how a non-native species contributes to microplastic
pollution, and (c) approaches to reduce these effects in the many
bays that harbor populations of boring isopods.
2.1. Observations of isopods attacking ﬂoats
Shoreline surveys were conducted in Yaquina Bay, Oregon, USA,
and in Budai Township and Tainan, Taiwan. Both the high tide lines
and docks and marinas in intertidal and shallow subtidal areas
were examined for damaged polystyrene ﬂotsam or ﬂoats; popula-
tions of Sphaeroma sp. occurring in adjacent substrata were also
noted. Search effort was focused on areas between 5 and 31 PSU,
where boring isopod populations are most often found (Davidson,
2008; Davidson et al., 2008). Polystyrene ﬂoats and ﬂotsam were
considered burrowed by sphaeromatids if they harbored living or
dead individuals in their burrows or if vacant burrows were consis-
tent with the morphology of burrows created by sphaeromatid iso-
pods: (i) vermiform burrows with smooth walls, (ii) circular
diameters between 2 and 10 mm, and (iii) up to 77 mm deep,
and (iv) mostly straight without abrupt changes in direction
(Barrows, 1919; Davidson and de Rivera, 2012; Talley et al.,
2001). To my knowledge, no other boring organism creates bur-
rows consistent with this morphology and the burrows of other
organisms (e.g. small grapsid crabs) are rare (per. obs.). Further-
more, these surveys are supplemented with additional reports of
isopod burrowing from both published sources and unpublished
2.2. Mean density of individuals and burrows of S. quoianum in
expanded polystyrene ﬂotsam
Between February 2005 and May 2006, samples of burrowed
expanded polystyrene ﬂoating dock ﬂotsam encountered during
surveys of Coos Bay (n= 18 pieces) were collected. Each piece
was photographed with a haphazardly placed 10 10 cm quadrat
in the burrowed area of the ﬂoat. The number of quadrats photo-
graphed varied concomitantly with the size of the expanded poly-
styrene ﬂotsam found. One quadrat was used to estimate the
burrow density of small pieces (30–60 cm long) and between 6
and 50 quadrats for larger pieces (entire ﬂoats >60–100 cm long).
Digital analysis software, ImageJ 3.0 version 1.49u, was then used
to count the total numbers of burrows per quadrat.
Burrowed expanded polystyrene ﬂoat mimics were deployed in
Coos Bay, Oregon for 1 year (2005–2006) to provide an estimate for
how many isopods inhabit expanded polystyrene ﬂoats. The ﬂoat
mimics were constructed of a burrowed expanded polystyrene
ﬂoat found in the ﬁeld (devoid of isopods). Blocks were cut to
10 10 8 cm (length, width, depth). Each block was surrounded
in polyethylene tape exposing only the burrowed 100 cm
Burrow densities in these mimics were 64.2 ± 2.3 burrows per
(mean ± 95% CI). The blocks were afﬁxed facing down-
wards to weighted PVC tubing and placed around a length of rebar
planted into the ground. The weighted PVC tube kept the orienta-
tion of the blocks pointing downward while allowing the ﬂoats to
move up and down with the tide along the rebar pole. These ex-
panded polystyrene dock mimics were deployed in six different
locations with salinity between 10 and 31 PSU in Coos Bay, Oregon.
2.3. Quantity and morphology of the plastic particles created during
boring by S. quoianum
A lab experiment was conducted to quantify the numbers of par-
ticles created by S. quoianum during the boring process (methodol-
ogy described in detail in Davidson et al., in preparation). Small
colonies of 20 adult isopods (7–12 mm in length) from Coos Bay,
Oregon were placed inside cages with an expanded polystyrene
foam block (800 cm
) with one exposed surface (100 cm
small holes (4 mm deep) were created in each block to prompt iso-
pods to begin burrowing; these values were not included in the
measurements of burrow length. Each cage was then submerged
in a closed aerated aquarium at one of 13 water temperatures
(7.5–25.2 °C) to vary burrowing intensity (Davidson et al., in prep-
aration). Isopods were allowed to burrow for 2 months. At the end
of the experiment, the number of burrows created and mean
lengths were measured in each foam block and the plastic particles
were collected by discharging the aquarium water through a 63
sieve. The particles were placed on a gridded paper ﬁlter (1 cm
grid) and agitated to help homogenize the distribution of particles
on the grid. The total number of grid squares occupied by plastic
particles was counted and then ﬁve subsamples (1 cm
were randomly selected to be photographed using a digital micro-
scope camera. The numbers of particles in each square subsample
were counted using digital analysis software. The total number of
particles created during the boring process in the different blocks
was calculated by multiplying the mean number of particles per
subsample (1 cm
) by the total number of squares occupied by plas-
tic particles. The relationship between the number of particles cre-
ated per burrow and mean burrow length (total burrow length in a
block/number of burrows created) was examined using ordinary
least squares regression. The data were square-root transformed
to meet assumptions of linearity, homogenous variance, normality,
and reduce the inﬂuence of outliers. The lowest value appeared to
be inﬂuential (Cook’s Distance = 0.81), however, its removal did
not substantially change the shape of the relationship (but reduced
1822 T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828
to 0.67). This potentially inﬂuential value was retained since
there was not a non-statistical reason to merit its removal.
To examine the morphology of the particles, particles from
aquaria where adult individuals of S. quoianum (7–12 mm long)
had burrowed into a block of expanded polystyrene were collected.
