Population and Spatial Dynamics Mangrove Jellyfish Cassiopeia sp at Kenya��s Gazi Bay

Abstract

Cassiopeia, the upside-down or mangrove jellyfish is a bottom-dwelling, shallow water marine sycophozoan of the phylum Cnidaria. It is commonly referred to as jellyfish because of its jelly like appearance. The medusa is the dominant phase in its life history. They have a radial symmetry and occur in shallow, tropical lagoons, mangrove swamps and sandy mud falls in tropical and temperate regions. In coastal Kenya, they are found only in one specific location in the Gazi Bay of the south coast. There are no documented studies on this species in Kenya. The objective of this study was to quantify the spatial and size-class distribution, and recruitment of Cassiopeia at the Gazi Bay. Ten 50mx50m quadrats were randomly placed in an estimated study area of 6.4ha to cover about 40 percent of the total study area. A total of 1043 individual upside-down jellyfish were sampled. In each quadrat, all jellyfish encountered were sampled individually. For each individual jellyfish, the diameter was determined using a standard ruler. The colour and number of tentacles on each individual was determined and its activity at the time, recorded. Because jellyfish occurred in tidal pools, the number of individuals in each pool was determined. The percentage vegetation cover in each pool was estimated and recorded to the nearest whole number. There were predominantly more light brown individuals followed by dark brown with the least number of individuals being grey. Majority of the individuals were feeding. Most individuals had a mean diameter of 8.0-8.9 cm. Individuals were well represented in the size-classes 4.0-4.9cm; 5.0-5.9; 6.0-6.9cm and 7.0-7.9cm. Very few individuals occurred in the 12.0-12.9cm class. More individuals were found in clumps than singly more so where the sea grass dominated. If the diameter of the jellyfish correlates with the age, then the population structure of the jellyfish at Gazi Bay is that of a growing population. Diameter appears to have been the single most factors that influenced spacing. The smaller the individuals, the more likely they were to be found in clumps compared to larger individuals. It is possible that clumping was also related to the size of the tidal pools. This study is significant in being the first study to document the population biology of the jellyfish at the Kenyan coast. Future studies are recommended to determine factors that restrict the distribution of this species to a specific location at the south coast of Kenya. The presence of Cassiopeia at the Gazi lagoon adds value to tourism attraction of the area.