Morphological heterogeneity in the giant African River Prawn, Macrobrachium vollenhovenii from three rivers systems in the Niger Delta Region, Nigeria

The genus Macrobrachium (Bate, 1868) belongs to a group of freshwater prawns (Crustacea, Decapoda, and Palaemonidae). They constitute one of the most diverse, abundant, and widespread crustacean genera (Murphy and Austin, 2005), occurring throughout the tropical and subtropical zones of the world with the exception of Europe (Holthuis, 1980; Fossati et al., 2002; March et al., 2002). Several studies have reported the existence of about 74 species (Chen et al., 2009; Holthuis and Ng, 2010; de Grave and Fransen, 2011). However, the number of species in the Niger Delta Region (NDR) of Nigeria are not well-understood and poorly known. The species that have been documented sofar in the NDR include M. macrobrachion, M. vollenhovenii, M. dux, M. felicinum and the invasive species of M. equidens.

The most economically important species for NDR are M. macrobrachion and M. vollenhovenii. The African river prawn, M. vollenhovenii is endemic to the west coast of Africa stretching from the Senegal River in the north to Angola in the south (Holthuis, 1980; Willfuhr-Nast et al., 1993; Paterson, 2007). In Nigeria, It does not, however, occur as plentiful in nature as M. macrobrachion even though it has a higher fecundity (Lawal-Are and Owolabi, 2012; George et al., 2013; Nwosu and Holzlöhner, 2016). In the Niger Delta, M. vollenhovenii is an important economic resource in the rural areas, supporting and sustaining viable artisanal fisheries in some rivers and estuaries within the region. Because it is the biggest of all the Macrobrachium species in West Africa (Konan et al., 2008) and grows at relatively fast rates in addition to a number of other favourable culture characteristics, it is considered a good candidate for aquaculture (Marioghae, 1987; Willfuhr-Nast et al., 1993; Niass and Fall, 2015). The African river prawn is also hardy in many parameters, it thrives in murky waters with dissolved oxygen as low as one part per million (Jimoh et al., 2011)

To develop the culture of the species, knowledge of the degree of genetic variation is crucial and must be understood because it will provide information on the condition or status of a population with respect to long-term survival of a species and the fitness of the population to adapt to environmental dynamics (Dunham, 2002). Also, natural population are perhaps the best gene bank,a critical resource for genetic variation for current and future application in genetic improvement for farmed species and specialized sports fish application (Dunham, 2011).

One of the major requirements for shrimp aquaculture is the initial selection of breeders which involves analyses of external morphology. Morphological variability is not only used to explore differences among geographically distinct populations but also to ascribe distinct genetic structures or environmental conditions to each geographic location (Kinsey et al., 1994). In order to effectively manage fishery resources and exploit the aquaculture potentials, identification of the population structure of an explored species is necessary (Grimes et al., 1987).

At the present, scant information is available on the morphological variations of M. vollenhoveniii from the Niger Delta Region of Nigeria thereby creating a knowledge gap. Most importantly, no study has compared morphological traits of populations of the animal in the Niger Delta. Hence, the purpose of this study was to determine the morphological variations of M. vollenhovenii caught in three locations from the Niger Delta Region.