Sudden Ray Death

I lost the female Potamotrygon sp. 'pearl' today. I found it lying still next to the other one midday and couldn't comprehend until now what happened. The water is perfectly normal that its other tankmates, my lone surviving show-grade discus and a chocolate colored goldfish are doing quite well. For most, this is what one would call a 'sudden ray death.'

Yes, that is the term and it is something that has been quite known among ray enthusiasts (with P. schroederi having the highest mortality rate among all the commonly exported freshwater stingrays) that the only advice is to get the healthiest ray to keep and place them in the biggest fish tank one can afford. Recommended tank length is 5 to 6 x and width 2 x of mature stingray size. Still it happens and a variety of possible reasons for its occurrence which I will be discussing here. I am not in a way pointing to a single reason as it most probably is multi-factorial.

Ammonia

Freshwater stingrays are very sensitive to even the minutest levels of ammonia after evolving from their marine ancestors millions of years ago. This has something with the inability to produce urea from ammonia (which is toxic as it interferes with oxygen extraction in the gills, also the same site where ammonia is excreted in fishes) and its excretion in the presence of ammonia in the water (the process is part of osmoregulation just in case you are interested to read further). Remembering that freshwater stingrays also have high metabolic rates which translate to higher ammonia production then it becomes obvious why even low levels of ammonia in your tank water will be harmful specially to ray pups.


Unhealthy Ray to Start With

Oftentimes, the freshwater stingrays that are imported are tea-cup in size (tea-cup refers to size of pups and not a common name for any freshwater stingray as they grow roughly to a foot in diameter even among the smaller Potamotrygon species) and are quite young really. In transit from their place of origin, they have been subjected to many stresses (stress itself lowers immunity) and may not have been fed to lessen the risk during transport (pups need a lot of food as they have high metabolism for growth) and thus the pups can become 'skinny' (concave between the eyes and protruding bones right before the tail) and susceptible to disease(s) because of the lowered immunity associated with stress.

To be continued....