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Philippine Journal of Science141 (1): 1-11, June 2012ISSN 0031 - 7683Date Received: 18 Jan 2011

Key Words: Arius dispar, Arius manillensis, geometric morphometrics, kanduli, Laguna de Bay, Siluriformes

*Corresponding author: [email protected]

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Brian S. Santos and Jonas P. Quilang*

Genetics Research Laboratory, Institute of Biology, College of Science,University of the Philippines, 1101 Diliman, Quezon City, Philippines

Geometric Morphometric Analysis of Arius manillensisand Arius dispar (Siluriformes: Ariidae) Populations

in Laguna de Bay, Philippines

Geometric morphometrics has become a widely used tool in studying shape variation in fish populations. In this study, geometric morphometrics was used to examine shape variation in the sea catfishes Arius manillensis and A. dispar. The endemic species A. manillensis and the native species A. dispar constitute major fishery resources in Laguna de Bay. Thus, they are economically important species. The two species look very similar externally, but they can be distinguished by examining the tooth patch morphology on the palate. However, within each species, there are variants of tooth patch morphology. Shape differences between A. manillensis and A. dispar, between populations, and between variants within each species, were determined. Samples were obtained from Binangonan, Tanay, and Calamba areas of Laguna de Bay. Shape differences between species, between morphs within species, and among specimens of different sites were significant, but the groups were difficult to differentiate due to high overlaps in Canonical Variate Analysis (CVA) plots and low Mahalanobis distance-based correct classification percentages. This was attributed to possible introgression between A. manillensis and A. dispar.

INTRODUCTIONThe sea catfishes Arius manillensis Valenciennes, 1840 and Arius dispar Herre, 1926 are commercially important to local fishery in the Philippines. A. manillensis is endemic to the Philippines and although A. dispar is native to this country, it is also found in other parts of the world. These two species are locally known as kanduli. In the 1920s kanduli used to be the most abundant fishes in Laguna de Bay, a 90,000-ha lake which is the largest in the Philippines (Mane 1929; Aldaba 1931; Mercene 1978). A downward trend in the abundance of kanduli, however, was noted in the 1930s (Villadolid 1932; Villadolid 1934) until the 1960s when kanduli was reported to be very rare (Delmendo & Bustillo 1968). The depletion of this

fishery resource was attributed largely to the use of drag seine, a destructive fishing method, which catches even the immature fish and disturbs the habitat of bottom-dwellers including kanduli and other benthos that serve as food for kanduli (Villadolid 1932; Villadolid 1934; Mercene 1978). In the mid 1990s, it seemed that the population of kanduli in Laguna de Bay has recovered because in terms of yield, kanduli ranked third after tilapia and bighead carp (Palma et al. 2002). Although the population of kanduli has recovered, the yield to biomass ratio indicated that it is being overfished (Palma et al. 2002). Hence, there is a need to conduct more studies on the populations of these economically important fishery resources for the construction of proper management and conservation programs. To date, there are only a few studies that have been conducted on these species. Mane

Santos BS & Quilang JP : Geometric Morphometrics of Arius Populations

Philippine Journal of ScienceVol. 141 No. 1, June 2012

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(1929) conducted a preliminary study on its life history and habits, but all the other studies were on its catch landing statistics and fisheries (Aldaba 1931; Villadolid 1932; Villadolid 1934; Mercene 1978). This study used geometric morphometrics to investigate the populations of these two species in Laguna de Bay.

Geometric morphometrics (GM) is a widely used technique in determining shape variation. Instead of using linear measurements, counts, and ratios, as in traditional morphometrics (Adams et al. 2004), data in GM are recorded in the form of coordinates of landmark points (Rohlf and Marcus 1993), which are morphological points of specimens that are of biological interest (Richtsmeier et al. 2002). With GM, graphical displays of the results can be done using either difference-vector diagrams or thin-plate spline (Slice 2007). Image processing techniques have greatly enhanced morphometric analysis and have greatly improved stock identification and discrimination in fishes (Cadrin & Friedland 1999). Thus, GM have been used in various studies on fish population biology and stock identification and discrimination.

