Tooth use and wear in three iron-biomineralizing mollusc species.
Chitons (Polyplacophora) and patellid limpets (Gastropoda) have been studied extensively due to the significant amounts of iron biominerals incorporated into their teeth. These minerals are used to reinforce the tooth cusps, increasing their resilience when feeding on hard substrates such as rock (Lowenstam, 1962; Weiner and Addadi, 2002; Shaw et al., 2009a). The main focus of previous research on these animals has been to elucidate the composition, structure, and development of the teeth (Runham et al., 1969; Lowenstam and Weiner, 1989; Macey et al., 1996; Saunders et al., 2009; Shaw et al., 2009b), while far less attention has been paid to how these factors relate to tooth function.
Chiton and limpet teeth form part of the radula, a highly adaptive feeding organ common to most molluscs, consisting of an elongated flexible but inelastic membrane, which is manipulated during the feeding process by various muscles and cartilages (Guralnick and Smith, 1999). Each tooth consists of a base (stylus) and a cusp, the latter of which may possess a number of denticles (Runham and Thornton, 1967; Brooker and Macey, 2001). Chitons have 17 teeth in each transverse row, with a single pair being iron-mineralized. Limpets typically have fewer teeth than chitons but have more mineralized tooth pairs. The tooth bases commonly interlock with those of adjacent tooth rows, acting to distribute and transfer stress from the cutting edge of the cusps (Solem, 1972; Hickman, 1980). Although the basic design of chiton and limpet radulae is similar, the radular anatomy of chitons provides a far greater degree of lateral flexibility compared to limpets.
Tooth formation is achieved by first constructing an organic framework, or matrix, which acts as a structural blueprint for the overall shape of the tooth and its internal micro-architecture. In iron-mineralizing species, the elements needed for mineralization are delivered to the teeth via an overlying epithelial tissue, which moves forward at the same rate as the teeth as they progress from the unmineralized to the mineralized state (Isarankura and Runham, 1968; Shaw et al., 2008, 2009b). Once fully mature, the teeth move into position within the mouth where they can then be used for feeding. Tooth formation, maturation, and subsequent degradation occur continuously, in a process that resembles a production line, with old teeth being constantly replaced with newly formed ones. The sequential nature of this mineralization process makes chiton and limpet radulae highly useful as models for the study of matrix-mediated biomineralization, as each phase of mineral deposition can be observed in a single specimen.
The teeth of chitons and limpets are among the most complex wear-resistant structures to have evolved within the animal kingdom. This is mainly owing to the composite organic and mineral architecture of the teeth, the structural hierarchy of which spans scales from the millimeter to the nanometer. Biomineral formation is a common strategy used by invertebrates for hardening body parts. However, certain invertebrates are known to use alternative mechanisms, such as incorporation of zinc or copper ions in the mandibles of polychaetes and termites (Lichtenegger et al., 2003; Cribb et al., 2008) or the sclerotization of insect cuticles (Broomell et al., 2007). In all cases, the resulting structures possess exceptional resistance to mechanical wear by modulating stiffness and hardness properties. Synthetic analogs based on the design principles of such biologically inspired structures have a great deal of applied potential. Additionally, observations of how these structures are used and subsequently degraded can provide information about the diet, habitat, and life history of both extant and extinct animals.
The size, structural organization, and mineral composition of chiton and limpet teeth contribute significantly toward maximizing the working life of each tooth cusp and in maintaining optimal cutting characteristics (Runham and Thornton, 1967; Runham et al., 1969; van der Wal et al., 2000; Wealthall et al., 2005). For this reason, the structural properties of the teeth have been investigated for the prospective design of new cutting tools and other biomimetic materials applications (van der Wal et al., 2000; Mann, 2001). The excavating ability of chiton teeth has also been investigated in relation to coastal and reef erosion processes, where they have been found to have substantial bioerosive potential (Rasmussen and Frankenberg, 1990; Barbosa et al., 2008). Accordingly, studies of the teeth of iron-mineralizing molluscan species are important from both materials engineering and ecological perspectives.
