Livestock Morphological variations of Tilapia guineensis (Bleeker 1862) and Sarotherodon melanotheron (Ruppell1852) (Pisces: Cichlidea) from Badagry and Lagos lagoon, South-West, Nigeria

Sarotherodon melanotheron (Ruppell) and Tilapia guineensis (Bleeker) are two dominant cichlids in Badagry and Lagos Lagoon, South-western Nigeria. Comparative racial studies were investigated among these species using multivariate analysis of nine morphometric characters and nine meristic counts of 100 specimens each. The results showed that they were phenotypically separable populations of the same species with some level of divergence in morphometric characters. The data were analysed using independent sample t-test after allometric test revealed significant differences ( p < 0.05) in body depth, caudal peduncle depth, and number of gill rakers in Sarotherodon melanotheron while vertebrae, caudal peduncle depth and right gill raker in Tilapia guineensis which were suggested to have occurred as a result of environmental fluctuations, genetic diversity and difference in salinity in the two water bodies.


Introduction
Tilapia guineensis (Bleeker, 1862) and Sarotherodon melanotheron (Ruppell, 1852) are typical estuarine species which can be found in abundance in most of the lagoons and estuaries of West Africa, supports a major lagoon fishery. They can live and reproduce in a wide range of salinities. They have adapted to diverse habitats: permanent and temporary rivers, large equatorial lakes, tropical and subtropical rivers, open and closed estuaries, lagoons, swampy lakes, deep lakes and coastal brackish lakes [1]. For instance, the brackish Saratherodon melanotheron and Tilapia guinensis were the last species recorded during droughts in the very saline waters of the Casamance River [2]. They are however not found at high elevations and generally require water warmer than 20°C [3].
Fagade [4] reported the relative abundance of Tilapia guineensis and Tilapia mariae in Lekki Lagoon. Kuton and Kusemiju, [5] studied their species diversity and richness of these species and others in the Badagry, Lagos and Lekki Lagoons. They are euryhaline species found along the West Coast of Africa [6]. These fishes have a great economic importance, ranking among the major species caught within the lagoon fisheries and there is an increasing interest in this fish for aquaculture purposes, particularly in areas of high or variable salinities, characteristic of the estuaries and extensive lagoon systems which constitute its natural range. In this habitat, other species more traditionally used in 'tilapia' culture are either not locally available (Oreochromis mosambicus) or do not tolerate the prevailing saline conditions (O. niloticus). T. guineensis shares much the same range and habitat as Sarotherodon melanotheron, but neither species is well known for aquaculture purposes.
Analysis of phenotypic variation in morphometric characters or meristic counts remains the most commonly used method to delineate stocks of fish despite the advent of techniques which directly examines biochemical or molecular genetic variation, these conventional methods continues to have an important role in stock identification even to date [7]. It remains the simplest and most direct method of species identification as confirmed in the previous studies [8,9].
Hence, the objective of this study is to Assessing the morphological variations that occur in Tilapia guineensis (Bleeker, 1862) and Sarotherodon melanotheron (Ruppell,1852) in Badagry and Lagos lagoon in Lagos States, Southwest, Nigeria using morphometric and meristic features in view of possible ecological changes and to estimating the level of divergence in this species among the two lagoons.

Study areas
The study sites were Badagry and the Lagos lagoon, all along the coast of Nigeria. The Badagry Lagoon lies between longitudes 3°0' and 3°45' E and between latitudes 6°25' and 6°30' N. It is part of a continuous system of lagoons and creeks along the coast of Nigeria from the border with the Republic of Benin to the Niger delta.
Lagos lagoon, on the other hand, lies between longitude 3º 23´ and 3º 43´ E and between latitude 6º 22´ and 6º 38´ N. It also forms part of an intricate system of waterways made up of lagoons and creeks that are found along the coast of Nigeria from the Republic of Benin border to the Niger Delta. The substratum was fine sand and mixed mud. Some of the physico-chemical features have been described by some authors [10][11][12][13] provided information on its sediments and communities.

Collection of specimens
A total of 200 samples of Sarotherodon melanotheron and Tilapia guineensis ranging between 12 cm and 15 cm standard length were collected from both sites within intervals of 4 weeks for three months, to ensure that samples were obtained as close to same period of the year as possible. The size ranges of fish used were likely to be of approximately the same age. The specimens were transported in ice chest to the laboratory, where measurements started immediately to avoid shrinkage.
Identification of these species of cichlid was done using Daget [14]; the confirmation of the identified species was carried out based on their colour in life and meristic characters. Analysis of the morphometric features and meristic counts was based on independent sample T-test.

Results
A total of 200 specimens of S. melanotheron,and T. guineensis made up of 50 each from the two study sites, were examined and analyzed. The statistical analysis of the morphometric features, as shown in Table  1, indicates that there were significant differences (p < 0.05) in the Body Depth (BD), Caudal Peduncle Depth (CPD) and number of Gill Rakers in Sarotherodon melanotheron while in vertebrae, as shown in Table 3, Caudal Peduncle Depth and Right Gill Raker in Tilapia guineensis, other features -SL, HL, HD, Snl, ED, CPL and HDO -showed that the fish were, in all probability, obtained from two statistically indistinguishable races or stocks. The mean values of meristic characters from Badagry and Lagos lagoon of two populations were shown in Tables 2 and 4. The fin rays including brachiostergal rays were fairly constant which required no further statistical analysis but other characters such as Vertebrae and Gill Raker counts revealed significant differences (p < 0.05) between the two populations.  Table 3: Mean values and T-value on morphometric measurements of T. guineensis from Badagry and Lagos lagoon.

Discussion
In this study, meristics, morphometric, biochemical and molecular features were used as they still remain dependable tools to characterize fish species. The fairly constant values of fin rays observed in the two populations agree with the findings [15] that fin rays of the tribe Tilapiini does not vary much.
The significant variations in Body Depth (BD), Caudal Pundicle Depth (CPD), the number of Gill Rakers and Vertebrae (Vtr) might have occurred as a result of environmental fluctuations, especially water temperature and salinity. The water temperature of Lagos lagoon varied between 24.6 and 31.8ºC while that of Badagry ranged from 28.0 to 31.5ºC. Though, they are both brackish water bodies, salinity ranged between 0.5‰ and 15‰ as reported by Ajao and Fagade [13]. Also, Kusemiju [11] reported that the differences in Gill Raker counts of Chrysichthys nigror5digitatus might have occurred as a result of isolation caused by differences in salinity gradients between Lagos and Lekki lagoons in Nigeria. The natures of substratum, as well as variation in dietary items, have been observed to influence morphometric features of species populations [16]. Hence, the significant differences observed in the morphometrics may have occurred as a result of isolation caused by variation in these ecological factors in the two habitats just as genetic diversity cannot be rule out.

Conclusion
The potential shown by Tilapia guineensis and Sarotherodon melanotheron for aquaculture breeding programme, especially their ability to live and reproduce in a wide range of salinity, coupled with their abundance in these water bodies, should be exploited in the near future.