*Vice Chancellor, Thiruvalluvar University Centre of Advanced Study in Marine Biology, Annamalai University

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1 Seagrasses Prof. L. Kannan* and Dr. T. Thangaradjou *Vice Chancellor, Thiruvalluvar University Centre of Advanced Study in Marine Biology, Annamalai University S eagrasses are the marine flowering plants. They are the only angiosperms that successfully grow in tidal and subtidal marine environment. Seagrasses belong to the families, Hydrocharitaceae and Potamogetonaceae and they are in no way related to the terrestrial grasses of Poaceae. There are 13 genera and 58 species available all over the world. Of these, six genera (Amphibolis, Heterozostera, Phyllospadix, Posidonia, Pseudalthenia and Zostera) are mostly restricted to temperate seas and the remaining seven genera (Cymodocea, Enhalus, Halodule, Halophila, Syringodium, Thalassia and Thalassodendron) are distributed in tropical seas. Seagrass meadows are conspicuous and wide spread in the shallow marine environs throughout the world, producing a greater amount of organic matter and serving as a good substratum for a variety of epiphytic algae including diatoms (Smith, 1991) and sessile fauna. As mangrove and coral reef ecosystems are closely associated with the seagrass ecosystem, there is a lot of export of organic matter and nutrients form the latter. Seagrass meadows are highly productive and dynamic ecosystems, which rank among the most productive ecosystems of the oceans (McRoy and McMillan, 1977; Hillman et al., 1989). Higher primary production rates of seagrasses are closely linked to the higher production rates of associated fisheries and thus the seagrass communities make significant contributions to the coastal productivity. Several reports indicate that seagrass biomass is the prime factor influencing the organization of marine macrofaunal communities. They also control the habitat complexity, species diversity and abundance of associated invertebrates and thereby shaping the structure of the marine communities.

2 223 Seagrasses Importance The true importance of seagrass meadows to the coastal marine ecosystem is not fully understood and generally under estimated. The rapidly expanding scientific knowledge on seagrasses has led to a growing awareness that seagrasses are valuable coastal resources. Where seagrasses abound, humans benefit directly and indirectly from the presence of this marine vegetation (Marten and Carlos, 2000). Eventhough, they contribute a smaller part to taxonomy, these plants are important for structuring a number of ecosystems, stabilizing coastlines, providing food and shelter for diverse marine organisms and act as a nursery ground for many fishes of commercial importance. 1. Seagrass meadows enhance the biodiversity and habitat diversity of coastal waters. It has been estimated that over 153 species of microalgae (mostly diatoms), 359 species of macroalgae and 178 species of invertebrates are found on the seagrass blades as epiphytes and epizooties (Phillips and McRoy, 1980). 2. A Seagrass meadow also acts as nursery and foraging area for a number of commercially and recreationally important fish and shellfish and other organisms. There are about 340 animals including green turtles which directly feed on the seagrasses and their epiphytes. Besides, the marine mammal Dugong, solely feeds on seagrasses. 3. Seagrasses improve water quality by acting as roughness elements that deflect currents and dissipate the kinetic energy of the water and thereby creating a relatively quiet environment favourable for sediment deposition and retention. With the help of their well developed root system, they bind the sediments and stabilize them. 4. Some of the species form reef like structures close to the water surface that dissipate the wave energy before it reaches the shoreline. 5. Seagrasses play an important role in carbon and nutrient cycling in the marine environment. The large biomass produced by the seagrasses and their epiphytes act as sink for carbon in the oceans. Seagrass meadows are also involved in nitrogen cycling through nitrogen fixation (eg. Posidonia oceanica meadows account for an annual input of 57x10 10 gn in the Mediterranean).

