Journal of Advanced Marine Science and Technology Society. Vol.14. No.2. pp. 1-8. 2008

Original paper

Dugong grazing scars confirmed

in Enhalus acoroides meadows

Yoshiei Nakanishi *', Kanjana Adulyanukosol *", Nobuaki Arai *3, Seiichi Hosoya *',

Yoshiko Nakanishi*1, Kazuhiko Kotani*1 and Kiyonori Katsukoshi*1

* 1 Okinawa Branch, Idea Consultants Inc., 2-6-19 Aja, Naha, Okinawa 900-0003, Japan

*2 Phuket Marine Biological Center, P.O. Box 60 Phuket 83000, Thailand

*3 Graduate School of Informatics, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan

*4 Okinawa Environmental Research Inc., 2-6-19 Aja, Naha, Okinawa 900-0003, Japan

Received : July, 1. 2008. Accepted : March. 6. 2009

Abstract

We performed a survey from 27 to 30 October 2005 in the seagrass meadows along the southeast coast of Talibong Island. Trangprovince. Thailand. We found three typical characteristics of dugong grazing scars on Enhalus acoroides leaves. The bladeswere unevenly cut (not in straight lines), and these same general shapes were observed at the same abovegroiind height on allleaves. Tooth marks consistent with dugong molars were observed on E. acoroides leaves, and the shapes of grazing scars in E.acoroides meadows were elliptical, whereas scars found in meadows of small seagrass species were meandering lines.

Keywords : Dugong dugon, Enhalusacoroides. grazing scar, dugong feeding trail

1. Introduction

Dugongs (Dugong dugon) occur in the tropical/

subtropical areas of the Indo-Pacific Ocean (Nishiwaki

and Marsh. 1985). and these herbivorous marine

mammals feed on seagrasses (Heinsohn and Birch.

1972; Lipkin. 1975: Johnstone and Hudson. 1981:

Marsh et al.. 1982: Erftemeijer et al.. 1993: Preen.

1995: Andre et al., 2005). Seagrass meadows are very

important dugong feeding grounds (Anderson, 1994;

Preen and Marsh, 1995).

Analysis of stomach and mouth contents revealed

that dugongs feed on Enhalus acoroides, a seagrass

(Heinsohn and Birch. 1972: Johnstone and Hudson.

1981: Erftemeijer et al., 1993: Adulyanukosol et al.,

2001. 2005). However, although field observations have

confirmed that dugongs feed on a variety of seagrasses

(Aragones. 1994: De Iongh et al.. 1995. 1997: Mukai et

al., 2000: Adulyanukosol et al., 2003: Nakanishi et al.,

2005), few studies have observed dugongs feeding on

E. acoroides specifically. In one study, dugongs were

reported to feed primarily upon small seagrass species

such as Halophila ovalis, but not on E. acoroides,

in the Philippines (Aragones, 1994). In that report,

however, the author provided no information about

dugong grazing scars on E. acoroides.

A thorough description of the shape and

characteristics of dugong grazing scars is essential to

understanding feeding ecology in seagrass meadows.

This paper describes the shape and characteristics of

dugong grazing scars in E. acoroides meadows.

2. Materials and methods

2.1 Study area

In Thailand, dugongs have been found along

the coastlines of both the Gulf of Thailand and the

Andaman Sea. The largest group of dugongs inhabits

the waters around Talibong Island, in Trang province

(Adulyanukosol et al., 1997: Adulyanukosol and

Chantrapornsyl, 1999: Adulyanukosol. 2000; Hines

et al., 2005). where the largest seagrass bed in Thai

waters is also located (Chansang and Poovachiranon.

1994: Poovachiranon. 2000).

This survey was performed around Talibong Island.

The substrate around the island is sandy mud. and

the seagrass meadows surrounding the island are

reportedly composed of 11 species of seagrass: E.

acoroides, Halophila heccarii, Halophila decipiens, H.

ovalis, Halophila minor, Thalassia hemprichii, Cymodocea

serrulata, Cymodocea rotundata, Halodule pinifolia,

Halodule uninervis, and Syringodium isoetifolium

(Nakanishi et al., 2005. 2006). Dugongs have been

observed feeding on these seagrasses (Nakanishi et al.,

2005.2006).

