A novel pathway of nutrient absorption in crustaceans: branchial amino acid uptake in the green shore crab (Carcinus maenas)

Abstract

Estuaries are environments enriched with dissolved nutrients such as amino acids. To date, marine arthropods are the only invertebrate group that have not been demonstrated to access this potentially important nutrient resource. Using in vitro gill perfusion techniques, we sought to investigate the ability of the green shore crab (Carcinus maenas) to take up the amino acid l-leucine directly from the water. Investigation of the concentration-dependent transport kinetics of radiolabelled l-leucine showed that there are two specific transport pathways across Carcinus gills, one with high affinity and low capacity, and the other with high capacity and low affinity. Using putative competitive substrates and reduced sodium preparations, we were able to identify the putative amino acid transport system associated with high-affinity uptake. This is the first study to demonstrate the absorption of dissolved organic nutrients across the gill epithelium of a marine arthropod.

Keywords: amino acid; arthropod; crustacea; gills; leucine; marine.

Figure 1.

Concentration-dependent ³H-l-leucine uptake in the posterior gill (8) of Carcinus maenas across (a) the entire substrate concentration range (0.5 to 500 µM) and (b) substrate concentrations from 0.5 to 10 µM, as determined via a gill perfusion technique. Values represent the mean ± s.e.m. of four to six replicates. Fitted lines were modelled on raw data using SigmaPlot (11.0).

Figure 2.

Effect of (a) replacing sodium chloride with choline chloride in the perfusate and (b) the presence of 10-fold higher concentration of putative competitive substrates (d-leucine, threonine and lysine) on branchial uptake (gill 8) of ³H l-leucine in the green shore crab (Carcinus maenas). Bars represent mean ± s.e.m of four to six replicates. Bars sharing letters are not significantly different at α = 0.05 using a one-way ANOVA followed by a Tukey post hoc assessment.

Figure 3.

Uptake of ³H l-leucine at a concentration of 50 µM in posterior (8) and anterior (5) gills of Carcinus maenas using an in vitro gill perfusion techniques. Significant differences (*) in uptake between the gills were assessed using a Student's two-tailed t-test with α = 0.05. All data are represented as means ± s.e.m. of four to six replicates.