Sustainable Ocean Resources Network

Abstract

 

A freshwater dwelling medusa was discovered in March 2009 from an artificial well in Gal-Amuna, Medirigirya in Sri Lanka (N 8⁰ 05.0801, E 81⁰01.4270). It was identified as Craspedacusta sowerbyi (Phylum:Cnidaria, Class: Hydrozoa, Family: Olindiidae). They are commonly known as freshwater jellyfish or hydromedusae. Occurrence of such hydromedusa had never been documented in Sri Lanka, although they have been recorded from other countries. The freshwater “jellyfish” is not a true jellyfish (schyphomedusae of Class: Schyphozoa). A muscular flap of tissue surrounding the margin of the bell (umbrella), known as the velum, is one feature that distinguishes hydromedusae from scyphomedusae.

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A total of 23 individuals were observed in the well. Collections of C.sowebyi were performed during daylight hours. Some physico-chemical characteristics such as temperature, pH, conductivity, dissolved oxygen and hardness were measured. Biological oxygen demand and qualitative and quantitative analysis of zooplankton were also recorded. Similarly, an identical well just 200m away from the jelly well, was also studied for comparison. However, there were no signs of occurrence of hydromedusae in that well. Introduced C.sowebyi had also not survived in this second well. Two individuals were wet preserved in 70% ethanol whereas permanent preparations were made with another two individuals for microscopic investigations. Microscopic characters were used for proper identification of the species. 10 individuals were caught and reared in the laboratory for up to 6 weeks. The size of the individuals was reduced with rearing time in the laboratory and it was not possible to obtain a second generation from the reared samples.

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C.sowebyi has been observed from December 2008 with very large numbers and they had completely disappeared from the well after May 2009 (which seems appearance is sporadic). It was observed that C.sowebyi were 12-25 mm in diameter and possesses very delicate transparent tissue with a whitish tinge that renders them difficult of nematocysts, were also visible. Tentacles of varying lengths protrude from the upper margin of the velum, arranged with three to seven short tentacles between longer ones. In the water sampled from the well, zooplanktons such as calanoid copepods (2000 I-1) and nauplii (2500 I-1) stages are the most abundant. Larvae produced asexually by budding (frustules) were also found among plankton samples. There were no obvious differences in physico-chemical parameters between hydromedusae inhabiting well and a normal well except for dense zooplankton population found in jelly well.