Our study sites are indicated in Fig. 1. At Site A, E.

acoroides was scattered among H. ovalis communities.

At sites 1-9, there were a lot of E. acoroides populations.

Yoshiei Nakanishi. Kanjana Adulyanukosol. Nobuaki Arai. Seiichi Hosoya, Yoshiko Nakanishi. Kazuhiko Kotani and Kiyonori Katsukoshi

— N7" 20. 000'

A.

•N7' 10 500'

E99: 20. 000'

•'ig. I Map of the study site around Talibong Island. Trang province. Thailand. Closed circles indicate dugong grazing study sitesin E. acoroides meadows. A opened triangle denotes a study site in which dugongs grazed on E. acoroides in /-/. ovalis-dominated meadows.

2.2 Methods

Field observations were made by scuba divers

from 27 to 30 October 2005. At Site A. which was

primarily populated by H. ovalis. we searched for

dugong grazing scars on scattered /;.'. acoroides leaves

that were left when dugongs fed on both E. acoroides

and H. ovalis. inside H. ovalis feeding trails. Within

these feeding trails, we measured the abovegroiind E.

acoroides leaf lengths, sampled plants, and observed

grazed sections of leaves.

Second, we searched for grazing scars on E.

acoroides leaves in E. acoroides meadows for 20 min

at Sites 1-9. At these sites, we measured aboveground

leaf lengths, sampled plants, and observed grazed

sections of leaves. We then determined whether these

grazing scars were caused by dugongs based on

comparisons to dugong grazing scars on E. acoroides

leaves found inside the H. ovalis feeding trails (Site A)

and key dugong grazing characteristics identified from

headframe and tooth samples.

In the laboratory, we determined the mean length

from the tip of the mouth to the molars based on

three dugong headframe specimens, which were

obtained from animals that drowned in fishing nets in

the Andaman Sea between 1997 and 2001. We then

measured and photographed the teeth of two dugong

specimens from the Andaman Sea collected in August

and November 2005(Table 1). The measurements and

photographs were used to identify whether dugongs

graze on /:'. acoroides and to verify how dugongs

graze on E. acoroides leaves.

3. Results

The mean abovegroiind grazed E. acoroides leaf

length from plants sampled within H. ovalis feeding

trails at Site A was 15.6 ± 5.3 cm (n = 17: Table 2).

The characteristics of dugong-grazed sections of E.

acoroides were as follows (Fig. 2) : 1) seagrass blades

were not usually cut in straight lines: 2) dugong-

grazed blade sections had similar shapes al the

same abovegroiind height, suggesting that dugongs

grazed several leaves at the same time: and 3) marks

Dugong grazing scars confirmed in Enhalus acoroides meadows

Table 1 I'holographs of two dugong specimens showing measurements of lhe molar teeth. UR. upper right: LL. upper leftlower left: LR, lower right.

LL.

Sample Information

Recorded Date

Area

Body Length

Body Weight

Recorded Date

Area

Body Length

Body Weight

9 August 2005

Phang-nga Province,Thailand

1.70 m

150 kg

27 November 2005

Trang Province,Thailand

2.0 m

180 kg

UR

Upper Jaw

1 2

| wide

Table 2 The abovegroiind leaf lengths of grazed /•.". acoroidessampled inside H. ovalis feeding trails (Site A) andE. acoroides at Sites 1 and 9 with grazing scars.

Study siteSite A

H. ovalismeadow

Site 1 Site 9E. acoroides £ acoroides

meadow meadow

Aboveground

leaf length

(cm)

5.4 8.2 17.0

6.5 11.4 17.0

8.5 11.7 17.1

11.5 11.9 17.5

13.6 12.1 17.6

14.0 12.2 17.9

14.3 12.3 18.3

15.8 12.7 20.1

16.5 13.5 20.2

16.8 14.1 21.0

17.3 14.2 23.2

18.5 20.1 23.6

19.5 21.0 24.5

20.8 30.2 24.6

21.5 24.8

22.1 24.9

23.2 25.0

26.2

26.7

N 17 14 19

Mean 15.6 14.7 21.4

SD 5.3 5.6 3.6

Lower Jaw Teeth

UR

UL

LL

LR

UR

UL

LL

LR

1(WXL)!2(WXL)(cm) | (cm)

