Abstract
Macrobrachium australe is an amphidromous prawn living in the insular freshwater systems of the Indo-Pacific. Because it possesses few informative morphological characters, that often vary from one habitat to another, M. australe has produced much taxonomic confusion and has historically been described under eight synonyms. Here, 53 specimens collected throughout the Indo-Pacific under the name M. australe were phylogenetically and morphologically examined. Results revealed that what has been called M. australe belongs to at least two distinct species: M. australe, distributed from the Southwest Indian Ocean to the Central Pacific Ocean, and a cryptic species potentially restricted to the Northwest Pacific Ocean, here identified as M. ustulatum, which until now was considered as a junior synonym. Although they are not quite found in the same habitat (lentic-lotic), the presence of these distinct, and reciprocally monophyletic entities in the same rivers on the islands of Palau and Santo strongly favors the hypothesis of two reproductively isolated entities. Six morphological characters, including the proportions of the joints of the male second pereiopod, the shape of the epistome lobe and the armature of the fourth thoracic sternite, are evidenced as diagnostic. A neotype of M. australe is designated and deposited in the Muséum national d’Histoire naturelle in Paris.
Highlights
Macrobrachium australe (Guérin-Méneville, 1838 in Guérin-Méneville 1829–1838) is a large-sized prawn species living in the insular freshwater systems of the tropical Indo-Pacific
The DNA sequencing of two unlinked molecular markers in specimens collected from a large geographic range throughout the Indo-Pacific, covering the entire known distribution range of M. australe and including localities close to most of the type localities of its synonyms, suggested the existence of at least two distinct species under what is commonly assigned to “M. australe”: M. australe (GuérinMéneville, 1838 in Guérin-Méneville 1829–1838), with a large distribution range from the Southwest Indian Ocean to the Central Pacific Ocean, and a cryptic species found syntopically at some islands of the Western Pacific Ocean (Taiwan, Philippines, Indonesia, Papua New Guinea, Palau and Vanuatu)
P. australis was described based on the shape of the rostrum; P. danae on the proportions of the various joints of the minor second pereiopod; P. dispar on the proportions of the major second pereiopod; P. alphonsianus on the second pereiopods; P. parvus on the rostrum, the position of the hepatic spine and the proportions of the minor second pereiopod; P. malliardi on the rostrum and the minor second pereiopod; P. ustulatus on the minor second cheliped; and L. lepidus on the rostrum, the cephalic region and the pereiopods I, II, III and IV
Summary
Macrobrachium australe (Guérin-Méneville, 1838 in Guérin-Méneville 1829–1838) is a large-sized prawn species living in the insular freshwater systems of the tropical Indo-Pacific. The species possesses an amphidromous life cycle in which the larvae develop in the marine plankton before returning as juveniles to rivers to grow and reproduce This specific life cycle is considered a critical element to the persistence of the populations and the viability of the species in insular freshwater habitats (Keith 2003), which are usually sparse and of an unstable nature. An unpublished primary molecular sequencing of specimens collected by the Muséum national d’Histoire naturelle (MNHN) over a range that spans the Indo-Pacific suggested the possible existence of distinct evolutionary units (species) within what was morphologically assigned to M. australe. This raised questions about the exact distribution of M. australe and its actual conservation status
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