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Quality of the Seagrass Halophila Ovalis on a Thai Intertidal Flat As Food for the Dugong

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Quality of the Seagrass Halophila Ovalis on a Thai Intertidal Flat As Food for the Dugong Journal of Oceanography, Vol. 61, pp. 183 to 186, 2005 Short Contribution Quality of the Seagrass Halophila ovalis on a Thai Intertidal Flat as Food for the Dugong 1 2 MASUMI YAMAMURO * and ANONG CHIRAPART 1Geological Survey of Japan, AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba 305-8567, Japan 2Department of Fishery Biology, Faculty of Fisheries, Kasetsart University, Chatuchak, Bangkok 10900, Thailand (Received 26 September 2003; in revised form 28 May 2004; accepted 28 May 2004) To determine whether dugongs along the Thai coast prefer relatively faster-growing Keywords: Halophila ovalis due to its nutritional value, we analyzed the contents of carbon (C), ⋅ Growth rate, nitrogen (N), phosphorus (P), ash, fiber, and lipids in several species of seagrass ⋅ lipid, ⋅ collected near dugong feeding trails. CNP concentrations in the faster-growing H. fiber, ⋅ ovalis were distinctly lower than those in the slower-growing species. Lipid concen- nitrogen, ⋅ phosphorus. trations were comparatively not as low; they were equivalent to the values of the slower-growing Enhalus acoroides in the leaves and to Thalassia hemprichii and Cymodocea rotundata in the rhizomes and roots. The ash content of H. ovalis was the highest of all species in both the above and below ground parts. The reason that dugong feeds exclusively on H. ovalis at this site may be the potentially large supply due to its high growth rate, rather than its nutritive qualities. 1. Introduction carbohydrate content are good descriptors of nutrient The dugong (Dugong dugon) is a seagrass-depend- value. De Iongh et al. (1995) further suggested the possi- ent marine mammal of tropical and subtropical coastal bility that dugongs preferentially graze seagrasses with waters. It feeds mainly on seagrass species in the genera higher rhizome organic C content. Zostera, Posidonia, Thalassia, Enhalus, Cymodocea, No study has so far compared the nutritional values— Halodule, Syringodium, and Halophila (Heinsohn and CNP, fiber (=partially digestible), and lipid (=easily di- Birch, 1972; Heinsohn et al., 1977; McRoy and gestible) content—of faster- and slower-growing Helfferich, 1980), generally selecting the faster-growing seagrasses in areas where dugongs selectively forage on species Halodule uninervis and Halophila ovalis over the the former. It is therefore not clear whether the animals sympatric, slower-growing Cymodocea rotundata, prefer faster-growing seagrasses due to their nutritional Thalassia hemprichii, and Zostera capricorni (De Iongh quality or their greater availability. et al., 1995; Preen, 1995). Relative leaf growth rates were In 1998, a seagrass bed of approximately 3-ha ex- similar between C. rotundata and T. hemprichii, while tended over the intertidal flat at Khao Bae Na, located in the growth rate of Enhalus acoroides was 20% lower the Haad Chao Mai Marine National Park on the south- (Cebrián et al., 1998). western coast of Thailand (Fig. 1). Mukai et al. (1999) Cebrián and Duarte (1998) hypothesized that herbiv- estimated the total biomass of H. ovalis and C. rotundata ores feed selectively on faster-growing seagrass species in that bed to be 1300 kg and 545 kg dry weight, respec- because of the higher nutritional quality compared with tively. From 11 to 23 March 1998, they also observed the slower-growing species. They suggested that herbivore behavior of a dugong, the seagrass distribution, and the preference for faster growing species is independent of grazing trails left by the dugong in the seagrass bed. At leaf nutrient concentrations (i.e., C:N, C:P), and that a high tide during the daytime, the dugong (probably the low content of indigestible fiber (i.e., lignin) and a high same individual) left 6 to 29 (15 on average) new feeding trails per day in the 1-ha quadrat, mostly feeding in the H. ovalis bed and avoiding the slower-growing C. * Corresponding author. E-mail: [email protected] rotundata and E. acoroides (Mukai et al., 1999). Copyright © The Oceanographic Society of Japan. By the end of October 1998, the bed had become 183 3. Results and Discussion CNP concentrations (Table 1) were within the ranges reported for tropical seagrasses (Duarte, 1990). Among the analyzed seagrasses, CNP concentrations in leaves and petioles were lower in the faster-growing H. ovalis than in the slower-growing species (Table 1). The ash content of H. ovalis was highest in both the above and below ground parts. Leaf blades and sheaths (leaves and peti- oles for H. ovalis) had higher concentrations of N and P than in rhizomes and roots, both in slower-growing spe- cies (E. acoroides, T. hemprichii, and C. rotundata) and in faster-growing