Blackworm behaviour is interesting - in a smooth container they form a large mass of worms which coil together. When I dropped them into my tank they initially clumped together - the ones that landed on plant stems turned into a mass the reminds of a group of swarming bees which have settled on a tree trunk.
|Cluster of blackworms on a Ludwigia stem.|
In normal circumstances, this is probably a good way for the blackworms to forage while providing themselves with an escape option. From the Wikipedia article about Lumbriculus vaiegatus;
When touched, L. variegatus will attempt to escape, either by swimming in a helical ("cork-screw") fashion, or by reversing its body. The escape pattern used depends on where the worm is touched: anterior touch elicits body reversal, whereas posterior touch triggers helical swimming. L. variegatus has quick reflexes, and uses its photoreceptors to detect shadows and movement, both used to escape threats. The posterior end lifts out of the water and forms a right angle. It is then exposed to air and is used to exchange oxygen and carbon dioxide, although this exposes its posterior to its enemies. If the photoreceptors detect a shadow or movement, the posterior rapidly shortens in response to a threat.Given the way that the blackworms positioned themselves on the bottom of the tank, it seems likely that the reversal behaviour draws the organism down into the substrate where it would be safe from predators.
Absent the corys and kuhli loaches, the blackworms which made it to the bottom of my tank would probably have been safe. While the tetras greedily attacked any floating worms (and eventually found the worms that had settled on the vegetation) the worms that made it to the bottom of the tank seems to have been safe from them. Even the large mass of worms in the second picture (above) elicited no response from the tetras (although they were eventually find the worms which had settled on the vegetation. The corys, on the other hand, were not only able to find the worms on the bottom of the tank, they also seemed to be quite successful at digging them out of the gravel (and in so doing, managed to uproot several plants). A day later there were still a few worms sticking out of the gravel where the thickest clump had been located, but I don't see any left. It's entirely likely that some - maybe even many - survived by burrowing into the substrate where they can grow and reproduce (and hopefully, continue to become cory food). But that might just be wishful thinking,