The plastic particles and surrounding water were haphazardly col-
lected from aquarium water and placed on a microscope slide. The
particles were photographed using a digital camera attached to a
light microscope (with a calibrated scale bar). Image analysis and
preprocessing were completed using ImageJ. Images were prepro-
cessed using the Sharpen and Find Edges functions to make the
main body of the particle more conspicuous. The area (as measured
by ImageJ), perimeter, longest axis (length), and the widest axis
(orthogonal to the longest axis, width) of each of 200 particles
was measured and the equivalent circular diameter calculated
q;Russ, 2007; Sprules et al., 1998). The ECD stan-
dardizes irregular objects to a standard circle to allow comparisons
of objects of variable shape and orientation (Russ, 2007). All mea-
surements were recorded from the areas of the particles that were
solid and opaque; the many light, short, diaphanous plastic threads
exuding from most sides were not included in measurements.
2.4. The effects of polystyrene ﬂoat type on colonization by a non-
native boring isopod
The effects of three different types of polystyrene ﬂoats on bur-
rowing by S. quoianum were tested using a lab experiment in a La-
tin square design. One block of polystyrene (5 612 cm) was
afﬁxed vertically per cylindrical microcosm (946 ml). The treat-
ments were: (a) expanded polystyrene (EPS; n= 23), (b) extruded
polystyrene (XPS; n= 22), or (c) expanded polystyrene encapsu-
lated with a damaged polyethylene cover (encapsulated EPS;
4 cm single tear at the bottom, mimicking wear and tear of encap-
sulation material exposed to boats and ﬂoating debris, n= 23). Thin
polyethylene encapsulation sheeting was often used to encapsu-
late ﬂoats in ﬂoating docks in Coos Bay and other Paciﬁc coast estu-
aries (per. obs.). While the use of encapsulation material over foam
is mandatory in Oregon (G. Dolphin, per. comm.; Oregon Adminis-
trative Rule 250-014-0030), many docks do not use it or the mate-
rial is degraded and torn (per. obs.). Each microcosm was ﬁlled
with saltwater (25 PSU) and phytoplankton, their primary food
source in nature, was periodically added for sustenance (Rotramel,
1975). Since S. quoianum is thigmotactic, a small divot was created
in the bottom of each block to prompt isopods to start burrowing.
One adult isopod between 7 and 12 mm in length was then added
to each microcosm and allowed to burrow for 24 days. The status
of the isopods (burrowing, not burrowing, moribund, or dead)
was recorded each day for 15 days. The status of the isopods on
days 19 and 24 (the last day of the experiment) were also noted.
A chi-square test (with 10,000 randomizations) was used to test
if the total number of isopods that burrowed in a block differed
among treatments. The differences in total burrow length and bur-
row use (the percent of the time isopods were present in burrows
they created) between EPS and encapsulated EPS treatments were
analyzed using Mann–Whitney tests since transformations failed
to normalize the data. Statistical analysis of the XPS treatment data
was unnecessary as burrowing was not observed.
3.1. Damage to the dock ﬂoats of aquaculture facilities and marinas by
Damage from dense colonies of boring isopods was observed in
aquaculture facilities in Yaquina Bay, Oregon, USA and Tainan,
Taiwan. In Yaquina Bay, Oregon, colonies of the non-native isopod
S. quoianum damaged the expanded polystyrene ﬂoats used by an
aquaculture facility to raise oysters. Repair of the docks required
removing around 60 heavily-riddled ﬂoats (each 1 m long;
Fig. 1A–C). Some ﬂoats still harbored dead isopods. The outer sur-
face of many of the ﬂoats had become eroded, vacuous, and easily
ablated by touching the surface. The attack was so concentrated in
some ﬂoats that it reduced the normally rectangular shaped ﬂoat
to a t-shaped cross-section (Fig. 1B).
Similar patterns were observed in Tainan, Taiwan; ﬁfteen ﬂoats
removed from an adjacent aquaculture facility were found onshore
and heavily riddled with isopod burrows (Fig. 1D–F). In Taiwan, the
native isopod S. terebrans was likely responsible for the damage
since this species was abundant in the mangroves lining the pond.
In addition to the above observations of isopods impacting
these aquaculture facilities, ﬁeld surveys revealed the presence of
burrowed foam ﬂotsam or ﬂoats in Yaquina Bay, several sites in
Taiwan, and one site in Caribbean Panama. Five out of six sites in
Yaquina Bay with foam ﬂotsam or exposed ﬂoats accessible for
examination were damaged by isopods. Burrowed ﬂoats were also
observed in Taiwanese sites including ﬂotsam in Kinmen Island
(presumably washed ashore from neighboring Xiamen, mainland
China), and two burrowed ﬂoats each in Budai Township and
Tainan. Two small lightly burrowed ﬂoats were also observed at
Galeta Point near Colon in Caribbean Panama. These ﬂoats were
likely burrowed by S. terebrans since these isopods were abundant
in the adjacent red mangroves.
Burrows and isopods were also found in high densities in ex-
panded polystyrene foam ﬂotsam and in ﬂoat mimics. The foam
collected from Coos Bay harbored thousands of burrows of S. quoia-
num per square meter (Table 1). Similarly, isopods were found in
high densities in the experimental ﬂoat mimics.