To date, observations of tooth wear in iron-mineralizing molluscan species have been made only on dissected animals (Runham and Thornton, 1967; van der Wal et al., 2000), and are thus without the benefit of comparative data from actual tooth-substrate interactions in the living specimen. The latter information is critical for identifying the relationships between matrix structure and the mineral architecture of the teeth. Accordingly, a detailed light and scanning electron microscope study of radular tooth wear, and the grazing stroke patterns produced by teeth on wax during the feeding process, has been undertaken for three iron-mineralizing molluscan species. The chitons Acanthopleura hirtosa (Blainville, 1825) and Plaxiphora albida (Blainville, 1825), and the limpet Patelloida alticostata (Angas, 1865) were examined, each being key inhabitants within the mid-to upper-intertidal region along the coast of Western Australia (Wells and Bryce, 1986). These species have been studied extensively in relation to the processes of tooth biomineralization (see, for example, Macey et al., 1996; Liddiard et al., 2004; Shaw et al., 2009a, b; Saunders et al., 2009). The progressive wear of the tooth cusps and the associated grazing stroke patterns are discussed in the context of the functional morphology of the radula and the mineral architecture of the teeth.
Materials and Methods
Adult specimens (determined by size) of Acanthopleura hirtosa, Plaxiphora albida, and Patelloida alticostata were collected from Woodman Point, Perth, on Australia's southwest coast (Lat. 32[degrees]S, Long. 116[degrees]E).
Tooth size measurements
For each species, radulae from six individuals were excised, and maximum width and height were measured for each mineralized tooth pair (n = 12 teeth for each species). For the chitons A. hirtosa and P. albida, these measurements were made from the discoid-shaped unicuspid major lateral teeth and the tricuspid major lateral teeth respectively (Figs. 1 and 2). For Pa. alticostata, measurements were taken from the narrow and curved medial teeth and the broad square lateral teeth, which possess a secondary denticle (spur) on the lateral margin. To ensure the cusps were unworn, teeth were obtained posterior to the radular hyaline shield, a region where the epithelial tissue remains adhered to the teeth (Fig. 1). Double-sided tape was used to mount the teeth on a flat glass rod in a manner that presented the full posterior surface of each tooth. Tooth measurements were made from photographs taken with a digital camera (Olympus, DP10) mounted on a dissecting microscope (Olympus, SZH10).
[FIGURE 1 OMITTED]
The degree of tooth wear was evaluated for a further set of animals by scoring the teeth according to a key developed from detailed microscope (Olympus, SZH10) observations of radulae from each species (Table 1). For A. hirtosa and P. albida this scoring system was applied to both left and right major lateral tooth rows so as to determine whether the tooth pairs wear asymmetrically. Data from left and right tooth rows were compared using ANOVA. For Pa. alticostata, both the medial and lateral teeth were treated as a single unit owing to the limited lateral flexure in limpet radulae. To determine the number of functioning tooth rows, the number of transverse rows protruding from the superior epithelial tissue to the radula tip was determined for each species.
Table 1 Definitions of tooth wear in three species of iron-mineralizing mollusc Wear type Species * Light Moderate Heavy Detached Acanthopleura Tips of The core Teeth are Any tooth missing hirtosa one or of the stublike, that is in line with, more tooth is with the or posterior to, the cusps partially core of anteriormost major show exposed the tooth lateral cusp or slight due to fully cusps signs some exposed of removal of due to wear the complete anterior removal of tooth the surface, anterior and the tooth lab is surface; partially there is worn no evidence of the tab Plaxiphora As The three Teeth are As above albida above tips of stublike, each cusp with are extensive rounded, removal of but wear the does not anterior extend tooth between surface, the cusps; and the most of entire the cusp edge anterior is worn tooth surface is present Patelloida As The Wear As above alticostata above anterior extends surfaces between of the the tooth lateral cusps are tooth and partially the spur; worn; medial lateral teeth are cusps are worn to rounded stubs but wear does not extend between the cusp and spur * n = 10 animals from each species.
The anterior portion of radulae (severed posterior to the hyaline shield) from each species (n = 2) were examined by scanning electron microscopy (SEM). In each case, radulae were dehydrated through a graded series of ethanol followed by propylene oxide, prior to critical-point-drying. Dried samples were then mounted, using carbon tape, onto aluminium stubs and photographed (Olympus, DP10) before being coated with 30 nm of Au. Radulae were imaged with SEM at 10 kV (Philips, XL20).