This study aimed to determine if there are differences in shape between A. manillensis and A. dispar since the two species are difficult to tell apart based on external morphology. The study also aimed to investigate shape variation between populations and between morphs

within each species. The three populations of each species came from Binangonan, Tanay, and Calamba areas of Laguna de Bay.

MATERIALS AND METHODS

Collection of SpecimensKanduli specimens were obtained from Binangonan, Tanay, and Calamba areas of Laguna de Bay. Binangonan, Tanay, and Calamba represent the west, central, and south regions of the 90,000-ha lake, respectively (Figure 1). Each specimen was identified based on tooth patch morphology (Conlu 1986; Kailola 1999). Those individuals that have tooth patches in both the upper and lower palate were designated as A. manillensis while those with tooth patches in only the upper area of the palate were designated as A. dispar (Figure 2). Variants of the tooth patch morphology in each species were also noted (Figure 2). Each specimen was photographed using a Nikon D90 SLR camera. The weight of each fish was determined. The standard length of each specimen was measured using a ruler. The measurements were taken from the left body side of each fish. The sex of each specimen was determined by internal inspection. Significant differences in

Figure 1. Location of sampling sites. (1) Binangonan; (2) Tanay; (3) Calamba.



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weight and length between species, sites and morphs were determined using SPSS software (SPSS, Chicago, IL, USA).

Digitization and Data AnalysesTwenty-two (22) landmarks from the left body side and 10 landmarks from the dorsal head view (Figure 3) were digitized on each digital image of the specimen using the tpsDig2 software (Rohlf 2010). The left body side landmarks chosen for this study were based on those used by Trapani (2003) and Maderbacher et al. (2008) while the dorsal head view landmarks chosen for this study were based on those used by Reis et al. (2006).

Generalized Procrustes Analysis (GPA) was performed to superimpose landmark coordinates as shape variables. This was done so that the differences between landmarks would reflect only the differences in shape independent of size, position, or orientation (Slice 2007). Programs from the Integrated Morphometrics Package (Sheets 2004) were used for this and for the succeeding analyses. GPA was performed using CoordGen6f.

Principal Component Analysis (PCA) of all specimens was performed separately for each view using the PCAGen6n application of IMP to determine

Figure 2. Tooth patch morphology in A. manillensis and A. dispar as documented by Kailola (1999) (A) and as observed in this study (B, C).

intrapopulation shape variation. Multivariate Analysis of Variance (MANOVA) was done on the principal components (PC) scores of the significant PCs to differentiate the species and populations. MANOVA was conducted using SPSS software (SPSS, Chicago, IL, USA). Shape variations between species, between populations, and between groups of different tooth patch morphology (Figure 2) were analyzed using MANOVA and were summarized using Canonical Variate Analysis (CVA) and discriminant analysis using the program CVAGen6l. To determine whether the fish exhibits isometric or allometric growth pattern, partial warp scores were computed to correlate shape variables with the logarithm of centroid size. The centroid size is the square root of the sum of squared distances of the landmarks in a configuration to their average location (Slice 2007). To visualize patterns in shape variability, deformation grids were generated. Deformation grids provide an exact mapping of one configuration onto another and involve the use of thin-plate functions to fit differences in the landmark positions in one specimen relative to the positions in another (Rohlf & Marcus 1993; Slice 2007).



Figure 3. Landmarks digitized on the left (A) and dorsal (B) views. Left side landmarks: (1) anterior tip of upper jaw; (2) posterior end of upper lip; (3) posterior end of lower jaw; (4) top of supraoccipital; (5) dorsal extreme of operculum; (6) posterior extreme of operculum; (7) ventral extreme of operculum; (8) center of eye; (9) anterior extreme of the orbit; (10) posterior extreme of the orbit; (11) anterior insertion of dorsal fin; (12) posterior insertion of dorsal fin; (13) anterior insertion of the adipose fin; (14) top of caudal fin; (15) center of caudal fin; (16) base of caudal fin; (17) posterior insertion of anal fin; (18) anterior insertion of anal fin; (19) posterior insertion of pelvic fin; (20) anterior insertion of pelvic fin; (21) ventral insertion of pectoral fin; and, (22) dorsal insertion of pectoral fin. Dorsal head view landmarks: (1) snout tip; (2) right lateral margin of snout; (3) left lateral margin of snout; (4) right nostril; (5) left nostril; (6) right orbit; (7) left orbit; (8) posterior tip of right opercle; (9) posterior tip of left opercle; 10) posterior tip of supraoccipital bone process.