3.2. Quantity and morphology of the microplastic created by S.
The numbers of particles created per burrow were strongly re-
lated to the length of the burrow (R
= 0.89, F= 85.8, P< 0.001;
Fig. 2). A minimum of 89 particles were created from a burrow
1.6 mm long and a maximum of 4630 particles were created from
a burrow 17.4 mm long. The plastic particles created by S. quoia-
num were variable and irregular in shape (Fig. 3). Most of the plas-
tic particles were roughly globular or rectangular in shape and
lined with ﬁne strands; others were highly irregular. The mean
(±95% CI) maximum length of the particles was 462.6 ± 29.2
and mean maximum width (orthogonal to the maximum length)
was 283.0 ± 19.0
m(Fig. 4). The mean ECD was 255.1 ± 12.4
The mean perimeter-area ratio was 0.033 ± 0.002, which was
200% higher than a similar-sized circle (diameter = 255
perimeter-area ratio = 0.016). While the histograms of the mor-
phological characteristics were centered around the means de-
scribed above, they were skewed to the right due to a few high
values (Fig. 4).
3.3. The effects of polystyrene ﬂoat type on colonization by a non-
native boring isopod
The type of polystyrene ﬂoat affected the frequency of burrow-
ing, burrow length, and burrow use by S. quoianum. Isopods
burrowed more often in expanded polystyrene (10 of 23 ﬂoats
were burrowed, 43.5%) than damaged encapsulated expanded
polystyrene (7/23, 30.4%) and extruded polystyrene (0/22, 0%;
= 10.3, df = 2, P= 0.006). There was no difference in the fre-
quency of burrowing between EPS and damaged encapsulated
= 0.89, df = 1, P= 0.35). Isopods did not burrow into (hence
did not use burrows) in the extruded foam treatment. While the
T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828 1823
expanded polystyrene had greater values in all measures than the
encapsulated treatment, mean burrow length (±95% CI) was not
signiﬁcantly greater in the EPS (3.23 ± 1.49) than in the damaged
encapsulated EPS (2.21 ± 1.49; U= 311, P= 0.26). Likewise, isopods
did not use burrows signiﬁcantly more in the EPS (13.52 ± 8.09)
than the encapsulated EPS (9.14 ± 7.00; U= 318, P= 0.19).
4.1. Damage to foam ﬂoats in aquaculture facilities and marinas by
non-native and native boring isopods
The ﬂoats of docks and facilities in Asia, Australia, Central Amer-
ica, and North America suffered damage from burrowing sphaero-
matids (Fig. 5,Table 2). These damaging effects are exempliﬁed in
the two aquaculture facilities examined. Dense colonies of boring
isopods attacked the ﬂoats used in aquaculture facilities in Yaquina
Bay, Oregon, USA and Tainan, Taiwan, forcing the replacement of
ﬂoats and incurring economic costs. The burrow densities in these
ﬂoats, foam ﬂotsam, and ﬂoat mimics exceeded many thousands
per exposed square meter of foam. Floats inhabited by high densi-
ties of isopods were noticeably weaker and vacuous; the outermost
surface was easily removed by hand. Given such a weakened sur-
face, additional damage occurs to heavily burrowed ﬂoats when
they are scoured by water movement or abraded by debris (per.
obs.). Docks damaged by isopods have also been reported from
Coos Bay and San Francisco, California (Carlton, per comm.; Cohen
and Carlton, 1995; Davidson, 2008) with non-native populations of
S. quoianum. Previous surveys of Coos Bay revealed a ten-meter
section of a derelict dock riddled with burrows (Davidson, 2008)
and another dock in a state of disrepair with the exposed ﬂoats
burrowed by isopods (per. obs.). Likewise, a tugboat terminal in
Coos Bay was abandoned when severe burrowing by isopods ren-
dered it virtually inoperable (Carlton, per comm). While a previous
study reported polystyrene foam (Styrofoam) was rarely inhabited
compared to other substrata (Davidson, 2008), few docks accessi-
ble to surveying were available and thus may reﬂect low sampling
effort. Four out of ﬁve surveyed docks with exposed ﬂoats exhib-
ited burrowing damage consistent with S. quoianum (unpublished
data from Davidson, 2008). Furthermore, Cohen and Carlton
(1995) report the dock ﬂoats in marinas of San Francisco Bay were
frequently riddled by S. quoianum. This report is consistent with
accounts by Rotramel (1971), per comm who ﬁrst observed exten-
sive damage by S. quoianum in ﬂoating docks at Berkeley Marina
(San Francisco, CA) in 1966. Damage to ﬂoats under docks was also
noted in Moss Landing Harbor in 1998 (Elkhorn Slough, CA;
Wasson, per comm).
Fig. 1. Extensive burrowing by populations of boring isopods damaged the polystyrene ﬂoats in the docks used by aquaculture facilities in (A–C) Yaquina Bay, Oregon, USA
(Sphaeroma quoianum; 7/15/2007) and (D–F) Tainan, Taiwan (presumably Sphaeroma terebrans; 8/5/2010). The ﬂoats in A and D were approximately 1 m and 2 m in length,
respectively. Images in C and F are at differing scales, but the burrows pictured in these images are similar in size (8–10 mm).
Fig. 2. The relationship between the numbers of plastic particles created (square-
root transformed) per burrow and the mean length of burrows created by
Mean, maximum, and minimum of densities of burrows and isopods (Sphaeroma
quoianum) collected from expanded polystyrene ﬂoats (n= 18; burrow densities) and
ﬂoat mimics (n= 6; isopod densities) in Coos Bay, Oregon.
Density Mean (±95% CI) Max. Min.
Burrows per m
7875 (±1687) 25,000 2400
Burrows per ﬂoat
23,413 (±5016) 74,322 7134
14,900 (±7576) 32,000 2400
Isopods per ﬂoat 44,296 (±22,523) 95,133 7135
Calculations were based on a ﬂoat with the following dimensions:
244 122 46 cm; surface area 3m
assuming the outer 6 cm was vulnerable to
Isopod densities are based on the colonization of the outer 6 cm of expanded
polystyrene ﬂoat mimics (n= 6) deployed for 1 year.
