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2. Aquatic Plant Communities of Poikilosaline Waters

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2. Aquatic Plant Communities of Poikilosaline Waters 2. Aquatic plant communities of poikilosaline waters C. den Hartog Lab. of Aquatic Ecology, Catholic University, Toernooiveld, 6525 ED Nijmegen, The Netherlands The euryhaline plant group Table 2.1 Taxonomic composition of the eurysaline group of the Potamogetonaceae. In 1920, Arber formulated a set of four properties Subfamily Genus indispensable for a marine water plant. These properties are: (1), adaptation to life in a saline Ruppioideae Ruppia medium; (2), ability to grow when fully submerged; Zannichellioideae Zannichellia (3), possession of a secure anchoring system; and Althenia (4), hydrophilous pollination. In his monograph of Pseudalthenia Lepilaena the seagrasses of the world, den Hartog (1970) Potamogetonoideae Potamogeton subgen. stated that this set of properties is insufficient for Coleogeton the delimitation of these marine angiosperms, because there is a second group of species which shares these properties with the seagrasses. This group is rather restricted; all representatives belong second group, which is here referred to as the to the family of the Potamogetonaceae s.l. Two eurysaline group, consists of a number of aquatic subfamilies are completely confined to poikilosaline plant species occurring in various fresh waters, habitats, and in a third subfamily only a subgenus is mixo- and hyperhaline brackish waters, and in involved. In fact, the whole group consists of 5 continental waters of very different chemical com- genera and 1 subgenus (Table 2.1). At the species position and concentration. These plants can toler- level there are still many uncertainties and a reliable ate considerable fluctuations in the salt content, estimation of the number of species is not yet and their salinity range of occurrence is very wide in possible. comparison to that of seagrasses (Bourn, 1935; St. John & Courtney, 1924; Davis & Tomlinson, 1974). These plants are also in other respects very tolerant, A review of the genera particularly to high temperatures. Their absence from the marine environment is striking; only 1. Ruppia under very special conditions one can find represen- tatives of this eurysaline group in the sea. Probably The genus Ruppia is almost cosmopolitan in this may be ascribed to the basic rule in ecology that brackish and continental salt waters; there is even a a wide tolerance with regard to environmental number of freshwater records of this genus. fluctuations is coupled with a reduced capacity to The taxonomy of the genus at the species level is compete with more stenobiontic taxa in more or still poorly understood, in spite of various local or less stable habitats (men being the only exception to more general monographic treatments (Ascherson this rule) (See Dahl 1956; den Hartog 1967). & Graebner 1907, 1913; Fernald & Wiegand 1914; From a taxonomic point of view the eurysaline Setchell 1946; Luther 1947; Reese 1962, 1963; Hydrobiologia 81, 15-22(1981). 0018 8158/81/0811-0015/$ 01.60. © Dr W. Junk Publishers, The Hague. Printed in the Netherlands. 16 Mason 1967; Gamerro 1968; den Hartog 1971). 4. Althenia For a long time it has been generally accepted that the genus was monotypic, i.e. consisting of one The genus Althenia is monotypic, as far as is very variable species. Many taxa have been de- known today. Originally the genus was thought to scribed at the variety level. Nowadays it appears be restricted to the western mediterranean coastal that there are at least 7 species, but due to the wide lagoons (Onnis 1967), but in the last 15 years variety of biotopes they occupy, geno- and pheno- records have been published from Iran (Dandy typic variation can only be separated by culture of 1971), Turkey (den Hartog 1975), southern Russia the plants under controlled conditions. From an and Central Asia up to the foothills of the Altai extensive study of herbarium material, it is now Mountains (Tvelev 1975), and South Africa clear that there is one worldwide distributed spe- (Obermeyer 1966). The genus has been found in cies, Ruppia maritima L., characterized by the typical brackish-water situations, but the occur- possession of a short peduncle. The other taxa, rence in Central Asia and Iran is probably not which possess long, spirally coiled or straight confined to relic brackish waters (den Hartog peduncles, are restricted to the temperate zone; 1974); it may also concern continental salt waters. some of them have been found in the tropics, but only at high altitudes. 