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RESULTS

The descriptive statistics for standard length and weight of specimens of each of the two species from three areas of Laguna de Bay are given in Table 1. Shape variations between species, between populations, and between different morphs were significant based on CVA-MANOVA (Table 2). Variation was greater in the left body side compared to the dorsal head view. Based on the CVA classifications (Table 3), the percent correct classifications were higher for the left body side compared to the dorsal head view. Correct classification rates based on Mahalanobis distances were higher for analyses between

species and between sites compared to analyses between morphs. Of the groups classified, however, only between species classification for A. manillensis and A. dispar and a posteriori assignment of A. manillensis specimens from Binangonan exhibited a percent correct classification of 90% or higher. There were three significant principal components identified for the left side (accounting for 49.2% of the total variance explained) and two significant principal components for the dorsal head view (accounting for 54.8% of the total variance explained). Of these PCs, only the third left side PC (12.4% of the total variance explained) and the first two dorsal view PCs significantly differentiated the species and their respective sites based on MANOVA of PC scores.



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Table 1. ANOVA of length and weight measurements from specimens of A. manillensis and A. dispar from three areas of Laguna de Bay. Values are mean ± standard deviation and range (enclosed in parenthesis). Means followed by the same letter are not significantly different at α= 0.05 using Duncan Multiple Range Test. n, sample size; wt, weight (g); SL, standard length in millimeters (mm).

Groups Compared F-value P-value

Between Species

A. manillensis (n = 245) A. dispar (n = 239)

SL 20.3a ± 3.2 (16.6–52.6) 19.0b ± 1.9 (15.4–27.9) 27.55 0.00

Wt 126.1a ± 59.9 (59.8–573.0) 100.0b ± 37.5 (47.9–326.6) 32.71 0.00

Within A. manillensis Between Sites

Binangonan (n = 35) Tanay (n = 103) Calamba (n = 107)

SL 20.3 ± 2.6 (16.9–27.4) 20.5 ± 2.3 (16.6–28) 20.1 ± 4.0 (16.6–52.6) 0.32 0.73

Wt 126.2 ± 58.2 (59.8–325.1) 129.6 ± 45.8 (60.9–268) 122.7 ± 71.7 (61.4–573) 0.35 0.71

Within A. manillensis Between Morphs

A. manillensis A (n = 71) A. manillensis B (n = 66) A. manillensis C (n = 53) A. manillensis D (n = 31)

SL 20.7 ± 2.6 (17.3–31.2) 19.8 ± 2.2 (16.6–26.5) 20.6 ± 2.8 (16.6–30.4) 20.6 ± 6.1 (17.4–52.6) 0.94 0.42

Wt 137.9 ± 63.3 (72.7–477.1) 118.4 ± 44.1 (61.4–243.8) 137.2 ± 81.6 (60.9–573.0) 112.4 ± 40.1 (70.8–245.5) 2.26 0.08

Within A. dispar Between Sites

Binangonan (n = 94) Tanay (n = 58) Calamba (n = 87)

SL 18.6a ± 1.7 (15.7–24.5) 20.3b ± 1.8 (15.8–25.9) 18.7a ± 1.8 (15.4–27.9) 20.06 0.00

Wt 89.5a ± 27.9 (53.5–177.1) 125.3b ± 36.6 (58.8–256.3) 94.8a ± 39.9 (47.9–326.6) 20.42 0.00

Within A. dispar Between Morphs

A. dispar A (n = 51) A. dispar B (n = 65) A. dispar C (n = 29)

SL 19.7b ± 2.4 (15.4–27.0) 19.4b ± 1.7 (16.0–27.9) 18.5a ± 1.8 (15.8–23.2) 3.78 0.03

Wt 113.9 ± 48.2 (47.9–310.8) 107.2 ± 37.2 (56.1–326.6) 94.0 ± 34.6 (54.5–206.7) 2.19 0.12

Table 2. Summary of distinct canonical variates (CV) based on CVA-MANOVA between species, between localities of the same species, and between different morphs for both dorsal and left body sides of A. manillensis and A. dispar specimens.

