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

Seasonal shell growth, reproduction and mortality of the eggcowry Ovula ovum were studied from May 2004 to October 2006in southern Kyushu, Japan, an area at the northern margin ofits geographic distribution. Rapid shell growth was observedduring 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 yearexcept during the coldest period (March–April). Egg cowriesreach maturity at approximately 7 cm in shell length, and at1 year of age. Average shell length of brooding females andcopulating 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%) wereobserved from December to May. The maximum numbers of emptyshells in the field (April 2005, May 2006) were seen duringthe period of the lowest ocean temperatures (March 24, 2005;March 7, 2006). This suggests that ocean temperature may bean important factor for egg cowry populations at the northernmargins of their geographical distribution, and that O. ovummay increase fitness with rapid shell growth and early age ofmaturation. (Received 11 January 2008; accepted 22 August 2008)