0.88 X

1.21

0.90 X

1.17

0.88 X

1.21

0.87 X

0.95

0.90 X

0.97

0.71 X

0.96

3(WXL)(cm)

0.82 X

0.76

0.83 X

0.80

0.74 X

0.74

0.82 X

1.17

0.74 X 0.72 X

0.95 ' 0.74

1.37 X

2.25

1.44 X

2.10

1.40 X

2.25

1.44 X

2.27

1.47 X

1.24

1.40 X

1.51

1.34 X

1.32

1.40 X

1.50

4(WXL) 5(WXL)(cm) (cm)

0.55 X

0.70

0.60 X

0.70

0.60 X 0.34 X

0.66 0.36

0.62 X 0.30 X

0.61 0.34

resembling dugong molars were often observed on

/:'. acoroides leaves. In addition, the hard part of leaf

edges tended to remain on E. acoroides leaves grazed

by dugongs.

Grazing scars on E. acoroides blades, which were

cut al almost the same aboveground height, were

observed at Sites 1 (Fig. 3) and 9 (Fig. 1). The mean

aboveground grazed E. acoroides leaf length was 14.7 ±

5.6 cm at Site 1 (n= 14: Table 2) and 21,1 ± 3.6 cm al

Site 9(n= 19: Table 2). No significant difference was

observed between Site 1 and Site A in the lengths of

grazed E. acoroides leaves (Mann -Whitney U-tesl. p >

0.01). whereas the difference between Site 9 and Site

A was significant (Mann - Whitney U-test. p < 0.01).

However, the three longest grazed E. acoroides leaves

sampled within feeding trails at Site A ranged from

21.5 to 23.2 cm (Table 2); these leaves were longer

than the mean length of grazed E. acoroides leaves at

Site 9. The shapes of grazed sections of E. acoroides

leaves at Sites 1 ( Fig. 3) and 9 (Fig. A) were very-

similar to those at Site A (Fig. 2). The grazing scars

in /:'. acoroides meadows were elliptical, with longer

and shorter axes of 23 and 13 cm at Site 1. and 15 and

13 cm at Site 9, respectively.

Measurements and photographs of the molar teeth

Yoshiei Nakanishi. Kanjana Adulyanukosol. Nobuaki Arai. Seiichi Hosoya. Yoshiko Nakanishi. Kazuhiko Kolani and Kiyonori Katsukoshi

Table 3 The approximate length from the lip of the mouth to the molar and individual information on each bone specimen.

Sex Length Weight Date

Length from the

tip of the mouth

to the molar (cm)

Mean SD

Male 209 cm 166 kg 6 Mar. 2001 15

16.7 2.9Male 235 cm 228 kg 30 Aug. 1998 20

Male 192 cm 120 kg 18 Mar. 1997 15

d

I

HHRhHI•HHii^ni

Fig. 2 A variety of grazing scars on E. acoroides leaves. Dugong-grazed seagrass blade sections were not usually cul in straightlines, but had similar shapes at the same aboveground height, a. Dugong grazing scars on E. acoroides leaves found insidefeeding trails dominated by H. ovalis. b-f. Marks resembling dugong molars were observed on E. acoroides leaves, c-f.The hard part of leaf edges tended to remain on E. acoroides leaves grazed by dugongs.

Dugong grazing scars confirmed in Enhalus acoroides meadows

of two dugongs are shown in Table 1. Similar marks of

dugong molars were also observed on grazed sections

of E. acoroides leaves at Sites 1 and 9. The mean

length from the tip of the mouth to the molar of the

three headframe specimens was 16.7 ± 2.9 cm (Table

3). which was similar to the mean length of grazed

/:'. acoroides leaves at Site 1 (14.7 ± 5.6 cm; Table

2: Mann - Whitney U-lesl. p> 0.01). In addition, the

lengths of grazed E. acoroides leaves at Site 9(21.4 ±

3.6 cm: Table 2) did not differ significantly from mean

dugong mouth length (Mann-Whitney U-test. p>().()!).

c

'"•'•-.