Comparisons

Left Body Side Dorsal head view

CV* Wilk’s λ df P-value CV* Wilk’s λ df P-value

Between species

CV1 0.342 40 0.000 CV1 0.929 16 0.014

Within A. manillensis Between Sites

CV1 0.135 80 0.000 CV1 0.462 32 0.000

CV2 0.444 39 0.000 CV2 0.778 15 0.000

Within A. manillensis Between Morphs

CV1 0.307 120 0.000 CV1 0.606 48 0.000

CV2 0.577 78 0.012 CV2 0.794 33 0.018

Within A. dispar Between Sites

CV1 0.265 80 0.000 CV1 0.451 32 0.000

CV2 0.714 39 0.000 CV2 0.794 15 0.001

Within A. dispar Between Morphs

CV1 0.347 80 0.000 CV1 0.628 32 0.001

*only distinct canonical variates are shown



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Shape variation between speciesBased on the deformation grids (Figure 4) generated, A. manillensis and A. dispar differ in terms of the position of the top of the supraoccipital, which explains the strong deformation observed in that region. A. manillensis has a more anteriorly positioned top of supraoccipital while A. dispar has a more anteriorly positioned dorsal spine. There is also a slight counterclockwise rotation of the head region with respect to the body for A. manillensis specimens compared to A. dispar. These differences, however, were not very distinct in the specimens as the vector diagrams were already exaggerated. In the CVA plot generated (Figure 4), there was a distinct cluster formed by A. manillensis in the negative end of the CV1 axis while A. dispar specimens grouped in the positive end of the CV1 axis. There was little overlap on the CVA plot generated for the left body side but the overlap was huge for the plot generated for the dorsal head view. This was supported by the classification matrix (Table 3) wherein 91.9% of the specimens were correctly classified to their

species based on the CV scores of the left body side but only 53.6% were correctly classified based on the CV scores of the dorsal head view.

Shape variation in A. manillensis specimensThere were two distinct CVs on the left body side that distinguishes samples of A. manillensis from the different sites (Table 2; Figure 5). Shape differences along the CV1 axis was interpreted as variation in the position of the top of the supraoccipital while shape differences along the CV2 axis was interpreted as variation in body depth. The CV1 axis discriminated Binangonan specimens from the rest, while the CV2 axis discriminated Calamba specimens from Tanay. Binangonan specimens had a more posteriorly positioned top of supraoccipital as compared to Tanay and Calamba specimens while Binangonan and Tanay specimens were deeper bodied compared to Calamba specimens. Based on the classification matrix (Table 3; Figure 5), all Binangonan specimens were correctly

Table 3. Canonical Variate Analysis (CVA) classification for A. manillensis and A. dispar populations.

A priori assignment*

A posteriori assignment

Left Body Side Dorsal head view

% Correct classification % Misclassification % Correct classification % MisclassificationBetween speciesA. manillensis (n = 245) 91.0 9.0 66.3 33.7A. dispar (n = 239) 92.9 7.1 59.2 40.8Average 91.9 8.1 53.6 46.4

Within A. manillensis Between SitesBinangonan (n = 35) 100 0 80.0 20.0Tanay (n = 103) 89.7 10.3 64.5 35.5Calamba (n = 107) 87.4 12.6 68.9 31.1Average 90.2 9.8 71.1 28.9

Within A. manillensis Between MorphsA. manillensis A (n = 71) 74.6 25.4 62.0 38.0A. manillensis B (n = 66) 68.2 31.8 42.4 57.6A. manillensis C (n = 53) 20.8 79.2 50.9 49.1A. manillensis D (n = 31) 77.4 22.6 19.4 80.6Average 60.2 39.8 43.7 56.3

Within A. dispar Between SitesBinangonan (n = 94) 86.2 23.8 64.1 35.9Tanay (n = 58) 75.9 24.1 65.5 34.5Calamba (n = 87) 75.9 24.1 74.1 25.9Average 80.0 20.0 67.9 32.1

Within A. dispar Between MorphsA. dispar A (n = 51) 60.8 39.2 72.5 27.5A. dispar B (n = 65) 76.9 23.1 29.2 70.8A. dispar C (n = 29) 37.9 62.1 55.2 44.8Average 63.4 36.6 52.3 47.7*The a priori assignments are based on the species, population, or morph to which the specimens belong, while the a posteriori assignments make use of the Mahalanobis-based approach to predict the group membership of each specimen based on the CVA.



