Tumors, or neoplasms, have been found in a variety of invertebrates such as molluscs, insects, and flatworms, but reports of tumors on crustaceans are rare. Earlier research speculated that tumors might form in zooplankton, but the organisms would die at molting before tumors would be obvious. The team found large tumors on juveniles and on young, recently molted adult zooplankton in Lake Michigan. Researchers found tumors in several species of copepods and Cladocera. Predatory species were more likely to have tumors than herbivorous species.
Preliminary analyses suggest that tumors are more common nearshore than offshore.
Analysis of the tumors by T. Rizki showed a high frequency of dividing cells, which is suggestive of cancerous growths. Only a few samples have been analyzed so far. Because such tumors have never been seen until now, despite the many programs on the Great Lakes that have monitored or collected zooplankton, we suspect the occurrence of the tumors is a recent phenomenon in the Great Lakes in general and in Lake Michigan in particular. It is also possible that previous monitoring efforts, restricted to a few spring and late summer collections, would have missed the tumors because of their limited seasonal coverage and focus on offshore surface waters. The research team plans to examine archived collections and make new collections at many nearshore and offshore sites to find hot spots that might be associated with some environmental factor.