Taxonomy

2.3.1 The genus Macrobrachium

The genus Macrobrachium is rather easily distinguished from allied genera. It can be characterised by the presence of a hepatic spine, absence of supraorbital and branchiostegal spines and a simple dactylus of the last three legs. The rostrum is well developed, compressed and toothed. The telson is provided with two pairs of dorsal and two pairs of posterior spines. Some authors synonymised Macrobrachium with Cryphiops Dana, 1852, the name of a South American genus. However, Cryphiops can immediately be distinguished from Macrobrachium by the fact that the hepatic spine of the carapace is absent. This spine is always present in Macrobrachium, except in abnormal specimens.

Efforts to divide Macrobrachium into subgenera undertaken at the end of the 19th century, for example, by Ortmann (1891), proved to be unsuccessful. Even though a few natural groups of species can be recognised within the genus, a complete subdivision has so far proved impossible. With such a wide distribution across the world and with so many exclusively freshwater species, the general belief is that Macrobrachium is unlikely to be one monophyletic genus. Even within one continent like Asia, there is such a diversity of form among the many species that is difficult to imagine the genus is monophyletic. Unfortunately, biologists have found few effective or reliable characters to separate most of them. A few more characteristic species such as M. mirabile (Kemp, 1917) and M. palaemonoides Holthuis, 1950, will be separated out into their own genera (D. Wowor, pers. comm. 2008). A practical and more natural subdivision can probably only be realised when the many species are better collected, better studied and more characters elucidated and analysed.

M. rosenbergii (Fig. 2.2) can be distinguished from other species of the genus by the combination of the following characteristics:

• It is the largest known of all Macrobrachium species (total body length up to 320 mm).

Céphalothorax

Flagella of first antenna

Antennal scale Rostrum i PeduncleEye First, tooth Î ; of first [Orbit • antenna

Abdomen

Reptilien Rperbau

Flagellum of second antenna

Mobile finger

Palm

Fig. 2.2 Morphology of M. rosenbergii (De Man). (Source: after Cowles 1914.)

Flagellum of second antenna

Merus

^ Carpus

Mobile finger

Palm

Pr°p°dus ; Merus Dactylus Carpus ischium

Fig. 2.2 Morphology of M. rosenbergii (De Man). (Source: after Cowles 1914.)

• It is characterised by the long rostrum with 8 to 15 dorsal teeth and 6 to 16 ventral teeth.

• The tip of the telson reaches distinctly beyond the posterior spines of the telson.

• The adult male has very long second chelipeds in which all segments are elongate and provided with blunt spines.

• The movable finger of the second chelipeds of the adult male is covered by a dense velvet-like fur (except the extreme tip), while such fur is absent from the fixed finger and the rest of the cheliped.

2.3.2 The identity of Macrobrachium rosenbergii

To understand the correct scientific name for the giant freshwater prawn, it is important to understand its convoluted taxonomic history - the name Macrobrachium rosenbergii (De Man, 1879) has 15 synonyms associated with it! When De Man (1879) obtained a specimen from Andai, New Guinea, he thought its differences from the western form (which he indicated as Palaemon carcinus) were so significant that he described it as a new species, Palaemon rosenbergii. Ortmann (1891) reduced the status of M. rosenbergii to that of a subspecies (or variety as he called it) and was followed in this by De Man himself (1892) and by the carcinologist Jean Roux in various papers dealing with New Guinean material. Most subsequent authors did away with the subspecific status andrecognisedjust a single undivided species. It was Johnson (1960, 1973) who discussed the two forms again, one western and one eastern, and made it clear that he considered them to be subspecies of M. rosenbergii. The range of the western subspecies includes the east coast of India, the Bay of Bengal, the Gulf of Thailand, Malaysia, and western Indonesia (east to Borneo and Java); while the eastern subspecies inhabits the Philippines, eastern Indonesia namely Sulawesi (= Celebes) and Irian Jaya (= western New Guinea), as well as Papua New Guinea and Australia. Since the type locality of M. rosenbergii is New Guinea, the eastern form should therefore be named M. rosenbergii rosenbergii. For the western form, Johnson (1973) proposed a new name, M. rosenbergii schenkeli. Since then, many independent studies using morphological and/or biochemical studies have provided strong evidence that there are in fact two distinct populations of what is traditionally called 'M. rosenbergii' (Hedgecock et al. 1979, Sarver et al. 1979, Lindenfelser 1984, Wowor & Choy 2001, Cai & Ng 2001, 2002, Cai et al. 2004, Wowor et al. 2004, De Bruyn et al. 2004, Chand etal. 2005).

Holthuis (1995) had commented that if two subspecies were indeed to be recognised, the correct name for the western subspecies should be M. rosenbergii dacqueti (Sunier, 1925), with the name of the eastern subspecies remaining as M. rosenbergii rosenbergii (De Man, 1879). The type locality for dacqueti is Java in Indonesia while that of rosenbergii is New Guinea. Macrobrachium rosenbergii schenkeli Johnson, 1973, is clearly a junior synonym of dacqueti.

A major taxonomic change arose with a brief report by Wowor & Ng (2001) that they had strong morphological evidence that the two subspecies were actually separate species. The complete evidence, including larval information, was summarised andpresented in Wowor &Ng (2007). The recognition of two distinct species, M. rosenbergii (De Man, 1879) and M. dacqueti (Sunier, 1925), presents a major nomenclatural conundrum. Because the most widely cultured and studied species is actually the 'western subspecies', the name that should be used is M. dacqueti and not the better known M. rosenbergii. This is clearly a very unsatisfactory situation as the Giant Freshwater Prawn has long been known as 'M. rosenbergii'. Having its scientific name changed because of a twist of history is clearly not in the interests of nomenclatural stability since almost all aquacultural and fisheries studies (and the thousands of associated reports and papers) have been using material from Thailand and Malaysia identified as 'M. rosenbergii'. To this effect, Wowor & Ng (2008) submitted an application to the International Commission for Zoological Nomenclature to have the type of M. rosenbergii changed so the name can remain with the commercially more important and scientifically better known species. Because of this action, a new name will need to be used for the eastern species from Philippines, Sulawesi, New Guinea and Australia (Wowor & Ng 2008).

Comparing the two species that have been confused under 'M. rosenbergii', adults of the scientifically more important Macrobrachium rosenbergii, as defined here, usually having a blue to bluish-green colour patterns in life (versus dark brown to almost black), a basal rostral crest which is always relatively high to moderately high (versus usually lower), the tip of the rostrum either reaches or clearly exceeds the tip of the scaphocerite (versus relatively shorter, often not reaching end of scaphocerite), and all the surfaces of the segments of the second pereopods (except for dactylus and ischium) are covered with widely-spaced and elevated large spines but without interspersed spinules (versus surfaces abundantly covered with spinules interspersed with widely-spaced, elevated large spines).

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