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Figure 4. Canonical variate analysis on shape variables of specimens of A. manillensis (n = 245) and A. dispar (n = 239) based on (A) the left body side and (B) the dorsal head view. The CV1 axis accounts for 100% of the total variance of both left body side and dorsal head view. The Procrustes deformations between A. manillensis and A. dispar based on the first canonical variate are also shown for each body side. Axis units for deformation grids are shown.



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Figure 5. Canonical variate analysis on shape variables of specimens of A. manillensis from Binangonan (n = 35), Tanay (n = 103), and Calamba (n = 107) based on (A) the left body side and (B) the dorsal head view. The CV1 and the CV2 axes account for 64% and 36%, respectively, of the total variance of the left body side and 63% and 37%, respectively, of the total variance of the dorsal head view. The Procrustes deformations between sites based on the first two canonical variates are also shown for each body side. Axis units for deformation grids are shown.

classified. Likewise, Binangonan specimens also had the highest percent correct classification based on the CV scores of the dorsal head view.

Specimens of A. manillensis, when grouped to their respective morphs based on tooth patch morphology (Figure 2), showed significant shape variations based on CVA-MANOVA (Table 2). Only the pair of A. manillensis A and A. manillensis B did not significantly differ based on the dorsal head. These shape differences among the specimens, however, were not distinct based on the low correct classification rates (60.2% for the left body side; 43.7% for the dorsal head view; Table 3).

Shape variation in A. dispar specimensThere were two distinct CVs on the left body side that distinguishes samples of A. dispar from the different sites (Table 2; Figure 6). Shape differences along the CV1

axis was interpreted as variation in body depth while shape differences along the CV2 axis was interpreted as variation in head size relative to body size (deformations not shown). The CV1 axis discriminated Binangonan specimens from the rest, while the CV2 axis discriminated Calamba specimens from Tanay. Binangonan specimens had a greater body depth while Calamba specimens had smaller head size relative to body size. Based on the classification matrix (Table 3), Binangonan specimens had a relatively high percent correct classification (86.2%).

Specimens of A. dispar, when grouped to their respective morphs based on tooth patch morphology (Figure 2), yielded only one distinct CV based on CVA-MANOVA of both left and dorsal views (Table 2). Correct classification rates were low for A. dispar morphs (63.4% for the left body side; 52.3% for the dorsal head view; Table 3).



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Figure 6. Canonical variate analysis on shape variables of specimens of A. dispar from Binangonan (n = 94), Tanay (n = 58), and Calamba (n = 87) based on (A) the left body side and (B) the dorsal head view. The CV1 and the CV2 axes account for 81% and 19%, respectively, of the total variance of the left body side and 72% and 28%, respectively, of the total variance of the dorsal head view. The Procrustes deformations between sites based on the first two canonical variates are also shown for each body side. Axis units for deformation grids are shown.

Allometry and Sexual DimorphismAllometry occurs when variation in size among the specimens corresponds to variation in shape. Regression of the PC scores of the first principal component on the logarithm of centroid size showed that allometry in the sampled specimens was statistically insignificant for both the left body side (P = 0.59) and the dorsal head view (P = 0.85). No sexual dimorphism was observed. No distinct canonical variates were detected in comparing the shapes of female and male specimens.

DISCUSSIONAlthough significant differences were observed for most analyses in this study, the shape variations observed were not very pronounced based on the diagrams. There was

also high level of overlaps observed in the CVA plots, and low correct classification percentages. Shape differences can only be visualized when the vector diagrams are exaggerated. This is true even for the deformation between A. manillensis and A. dispar, even though the distance between them is high. As such, the shape differences observed in this study offer little practical use in terms of discriminating fish species and populations.