1824 T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828
Furthermore, the presence of large pieces of foam ﬂoats found
throughout Coos Bay suggests rafting may be an important dis-
persal mechanism for the non-native S. quoianum and likely other
sphaeromatids. Since isopod boring may facilitate the breakage of
ﬂoats, large ﬂoating colonies may potentially be dispersed to new
areas within a bay or possibly between bays. The movement of
large colonies of hundreds or thousands of direct developing iso-
pods may enhance invasion success in new locations (Johannesson,
1988; Thiel and Gutow, 2005).
The spread of S. quoianum to new estuaries may result in
damage to ﬂoats under docks and facilities but may also have
destructive effects to other estuarine habitats and substrata. By
perforating saltmarsh banks with burrows, populations of S.
quoianum appear to exacerbate erosion rates of saltmarshes
(Carlton, 1979; Davidson and de Rivera, 2010; Talley et al.,
2001); areas in saltmarsh banks inhabited by S. quoianum experi-
ence erosion rates 300% higher than adjacent unburrowed areas
(Davidson and de Rivera, 2010). Burrowing by isopods also alters
and damages other estuarine substrata (e.g. friable rocks, wood),
and provides a novel habitat for other organisms including dispro-
portionately large numbers of non-native species compared to
other habitats (Davidson et al., 2010).
4.2. Microplastic pollution created by a non-native boring isopod
Boring by colonies of sphaeromatid isopods in expanded poly-
styrene ﬂoats can create millions of microplastic particles and
may have negative effects to marine organisms. An individual of
S. quoianum can create up to 4630 plastic particles when excavat-
ing a burrow 17.4 mm long. Extrapolating that estimate to a popu-
lation of 100,000 (a density observed in a cubic meter of substrata,
Davidson et al., 2010; or two ﬂoats, Table 1), the total number of
particles created by 100,000 isopods each creating a burrow is
416.7 million. However, the mean burrow length (±95% CI) created
by S. quoianum in the lab (22.6 ± 2.2 mm in the lab at 14 °C) and
from ﬁeld measurements (25.3 ± 17.5 mm; Davidson and de
Rivera, 2012) are longer than the burrow lengths observed in this
experiment. When estimating the number of plastic particles
created using these mean values and the equation presented in
Fig. 2 (and back transforming), one adult of S. quoianum would
create between 4900 (±1.1) and 6300 (±2801) particles during
the boring process (490–630 million per 100,000 isopods). While
there is variation in the speciﬁc number of particles created in
the boring process, these estimates reveal the extremely high mag-
nitude of microplastic that is created through the boring process by
this non-native isopod and likely other boring isopods.
4.3. Potential implications of microplastic pollution
Microplastics, similar in size to those created by this bioeroder,
persist in the marine environment (Barnes et al., 2009) and may be
consumed or colonized by numerous species (Gregory, 2009; Cole
Fig. 3. Microscope images of the plastic particles created by Sphaeroma quoianum
during the burrowing process into expanded polystyrene ﬂoats. The images are
shown at two magniﬁcations: (A) Each square in the image is 0.25 cm
. (B) The scale
bar in this image is 500
Fig. 4. Frequency histograms of the (A) area, (B) perimeter, (C) maximum length,
(D) maximum width (orthogonal to the length measurement), (E) equivalent
circular diameter (ECD) and (F) perimeter-area ratio of the microplastic particles
created during burrowing by Sphaeroma quoianum in expanded polystyrene ﬂoats
T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828 1825
et al., 2011). These particles were similar in size to numerous spe-
cies of zooplankton and some phytoplankton (Hansen et al., 1994;
Sprules et al., 1998) and thus may be confused for planktonic food.
Microplastics are ingested by species in a variety of trophic levels,
habitats, and feeding modes. They have been ingested by
detritivorous amphipods (Thompson et al., 2004); deposit feeding
echinoderms (Graham and Thompson, 2009) and polychaetes
(Thompson et al., 2004); ﬁlter feeding mussels (Browne et al.,
2008), crustaceans (Thompson et al., 2004), and echinoderms
(Graham and Thompson, 2009); omnivorous lobsters (Murray
and Cowie, 2011); and small planktivorous ﬁsh (Boerger et al.,
2010; Davison and Asch, 2011). Larger predators such as birds
(Laist, 1997), turtles (Laist, 1997), numerous species of ﬁsh
(Carpenter et al., 1972; Kartar et al., 1976; Laist, 1997) and mam-
mals (Ericsson and Burton, 2003) were also found with microplas-
tics inside their guts. Since isopods damage the ﬂoats used in
aquaculture facilities, the microplastic pollution created may even
become ingested by the cultured species (for example, oysters) and
thus may be transferred to humans.
There are three primary effects of microplastics to marine life
including facilitating the spread of non-native or toxic species
and both physical and toxicological effects when ingested.
Microplastics may facilitate the spread of non-native species
(Barnes, 2002; Gregory, 2009) by providing a surface to which
Fig. 5. Global occurrences of boring isopod damage to expanded polystyrene ﬂoats. The open circles in North America and Central America represent areas with known
damage from non-native sphaeromatid isopods. The closed circles in Asia and Australia denote areas damaged by native populations. See Table 2 for details.
Locations where boring sphaeromatid isopods have attacked expanded polystyrene ﬂoats.
