Reproductive periodicity of the sea urchin Tripneustes gratilla (Linnaeus, 1758) in Ahangama rocky reef, Sri Lanka.

Abstract

Sea urchins are ecologically, and economically important species distributed in many shallow marine benthic habitats. Among edible sea urchins, Tripneustes gratilla is abundant and crucial for controlling macroalgae on Sri Lankan coastal reefs. This study focused on the reproduction periodicity of T gratilla in the Ahangama Reef, Sri Lanka. Monthly samples (approx. 24) of sea urchins were collected from February to October 2023. Samples were dissected to extract gonads. The Ganado somatic index (GSI (%)) was calculated and histological examinations of gonads were carried out. T gratilla at Ahangama Reef exhibits continuous asynchronous reproduction with peak spawning periods in April, August, and September. Male and female reproductive cycles were synchronized. The highest mean GSI (%) values in April (male: 5.769 ± 0.581%, female: 5.022 ± 0.535%) suggests the maturation of the majority of urchins in this population, and the subsequent decline in mean GSI (%) in May (male: 2.729 ± 0.581%, female: 1.847 ± 0.535%) reflects the spawning. Gametogenesis consists of five gametogenic stages; growing, premature, mature, spent, and recovering. All stages of gametogenesis were present every month throughout the study period, indicating the continuous reproduction of T gratilla population at Ahangama rocky reef. Peak GSI (%) of males in April coincided with the highest mature stage frequency (36.36%). Similarly, The sudden drop in GSI (%) in May coincided with the high spent and recovering stage frequencies of both males (spent – 27.27%, recovering - 36.36%) and females (spent - 38.46%, recovering - 61.54%) which indicate the accumulation of nutritive phagocytes immediately after spawning. Both GSI % values and histological evidence revealed that T gratilla has a continuous asynchronous reproductive cycle, with the majority of the T gratilla population at Ahangama rocky reef spawning in April which is the first inter-monsoon period in Sri Lanka.