The two species belonging to the genus Thalassia (Family: Hydrocharitaceae), T. testudinum Banks ex Konig and T. hemprichii (Ehrenberg) Ascherson are widely distributed in shallow coastal areas in the tropics and subtropics of the Western Atlantic and Indo-Pacific, respectively (den Hartog, 1970; Phillips andMefiez, 1988; Spalding etal., 2003), and they are considered to be 'twin species'. The current hypothesis, based on paleographical data, is that they
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derived from a common ancestor in the Tethys Sea, until continental drift separated the modern West Pacific and West Atlantic (before the Miocene, >24 million years ago). Lumbert et al. (1984) reported a fossil leaf of Thalassia in the Avon Park Formation, Florida from the late Middle Eocene, which was identical to leaves of present-day Thalassia. Both species are very similar morphologically, and on a macro-scale, they can only be separated on the basis of counts and dimensions of the styles and stamens of the flowers (see section VI.B). However, genetically, the two species show divergence, as was shown by McMillan (1980) using allozymes, and very recently by Waycott and Les (2000) using chloroplast ribo-somal (trnL) DNA loci and nuclear ITS sequences.
A. WD. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp. 409-439. © 2006 Springer. Printed in the Netherlands.
Considering the habitat types where the two species of Thalassia occur, their beds could potentially extend over huge areas (hundreds of thousands of square km). However, no serious attempts at mapping their total surface area have been undertaken. Even without detailed knowledge of their exact distribution and abundance, both species are known to play an enormous role in the ecological equilibrium of many tropical coasts. Thalassia-dominated-beds can either be found in marine lagoons or coastal estuaries, which are often fringed by mangroves or coral reefs, and it is very likely that narrow interactions exist between seagrass, coral reef, and mangrove systems. These interactions include modification of hydrodynamic environment (the reefs provide low wave-energy environments necessary for the establishment of seagrasses and mangroves, and the seagrasses reduce water motion on a smaller scale which allows stabilization of sediments), sediment production (by the reef organisms and calcareous algae associated with the seagrasses), export of organic material (mainly by seagrasses and mangroves toward the reef), and migratory movements of fauna between the three systems (Ogden and Glad-felter, 1983; Bosence, 1989; Hemminga et al., 1994; Holmer et al., 1999; Nagelkerken et al., 2000). Tha-lassia has the potential to attain high biomass and production rates, and Duarte and Chiscano (1999) who compiled data from many sites, reported an average aboveground biomass of 519 and 87 g dry mass m-2 for T. testudinum and T. hemprichii, respectively. The differences in mean aboveground biomass between the two species are most likely not due to differences in the plant body but caused by differences in the environmental settings and co-occurring seagrass and macroalgae species in the Atlantic and Indo-Pacific (see also Section VIII). These two species have the best developed root and rhizome system of all seagrass species in the western Atlantic, which is better developed than most other grasses in the Indo-Pacific. Belowground biomass (which includes all tissues except for the leaves) accounts for ~80-92% of the total biomass (Brouns, 1985; van Tussenbroek, 1996; Kaldy and Dunton, 2000). Duarte and Chiscano (1999) reported mean aboveground production rates of 5.0 and 3.7 g dry mass m-2 day-1 for T. testudinum and T. hemprichii, respectively; belowground tissues (excluding roots) only contribute between 5 and 27% (generally varying between 10 and 20%) of the total production for this genus (Brouns, 1985; Gallegos et al., 1993; Ver-
maatetal., 1995; van Tussenbroek, 1996), but Kaldy and Dunton (2000) reported an exceptionally high contribution of 35% by rhizomes in the total plant production for T. testudinum in Laguna Madre. Although Thalassia is often the dominant primary producer in tropical coastal seagrass communities, other macrophytes (seagrasses and macroalgae), benthic and epiphytic diatoms and phytoplankton also contribute, to a greater or lesser degree, to the total community production, and Thalassia-dominated meadows are amongst the most highly productive marine systems on Earth (Westlake, 1963; Erftemeijer, 1993; Duarte and Chiscano, 1999).
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