Sublethal effects of malathion insecticide on growth of the freshwater crab Poppiana dentata (Randall, 1840) (Decapoda: Trichodactylidae)

Abstract

Pesticides can interfere with various aspects of growth and the normal molt cycle of a crustacean. Poppiana dentata (Randall, 1840), an indigenous crab species, spends most of its life cycle in, and proximal to, benthic sediments in which pesticide residues can reside. This study sought to assess the sublethal effects of a locally-used, commercial malathion insecticide on growth aspects of P. dentata. Juvenile crabs were obtained from berried females collected in northwest Trinidad. Young crabs were placed in a control (insecticide-free) treatment and an exposure treatment involving continuous exposure to the malathion insecticide, at 10 µg/L concentration over five months (n = 4 crabs/treatment). Carapace width (CW), length (CL) and intermolt period were recorded and used to derive size increment, specific growth rate (SGR), growth curves and logistic equations. Malathion-exposed crabs exhibited irregular patterns in SGR and size increment. Exposed crabs also exhibited a delay in molting and longer intermolt periods, compared to the control crabs (p < 0.05). Breakpoint (17.5 mm CW) and maximum size (CW = 25.77 (1+exp (1.500-0.056t))-1) for exposed crabs were relatively smaller than those of the control (22.11 mm CW; CW = 34.30 (1+exp (1.774-0.035t))-1). Findings indicate that sublethal exposure to malathion insecticide altered growth patterns in P. dentata, some of which can influence maturity and later cascade into secondary consequences for local populations.