3 L. Kannan and T. Thangaradjou Seeds of Enhalus acoroides are used as food by the coastal populations as the nutritional value of the flour derived from the seeds is comparable to that of wheat and rice in terms of carbohydrate and protein content and in energetic value and even it surpasses these types of flour in calcium, iron and phosphorous content (Montano et al., 1999). Coastal people use rhizomes of Cymodocea sp. (nicknamed as sea sugarcane) as food, for the preparation of salad. 7. Seagrasses are used as filling material for mattresses and shock absorbing materials for the transport of glasswares. 8. Seagrasses are also used as raw materials in paper industry and in the production of fertilizer, fodder and feed. Most of the seagrasses are used extensively as soil fertilizer for coconut and other plantations. 9. A variety of medicines and chemicals are also prepared from them. Agar like substance, zosterin is extracted form Zostera sp. Collection, Preservation and Identification of Seagrasses Flowering in seagrasses is rare and ephemeral and hence sufficient care should be taken to collect seagrasses with all the developmental stages of male and female flowers and fruits. During collection, seagrasses should be uprooted with care to keep the underground parts intact and washed in the field itself to remove sediments and epiphytes without making any damage to the plants. Then the specimens should be poisoned with 1% mercury chloride solution and pressed and dried for preservation. The preserved materials could be pasted on mounting boards. If the specimens are slender and fragile, then they should be spread neatly on a mounting board submerged in a tray containing water and the board should be gradually lifted allowing the excess of water to drain. The board with the specimen should then be kept in blotters for drying. After drying, rectified sprit saturated with mercuric chloride can be brushed on the plants and allowed to dry (Ramamurthy et al., 1992).

4 225 Seagrasses Basic components of seagrass architecture The poisoned, pressed and dried specimens can neatly be pasted on mounting boards. Fresh materials of various developmental stages with fruits can also be fixed in 25% rectified sprit mixed with seawater. The specimens can be identified using the standard seagrass works including the keys given by Hartog (1970) and Ramamurthy et al. (1992). Key to the Identification of Indian Seagrasses Based on Vegetative (modified from Ramamurthy et. al., 1992) 1a. Leaves ligulate; male flowers without tepals; fruits indehiscent Potamogetonaceae 2a. Leaves terete, fleshy, grooved along adaxial side for a short distance; nerves absent Syringodium isoetifolium 2b. Leaves flat, not fleshy, without grooves; nerves present

5 L. Kannan and T. Thangaradjou 226 3a. Rhizomes usually moniliferous, with scales; nerves 3, lamina mm broad Halodule 4a. Leaf tips obtuse, serrulate; lateral teeth poorly developed or absent H. pinifolia 4b. Leaf tips not obtuse, not serrulate; lateral teeth well developed: 5a. Leaf tips tridentate; median tooth present; lamina mm wide H. uninervis 5b. Leaf tips bidentate; median tooth absent; lamina mm wide H. wrightii 3b. Rhizomes not moniliferous, without scales; nerves 7 22; lamina 4 10 mm broad Cymodocea 6a. Leaf scars forming closed rings; sheaths persistent; lamina 3 6 mm broad, rarely serrulate at apex; nerves 9 14 C. rotundata 6b. Leaf scars forming opened rings; sheaths not persistent; lamina 4 10 mm broad, serrulate at apex; nerves C. serrulata 1b. Leaves eligulate; male flowers with 3 to 6 tepals; fruits dehiscent (except Halophila) Hydrocharitaceae 7a. Leaves differentiated into petioles and blades; lamina oblong, elliptic, linear, ovate, obovate or spathulate without tannin cells Halophila 8a. Leaves 6 12 at each node; cross veins absent. H. beccarii 8b. Leaves 2 at each node; cross veins present: 9a. Lamina hairy, margins serrulate:

6 227 Seagrasses 10a. Plants dioecious; leaves linear oblong 10b. Plants monoecious; leaves oblong elliptic H. stipulacea H. decipiens 9b. Lamina glabrous, margins entire: 11a. Seeds 6 12 H. ovalis subsp. ramamurthiana 11b. Seeds 20 or more: 12a. Lamina mm long; cross veins pairs H. ovalis subsp. ovalis 12b. Lamina 4 15 mm long; cross veins 3 9( 11) pairs H. ovata 7b. Leaves not differentiated into petioles and blades; lamina linear with tannin cells: 13a. Rhizomes 10 to 20 mm thick, without scales; roots stout; leaves ca 100 cm by 17 mm Enhalus acoroides 13b. Rhizomes 2 to 5 mm thick, with scales; roots thin; leaves ca 16 cm by 12 mm Thalassia hemprichii Description of Species Enhalus acoroides (L.F.) Royle Habit: Perennial; plants growing up to 2m in length and spreading by producing rhizomes which are covered with decayed old leaves; plants fixed in soil by numerous unbranched spongy white roots. Habitat : Purely marine forms, growing in shallow coastal areas, capable of withstanding higher salinity and wave actions.

