Fishes, like all other creatures endure diseases and parasites. Common freshwater parasites in this area are the “black spot” flatworm parasite which gets its name from the fact that it causes a black pigmented spot on the skin of the fish. The “yellow grub” and “white grub” are tiny flatworm parasites in the muscular and the interior of the fish respectively. Parasites in fish, and other creatures including humans, can often be tolerated if the individual is healthy and the number of parasites is small. If an individual is stressed by malnutrition or pollution, parasites may not be tolerated and the individual can sicken and die.
Parasites have fascinating and complex life cycles that can involve two or even three hosts or the life cycle cannot be completed. For example, in the life cycle of the black spot- parasite: adult worms in a fish eating bird drop eggs into the water that hatch and mature to a certain stage in snails. The partially mature worms exit the snail and embed in fish that they chance to encounter. The worm remains in the fish until the fish is eaten by a bird such as a kingfisher. The worms mature to adult stage in the bird and the cycle begins again.
We studied the geographic distribution and abundance of five parasites in two common species of fish of the Darby Creek watershed- the blacknose dace Rhinichthys atratulus which live in all but the lowest part of the watershed, and the mummichog Fundulus heteroclitus which lives only in the lower watershed. F. heteroclitus is better known as a saltmarsh fish, so it is interesting that it is found in the Darby and Muckinipattis Creeks; it is known to live in fresh water in a number of geographical areas throughout its range. We found that parasitism appears to be negatively affected by urban development in that definitive (bird) hosts were less available for white grub, black-spot, yellow grub, and the round worm parasite called Eustrongylides sp. to complete their life cycles in some parts of the watershed. We found parasites called the “thorny headed worm” in both fish species in the lower watershed. High levels of the above named parasites were observed at two highly developed sites in the lower watershed. These sites are near or within the John Heinz National Wildlife Refuge where there are abundant bird hosts. The pollution found in the area could be weakening the immune system of the fish, but none of the 500 individuals that we collected showed signs of tumors or malnutrition. In conclusion, the parasites in the lower watershed are a natural part of a complex aquatic community supported by the unique presence of the urban wildlife refuge.
Reference: Requa, E.R., Much, K., Bell, J., Bemis, K., Requa, L.C., Ordog, S., Chi, P. and Goddard, K., 2017. Geographic distribution of metazoan parasites of Rhinichthys atratulus atratulus and Fundulus heteroclitus throughout the suburban–urban watershed of Darby Creek, Pennsylvania. Journal of the Pennsylvania Academy of Science, 91(2), pp.112-133.