Population Ecology of a Tropical Oyster Saccostrea Cucullata Near the Northern Limit of Its Range and Its Temperate Congener Saccostrea kegaki during 20 Years

Abstract

Population traits of two species of oysters, Saccostrea cucullata (= S. mordax) and Saccostrea kegaki were investigated every year from 1991 to 2011. The study site was located near the northern limit and around the center of the geographic ranges of S. cucullata and S. kegaki, respectively. The recruitments of these species fluctuated significantly year to year. The new recruits of S. cucullata often disappeared before joining into the older-year cohort, whereas those of S. kegaki constantly merged with the older-year cohort. The growth increment in shell length of S. cucullata was 5–10 mm / year up to 40 mm in shell length and that of S. kegaki was 5 mm / year up to 15 mm, and the growth of the both species slowed down subsequently. The age of S. cucullata of 60 mm in size and that of S. kegakiof 20 mm in size were estimated to be 15 and 4 years, respectively. The ratio of dead empty shells of these species in the field decreased in winter when the density of their predator, muricid gastropods, decreased. The ratio of empty shells with holes drilled by the muricids was 36% for S. cucullata and 30% for S. kegaki. Of these shells, the proportion of juveniles (<10 mm) was 83% in S. cucullata and 8% in S. kegaki. The survivorship curves suggested that the mortality of juveniles of S. cucullata was higher than that of S. kegaki. Mass mortality of S. cucullata occurred after a period of extremely low air temperature in January, 2011, during which the temperature was the lowest in the last 30 years. In contrast, the effect of this cold wave on S. kegaki was not serious. The decrease in air temperature was drastic compared to that in the water temperature, and the mortality of S. cucullata was higher at higher littoral levels, suggesting a more serious effect from air temperature than from water temperature. The population ecology of Saccostrea cucullata thus shows characteristics of a tropical species at the northern front of its range, in that the recruitment was inconsistent and the adults were vulnerable to episodic cold weather, compared to the temperate congeneric species S. kegaki.