7 L. Kannan and T. Thangaradjou 228 Leaves: 4 6 at each erecting shoot, longer than broad (length: cm and width: 2 3 cm). Inflorescence: Spathed inflorescence, dioecious; male inflorescence always submerged; female inflorescence floating on water surface before fertilization. After fertilization, the peduncle becomes coiled and contracted below the water surface. Perianth: Tepals 3, elliptic, white in case of male flowers whereas in female flowers, reddish streaks are seen on white tepals. Ovary: Doom shaped, incompletely separated into six chambers. Fruit: Ovoid, covered with numerous black appendages, irregular opening during seed dispersal, seeds. Halophila beccarii Asch. Habit : Plants monocious, rhizome creeping; one root at each node; lateral shoots erect, growing up to 2 cm long. Habitat : H. beccarii occupies the lower part of the eulittoral zone in shallow river mouths, estuaries, backwaters and mangrove swamps. It grows as pure stands in muddy soils and as mixed population with H. ovalis and Halodule pinifolia in sandy soil. Leaves: 6 10, petiolate, arranged in pseudo whorl with 2 scales at the base of the whorl; leaves lanceolate, broadly acute at the tip with entire margin and a prominent midrib without any cross veins. Inflorescence: Solitary, axillary flowers. Perianth: Transparent white tepals with darker midrib. Ovary: Female flowers upto 1.7 cm long, ovary oblong with 2 4 ovules. Fruit: Ellipsoid to ovoid with 1 4 seeds. Seeds: Globose, reticulate, hard, reddish brown. Halophila decipiens Ostenf. Habit : Monocious plants with slender, branched and creepting rhizomes; roots solitary, unbranched arising from each node of the rhizome. Habitat : H. decipiens is a purely marine form found growing in soft muddy sand to fine coarse sand; mostly occurring as single stand, sometime associated with green algae.

8 229 Seagrasses Leaves: Two at each node, petiole faintly triquetrous, long up to 10 cm, lamina oblong with serulate margins and hairs on both surfaces, 5 9 pairs of cross veins joining the intramarginal nerves which run at a distance of about 0.5 mm from the margin. Inflorescence: Male flowers occur at the base of the subsessile female flowers, male flowers grow up to 3 mm long, female flowers, up to 6 mm long. Perianth: Tepals 3, ovate to elliptic. Ovary: Oblong to ovoid. Seeds: Globose, bluntly beaked at both the ends, seeds up to 26 in number. Halophila ovalis (R.Br.) Hook. subsp. ovalis Habit : Plant shows morphological diversity due to habitat variations; separate male and female plants with branched, creeping, slender rhizomes; root single with root hairs, at each node of the rhizome. Habitat : Plants occur in both purely marine environs and in backwater areas. Marine forms grow on coarse sands in the sea and on the muddy substratum in tidal and subtidal zones. The backwater forms grow along the shallow margins of estuaries and mangrove creeks where the substratum may vary from black mud to clay. Leaves: Paired at each node with long petiole; a leaf with petiole may measure upto 4 12 cm. Inflorescence: Flowers solitary, axillary, covered by two spathes. Perianth: Tepals 3, broad elliptic. Ovary: Ellipsoid in shape, 1 celled with pointed apex. Fruit: Ovoid to ellipsoid with seeds. Seeds: Globose, white in colour when young but brown when mature. Halophila ovalis subsp. ramamurthiana Ravikumar & Ganesan