Wax substrates have been commonly used to study molluscan feeding tracks that are made by the teeth during the rasping stroke of the feeding process (see, for example, Hickman and Morris, 1985; Thompson et al., 1997).The feeding mechanics of A. hirtosa, P. albida, and Pa. alticostata were indirectly observed by studying the characteristic imprints left on wax feeding platforms. Platforms consisted of petri dish lids that were coated with molten paraffin wax, which was then solidified. These wax-coated disks were suspended within an aerated saltwater aquarium that was illuminated by a fluorescent Glo-tube to promote algal growth on the disks. After 2 weeks, two specimens of each species were collected from the wild and placed on their respective disk in the aquarium. Following this, the wax was observed on a regular basis until it was sufficiently covered with grazing marks [approximately equal to]14 days). Animals were then removed, and the disks were carefully washed free of any algal material. The grazing marks left in the wax were observed uncoated using variable pressure mode with field emission SEM (Zeiss, VP-1555) with the following operational parameters: 20 kV, WD 10 mm, 120-[micro]m aperture, high current mode, 30-Pa chamber pressure (air), and a 380-V bias upon the variable pressure secondary electron detector.
Tooth size measurements
Despite the differences in tooth morphology, major lateral cusps from the chitons Acanthopleura hirtosa and Plaxiphora albida were similar in size, while both the medial and lateral teeth from the limpet Patelloida alticostata were considerably smaller (Table 2). For both chitons, the teeth are skewed toward the medial plane (Figs. 1 and 2).
Table 2 Summary of measurements taken from the anterior working region of the radula in three species of iron-mineralizing mollusc Species Tooth size Erupted teeth * ([mu]m) * (no.) length/width Acanthoplura 310 [+ or -] 10 [+ or -] 0.3 (n = 10) hirtosa 4/306 [+ or -] 6 (n = 12) Plaxiphora 310 [+ or -] 11[+ or -] 0.5 (n = 10) albida 7/309 [+ or -] 6 (n = 12) Patelloida M 143 [ + or - ] 5 [+ or -] 0.2 (n = 15) alticostata 8/8 [+ or -] 5 (n = 12)[double dagger] L 103[+ or -] 4/177 [+ or -] 11 (n = 12) Wear type number of rows (tooth row numbers)[dagger] Species Unknown Light Moderate Heavy Acanthoplura 1 (9) 1 (8) 3 (5 - 7) 4 (1 - 4) hirtosa Plaxiphora 2(8-9) 2(6-7) 3(4-6) 3(1-3) albida Patelloida No unworn rows 1(5) 1(4) 3(1-3) alticostata observer * Data for tooth size and number of erupted teeth are presented as mean [+ or -] 1 SE. [dagger] Tooth row numbers relate to the numbering system used in Fig. 3. [double dagger] M = medial teeth, L = lateral teeth.
The chiton P. albida possessed the largest number of erupted teeth, while the limpet Pa. alticostata had the least (Table 2). For all species, evidence of tooth wear was observed within 1 to 4 rows following their eruption from the superior epithelial tissue, with P. albida having the most unworn teeth post-eruption (Table 2, Fig. 3). Exposed teeth exhibited increasing wear toward the anterior end of the radula. Tooth wear is illustrated in Figure 3. In general, no significant difference in tooth wear was observed between the left and right major lateral tooth rows for the two chiton species (ANOVA: P > 0.05). However, for A. hirtosa, a significant difference was found at tooth row 1, where tooth detachment was greater on the right side than on the left (P = 0.02).
[FIGURE 3 OMITTED]
Compared to observations using light microscopy, SEM revealed further detail on the patterns of tooth wear. For A. hirtosa, wear was first observed at row 8, where a thin surface coating had been removed between the medial edge and anterior tip of the cusps, producing an irregular ridgeline about 10 [micro]m from the cusp circumference (Fig. 4a, b). In addition, the cusp periphery changed from being a smooth regular edge at row 8, to slightly rough and irregular at row 7 (Fig. 4b). For teeth 6 and 5, the ridgeline was deepened and the cusp edge was increasingly irregular in shape, especially on the medial edge (Fig. 4c). For teeth 4 and 3, the anterior surfaces were bevelled, with the ridge forming a border between the upper and lower cusp regions about 40 [micro]m from the cusp edge (Fig. 4d). In addition, the edges were rounded and the periphery of the cusp was returned to a more regular shape (Fig. 4d). At tooth 2, striations were visible, running vertically from the cusp edge toward the tooth stylus, and the anterior surface remained beveled (Fig. 4e). Tooth 1 appeared stublike, and the anterior surface was slightly concave (Fig. 4e).
