Reproductive Biology of Short-neck Clam Marcia recens (Holten, 1802) at Tambon Koh Sarai, Satun Province

Abstract

A study of the reproductive cycle of the Short-neck clam Marcia recens (Holten, 1802) was conducted at two different areas (Plao Yan and Chong Lad) of Koh Sarai sub-district, Satun Province on the Andaman sea coast of southem Thailand. A total of 1,177 samples (of between 12.25 - 48.50 mm shell length) were collected from November 2009 until February 2011. Qualitative techniques (histology analysis) as well as quantitative techniques (analysis of condition index and gonadal index) were used to investigate the monthly gonadal development stages, the reproductive cycle, and the size at first maturity of M. recens. The samples were analyzed to determine the relationship between the reproductive cycle and environmental factors. It was observed that M. recens is a dioecious clam. Six different gonad development stages in both males and females were identified: (0) resting stage, (1) early development stage, (2) late development stage, (3) ripe stage, (4) partially spent stage, and (5) spent stage. Monthly changes in the gonadal index in both sites indicated that gametogenesis occurred throughout the year, while the condition index in both sites did not explain gametogenesis. The gonad index of M. recens did not show any correlation to their condition index. The spawning season was identified as lasting all the year around, with a peak during November - February. The sex ratio (male: female) of the clam population was 1:1.07 (X= 1.429, d.f. = 1, P<0.05). Only three hermaphrodites were found at the Plao Yan site. Males attained their first sexual maturity at a shell length of 19.51 mm, while female maturity occurred at 20.09 mm. Environmental factors measured monthly in the habitat of M. recens included temperature, salinity, pH, dissolved oxygen, chlorophyll a, organic matter, and soil texture. The correlations between these factors and M. recens gametogenesis were calculated for both sampling sites. Only salinity was significantly negative correlated with the partially spent stage of females at Plao Yan (Pearson product-moment correlation, r = -0.543, n = 16, P = 0.03). Macia recens from Tambon Koh Sarai, Satun Province, southem Thailand, displayed continuous recruitment with two peaks, which suggested a seasonal pattern of reproductive activities.