9 L. Kannan and T. Thangaradjou 230 Habit : Male and female plants are separate; rhizomes, slender transparent growing up to 2.6 cm long; roots solitary at each node with root hairs. Habitat : Found growing well in the substratum having loose, soft black mud full of decayed plants. The male and female populations of this species are found growing in separate patches. Recorded only in the backwaters and not from open sea. Leaves: Paired at each node, petiolate, petiole 0.3 to 6 cm long, lamina oblong, glabrous, assymmetrical at base with entire margin, cross veins 7 16 pairs, sub opposite or alternate, mid rib never extend beyond the intramarginal nerve at the apex. Inflorescence: Solitary, flowers axillary, dioecious covered by two spathes; female flowers sessile. Perianth: Tepals three, ovate, mucronate at apex. Ovary: Ellipsoid and ovoid, papillose adaxially. Fruits: Ovoid normally with 6 12 seeds, sometimes with 18 seeds in viscous fluid. Seeds: Globose, beaked at both ends. Halophila ovata Gaud. Habit : Separate male and female plants. Fleshy, unbranched (rarely branched) rhizomes, unbranched solitary roots at the nodes upto 6cm long, roots with root hairs. Habitat : Usually inhabits sheltered localities, waveless open seas and backwaters. Prefers fine coarse sandy, soft muddy bottom and coral rubbles. Leaves: Paired at each node, lamina transparent, oblong, glabrous with 4 24 mm long petiole; lamina with entire margin, cross veins 3 9 pairs, sub opposite or alternate, merging with intramarginal nerves. Inflorescence: Solitary axillary flowers covered by spathes; female flowers subsessile. Perianth: Tepals 3, margins entire, concave and transparent. Ovary: Ellipsoid; styles 3, all inserted at the same point, papillose adaxially.

10 231 Seagrasses Fruits: Ovoid with 20 seeds. Seeds: Globose, beaked at both the ends, white when young and brown when mature. Halophila stipulacea (Forsk.) Asch. Habit : Plants dioecious, rhizomes creeping, branched and fleshy; roots solitary at each node of the rhizome, unbranced and thick with dense soft root hairs. Habitat : It is purely a marine form and not occurring in brackish waters. It grows in sheltered localities as isolated patches either as pure forms or mixed with Halophila ovalis, H.ovata, Cymodocea rotundata and Halodule sp. It prefers muddy bottom and coral rubbles. Leaves: 2 at each shoot node, petioles upto 12 mm long, flat and with sheathing leaf base, lamina linear oblong in shape with serrulate margin and hairy on both the surfaces, cross veins, 4 14 pairs. Inflorescence: Flowers solitary, axillary covered by spathes. Male flowers not found so far. Perianth: Absent. Ovary: Ellipsoid with 3 styles. Fruits: Ovoid to globose, seeds upto 27. Seeds: Globose, beaked at both the ends. Thalassia hemprichii (Ehrenb.) Asch. Habit : Male and female plants are separate, perennial with creeping rhizome; rhizome with scales and scale scars; shoot erect, with 2 6 leaves; shoot covered by old decayed leaves. Habitat : Purely a marine form, not seen in backwaters and estuaries; plants occur in tidal and subtidal zones, in black muddy and loose sandy soils. Leaves: 3 7 in each shoot, measuring up to 15.5 cm in length and 1.2 cm in width, leaf blade linear, sometimes leaf tips show some serration. Inflorescence: Flowers single, covered by spathe. Perianth: Tepals 3, elliptic in shape.

11 L. Kannan and T. Thangaradjou 232 Fruit: Globose, rough coated, showing 3 distinct ridges, bursting into 8 13 irregular valves. Seeds: Globose, white in colour when young but brown when mature. Cymodocea rotundata Ehrenb. & Hempr. ex Asch. Habit : Male and female plants are separate; rhizomes creeping, branched, jointed; roots single at each node, branched, 2 4 mm thick; shoots erect, up to 31 cm long, each shoot bearing 3 4 leaves with persistent leaf scars. Habitat : Purely a marine form, found growing in sheltered and shallow regions of tidal and subtidal zones; plants prefer fine sand to coarse sand and also substratum with mud and coral rubbles; usually grows as pure patches and often found along with C. serrulata, Halophila stipulacea, Halodule uninervis and H. pinifolia. Leaves: Linear, narrowed at base, emarginate, rarely faintly serrulate at apex, cm long, 9 14 nerved. Inflorescence: Stalked male flowers and sessile female flowers are axillary in position. Perianth: No organized tepals. Ovary: Very small, stigma spirally coiled, upto 30 mm long. Fruits: 1 or 2, sessile, semicircular in shape. Cymodocea serrulata (R.Br.) Asch. & Magnus Habit : Male and female plants are separate, perennial; plant with creeping rhizome, rhizome with scales and scale scars; shoot erect, with 2 6 leaves, shoot covered by old decayed leaves. Habitat : Purely a marine form, not seen in backwaters or estuaries; plants grow in shallow water areas up to 1m depth on fine to coarse sand with mud. Leaves: 2 5, in each branch; leaf sheaths broadly triangular. Inflorescence: Flowers solitary, terminal and become lateral in due course due to the production of successive lateral shoots. Perianth: No organized tepals.