Location Date Species Invasion status Substratum Reference
Yaquina Bay, Oregon 2007 Sphaeroma quoianum Non-native Floats, ﬂotsam This paper
Coos Bay, Oregon 1995 Sphaeroma quoianum Non-native Floats, ﬂotsam Cohen and Carlton (1995)
San Francisco Bay, California 1966 Sphaeroma quoianum Non-native Floats, ﬂotsam Rotramel (1975); per comm.
Elkhorn Slough, California 1998 Sphaeroma quoianum Non-native Floats, ﬂotsam Wasson, per comm.
Throughout southwest Florida 1978 Sphaeroma terebrans Non-native Floats, ﬂotsam Estevez (1978), per comm
Lake Pontchartrain, Louisiana 2004 Sphaeroma terebrans Non-native Flotsam Wilkinson (2004)
Colon, Panama 2012 Sphaeroma terebrans Non-native Flotsam This paper
Kinmen Island, Taiwan 2010 Sphaeroma terebrans Native Flotsam This paper
Budai township, Taiwan 2010 Sphaeroma terebrans Native Flotsam This paper
Tainan, Taiwan 2010 Sphaeroma terebrans Native Floats, ﬂotsam This paper
Tamar river, Tasmania 2006 Sphaeroma quoianum Native Flotsam Davidson et al. (2008)
Port Stephens, Australia 1986 Sphaeroma quoianum,
Native Floats Cookson et al. (1986)
1973 Sphaeroma triste Native Floats Harrison and Holdich (1984)
1986 Sphaeroma terebrans,
Native Floats Angell (1986)
Harrison and Holdich (1984) noted Sphaeroma triste in polystyrene blocks (in 1973) afﬁxed beside a dock in Townsville, Australia but is unclear if the dock used these
blocks as ﬂoats.
Angell (1986) reports the necessity of protecting expanded polystyrene ﬂoats used in oyster culture facilities in the Philippines from borers; while not explicitly stated,
these borers are likely S. terebrans or S. triste.
1826 T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828
those organisms can attach and subsequently ﬂoating to a new
area. Numerous non-native taxa have been found on plastics
including sponges, hydroids, bryozoans, mollusks, isopods, barna-
cles, polychaetes (Barnes, 2002; Gregory, 2009) and toxic microal-
gae (Masó et al., 2003). Even microplastics may be a viable vector;
plastics similar in size to the current study have been found trans-
porting non-native bryozoans (Barnes, 2002; Gregory, 2009). The
high surface area of these microplastics and high abundances cre-
ated through burrowing may provide additional opportunities for
small non-native taxa to colonize and disperse to new areas.
When ingested, microplastics can also accumulate in some
organisms (Browne et al., 2008; Murray and Cowie, 2011), which
may possibly lead to physical effects. The accumulation of plastics
may lead to intestinal obstructions (Carpenter et al., 1972) and
stomach ulcers (as with birds, Pettit et al., 1981). It also may cause
false indication of satiation, hence reduced growth and perhaps ﬁt-
ness (Connors and Smith, 1982; Ryan, 1988). Researchers have
found negative correlations between plastic load and body mass
(Ryan, 1987; Spear et al., 1995) and possibly the amount of fatty
deposits in seabirds (Connors and Smith, 1982). However, it is un-
clear if microplastics can also cause similar negative physical ef-
fects to biota; this question remains an important gap that needs
to be addressed (Cole et al., 2011). Browne et al. (2008) did not ﬁnd
a signiﬁcant short-term biological effect of the ingestion of micro-
plastic in the mussel Mytilus edulis; however, they caution addi-
tional longer-term studies with an array of different polymers
and organisms are necessary.
Microplastics are chemically inert (Andrady, 2011; Teuten et al.,
2009), yet may become toxic due to degradation or the accumula-
tion of toxins from the ambient environment. When the plastics
degrade, they release toxic additives including phthalates, organo-
tin, and nonylphenol (Mato et al., 2001; Teuten et al., 2009; Zitko,
1993). Other toxins, such as persistent organic pollutants, have a
higher afﬁnity for plastics than ambient seawater and accumulate
in very high concentrations (Mato et al., 2001; Teuten et al., 2007)
and may be absorbed into marine fauna (Ryan et al., 1988; Teuten
et al., 2007; Thompson, unpublished, as cited in Teuten et al.,
These persistent organic pollutants including polychlorinated
biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAH’s), chlo-
rinated phenols, organochlorine pesticides (DDT and DDE) and
Bisphenol-A (BPA) are of particular concern since they are endo-
crine disruptors and carcinogenic (Walker et al., 2006). Further-
more, heavy metals (Cadmium and Lead) can also accumulate in
microplastics (Ashton et al., 2010). Since the microplastics created
by isopods are small, irregular, and have a high perimeter to area
ratio (and likely a high surface area to volume ratio), it seems likely
they would accumulate toxins more rapidly than larger spherical
plastic particles and pellets.
4.4. Polystyrene ﬂoat type prevents colonization by a non-native
Isopods did not burrow into the XPS foam treatments during the
lab experiment, which suggests this foam type may prevent isopod
colonization and burrowing in the ﬁeld. These lab results are con-
sistent with observations from the ﬁeld. The XPS ﬂoats or ﬂotsam
encountered during surveys were never burrowed by isopods
(per. obs.). The XPS foam is noticeably harder than EPS foam and
it is likely this substratum is too hard for boring. In contrast, EPS
foam, such as the type used in many ﬂoating docks, was burrowed
more frequently and burrowed deeper than the other treatments.