H. ovalis (Table 1). Yamamuro et al. (2001) measured the CNP concen- trations of the leaf and petiole of H. ovalis at Khao Bae Na (Fig. 1) every month from April to October 1998. The Fig. 1. Map showing the study site. concentrations were 10.7–17.7% for C, 0.62–1.2% for N, and 0.70–1.4 mg g–1 for P. The CNP concentrations of H. ovalis at St-B were thus not exceptionally low for this coast. Similarly low concentrations (19.5% for C, 0.59% buried in silt, reducing the coverage of H. ovalis to 0– for N, and 1.5 mg g–1 for P) and high ash content (57– 10% of the area (Nakaoka and Supanwanid, 1999). After 60%) were also reported from intertidal H. ovalis in Aus- the siltation at Khao Bae Na, the dugong changed its feed- tralia (Birch, 1975), and Yamamuro et al. (2001) suggested ing site to Laem Yong Lam (Fig. 1). Again, its feeding that emersion during ebb tide caused the low nutrient tracks were observed only in H. ovalis patches (M. concentrations of H. ovalis growing on intertidal flats, Yamamuro, personal observation). presumably due to limited access to water column nutri- For the present study we collected several species of ents. The CNP concentrations of C. rotundata at St-B were seagrass near dugong trails on the tidal flat at Laem Yong no lower than the other species at St-A, which may sug- Lam and analyzed their nutritional values (CNP, ash, fiber, gest that limited access to water column nutrients does lipids) to determine whether the dugongs on the coast of not lower the CNP concentrations of slower-growing spe- Thailand prefer the faster-growing H. ovalis because of cies. its nutritional value. De Iongh et al. (1995) suggested the possibility that dugongs preferentially graze seagrass with higher rhizome 2. Material and Methods organic C concentration. Our analysis of seagrasses from We collected E. acoroides and T. hemprichii from the Thai coast did not support that hypothesis, because C the St-A and H. ovalis and C. rotundata from St-B at Laem concentration of rhizome and root in H. ovalis was less Yong Lam in December 1998 (Fig. 1). At St-A, the mean than other seagrasses (Table 1). water depth relative to the mean sea level was 1.3 m, while Cebrián and Duarte (1998) expected that nutritional St-B emerges completely during ebb tide. At St-A, E. quality would be higher in faster-growing seagrass spe- acoroides and T. hemprichii were the dominant seagrasses cies than in slower-growing species. They suggested that (Koike et al., 1999), while H. ovalis was the most abun- a low content of indigestible fiber (i.e., lignin) and a high dant species at site St-B, with some patches of C. carbohydrate content are good descriptors of nutrient rotundata (M. Yamamuro, personal observation). value. On the Thai coast, the fiber content of H. ovalis The samples were collected randomly. Epiphytic leaf was lower than other species. However, this does not materials were removed and the seagrass samples were imply a higher carbohydrate content because the carbon separated into leaf blades, leaf sheaths, and roots plus content of H. ovalis leaf was lower than other species. rhizomes, except in the case of H. ovalis, which was sepa- The lower fiber content of H. ovalis leaf was presumably rated into leaves plus petioles, and rhizomes plus roots. due to the higher content of ash. Thus, the nutritional C and N content were determined by the combustion quality of faster-growing H. ovalis would not be higher method using an elemental analyzer (EA1108, Fisons In- than other species, as far as fiber and carbohydrates are struments). Concentrations of P were determined concerned. colorimetrically after digestion following the method Lanyon and Marsh (1995) found that the mouth-to- described in Ohtsuki (1982). Each measurement was de- anus retention time (146–166 h) of dugongs is the long- termined twice. Ash, fiber, and lipid contents were deter- est among the hindgut fermenters. They further suggested mined once as described by the AOAC (1984). that dugongs may prefer seagrasses with lower fiber con- 184 M. Yamamuro and A. Chirapart Table 1. Nutrient concentrations of seagrasses collected along the coast of Thailand. Sample Carbon Nitrogen Phosphorus Ash Fiber Lipid (relative growth rate) (%dw) (%dw) (mg g−1) (%dw) (%dw) (%dw) Enhalus acoroides leaf blade 31.4 1.90 2.64 24.6 19.8 0.38 (slowest) leaf sheath 29.6 1.38 2.30 28.9 20.8 0.53 rhizome and root 36.0 0.70 0.97 13.8 16.3 0.17 Thalassia hemprichii leaf blade 33.5 2.33 2.41 22.6 17.9 0.29 (slow) leaf sheath 31.3 1.46 2.17 32.0 18.2 0.71 rhizome and root 32.0 0.83 1.80 19.8 15.5 0.34 Cymodocea rotundata leaf blade 38.9 3.02 2.36 13.9 23.6 0.28 (slow) leaf sheath 34.3 1.72 2.23 25.4 26.7 0.70 rhizome and root 34.3 0.86 0.74 16.1 19.1 0.34 Halophila ovalis leaf and petiole 25.1 1.30 1.31 42.9 13.5 0.38 (fast) rhizome and root 17.7 0.41 0.53 29.7 22.9 0.35 centration to almost completely digest the fiber.
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