12 233 Seagrasses Ovary: Globose. Fruits: 2, ellipsoid, 4 angular, rarely an immature fruit is seen on each developed fruit. Halodule pinifolia (Miki) Hartog Habit : Male and female plants are separate; rhizomes slender and branched, roots creeping, formed at each node, branched. Habitat : It generally grows on sandy to muddy soils along the coasts, mangrove creeks, coral platforms etc. Leaves: Measure about 1 6 cm long and 1 5 mm width, linear with entire margin and with 3 prominent midribs, lateral ribs form lateral teeth on leaf apex. Inflorescence: Two flowers enclosed by leaf sheaths. Tepals: Not well organised. Ovary: Ovoid with filiform terminal styles and becomes subterminal or lateral in fruits. Fruit: Globose, seen in pairs with persistent lateral styles; seed coat hard with ornamentations. Halodule uninervis (Forsk.) Asch. Habit : Male and female plants are separate, rhizome creeping, branched and moniliferous; roots unbranched, 1 6 at each node. Habitat : Plant prefers fine sand to coarse sand, black mud, rock and coral rubbles; found to occur in open seas, sheltered localities, backwaters, estuaries and margins of mangrove creeks. Leaves: Shoots upto 30cm long and erect, having 2 4 leaves in each branch, leaf linear, narrowed at base with sheath, margin entire, nerves 3, midrib conspicuous and lateral ribs inconspicuous, ending in well developed lateral teeth at leaf apex, teeth tridentate. Inflorescence: Male flowers 2, subequal, female flowers sessile and enclosed in leaf sheaths. Perianth: Absent. Ovary: Ovoid with 2 3 (8cm long) slender, smooth, terminal styles.

13 L. Kannan and T. Thangaradjou 234 Fruit: Subglobose to ovoid, usually in pairs with persistent styles. Halodule wrightii Asch. Habit : Plant dioecious, rhizome creeping, branched, often moniliferous; internodes upto 2.6 cm long with 1 7 roots at each node; roots upto 13cm long, unbranched. Habitat : Plants found growing in fine sands, black mud and coral platforms, usually growing as mixed populations with H. pinifolia. Leaves: 5 12 cm long, lamina linear, narrowed at base, covered; leaf margin entire, nerves ending in lateral teeth at the leaf tip, teeth bidentate. Inflorescence: Male flowers 2, subequal; female flowers sessile and enclosed by leafy sheaths. Perianth: Absent. Ovary: Ovoid, with upto 2.8 cm long styles. Fruit: Globose or ovoid, mostly found in pairs with persistent lateral styles. Syringodium isoetifolium (Asch.) Dandy Habit : Herbaceous plants; rhizomes creeping, shoots erect, branched, bearing 2 3 leaves; rhizomes and shoots having scars; rhizomes produce branched roots at each node. Habitat : It generally grows well on coral flats, but also grows on sandy to muddy bottoms. It is not seen in backwaters and estuaries. Leaves: Leaf tubular, narrowed at base and pointed at the apex. Inflorescence: Flowers in terminal cymes, growing upto 29 cm long. Perianth: Anthers and ovaries are protected by reduced leaves; no organised tepals. Ovary: 2, ovoid, one style with bifid stigma. Fruit: Ellipsoid with hard pericarp.

