Calcium carbonate supersaturation and precipitation in Chinese mitten crab (Eriocheir japonica sinensis) larval ponds in China: Mass mortality, crystal form analysis, and safety saturation index

Abstract

In recent years, mass mortalities of Chinese mitten crab (Eriocheir japonica sinensis) larvae have been reported in several crab culture regions in China and have resulted in great economic losses. The mass mortality usually occurs in seawater ponds in the afternoon of sunny days when water color suddenly changes from dark green or brown to cloudy white or light yellow, and mineral precipitate occurs. Moribund and dead larvae are typically covered with debris. A series of experiments was conducted to examine the cause of precipitation and mass mortality of crab larvae. Analysis of the precipitate from ponds with mass mortalities showed that CaCO3 was the main component of the precipitate, accounting for 70.8% of the total weight. In an animal experiment, mortalities were observed within 48 h in all groups with CaCO3 precipitate, and generally increased as pH and CaCO3 saturation rates increased. Mortalities in groups with CaCO3 precipitate were greater than those in groups without CaCO3 precipitate (P≤0.05). All larvae in groups with CaCO3 precipitate sank to the bottom of the beaker within 48 h. The animal experiment also showed that increased pH (up to 9.4) alone without CaCO3 precipitate did not result in larval mortality within 48 h. Results from the present investigation indicate that mass mortalities of the crab larvae are caused by CaCO3 supersaturation and rapid precipitation possibly triggered by increased pH as a result of intensive photosynthesis of phytoplankton in the seawater ponds. By comparing mean Ca2+ and CO32− ionic product obtained at the equilibrium with solubility product constants for various CaCO3 crystal forms, it appears that CaCO3·H2O is the main CaCO3 form. Based on the CaCO3·H2O solubility product constant and our field investigations, we suggest that CaCO3·H2O saturation indices of 2.5 and below be considered safe and 3.4 as warning signal for CaCO3 precipitation. Since several factors may cause CaCO3 supersaturation and precipitation in crab larval ponds, preventive measures should be implemented to stabilize pH and reduce the CaCO3 saturation rate.