4. Discussion

To our knowledge, this is the first study to describe

dugong grazing scars on E. acoroides. Observations in

/-.'. acoroides meadows indicate that grazing scars were

elliptical in shape, whereas dugong feeding trails found

in meadows composed of small seagrass species were

long meandering lines, as reported by Adulyanukosol

et al (2003) and Nakanishi et al. (2005). Crazed E.

acoroides leaves were cut at almost Ihe same height at

both Sites 1 and 9. The difference in length of grazed

E. acoroides leaves between Site 1 and Site A was not

Fig 3 Examples of dugong grazing scars on /•.. acoroides from Site 1. Dugong-grazed seagrass blade sections were not usuallycut in straight lines, but had similar shapes at the same aboveground height a. E. acoroides leaves were cut at almost thesame aboveground leaf length, i.e.. around I 1.7 em. b-f. Marks resembling dugong molars were observed on E. acoroidesleaves, f. The hard pari of leaf edges tended to remain on grazed /;'. acoroides leaves.

Yoshiei Nakanishi. Kanjana Adulyanukosol, Nobuaki Arai. Seiichi llosoya. Yoshiko Nakanishi. Kazuliiko Kolani and Kivonori Kalsukoshi

significant (Mann - Whitney U-test. p> 0.01). but that

between Site 9 and Site A was significant (Mann -

Whitney U-test. p < 0.01). However, the three longest

grazed E. acoroides leaves, which were sampled

within feeding trails at Site A. were longer than the

mean length of grazed E. acoroides leaves at Site 9.

Therefore, dugongs may graze on E. acoroides leaves

at Site 9. In addition, the characteristics of grazed

E. acoroides plants at Sites 1 (Fig. 3) and 9 (Fig. 4)

were almost the identical to those at Site A (Fig. 2).

and dugong tooth marks were identified. The lengths

of E. acoroides leaves left behind at Sites 1 and 9 did

not differ significantly from the lengths from the tip

of the mouth to the molars of the dugongs (Mann

- Whitney U-test. p> 0.01 ). Based on these results,

dugongs appear to feed upon E. acoroides plants

using their molar teeth. Green turtles are also large

marine animals that graze on seagrasses: however,

the characteristics of green turtle grazing are distinct

from those observed at Sites 1 and 9. In general,

green turtles graze in a clean, straight line, and green

turtle grazing scars are typically dented in the central

Fig. 1 Examples of dugong grazing scars on /;. acoroides from Site 9. Dugong-grazed seagrass blade sections were not usuallycut in straight lines, but had similar shapes at the same aboveground height, a. E. acoroides leaves were cut al almost thesame aboveground leaf length, i.e.. around 21.-1 cm. b-f. Marks resembling dugong molars were observed on /•;. acoroidesleaves, f. The hard pari of leaf edges tended to remain on grazed E. acoroides leaves.

Dugong grazing scars confirmed in Enhalus acoroides meadows

region, resembling a mortar (Tabata and Nakanishi.

2005). Therefore, we concluded that the grazing scars

at Sites 1 and 9 were created by dugongs. rather than

green turtles.

The teeth of hindgut-fermenting herbivores usually

play an important role in food breakdown: however,

the morphology, small size, and occlusal variability of

dugong cheek teeth may indicate the lack of strong

selective pressure to maintain effective dentition

(Layon and Sanson, 2006). The horny pads of dugongs

may play a major role in food comminution. However,

our study indicates that dugongs feed on E. acoroides

leaves using their molar teeth. Moreover, the hard,

cellular tissue at the leaf edge tended to remain on

grazed E. acoroides leaves. Enriquez et al. (1992)

reported that E. acoroides leaves were thicker than

those of small seagrasses. Therefore, dugongs may

graze on the soft, thin leaves of small seagrasses using

the horny plate alone, whereas the molars are required

to cut tougher E. acoroides blades. Further studies,

including underwater observation and observation of

captive animals, are required to further clarify dugong

feeding habits.

Acknowledgments

We thank Dr. Takeshi Hara, and all who helped in

this research.

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