14 235 Seagrasses Factors Affecting Seagrass Biodiversity As all the other natural resources, it has also become evident that seagrasses are a vulnerable resource and subjected to various kinds of destructive disturbances all over the world, due to both natural and man induced influences. It is due to both natural pressure and anthropogenic influences such as inputs of nutrients, discharge of industrial and other wastes etc. Natural causes of seagrass decline includes geological and meteorological events and specific biological interactions. Earthquakes cause rise in shoreline and subsequent exposure of the seagrass vegetation. The ash and debris rains that accompany volcanic eruptions may eradicate seagrass beds. Above all, hurricanes and cyclones repeatedly cause damage to the seagrass meadows. These windstorms coincide with huge waves and strong currents over the seafloor, which may uproot seagrasses and erode the sediment surfaces. Seagrass vegetation loss is also due to grazing by waterfowl, mortality due to low winter temperatures and prolonged exposures to sunlight. Sea urchins grazing on Posidonia beds and small limpet (Tectura depicta) which consumes the chloroplast containing epidermis of Zostera marina are also responsible for large scale seagrass destruction. Besides these, the wasting disease caused by the marine slime mould Labyrinthula zosterae (Muehlstein et al., 1991) is also responsible for large scale seagrass mortality. Massive use of fertilizers in agriculture and discharge of industrial effluents have led to an exponential increase in nutrient inputs in to the coastal zone which leads to eutrophication, resulting in the outspread of algal blooms. Being benthic plants, seagrasses are unable to compete with the phytoplankton and other algae for light harvesting. Due to poor light intensity, photosynthetic activity in seagrasses becomes lower and lower, finally leading to seagrass death. Higher rates of organic loading in the coastal sediments and mixing up of toxic chemicals are also responsible for large scale seagrass destruction. Siltation due to dredging, filling, channeling and diversion of streams also cause severe damage to the seagrass ecosystem by increasing the turbidity and decreasing the photosynthetic activity, leading to seagrass biomass decrease. Apart from this, fishing activities and anchoring of boats on the seagrass meadows also cause serious threats to the seagrass ecosystem. Such decline in seagrass biomass would adversely affect the commercial fisheries of that area since seagrass meadows are the

15 L. Kannan and T. Thangaradjou 236 nursery grounds for a plethora of economically important shell and finfish larvae. Comparison with Coral Reef and Mangrove Environments (UNESCO, 1983) Coral reefs, seagrasses and mangroves interact physically, chemically (nutrients, dissolved organic matter and particulate organic matter) and biologically (animal migration and human impact) in a number of ways. a) Physical Interaction Reduction of water energy, flow regulation and sediment relationships are important. Seagrasses and mangroves to some extent, depend on coral reefs, the hydrodynamic barriers, which dissipate wave energy and enhance the mangrove and seagrasses communities. Due to the wave action and water currents, the sediments in the reef lagoons form shoals and islands which in due course may be colonized by seagrasses and mangroves. On the other hand, seagrass community may influence the other two communities viz. coral reef and mangroves by trapping and stabilizing sediments and by producing sediments. Trapping and mobilizing sediments favour the coral growth and prevent burial of reefs. Production of (carbonate) sediments by seagrasses in association with calcareous algae, epiphytes and infauna can exceed in some places the contribution made by reefs to sediments. Mangroves act as depositional basins and sediment binders as a result of reduction of sediment load into the coastal waters. They also regulate fresh water flow into the coastal areas. The excess fresh water which flows into this ecosystem, dilutes the excess salts accumulated in the mangrove environment during drought and thus probably buffering the salinity changes. b) Nutrients, Dissolved Organic Matter (DOM) and Particulate Organic Matter (POM): Inorganic nutrients, phosphrous and nitrogen are essential for the primary producers of all the ecosystems. The mangrove, seagrass and coral reef ecosystems utilize dissolved nitrogen and reduce its concentration in water. It is estimated that the mangroves > seagrasses > coral reefs rank in that order in terms of their nutrient requirements. Mangroves are reported to be in high nutrient water. Seagrasses can tolerate eutrophication than the coral reefs which can withstand

