Pygmy Chain Sword & Chain Sword Plants Reclassification
Many planted tank aquarists are familiar with the pygmy chain sword plants. For many years they were deemed to be within the genus Echinodorus, and E. tenellus has been the most widely available of the several very similar species. In 2007 the young Finnish botanist Samuli Lehtonen completed his doctorate on the subject of the natural history of Echinodorus, and several scientific studies have followed from him since then, with the result that the Echinodorus genus has undergone a significant revision and the pygmy chain swords are now in the genus Helanthium. A number of aquarium plant nurseries are now using his revised names, along with the Royal Botanical Gardens at Kew (considered “the” authority among many botanists) and the International Plant Names Index. Dr. Lehtonen is a recognized authority on the Alismataceae; we are therefore following this classification henceforth in our plant profiles. The rest of this article will summarize these developments as they relate to the pygmy chain sword species; the reclassification of the species remaining in Echinodorus will follow later.
The Alismataceae is a family of aquatic herbs containing 12 genera with about 80 species that are distributed in the tropical and subtropical regions of both hemispheres. Three genera of interest to aquarists occur in the Neotropics [=tropical regions in the Americas]: Sagittaria, Echinodorus and Helanthium. With a few exceptions that grow fully submersed, the plants in these genera occur as amphibious bog plants, spending roughly half the year emersed during which time they flower, and the remainder submersed during the flooded period which lasts several months. Several of the species are quite similar in appearance, making it difficult for aquarists to differentiate between them. To add to the confusion, even within each species the plants can take on quite different leaf lengths depending upon the conditions in the aquarium.
The group Helanthium [the spelling Helianthium with the first “i” is incorrect] was described by Engelmann, Bentham and Hooker in 1883 as a section in the genus Alisma. In 1905, Engelmann and Britton elevated Helanthium as a distinct genus and they assigned to it the dwarf chain sword species from Echinodorus. Pichon (1946) accepted this and further elevated Albidella as a separate genus. In 1955, Fassett again considered the species within Helanthium to be Echinodorus; however, he divided the genus into two subgenera, Helanthium and Echinodorus. Helanthium held two sections, Nymphaeifolii (containing one species, Echinodorus nymphaeifolius) and Tenellii that contained the several closely-related species with E. tenellus as the type species. The subgenus Echinodorus held nine sections containing the remaining species within this genus. In his recent revision of the genus Echinodorus, Rataj (2004) followed Fassett (1955) in this respect, though he increased the number of distinct species considerably.
In phylogenetic analyses (Lehtonen 2006; Lehtonen & Myllys 2008), Echinodorus was found to be polyphyletic [=the last common ancestor is not included in the genus] and in order to obtain a monophyletic [=a clade (here genus) consisting of the last common ancestor and all descendant species] circumscription of the genus, the classification proposed by Pichon (1946) was followed by Lehtonen. E. nymphaeifolius was transferred into the genus Albidella, and E. bolivianus, E. tenellus and E. zombiensis were transferred into Helanthium.
The species now in Helanthium are those former Echinodorus species that have traditionally been considered within the generic common designation of dwarf chain sword plants and are smaller than the true Echinodorus species, though leaf length can vary greatly depending upon conditions in the aquarium. The same species grown in two aquaria can look different, and within the same aquarium two plants of the same species may appear slightly different. They are distributed from the temperate USA down to Argentina, and all species are amphibious bog plants that grow emersed and submersed. Regardless of whether they are cultivated emersed or submersed, these species propagate vegetatively via runners up to 50 cm in length from which plantlets arise at intervals of 2-5 cm. They also produce inflorescences when growing emersed which produce flowers but adventitious plants are rare.
The number of actual pygmy chain sword species has long been a matter of debate. Haynes and Holm-Nielsen (1994) proposed two species, a narrow-leaf sword (Echinodorus tenellus) with a leaf width of 4mm or less, and a wide-leaf sword (E. bolivianus) with a leaf width of 1-1.5 cm; the remaining “species” were deemed variants of one of these. Christel Kasselmann subsequently noted that some of the wide-leaf forms have distinctive genetic makeup and advocated that the distinct species should be retained. Rataj (2004) listed nine species in his Tenellii group. The IPNI (following Lehtonen & Myllys 2008) currently lists five species, now under the genus Helanthium, as follows:
There are two variants: the “narrow leaf” has a leaf width of 2mm (1/16 inch) and grows to 3 inches or sometimes a bit taller; under bright light the leaves may turn slightly reddish. The “wide leaf” has a leaf width of 5mm (1/8 inch) and attains 3-4 inches in length but in lower light it may grow to 10 inches. The species epithet is now tenellum rather than tenellus to agree with the gender of the genus name.
In this article and frequently in the profiles I mention cladistics, so at this point, a word on what cladistics is about, and why it is today such a vital part of the work to classify plants and fish, and why former classifications are changing as a result.
Cladistics is a method of systematic analysis for establishing and demonstrating phylogenetic relationships (=the degree of relationship) between taxa [the plural of taxon, which means a group or clade of organisms], and reflecting their origins. Phylogeny is sometimes referred to as the natural relationships and is an attempt to construct the history of all life based on the evidence from both living and fossil organisms. This method uses the following assumptions:
1. All species originate from other species, and are therefore related to other species in an ancestor-progeny relationship; and
2. Species change their features over time, and those features are passed on to the progeny.
The phylogenetic analysis is based on establishing which features found in the group under study are relatively primitive and which are more recently derived, as well as on grouping taxa on the basis of derived features shared by all members of the group. A group (taxon) is termed monophyletic if it consists of the last common ancestor and all descendants; whereas polyphyletic means that the last common ancestor is not included in the group. Many of the existing classifications of fish and plants are polyphyletic, and scientists are endeavouring to unravel the evolutionary maze to discover and identify the hierarchy with the goal of having monophyletic groups. When classifications are based on phylogenies we can ascertain (and predict) how that group of related fish function, and since this tells us something about their behaviours and requirements it is of interest to aquarists.
Britton, N.L. (1905), Alismaceae, Manual of the Flora of the northern states and Canada, 2nd ed, Holt & Co., New York.
Costa, J.Y., E.R. Forni-Martins and A.L.L. Vanzela (2005), “Karyotype characterization of five Brazilian species of Echinodorus (Alismataceae) with chromosomal banding and 45SrDNA FISH,” Plant Systematics and Evolution Vol. 257, Nos. 1-2.
Fassett, Norman C. (1955), “Echinodorus in the American Tropics,” Rhodora, Vol. 57, No. 677 (May 1955).
Frank, Neil (2000), “The Chain Sword Plants: History and Nomenclatural Perspectives,” Aquatic Gardeners Association [online].
Kasselmann, Christel (2002), Aquarium Plants [translated by Ulf Kotlenga].
Lehtonen, Samuli (2007), “An integrative approach to species delimitation in Echinodorus (Alismataceae) and the description of two new species,” Kew Bulletin Vol. 63, No. 4, pp. 525-563.
Lehtonen, Samuli and Leena Myllys (2008), “Cladistic analysis of Echinodorus (Alismataceae): simultaneous analysis of molecular and morphological data,” Cladistics, Vol. 24, No. 2 (April 2008), pp. 218-239.
Pichon, M. (1946), “Sur les Alismatacees et les Butomacees,” Notul. Syst. (Paris), No. 12, pp. 170-183.
Rataj, Karel (2004), “A New Revision of the Swordplant Genus Echinodorus Richard 1848 (Alismataceae),” Aqua—Journal of Ichthyology and Aquatic Biology, Special Publication No. 1, March 2004.
August 19, 2010
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