The condition and size of major lateral teeth from P. albida also deteriorated progressively from the posterior to the anterior working region of the radula (Fig. 5a to e). At row 8, the first signs of wear were apparent, with the central and medial cusp denticles showing some abrasion at the tips; by row 7, this abrasion extended to the lateral denticle (Fig. 5b). From rows 6 to 4, the three denticles of the major lateral teeth were rounded, with the prominent central denticle becoming similar in size to the medial and lateral denticles (Fig. 5c, d). Tooth wear at row 4 also extended between the denticles, baring the entire cusp periphery to abrasion and exposing the tooth core (Fig. 5d). At row 3, the cusp edges were blunt, with the core exposed on the anterior surface behind each denticle (Fig. 5e). The teeth of rows 2 and 1 were stublike. The continued removal of the anterior surface material behind the cusp edge resulted in this region becoming concave (Fig. 5a).
For the limpet Pa. alticostata, the medial and lateral teeth were increasingly worn from the point at which they emerged from the superior epithelium to the anterior tip of the radula (Fig. 6a to g). At tooth row 7, no wear was evident on the medial or lateral teeth, and epithelial tissue remained adhered to the tooth cusps (Fig. 6a). Wear was first observed on the medial teeth at row 6, in this case presenting as a small chip at the tip of the left cusp (Fig. 6b). At row 5, a number of large chips had broken off from the lateral teeth, and the anterior surfaces of both cusps were abraded (Fig. 6c). This abrasion extended from the lateral cusp edge for about 10 [micro]m at the outer tooth edge and widened to about 25 [micro]m at the inner tooth edge. At row 4, the periphery of each cusp had been significantly worn and now appeared rounded, without the initial sharp edges (Fig. 6d). At row 3, the profiles of both the medial and lateral cusps were greatly reduced, and wear extended between the lateral tooth cusp and its spur (Fig. 6e). At rows 2 and 1, the tooth cusps appeared stublike, and the medial cusps were almost completely worn away (Fig. 6f, g).
[FIGURE 6 OMITTED]
Specimens of A. hirtosa left distinct grazing strokes upon the artificial wax substrate; these strokes generally matched the broad unicuspid shape of the teeth (Figs. 2 and 4). Grazing strokes were varied, ranging from what appeared to be isolated single "bite" marks to intense excavation of the wax within a localized region (Fig. 7a). In addition, strokes often overlapped or were orientated obliquely to each other. Typically, stroke marks were about 600 [micro]m long and 200 [micro]m wide, and were arranged in pairs separated by a gap of about 20 to 100 [micro]m wide (Fig. 7b). The strokes were scooplike and canoe-shaped, being wider and deeper in the middle than at either end. Within the concave surface of each stroke a number of small parallel ridges or track marks were often observed.
[FIGURE 7 OMITTED]
Individuals of the chiton P. albida created clumps and curving patches of grazing marks on the wax disk, ranging from small sparsely grazed areas to large regions where the wax was entirely removed (Fig. 7c). Stroke marks consisted of a series of large and small parallel V-shaped trenches that match the shape of the three denticles in this species (Figs. 2 and 5). The large, deeply excavated trenches usually occurred in pairs about 250 [micro]m apart (Fig. 7d). The large stroke marks were about 400 [micro]m in length and 100 [micro]m in width, while the smaller and shallower stroke marks were about 300 [micro]m in length and 30 [micro]m in width. Parallel ridges were also observed on the concave surface in some of the deeper stroke marks.
Grazing strokes made by the medial and lateral teeth of the limpet Pa. alticostata occurred in overlapping bands radiating outward from a central position on the wax disk (Fig. 7e). In the outer bands, a series of evenly spaced individual grazing strokes were observed, forming arcs of strokes at about 1-mm intervals. Each grazing stroke consisted of four adjoining parallel and elongated gouges, about 1 mm in length and 300 [micro]m in width (Fig. 7f). The width of the two inner gouges each measured about 60 [micro]m across, while the two outer gouges were about 90 [micro]m each. These gouges are consistent with the spacing of the medial and lateral teeth in this species (Figs. 1 and 6). The inner gouges were often longer than the outer pair, by about 100--200 [micro]m (Fig. 7f).