Shape variation was greater in the left body side compared to the dorsal head view in almost all analyses. Since the sea catfish is not laterally flattened, there may be several sources of variation in the left body side. Although allometric growth patterns were not significant, other factors such as stomach fullness, gonadal status, and season may cause variation (Tesch 1971). These factors may affect body weight and size and cause lateral compression or expansion that may be



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observed as differences in shape of the left (or right) body sides. For these reasons, the left body side is expected to respond more to environmental conditions than the dorsal head view. Other factors that may contribute to shape variation are lifestyle, feeding habits, swimming patterns (Cullen et al. 2007; Rincón et al. 2007) of the fish, rearing temperature of the water (Marcil et al. 2006) and water depth (Rincón et al. 2007). In this study, variation was greatest in the head region, which was exemplified by the shift in the position of the supraoccipital, followed by body depth. Head shape variation has been explained by differences in feeding behavior (Cullen et al. 2007). Rotational variation in the head in particular suggests different patterns in prey capture kinematics (Leysen et al. 2011). Body depth, on the other hand is explained by differences in swimming patterns that may result from variation in water depth (Rincón et al. 2007). In this study, however, water depth is not expected to account for much variation since Laguna de Bay is a shallow lake with an average depth of 2.8 m (Baluyut 1983). Feeding lifestyle is also similar between the species. Both are carnivorous and their guts both include mostly shrimp and sometimes fish eggs, and smaller fish. The two species also occur together in all batches of fish catch in all samplings done in this study. This suggests that there is no ecological separation between the two species. In other words, the natural environment offers little variation to cause differentiation between the species and among populations. Such may lead to hybridization between the species. This was observed in the redfish Sebastes mentella, S. fasciatus, and their hybrids (Valentin et al. 2002). The two species are sympatric and are hard to distinguish morphologically. They were distinguished based on their isozyme patterns. GM was able to distinguish the two species and their hybrids. Shape differences were explained by differences in feeding lifestyle. A similar case of hybridization is suspected in this study. In this study, however, there seems to be no variability in the feeding ecology of A. manillensis and A. dispar that would lead to marked shaped differences. Also, introgression may be occurring at a greater extent between A. manillensis and A. dispar such that hybrids exhibit more than one shape conformation and shape variation is observed as a gradient. This is in contrast to the findings of Valentin et al. (2002) wherein there were only three distinct groups, one for S. mentella, one for S. fasciatus, and one for the hybrid. As mentioned, the dorsal head view is expected to respond less to environmental conditions compared to the left body side. Hence, the dorsal head view landmarks are suitable characters for determining shape variation as a function of locality or of morphology in laterally rounded fishes (Reis et al. 2006) and these are good markers for population differentiation. In this study, however, dorsal body shape variation offer little information due to low percentages of correct classification (Table 3).

In terms of aquaculture practices, there is a higher density of fish pens in Binangonan compared to Calamba or Tanay. Fish feeds that diffuse out of the pens may provide a greater fitness advantage to fishes near the pens compared to others. This advantage may translate to a shape that is distinct from the others. This is supported by the high correct classification percentages of specimens from Binangonan (Table 3). Variation between species may be explained, at least in part, by the greater number of A. dispar (n=94) specimens from Binangonan compared to A. manillensis (n=35).

In summary, GM revealed significant shape variations among the samples. Differentiation between species and among their populations and morphs, however, was poor based on large overlaps and low correct classification percentages. It is hypothesized that there is little or no genetic differentiation between the species and among the different populations. Introgression may also be occurring between the two species in the lake as suggested by their cohabitation, presence of variable tooth patch morphologies, and the low differentiation among the different morphs. As the species hybridize, variation between them is reduced. This can be verified in future studies using population genetic markers such as the mitochondrial control region and microsatellites. Distance-based measurements particularly involving the top of the supraoccipital, which was the most variable landmark in this study, is also recommended.

ACKNOWLEDGEMENTSThis study was funded through a PhD Incentive Grant (Project No: 090921) awarded to J.P. Quilang by the University of the Philippines Diliman Office of the Vice-Chancellor for Research and Development. Special thanks to the Institute of Biology, University of the Philippines Diliman for logistical support and to Reynand Canoy and Jazzlyn Tango for technical assistance.

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