The damaged encapsulated EPS ﬂoat mimics also exhibited lower
colonization rates, burrow use, and shorter burrows than the EPS
ﬂoats, although the results were not statistically different. While
these results suggest a thin encapsulation material may inhibit
boring to some degree, I recommend using a hardened polyethyl-
ene shell around an XPS foam core to prevent damage from borers
and debris and degradation from the ambient seawater. This labo-
ratory experiment, combined with ﬁeld and lab observations, sug-
gests that XPS is resistant to isopod damage and thus may be a
viable option to reduce the impacts of burrowing by S. quoianum
and other boring isopods.
The destruction of expanded polystyrene ﬂoats used in ﬂoating
docks and aquaculture facilities by boring isopods can be exten-
sive. Burrowing by dense colonies of isopods degrades ﬂoats,
reducing their longevity and function. Burrowing also releases mil-
lions of microplastic particles into the marine environment. These
particles are similar in morphology to other microplastic particles
(Carpenter et al., 1972; Gregory, 1996) and may have detrimental
effects to marine organisms. These negative effects, however,
may be prevented or mitigated by using extruded polystyrene
ﬂoats and/or a thick rigid encapsulation material under docks
I thank James Carlton, Andy Chang, Simon Cragg, Catherine de
Rivera, Glenn Dolphin, Ernie Estevez, George Rotramel, Gregory
Ruiz, Kerstin Wasson, and Elizabeth Wells for providing technical
advice, information on the distribution of borers, and helpful con-
versations. The comments from the reviewers helped improve a
previous version of this manuscript. I am grateful to Hwey Lian
Hsieh for providing lab space and transportation assistance while
in Taiwan. Aspects of this research were supported by the Western
Regional Panel on Aquatic Nuisance Species (under United States
Fish & Wildlife Service Grant Agreement number 60181-7G256
to C.E. de Rivera), the Estuarine Reserves Division, Ofﬁce of Ocean
and Coastal Resource Management, National Ocean Service,
National Oceanic and Atmospheric Administration (fellowship to
T.M. Davidson), and the National Science Foundation East Asia
and South Paciﬁc Summer Institute fellowship in Taiwan (fellow-
ship to T.M. Davidson).
Andrady, A., 2011. Microplastics in the marine environment. Mar. Pollut. Bull. 62,
Angell, C.L., 1986. The Biology and Culture of Tropical Oysters. ICLARM Studies and
Reviews 13. International center for living aquatic resources management,
Arthur, C., Baker, J., Bamford, H., 2009. In: Proceedings of the International Research
Workshop on the occurrence, effects and fate of micro-plastic marine debris,
September 9–11, 2008. NOAA Technical, Memorandum NOS-OR&R-30.
Ashton, K., Holmes, L., Turner, A., 2010. Association of metals with plastic
production pellets in the marine environment. Mar. Pollut. Bull. 60, 2050–2055.
Barnes, D.K.A., 2002. Invasions by marine life on plastic debris. Nature 416, 808–
Barnes, D.K.A., Galgani, F., Thompson, R.C., Barlaz, M., 2009. Accumulation and
fragmentation of plastic debris in global environments. Philos. Trans. R. Soc. B.
Barrows, A. L., 1919. The Occurrence of a Rock-boring Isopod Along the Shore of San
Francisco Bay, California, vol. 19, University of California Publications in
Zoology, pp. 299–316.
Boerger, C.M., Lattin, G.L., Moore, S.L., Moore, C.J., 2010. Plastic ingestion by
planktivorous ﬁshes in the North Paciﬁc Central Gyre. Mar. Pollut. Bull. 60,
Browne, M.A., Dissanayake, A., Galloway, T.S., Lowe, D.M., Thompson, R.C., 2008.
Ingested microscopic plastic translocates to the circulatory system of the
mussel Mytilus edulis (L.). Environ. Sci. Technol. 42, 5026–5031.
Carlton, J.T., 1979. History, biogeography, and ecology of the introduced marine and
estuarine invertebrates of the Paciﬁc coast of North America, PhD Dissertation,
University of California, Davis.
Carlton, J.T., Iverson, E.W., 1981. Biogeography and natural history of Sphaeroma
walkerii Stebbing (Crustacea, Isopoda) and its introduction into San Diego Bay.
Calif. J. Nat. Hist. 15, 31–48.
T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828 1827
Carlton, J.T., Ruckelshaus, M.H., 1997. Nonindigenous marine invertebrates and
algae. In: Simberloff, D., Schmitz, D.C., Brown, T.C. (Eds.), Strangers in Paradise,
Impact and Management of Nonindigenous Species in Florida. Island Press,
Washington, DC, pp. 187–202.
Carlton, J.T., Ruiz, G.M., 2005. The magnitude and consequences of bioinvasions in
marine ecosystems, implications for conservation biology. In: Norse, E.A.,
Crowder, L.B. (Eds.), Marine Conservation Biology, The Science of Maintaining
the Sea’s Biodiversity. Island Press, Washington, pp. 123–148.
Carpenter, E.J., Anderson, S.J., Harvey, G.R., Miklas, H.P., Peck, B.B., 1972. Polystyrene
spherules in coastal water. Science 178, 749–750.
Chilton, C., 1919. Destructive boring Crustacea in New Zealand. N Z J. Sci. Technol. 2,
Cohen, A.N., Carlton, J.T., 1995. Nonindigenous Aquatic Species in a United States
Estuary, a Case Study of the Biological Invasion of San Francisco Bay and Delta.
Biological study. University of California, Berkeley (Final report No. NOAA-
NA36RG0467, FWS -14-48-0009-93-9 61).
