Taxonomy of the Genus Halophila Thouars (Hydocharitaceae): A Review.

John Kuo

doi:10.3390/plants9121732

Abstract

The seagrass genus Halophila Thouars has more than twenty described species and is predominately distributed over a wide geographic range along the tropical and the warm temperate coastlines in the Indo-West Pacific Oceans. A brief history of the Halophila taxonomic development is presented. Based on reproductive and vegetative morphology, the genus is divided into eight sections including three new sections: section Australes, section Stipulaceae and section Decipientes. A rewritten taxonomic description of the type species for the genus Halophila, H. madagascariensis Steudel ex Doty et B.C. Stone, is provided. The lectotype of H. engelmannii Asch. as well as neotypes of H. hawaiiana Doty et B.C. Stone and H. spinulosa (Br.) Asch. are designated. Furthermore, H. ovalis ssp. bullosa, ssp. ramamurthiana and ssp. linearis together with H. balforurii have been recognised as distinct species. Nomenclature, typification, morphological description and botanical illustrations are presented for each taxon. Recent molecular phylogenetic surveys on certain Halophila taxa are also discussed. Field surveys for the deep water Halophila in West Pacific regions are suggested. Morphological studies combined with molecular investigations for the Halophila on the east coast of Africa and the West Indian Ocean are urgently needed and highly recommended.

Highlights

The seagrass Halophila is the smallest in size among the entire known seagrasses, but it is the most diverse group and represents more than one quarter of all recognised seagrass species [1].Halophila plants are minute and fragile, without stripe-like long leaf blades, and virtually unlike any other seagrasses
All three sections were originally under the section Halophila of den Hartog [6]; the genus Halophila contains a total of eight sections in this revision
The section Australes contains a single species, H. australis, and its female flowers, with six styles, which are uniquely formed on the extended floral shoots but not on the rhizome nodes

Summary

Introduction

Halophila plants are minute and fragile, without stripe-like long leaf blades, and virtually unlike any other seagrasses. As a result, their taxonomic position in the monocotyledonous classification has long been unsettled. Ascherson [2] placed Halophila under the tribe Halophileae in the family. Najadaceae and Ascherson and Gürke assigned it as the tribe Halophiloideae within the family. Den Hartog [5] considered Halophila as one of nine genera in the Hydrocharitaceae; in his monograph [6], he followed Nikai’s interpretation [7] and placed Halophila under the subfamily Halophiloideae (one genus only) in the family Hydrocharitaceae. Den Hartog and Kuo [1]

Methods

Findings

Discussion

Conclusion