Assessment of heavy metals and crude protein content of molluscs and crustaceans from two selected cities in Nigeria

Abstract

Crabs, shrimps, periwinkles and land snails form a source of aquatic food which is widely consumed in Nigeria. Therefore, there is need for comparison of nutritional information and heavy metals accumulation potentials of these organisms. The aim of this study was to quantify heavy metals (Cu, Zn, Pb and Cd) and crude protein content of these species that are sold in markets in Ibadan and Lagos, Nigeria. In addition, the influence of location, organism species, organism age, and processing on the levels of the analytes were also investigated. The results showed that mean Cu concentrations ranged from 4.88±0.54μg/g in edible land snail (Gabiella africana) to 34.1±3.0μg/g in raw guinea shrimp (Parapenaeopsis atlantica) and Zn concentrations from 21.4±1.6 in African giant snail (Archarcantina marginata) to 190±21μg/g in raw pink shrimp (Penaeus notialis). The concentration of Zn in all species was below the WHO maximum permissible limit of 100μg/g except in crabs and raw shrimps. Cu concentrations in all the samples except dry shrimps and snails were higher than the WHO permissible level of 10μg/g. Furthermore, highest Pb levels were found in crabs (Callinectes pallidus) (2.74±0.45μg/g) and the minimum mean level of 0.66±0.07μg/g was observed in G. africana. All the samples except dried shrimps contained Pb above the EC limit of 0.5μg/g. The highest mean level (0.20±0.17μg/g) of Cd was detected in crab while the least (0.03±0.01μg/g) was found in periwinkle. However, these levels were lower than the EC limit of 0.5μg/g. The concentrations of Cu, Zn, Pb and Cd in the organisms differed a little between the two sampling locations. The concentration of heavy metals (HMs) varied with studied organisms. For instance, gladiator swim crab (Callinectes pallidus) and raw pink shrimp (P. notalis) bioconcentrated remarkably higher levels of Cu, Zn, Pb and Cd than the other organisms. The process of drying significantly decreased bioavailable HMs in shrimps. Matured snails contained highest concentrations of Cu and Pb while baby snails contained highest levels of Zn. About 63% Cu, 70% Pb, 87% Zn and 10% protein content appeared to be lost during the drying process in shrimps. Crude protein contents ranged from 35.7±2.5% in crabs (C. pallidus) to 79.5±2.0% in snails (G. africana), implying that shrimps, crabs, periwinkles and snails are good sources of animal protein if they are found in contamination free environment.
Key words: Heavy metals, protein, crustaceans, molluscs