Cole, M., Lindeque, P., Halsband, C., Galloway, T.S., Smith, K.G., 2011. Microplastics
as contaminants in the marine environment: a review. Mar. Pollut. Bull. 62,
Connors, P.G., Smith, K.G., 1982. Oceanic plastic particle pollution, suspected effect
on fat deposition in red phalaropes. Mar. Pollut. Bull. 13, 18–20.
Cookson L.J., 1986. Marine borers and timber piling options. CSIRO, CSIRO
Research Review. CSIRO Printing Centre, Melbourne, Div. Chem. Wood
Cragg, S.M., Pitman, A.J., Henderson, S.M., 1999. Developments in the understanding
of the biology of marine wood boring crustaceans and in methods of controlling
them. Int. Biodeterior. Biodegrad. 43, 197–205.
Davidson, T.M., 2008. Prevalence and distribution of the introduced burrowing
isopod, Sphaeroma quoianum in the intertidal zone of a temperate northeast
Paciﬁc estuary (Isopoda, Flabellifera). Crustaceana 81, 155–167.
Davidson, T.M., de Rivera, C.E., 2010. Accelerated erosion of saltmarshes infested by
the non-native burrowing crustacean Sphaeroma quoianum. Mar. Ecol. Prog. Ser.
Davidson, T.M., de Rivera, C.E., 2012. Substratum composition affects per capita
burrowing impacts of a non-native isopod (Sphaeroma quoianum). J. Crust. Biol.
Davidson, T.M., de Rivera, C.E., Carlton, J.T., in preparation. Seawater temperature
mediates biological erosion by a non-native burrowing crustacean.
Davidson, T.M., Hewitt, C.L., Campbell, M., 2008. Distribution, density, and habitat
use among native and introduced populations of the Australasian burrowing
isopod Sphaeroma quoianum. Biol. Invasions 10, 399–410.
Davidson, T.M., Rumrill, S.S., Shanks, A.L., 2010. The composition and density of
fauna utilizing burrow microhabitats created by a non-native burrowing
crustacean (Sphaeroma quoianum). Biol. Invasions 12, 1403–1413.
Davison, P., Asch, R.G., 2011. Plastic ingestion by mesopelagic ﬁshes in the North
Paciﬁc Subtropical Gyre. Mar. Ecol. Prog. Ser. 432, 173–180.
Ericsson, C., Burton, H., 2003. Origins and biological accumulation of small plastic
particles in fur seals from Macquarie Island. AMBIO: J. Human Environ. 32, 380–
Estevez, E.D., 1978. Ecology of Sphaeroma terebrans Bate, a wood boring isopod, in a
Florida mangrove forest, PhD Dissertation, University of South Florida.
Estevez, E.D., 1994. Inhabitation of tidal salt marshes by the estuarine wood-boring
isopod Sphaeroma terebrans in Florida. In: Thompson, M.F., Nagabhushanam, R.,
Sarojini, R., Fingerman, M. (Eds.), Recent Developments in Biofouling Control.
Oxford & IBH Publishing Co., New Delhi, pp. 97–105.
Graham, E.R., Thompson, J.T., 2009. Deposit- and suspension-feeding sea cucumbers
(Echinodermata) ingest plastic fragments. J. Exp. Mar. Biol. Ecol. 368, 22–29.
Gregory, M.R., 1996. Plastic ‘scrubbers’ in hand cleansers, a further (and minor)
source for marine pollution identiﬁed. Mar. Pollut. Bull. 32, 867–871.
Gregory, M.R., 2009. Environmental implications of plastic debris in marine
settings- entanglement, ingestion, smothering, hangers-on, hitch-hiking and
alien invasions. Philos. Trans. R. Soc. B 364, 2013–2025.
Hansen, B., Bjornsen, P.K., Hansen, P.J., 1994. The size ratio between planktonic
predators and their prey. Limnol. Oceanogr. 39, 395–403.
Harrison, K., Holdich, D.M., 1984. Hemibranchiate sphaeromatids (Crustracea,
Isopoda) from Queensland, Australia, with a world-wide review of the genera
discussed. Zool. J. Linn. Soc. 81, 275–387.
Irwin, M., 1953. Science looks into it, Steel boring sea urchins. Paciﬁc Discov 6, 26–
Jansen, K.P., 1971. Ecological studies on intertidal New Zealand Sphaeromatidae
(Isopoda: Flabellifera). Mar. Biol. 11, 262–285.
Johannesson, K., 1988. The paradox of Rockall, why is a brooding gastropod
(Littorina Saxatilis) more widespread than one having a planktonic larval
dispersal stage (L. Littorea)? Mar. Biol. 99, 507–513.
Kartar, S., Abou-Seedo, F., Sainsbury, M., 1976. Polystyrene spherules in the Severn
Estuary – a progress report. Mar. Pollut. Bull. 7, 52.
Kofoid, C.A., Miller, R.C., 1927. Occurrence of rock boring molluscs in concrete. In:
Hill, C.L., Kofoid, C.A. (Eds.), Marine Borers and Their Relation to Marine
Construction on the Paciﬁc Coast. Final Report of the San Francisco Bay Marine
Piling Committee, San Francisco, pp. 301–305.
Laist, D.W., 1997. Impacts of marine debris: entanglement of marine life in debris
including a comprehensive list of species with entanglement and ingestion
records. In: Coe, J.M., Rogers, D.B. (Eds.), Marine Debris. Springer, Berlin.
Masó, M., Garcés, E., Pagès, F., Camp, J., 2003. Drifting plastic debris as a potential
vector for dispersing Harmful Algal Bloom (HAB) species. Sci. Mar. 67, 107–111.
