An enigma in white
A fairly unreported phenomenon and one of the many secrets yet to be unravelled by human beings, our writers found the occurrence of freshwater Jellyfish (Craspedacusta sowerbyii) in the Bhadra reservoir in Karnataka. We bring you their observations.
Freshwater Jellyfish (Craspedacusta sowerbyii) belongs to the phylum Cnidaria. C. sowerbyii, which is usually found in calm, freshwater reservoirs, lakes, impoundments, gravel pits or quarries. Only one of the few Freshwater Jellyfish species is known to exist and is among the least studied (and known) of its species. Since we’d heard about jellyfish inhabiting only marine environments, freshwater sightings at a reservoir in Bhadra, Karnataka, immediately aroused our interest.
From the classification or a science perspective, the Freshwater Jellyfish (Class: Hydrozoa; Order: Limnomedusae) is not a true jellyfish (Class: Scyphozoa) and it differs by the muscular, shelf-like structure called a velum on its ventral surface.
Craspedacusta sowerbyii is a hydrozoan and a dimorphic, alternating between both a polyp and medusa morphs in between generations. It shows two modes of reproduction, asexually in the polyp form, via budding, which can form polyps, frustules or medusa buds depending on the environmental conditions. Polyps can form podocysts if environmental conditions change suddenly and dramatically and persist in a dormant form till favourable conditions return. The medusa bud forms a free-living medusa, which shows sexual reproduction via fertilised eggs. But it will be interesting to note that most populations of Craspedacusta sowerbyii are either male or female only and quite understandably sexual reproduction is a rarely recorded phenomenon.
The jellyfish can move in any direction by the use of pulsating contractions along its bell surface. The tentacles face the direction opposite to its movement in general, however, in a few circumstances, the longer tentacles were seen in the direction of movement.
Where are they found?
Craspedacusta sowerbyii have a global distribution and are found on almost every continent except Antarctica. There are various theories about the origin of the species, that it came from the Amazon basin of South America or the Yangtze River in China. They have also been seen in river systems in the United States (Allegheny River, Ohio River and Tennessee River) and in Thailand (Wang Thong River). They prefer standing water and are not generally seen in fast-flowing streams or rivers. However, there are only a few reported sightings in India:
- Idukki Reservoir in Kerala, recorded in 1984 (by TC Khatri), probably one of the first ever recorded sighting in India.
- In an abandoned rock quarry near Kunnanpara near Thiruvananthapuram, Kerala (PK Sarkar & SN Mude, 2010)
- In a small pond carved out in rock on the banks of Cauvery River in Karnataka. (Amoghavarsha 2009)
- Bhadra reservoir near Tharikere, Karnataka, probably the only place with continuous sightings in large numbers since 2008.
Our observations on field
Our preliminary research and observations for over three years (2009-2011) were concentrated around Doddamakali and Bhadra Reservoir. In Doddamakali, these creatures were confined to a small pool created by rock escarpment on the banks of River Cauvery. Locals revealed that they were not found mainstream at both the rivers.
In contrast, they were widespread in the Bhadra reservoir. The commonality between both sites was that they were not found thriving in flowing water either upstream or downstream. There could be favourable habitats downstream with higher chances of survival, but we did not find any evidence to support this. Their fragility probably needs still waters to survive.
A typical phenomenon associated with jellyfishes at large is their ‘bloom’ wherein hundreds of jellyfishes are seen swarming together in a small patch. Though the correct term for a group of jellyfish is a ‘smack’, the word ‘bloom’ is widely used in literature. As per observations from researchers, Craspedacusta sowerbyii blooms happen between October to February, indicating the importance of temperature, which can affect population densities. Studies also reveal that Freshwater Jellyfish feed on zooplanktons, which are generally abundant on the surface of water-bodies in the evening, validating the phenomenon’s evening occurrence. Some reports also suggest that blooms are more frequent when water temperatures exceed 250C, a possibly accurate theory; temperatures at Bhadra ranged from 26.9 degree C to 29.9 degree C during blooms while sightings dropped in the mornings when temperatures ranged from 18 degree C to 23 degree C.
There is no conservation threat identified due to their occurrence in the Bhadra reservoir yet, but unless we understand more about their life cycle, it is unclear how their presence would affect local species.
According to a paper by Gravely & Agharkar, the first sighting was reported at a fish tank in Ranchi ((Firoz Ahmad et al, 1987). How it got there, no one knew. A zygote that attaches to substrates by sticky threads (Rao, 1932) may be capable of being transported in dry condition if attached to plant parts of other movable objects. The sudden appearance of Limnocnida in an aquarium is suggestive of this phenomenon.
Chaitra Ramaiah and Rajesh Puttaswamaiah are naturalists by passion and work in the software industry. Their notable documentation includes the breeding behaviour of Yellow-Throated Bulbuls and the commensalism behaviour between Short-Toed Snake Eagle and Indian Silver Bills during nesting season.
Read also: Revisiting the moments in the wild
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