Relationship between growth parameters of the amphipod Gammarus zaddachi (Sexton 1912) and the permeable body surface area determined by the acid-base titration method

More than 10000 specimens of the amphipod Gammarus zaddachiwere collected from the River Hunte (Huntebrück, Germany) in January, April, July and September 1999 and Jan 2000 to analyse relationships between body-length distributions, growth and the permeable body surface area (from which ions can enter the cell) as indicated by the acid–base titration method. Thus, body wet weights (BWWs), dry weights (BDWs), length (BL), surface area and specific surface area were measured. Mean BWWs were 56.0 mg, BDWs 11.1 mg and BLs 16.4 mm. The relationship between BDW and BL could be successfully described by the power-function: BDW=0.0029×BL ^2.88, probably indicating that the body volume of the test animals can be approximately expressed by an `equal-effectiveness ball'. The body-length distributions were analysed using two combined normal-distribution equations, suggesting that the population of G. zaddachi at Huntebrück was composed of two generations. A sigmoid logistic equation was applied to estimate the body length-based growth. It can be inferred that the reproduction must have occurred around October to December 1998 and that the size of juveniles in the brood pouch was 0.1 mm. The permeable body surface area (ABSA) of G. zaddachi was measured by applying the acid–base titration method, with an average measurement accuracy of 13.7%. ABSA ranged from 17.1 to 236.9 cm² animal^–1 in a variety of the body lengths studied. The specific surface areas (SSA), equivalent to the ratio of permeable body surface area to body volume, decreased from 14.2 to 8.5 cm² mg^–1 (dry wt) as BL increased from 8.1 to 18.6 mm and then remained nearly constant up to 24.1 mm, indicating a single exponential decay relationship between SSA and BL. Mean body length (BL _mean) of G. zaddachi collected simultaneously at Huntebrück increased seasonally from 0.1 mm in autumn–winter to 17.6 in next winter–spring, resulting in an increase of ABSA but a decrease of SSA. Our present results provide a sound and necessary basis for a quantitative study not only on the age-dependent biological/physiological conditions but also on size-dependent bioconcentration of metal by gammaridean amphipods.