Shell growth, reproduction and mortality of Ovula ovum in southern Kyushu, Japan

Seasonal shell growth, reproduction and mortality of the egg cowry Ovula ovum were studied from May 2004 to October 2006 in southern Kyushu, Japan, an area at the northern margin of its geographic distribution. Rapid shell growth was observed during the period of warmer ocean temperature (May– October). Average shell growth rate of small cowries was 2.84 mm/week. Reproductive activities were observed during most of the year except during the coldest period (March– April). Egg cowries reach maturity at approximately 7 cm in shell length, and at 1 year of age. Average shell length of brooding females and copulating pairs (female and male) were 8.14+ 0.40, 8.17+ 0.49 and 8.08+ 0.43 cm (mean+ SE), respectively. High mortality rates (average 67.5%) were observed from December to May. The maximum numbers of empty shells in the field (April 2005, May 2006) were seen during the period of the lowest ocean temperatures (March 24, 2005; March 7, 2006). This suggests that ocean temperature may be an important factor for egg cowry populations at the northern margins of their geographical distribution, and that O. ovum may increase fitness with rapid shell growth and early age of maturation.