Detailed observations of tooth wear in three species of iron-mineralizing mollusc have shown that the teeth are utilized for feeding shortly after they erupt from the superior epithelial tissue. From this point, the way in which the teeth wear strongly depends on tooth morphology, the underlying structure and composition of the tooth, and how the teeth are deployed in the feeding process. Each factor contributes toward maximizing the working life of the teeth and provides data critical for rationalizing the organic and mineral architecture of the teeth.
The first region of the tooth to make contact with the substratum for Acanthopleura hirtosa and Plaxiphora albida is the medial edge of the cusp and the medial and central denticle, respectively. This is due to the flexoglossate condition of the radula in chitons, where the teeth are forced to rotate laterally as they pass over the anteriorly positioned buccal cartilages prior to the feeding stroke (Morris and Hickman, 1981; Guralnick and Smith, 1999). This action repositions the teeth such that the medial edge of each major lateral cusp is aligned with the substrate. In contrast, limpet radulae are stereoglossate and lack the musculature and buccal morphology that allow the teeth to rotate laterally (Guralnick and Smith, 1999). The limpet Collisella asmi was found to make gouges on the shell of the gastropod Tegula funebralis that were similar to those created by Patelloida alticostata on wax (Hickman and Morris, 1985).
While the medially skewed nature of the major lateral teeth has been reported previously for A. hirtosa and a second unicuspid species A. echinata (Wealthall et al., 2005), P. albida is the first reported species with medially skewed tricuspid teeth, suggesting that this may be a morphological character common to chitons. For A. echinata, it was shown that the internal fine structure of the tooth cusp was also medially biased, and the teeth of A. hirtosa are likely to be structured in a similar way. Although less is known about the fine-scale structure of tricuspid chiton teeth, it is reasonable to suggest that the substructure of P. albida teeth is related to the gross morphology and functionality of the tooth.
The medial region of the cusp for both A. hirtosa and A. echinata is reinforced by a magnetite-mineralized accessory structure termed the tab, which extends from the cusp edge a short distance down the anterior surface (Fig. 2) (Wealthall et al., 2005). It is highly likely that this tab provides the softer apatite core with short-term protection from abrasion in these two species. It may also act as an additional anchor point to ensure that the two mineralogically distinct layers of the cusp remain strongly bonded, despite the shearing forces applied when feeding. As each of the three denticles of P. albida teeth are primarily composed of magnetite, there would be no need for such reinforcement.
[FIGURE 2 OMITTED]
The distribution of wear on the cusps and the grazing patterns on wax appear to be strongly associated with the structure and composition of the teeth for each of the three mollusc species. While certain aspects of radular structure and tooth composition differ between chitons and limpets, in general the processes of tooth mineralization are similar for both groups. For example, from what is known about the organic matrix, fibers in both are organized such that the orientation of the mineralized material is directed in a manner that controls the wear and fracture properties of the tooth (Runham et al., 1969; van der Wal et al., 1989; Wealthall et al., 2005). Chitons with unicuspid teeth, such as Chiton olivaceus and A. echinata, are reported to possess a rod-and-trough microstructure, while tricuspid species seem to possess a crossed system composed only of rods (van der Wal, 1990). Further comparative studies are needed to resolve the exact nature of this structural variation between species with different tooth morphologies.
It was evident that the major lateral teeth of A. hirtosa create broad trenchlike excavations of the substrate, whereas those of P. albida create rakelike marks along the surface. In addition to the obvious differences in tooth morphology, the internal architecture of the teeth in these two species appears to suit these two modes of food acquisition. For A. hirtosa, magnetite is mainly restricted to the posterior surface of the cusp and is completely backed by a softer core of apatite (Shaw et al., 2009a). The core may act to absorb shock resulting from the hard cutting edge of the magnetite cusp impacting on the substrate, and the interface between the two regions is likely to prevent cracks from propagating through the cusp. For P. albida, wear was not observed between each of its solid magnetite denticles until the tips had been worn down, demonstrating that the teeth are not pushed deeply into the substrate. This evidence supports the notion that A. hirtosa uses its unicuspid teeth in a spade-like manner to access microalgae living in pore spaces beneath the rock surface, while P. albida may use its tricuspid teeth in a rake-like manner to scrape larger algal species from the rock surface.
