Cymodoceaceae N. Taylor in N. Amer. Fl. 17 (1909)
Typus: Cymodocea Konig
Dioecious marine plants. Rhizome creeping, either herbaceous, monopodial, and rooting at the nodes (Cymodocea, Syringodium, Halodule) or ligneous, sympodial, and rooting from the internodes (Am-phibolis, Thalassodendron). Scales scarious, ovate or elliptic, marked with more or less small, dark, longitudinal stripes, and dots (tannin cells). Leaves distichous. Leaf sheath broad, completely or almost completely amplexicaulous, leaving open or closed circular scars when shed, bi-auriculate, ligulate; scar-ious flaps covered with numerous short dark, longitudinal stripes, and dots (tannin cells). Leaf blade linear or subulate with three to several parallel or pseudoparallel (Amphibolis) nerves; parallel with the nerves more or less, short, dark, longitudinal stripes, and dots (tannin cells); leaf-tip variable in outline. 'Flowers' without perigone, solitary, either terminal on a short branch or arranged in a cy-mose inflorescence (Syringodium). Male 'flowers' subsessile or stalked, consisting of two quadrilocular, extrorsely dehiscent anthers, which are dorsally connate over at least a part of their length and are attached either at the same height or at a slightly different level (Halodule). Pollen confervoid. Female 'flowers' sessile or shortly stalked, consisting of two free ovaries each with either a long style (Halod-ule) or a short style, which is divided into 2 or 3 loriform stigmata. Ovule 1, suborthotropous, pendulous. Fruit either with a stony pericarp, more or less compressed (Cymodocea, Halodule, Syringodium) or with a stony endocarp and a fleshy exocarp from which four cuneate spreading lobes grow out (Am-phibolis) or consisting of a fleshy bract which encloses the fertilized ovaries (Thalassodendron); not dehiscent. Seed 1. Embryo either consisting for the larger part of the plumula with a lateral primary root and a cylindrical hypocotyl, appressed to the upper part of the plumula (Cymodocea) or consisting of a long hypocotyl and a short plumula without a primary root (Amphibolis).
The family contains five genera: Halodule, Cymodocea, Syringodium, Thalassodendron, and Amphibolis.
From a morphological point of view the family is homogeneous, and monophyletic. In the past it has often been combined with the Zannichelliaceae, e.g. by Hutchinson (1934). The five genera are all well distinguished and there is no controversy about their status as is the case in the Zosteraceae. The family is old, as there are several fossil records of members of the genus Cymodocea from Eocene and Miocene deposits. Thalassocharis from the Cretaceous of The Netherlands and Germany has been considered as being a seagrass by Voigt and Domke (1955) and den Hartog (1970) did not reject this conclusion, but remarked that the stiff compact stems and the absence of aerenchymatic tissue show that Thalassocharis was not yet very well adapted to life in the aquatic environment. Kuo and den Hartog
(2000) did not regard Thalassocharis as a seagrass anymore.
In spite of the great differences in the morphology and the anatomy of their reproductive structures as well as their modes of pollination, Les et al. (1997) treated the families Cymodoceaceae, Posidoniaceae and Ruppiaceae together as one phylogenetic unit, the 'Cymodoceaceae complex', to distinguish it from the other seagrass groups such as the Zosteraceae and the marine Hydrocharitaceae.
Within the family two groups of genera can be recognized. Halodule, Cymodocea, and Syringodium have a monopodial rhizome, are herbaceous, and have leaf-blades that are shed before the leaf-sheaths. Thalassodendron and Amphibolis have a sympodial, ligneous rhizome, and the leaf-blades are shed with the sheaths as single units; further, these two genera show vivipary. There is, however, in our opinion no reason to give these groups a formal taxonomic status.
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