5. Lepilaena 2. Zannichellia The genus Lepilaena consists of at least 4 species (Willis, 1970; Aston, 1973), and is restricted in its The genus Zannichellia is almost cosmopolitan, distribution to temperate Australia and Tasmania. but was originally absent from Australia. Although One of the species, L. bilocularis Kirk ex Petrie, some records of the genus in high salinity waters are occurs also in New Zealand (Moore & Edgar 1970). known, its representatives occur generally in low The species inhabit a wide range of brackish and salinity waters, up to 6%0 Cl', and some are athalassic habitats; some occur also under fresh- characteristic of unstable freshwater habitats. water conditions, usually in temporary bodies of The taxonomy of Zannichellia suffers even more water, and there are also records of marine occur- from old ideas than that of Ruppia, because even rence. This genus needs further taxonomic investi- now the genus is considered to be monotypic gation. The chromosome number of L. bilocularis (Ascherson & Graebner 1907, 1913; Reese 1963, plants from New Zealand (Hair, Beuzenberg & 1967). Recent studies on European material, how- Person 1967) appears to be different from those of a ever, have shown that a number of taxa can be Victorian collection of this species (den Hartog, recognized on the basis of morphological, phys- unpublished). iological and cytological characteristics (Luther 1947; Reese 1963, 1967; W. van Vierssen, personal 6. Potamogeton communication). In Europe at least 4 taxa can be recognized, and it is by no means certain that the The genus Potamogeton consists of two sub- Asian, African and American representatives of genera, one of which is strictly confined to fresh Zannichellia are identical with these. waters, where it reaches a great differentiation at the species level (Ascherson & Graebner 1907, 1913; 3. Pseudalthenia Hagstr6m 1916). The other subgenus, Coleogeton, is small, and consists of 5, but possibly more, The genus Pseudalthenia,also known as Vleisia species. One of these species, Potamogetonpecti- (Tomlinson & Posluszny 1976; den Hartog 1980), natus L., has a worldwide distribution, and occurs accommodates only one species, P. aschersoniana in brackish, as well as continental salt waters, (Graebner) den Hartog. It is restricted to the although not usually at salinities higher than 10%o south-western point of Africa, where it inhabits Cl', and also in unstable freshwater habitats, even in so-called 'vleis', i.e. temporary estuarine lagoons very oligotrophic soft waters. Two other species, P. (Reinecke 1964). filiformis Pers. and P. vaginatus Turcz., are more restricted in their geographical distribution; they 17 occur only in the boreal areas of the northern nials, but, as a consequence of the harsh ecological hemisphere. They share with P. pectinatus, how- conditions to which they are exposed in their ever, the property that they inhabit extreme or natural environment, most of the species, especially physically unstable habitats. Nothing is known the finer ones, behave as annuals, and even show a about the ecology of the other species. tendency toward rhizome reduction. The flowering shoots are always sympodial, except in the case of Pseudalthenia(under the name Vleisia) which produces a monopodial flowering Taxonomy at the species level shoot, the flowering branches themselves being sympodia (Posluszny & Tomlinson 1977). From a The lack of knowledge of the taxonomy at the morphological point of view this genus forms a link species level is mainly due to the following factors: to Najas, which is monopodial, but in a number of a. Great morphological variability of the plants, other characteristics rather similar to the Zanni- ascribed to the variability of environmental chellioideae. factors, under which the plants occur. Geno- and The flowers are inconspicuous and very reduced. phenotypic variation were not distinguished. Pollination is hydrophilous, i.e. completely sub- b. Uncritical study of herbarium material. Descrip- merged or so-called semi-aquatic surface pollina- tions of taxa were often based on incomplete or tion takes place. In the latter case, pollen floats on immature material (e.g. unripe fruits). Several the surface and the gynoecia are at least partially in very useful characteristics were not recognized in contact with the surface. The latter mechanism is the past. characteristic of Potamogeton sect. Coleogeton c. No studies of living plants were carried out. (with oblong pollen) and Ruppia (with boomerang- d. Local development of taxonomy, often without shaped or oblong pollen). Lepilaena australis consultation and study of material from else- Drumm. ex Harv. and L. cylindrocarpa(Koernicke where, causing nomenclatural confusion. ex Walp.) Benth. form a special case; in these At present it is possible to recognize taxa also on species the male flowers, which are no more than other than morphological characteristics, by using the anthers, break off, rise to the surface, and chromosome numbers, biometrical and statistical release their spherical pollen which then sink and data, biochemical 'finger printing', etc. Culturing may be caught by the style of a female flower. In the plants under controlled conditions may lead to a Zannichellioideae the styles are long, while in better understanding of the causes of the variabil- Ruppia and Potamogeton sect. Coleogeton the ity, and to better definitions of the taxa. In order to styles are extremely reduced. finalize the taxonomic status of the representatives The fruits are sessile and arranged in small of the eurysaline group, such studies are strongly groups; in Potamogeton subgen. Coleogeton they recommended. occur more or less verticillate around the flowering axis.
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