16 237 Seagrasses oligotrophic conditions. Both mangroves and seagrasses tend to leak or export nutrients as DOM or POM which in turn nourish the reef organisms. Mangrove runoff and water flowing out of seagrass beds have appreciable DOM which on decomposition releases inorganic nutrients. Although the three ecosystems viz. seagrass, mangrove and coral reef ecosystem produce DOM, its net export is in this direction i.e. Mangroves Seagrasses Coral reefs. Likewise, large amount of POM also enters into sea as partially decomposed mangrove and seagrass leaf detritus. Many organisms (microbes, zooplankton, ciliates, nematodes etc.) act on this detritus and convert it into small fragments, which become a rich food source for larger marine organisms. c) Animal Migration Animal migration forms an important link between coral reefs, seagrass beds and mangroves. The different kinds of migration can be schematized in terms of gains or loss of energy for an ecosystem. Two types of migrations are noticed: (1) short term feeding migrations which are either diel or seasonal, and (2) Life history migrations between the systems. Coral reefs are known for the diversity and abundance of their faunal species. Seagrass beds and mangrove areas are vital nursery areas for many important commercial and forage organisms, as well as some reef species. The nursery functions of the mangroves and seagrasses are due mainly to the availability of shelter for juvenile organisms and abundant supply of food in the form of organic detritus. Numerous organisms have their early life cycle stages completed in the seagrass or mangrove regions and as they grow they show more of schooling habit. However, the animals are forced to migrate as they mature, because they become sufficiently large that the mangroves and seagrasses can no longer supply sufficient protection. Further, a flow of dissolved nutrients from mangroves enhances primary productivity of seagrasses whereas seagrass meadows and mangroves enhance the secondary productivity of the coral reefs by providing alternative feeding sites. Thus, coral reefs, mangroves and seagrass systems are inter connected in many ways. References Hartog, den, C. (1970). The seagrasses of the world. North Holland Publishing Company, Amsterdam, London, 272pp.

17 L. Kannan and T. Thangaradjou 238 Hillman, K., Walker, D.I., Larkum, A.W.D. and McComb, A.J. (1989). Productivity and nutrient enrichment. In: A.W.D. Larkam, McComb, A.J. and Shephered, S.A. (Eds.), Biology of seagrasses A treatise on the biology of seagrasses with special reference to the Australian region. Elsevir, Amsterdam, Kannan, L. and Veluswamy, K. (1989). Seagrasses Novel marine plants. Biol. Edun., 6 (4) : Kannan, L., Thangaradjou, T. and Anantharaman, P. (1999). Status of seagrasses of India. Seaweed Res. Utiln., 21 (1 & 2) : 25c 33. Marten A. Hemminga and Carlos M. Duarte (2000). Seagrass ecology. Cambridge University Press, 298pp. McRoy, C.P, and McMillan, C. (1977). Production ecology and physiology of seagrasses. In: P.C. McRoy and C Helfferich (eds.) Seagrass ecosystems: A scientific Prospective, Marcel Dekker, New York, pp Montano, N.M., Bonifacio, R.S., Rumbaoa, G.O. (1999). Proximate analysis of the flour and starch from Enhalus acoroides (L.f.) Royle seeds. Aquat. Bot., 65 : Muehlstein, L.K., Porter, D. and Short, F.T. (1991). Labyrinthula zosterae sp. nov., the causal agent of the wasting disease of eelgrass, Zostera marina. Mycologia, 83 : Phillips, R.C. and McRoy, C.P. (1980). Handbook of seagrass biology. Garland STPM press, New York, 353pp. Ramamurthy, K., Balakrishnan, N.P., Ravikumar, K. and Ganesan, R. (1992). Seagrasses of Coromandel coast, India. Flora of India Series 4, Botanical Survey of India, pp. 80. Smith, J.J. (1991). Factors influencing the standing crop of diatom epiphytes of seagrass Halodule wrightii Aschers. In South Texas seagrass beds. Contrib. mar. Sci., 32 : UNESCO (1983). Coral reefs, seagrass beds and mangroves: Their interaction in the coastal zones of the Caribbean, Report of a workshop held at West Indies Laboratory, St. Croia, U.S. Virgin islands, 133pp.

18 239 Seagrasses Enhalus acoroides Halophila beccarii Halophila decipiens Halophila ovalis Halophila ovata Halophila stipulacea Thalassia hemprichii Cymodocea rotundata Cymodocea serrulata Halodule pinifolia Halodule uninervis Syringodium isoetifolium