Diet and food preference of sea urchin; Tripneustus gratilla (Linnaeus, 1758) in a seaweed cultivated and non cultivated seagrass beds at Kibuyuni-Shimoni, Kenya

Abstract

A study to compare the diet of the sea urchin Tripneustus gratilla (linnaeus, 1758) in seaweed cultivated and non-cultivated seagrass beds at Kibuyuni-Shimoni, Kenya showed that he most preferred food item by sea urchin; T. gratilla at Kijiweni seagrass bed was seagrass; Thalassodendron. ciliatum (E*= 0.12) while the urchins at the seaweed farm showed no strong preference( E = -ve) for any of the food items found in the gut. Higher densities of sea urchin; T. gratilla were found in the seaweed farm compared to the seagrass bed. However, the population density of sea urchin; T. gratilla observed in this study was not a threat to the seagrass ecosystem due to overgrazing. There was positive significant correlation (R = 0.225, P = 0.233) between the population density of sea urchin; T. gratilla and the relative abundance of the cultivated seaweed Kappaphycus alverezii. Similar relationship was observed between the population densities of sea urchin; T. gratilla and seagrass; T. ciliatum in seaweed farm. However there was significant negative correlation between the population density of sea urchin; T. gratilla and seagrass; T. ciliatum in the seagrass bed. Based on the variations in values of relativized index of electivity obtained in this study and the correlation analysis it is concluded that the introduction of cultivated seaweed had influence on the feeding preference of sea urchin; T. gratilla. The morphometric measurements of sea urchin; T. gratilla at Kibuyuni seaweed farrm were relatively larger and heavier than those at Kijiweni seagrass bed (t-test = P 0.05). However, there was no significant differences in the depth and gut weight of sea urchin; T.gratilla between the sites. These results indeed suggest a degree of distinctiveness of the two sites as well as the supported population. Following the revealation that the introduction of cultivated seaweeds in a seagrass bed had effect on growth, this study recommends the conduction of further studies; to determine the the consumed quantities in weight per unit time per area and the chemical composition of seaweed; K. alverezii and the their impact on the performance of sea urchin; T. gratilla.