Although SEM clearly revealed that teeth from freshly collected animals progressively wear down to stublike structures through the abrasive nature of the feeding process, it is less obvious how these worn teeth interact with the substratum. A number of studies have documented the self-sharpening characteristics of chiton and limpet teeth, which are designed to wear preferentially on the anterior surface of the cusp, thereby maintaining a sharp posterior cutting edge (Runham and Thornton, 1967; Runham et al., 1969; Hickman, 1980; Padilla, 1985; van der Wal et al., 2000). This trait was clearly demonstrated by the beveled edges of worn A. hirtosa teeth. Although this self-sharpening effect was less obvious for P. albida, once each of its three denticles are worn down, the cusps may function in a similar way to A. hirtosa teeth.
A variety of factors dictate the effective working life of chiton and limpet teeth, including tooth size, the rate at which the teeth are replaced, the structural mechanisms controlling tooth wear, and the substrate upon which the animals feed. Radulae from the limpet Pa. alticostata possess far smaller teeth and have fewer tooth rows in use at any one time compared to the two chitons. It has been demonstrated (Shaw et al., 2008) that Pa. alticostata replaces its teeth at a significantly faster rate (0.51 tooth rows per day) than A. hirtosa or P. albida (0.40 and 0.36 tooth rows per day, respectively). Furthermore, limpet teeth are mineralized with goethite, a softer iron oxide than the magnetite of chiton teeth (Liddiard et al., 2004). Because Pa. alticostata grazes over the same substrate as the two chitons and possesses fewer, smaller, and softer teeth, it is likely that the limpet's tooth cusps become functionally ineffective more quickly than those of the chitons, therefore necessitating a faster rate of radular production.
The combination of SEM observations of teeth and grazing marks on wax has proved useful for understanding structure-function relationships in iron-mineralizing molluscan species. While SEM remains a powerful technique for the study of molluscan radulae (see, for example, Hickman, 1977; Geiger et al., 2007), other imaging modalities could be used to further resolve how the teeth are used during the feeding stroke. In particular, Padilla (2003) has used video footage to demonstrate interactions between individual teeth in both chitons and limpets. Such a tool would be particularly useful for observing interactions between worn teeth and the substrate. Other imaging methods, such as microscopic computed tomography (Micro-CT), have been used to reveal stunning three-dimensional detail of radular morphology (Golding and Jones, 2007); imaging of live animals is also available using this technique. The constant development of new instruments and methods will continue to provide new information on the dynamics of radular structure and, importantly, relate this to function.
The focus of current research on chitons and limpets is to elucidate the structural and biochemical properties of the organic matrix, its role in mineral formation, and its fate within the final mineralized composite. By studying the relationships between tooth wear and tooth structure, useful data have been generated for interpreting the fine-scale internal architecture of chiton and limpet teeth. However, our present understanding of tooth substructure is based on data obtained from relatively few species and from only a small number of specific tooth orientations, leaving large gaps in our understanding of matrix structure at the whole-tooth level. Clearly, future studies should focus on the overall arrangement of the organic matrix in teeth of various morphologies. Such information will form the basis for resolving the complex mechanisms involved in the formation of the teeth, which are highly desirable analogs for the development of synthetic materials based on these biomimetic principles.
This research was funded by an Australian Research Council Discovery Grant (DP0559858) to David J. Macey. The authors acknowledge the facilities and scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by The University, State and Commonwealth Governments.
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Received 20 October 2009; accepted 20 January 2010.
* To whom correspondence should be addressed. E-mail: email@example.com
JEREMY A. SHAW (1),*, DAVID J. MACEY (2), LESLEY R. BROOKER (3), AND PETA L. CLODE (1)
(1) Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley, 6009, Australia; (2) School of Biological Sciences & Biotechnology, Murdoch University, Murdoch, 6150, Australia; and (3) Faculty of Science, Health and Education, University of the Sunshine Coast, Maroochydore DC, 4558, Australia
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|Author:||Shaw, Jeremy A.; Macey, David J.; Brooker, Lesley R.; Clode, Peta L.|
|Publication:||The Biological Bulletin|
|Date:||Apr 1, 2010|
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