- The North American host of the introduced Asian isopod parasite, Orthione griffenis, is the burrowing Blue Mud Shrimp, Upogebia pugettensis. The shrimp are the predominant species in the Yaquina Bay estuary intertidal mudflats, and are essential ecosystem engineers. The parasite effectively castrates the shrimp host without causing mortality which allows for a widespread infestation of a large population of mud shrimp. The presence of the introduced parasite has caused a 58% reproduction loss in Upogebia pugettensis over a five year period. It is known that the maximum parasite size increases with shrimp size, however parasites are not found at all in shrimp with a carapace length of less than 12mm. Reproductive-size parasites are not found in shrimp with a carapace length of less than 15mm. This shows that there must be a relationship between the shrimp and the energetic needs of the parasite. In order to better understand the energetic exchange that occurs between O. griffenis and the host U. pugettensis, a comparison of size and weight of the isopod and host to the size and weight of the un-infested host was completed in this study. I wanted to see if over time, a measurable amount of weight loss could be detected in the parasitized shrimp versus the un-parasitized shrimp. I also wanted to see if any growth could be detected in the parasite as the shrimp lost weight due to the energetic needs of the parasite over time. It was predicted that the growth of the smaller isopods in a large shrimp would be greater than the growth of a large parasite in a large shrimp. It was also predicted that shrimp infested with the parasite would lose more weight over time than a shrimp that was not infested with the parasite. Shrimp were collected from the Sally’s Bend mudflat in the Yaquina Bay Estuary on July 26th, 2014 and were kept in the lab in individual containers with continuous water flow, but with no food, for 14 days. Three test categories were used: Un-parasitized, parasitized, and implanted. The implanted category included shrimp that were un-parasitized from the field but were parasitized by hand with isopods that were collected separately, and the parasitized category were shrimp that came from the field already with a parasite. Throughout the 14 days that the shrimp were kept on their starvation diet, every 3 days the shrimp and parasites where weighed and measured to keep track of their weight and size over time. After 14 days, there was no measurable parasite growth, and the average weight loss among all three categories was uniform. Because the shrimp are cyclical feeders, it is possible that because the shrimp were not eating at the time of collection, that the parasites were not feeding either, and thus, not growing. Another study done at the same time as this one noted that there seemed to be a constant population of cryptoniscans (the final developmental stage for O. griffenis before the reproductive stage) in the estuary water column. The cryptoniscans did not seem to be settling into their shrimp hosts like it was originally thought they would. This could be further evidence that the shrimp were not fit hosts to support the growth of the parasites during this time. Maybe the cryptoniscans were waiting to settle into the hosts and the already settled parasites were waiting to feed until the shrimp host became a better source of energy. Further studies such as this one need to be done during different times of the year in order to better understand the feeding patterns and energetic needs of both the shrimp Upogebia pugettensis and the parasite Orthione griffenis.