Growth pattern, condition factor, trace metal studies and ectoparasitic load of the blue crab, Callinectes amnicola from Lagos Lagoon, Badore, Ajah, Lagos, Nigeria

Abstract

Aspects of the biology of an economically important crab, Callinectes amnicola (De Rochebrune, 1883), were studied to determine the growth pattern, condition factor, ectoparasitic load and trace metal (Manganese, Zinc, Lead, Cadmium and Mercury) levels. Water and sediment samples were also collected for trace metal analysis and to determine bioconcentration factor in crab samples. Males recorded significantly longer carapace (p<0.05) and were heavier (p<0.01) compared to female crabs. C. amnicola showed negative allometric growth (p<0.05; b<3) regardless of sex or size using carapace width; however, with carapace length, the crab exhibited isometric growth for male (p>0.05; b=3) and positive allometric growth for female and sex combined (p<0.05; b>3). Males were better conditioned than females and condition increased with size for males. Only male C. amnicola were infected with ectoparasitic barnacles with a prevalence of 13.46%. The ectoparasites select better conditioned crabs for infestation and this colonization subsequently reduces the condition with increased size. Mean trace metal values recorded for C. amnicola, water and sediment were in the order Manganese>Zinc>Lead>Cadmium>Mercury. The gills had the highest mean values for all the trace metals measured and also had the highest bioconcentration factor with lead recording a factor of 42 from water. However, the edible portion such as muscle, chelipeds and carapace bioaccumulated Manganese, Zinc and Cadmium from water by a maximum factor of 4. Carapace length, growth pattern, condition factor and ectoparasitic infestation may be used to discriminate among sexes of C. amnicola. This study shows relatively low concentrations of trace metals compared to the WHO limits and C. amnicola may be used to monitor the bioavailability of trace metals in the Lagos Lagoon.