A growth model for a hyperiid amphipodThemisto japonica (bovallius) in the Japan Sea, based on its intermoult period and moult increment

Abstract

Themisto japonica was reared at 1, 5, 8, and 12°C in the laboratory to estimate its intermoult period (IP) and increase in body length (BL) at each moulting (ΔBL).IP was found to be a function of temperature andBL of the specimens, longerIPs being associated with lower temperature and larger specimens.ΔBL was not affected by temperature but increased with growth of the specimens. Observations on consecutive moults indicated that one new segment was added to pleopod rami at each moulting.ΔBLs obtained from the measurement of the segment number of pleopod rami andBL of wild specimens were slightly larger than values obtained from laboratory-raised specimens.IP data obtained from laboratory-reared specimens are combined withΔBL data from wild specimens to establish a growth model forT. japonica from its release from the marsupium (1.31 mmBL) to the maximum size (17 mmBL) as a function of temperature. This growth model predicts that a total of of 18 moultings is needed forT. japonica to reach the maximum size regardless of temperature, although the time needed to reach the maximum size is highly dependent on temperature. The life cycle, from the newly released larvae (1.31 mmBL) to the spent females (10–17 mmBL), was estimated as 333–593 days at 1°C, 195–347 days at 5°C, 118–210 days at 10°C and 82–146 days at 15°C; the last may be the upper temperature limit forT. japonica. Growth rates ofT. japonica expressed on the basis of body mass are comparable to the rates of euphausiids of equivalent size when the effect of temperature is accounted for. Feeding conditions ofT. japonica in the field are also discussed.