Mato, Y., Isobe, T., Takada, H., Kanehiro, H., Ohtake, C., Kaminuma, T., 2001. Plastic
resin pellets as a transport medium for toxic chemicals in the marine
environment. Environ. Sci. Technol. 35, 308–324.
Messana, G., Bartolucci, V., Mwaluma, J., Osore, M., 1994. Preliminary observations
on parental care in Sphaeroma terebrans Bate 1866 (Isopoda Sphaeromatidae), a
mangrove wood borer from Kenya. Ethol. Ecol. Evol. 3, 125–129.
Miller, R.C., 1926. Ecological relations of marine wood-boring organisms in San
Francisco Bay. Ecology 7, 247–254.
Murray, F., Cowie, P.R., 2011. Plastic contamination in the decapod crustacean
Nephrops norvegicus (Linnaeus, 1758). Mar. Pollut. Bull. 62, 1207–1217.
Neily, R.M., 1927. Historical development of marine structures in San Francisco Bay.
In: Hill, C.L., Kofoid, C.A. (Eds.), Marine Borers and Their Relation to Marine
Construction on the Paciﬁc Coast. Final Report of the San Francisco Bay Marine
Piling Committee, San Francisco, pp. 13–32.
Perry, D., Brusca, R.C., 1989. Effects of the root-boring isopod Sphaeroma peruvianum
on red mangrove forests. Mar. Ecol. Prog. Ser. 57, 287–292.
Pettit, T.N., Grant, G.S., Whittow, G.C., 1981. Ingestions of plastics by Laysan
ablatross. AUK 98, 839–840.
Riegel, J., 1959. Some aspects of osmoregulation in two species of sphaeromid
isopod crustacea. Biol. Bull. 116, 272–284.
Rotramel, G.L., 1971. Symbiotic relationships of Sphaeroma quoyanum and Iais
californica (Crustcea, Isopoda), PhD Dissertation, University of California,
Rotramel, G.L., 1975. Filter-feeding by the marine boring isopod, Sphaeroma
quoyanum H. Milne Edwards, 1840 (Isopoda, Sphaeromatidae). Crustaceana
Russ, J.C., 2007. The Image Processing Handbook. CRC Press, Boca Raton, FL.
Ryan, P.G., 1987. The effects of ingested plastic on seabirds, correlations between
plastic load and body condition. Environ. Pollut. 46, 119–125.
Ryan, P.G., 1988. Effects of ingested plastic on seabird feeding, evidence from
chickens. Mar. Pollut. Bull. 19, 125–128.
Ryan, P.G., Connell, A.D., Gardener, B.D., 1988. Plastic ingestion and PCBs in seabirds,
Is there a relationship? Mar. Pollut. Bull. 19, 174–176.
Schneider, M.R., 1976. Population dynamics of the symbiotic marine isopods,
Sphaeroma quoyana and Iais californica. MS Thesis, San Francisco State
Si, A., Bellwood, O., Alexander, C.G., 2002. Evidence for ﬁlter-feeding by the wood-
boring isopod, Sphaeroma terebrans (Crustacea, Peracarida). J. Zool. Lond. 256,
Spear, L.B., Ainley, D.G., Ribic, C.A., 1995. Incidence of plastic in seabirds from the
tropical Paciﬁc, 1984–91, relation with distribution of species, sex, age, season,
year and body weight. Mar. Environ Res. 40, 123–146.
Sprules, W.G., Jin, E.H., Herman, A.W., Stockwell, J.D., 1998. Calibration of an optical
plankton counter for use in fresh water. Limnol. Oceanogr. 43, 726–733.
Talley, T.S., Crooks, J.A., Levin, L.A., 2001. Habitat utilization and alteration by the
invasive burrowing isopod, Sphaeroma quoyanum, in California salt marshes.
Mar. Biol. 138, 561–573.
Teuten, E.L., Rowland, S.J., Galloway, T.S., Thompson, R.C., 2007. Potential for plastics
to transport hydrophobic contaminants. Environ. Sci. Technol. 41, 7759–7764.
Teuten, E.L., Saquing, J.M., Knappe, D.R.U., et al., 2009. Transport and release of
chemicals from plastics to the environment and to wildlife. Philos. Trans. R. Soc.
B 364, 2027–2045.
Thiel, M., 1999. Reproductive biology of a wood-boring isopod, Sphaeroma terebrans,
with extended parental care. Mar. Biol. 135, 321–333.
Thiel, M., Gutow, L., 2005. The ecology of rafting in the marine environment — II. the
rafting organisms. Oceanogr. Mar. Biol. Annu. Rev. 43, 281–420.
Thompson, R.C., Olsen, Y., Mitchell, R.P., Davis, A., Rowland, S.J., John, A.W.G.,
McGonigle, D., Russell, A., 2004. Lost at sea, where is all the plastic? Science 304,
Walker, C.H., Sibly, R.M., Hopkin, S.P., Peakall, D.B., 2006. Principles of
Ecotoxicology, third ed. CRC Press, Boca Raton, FL.
Wilkinson, L.L., 2004. The biology of Sphaeroma terebrans in Lake Ponchartrain,
Louisiana with emphasis on burrowing, MS thesis, University of New Orleans.
Ye, S., Andrady, A.L., 1991. Fouling of ﬂoating plastic debris under Biscayne Bay
exposure conditions. Mar. Pollut. Bull. 22, 608–613.
Zitko, V., 1993. Expanded polystyrene as a source of contaminants. Mar. Pollut. Bull.
1828 T.M. Davidson / Marine Pollution Bulletin 64 (2012) 1821–1828