The last couple of weeks we had some unusually cold weather. Some days, daytime temperature was 20 degrees below the average for this time of the year. The graph below compares average maximum daily temperatures with actual 2011 maximum daily temperatures during April and May in Colusa County. 
Because of the cold weather, rice seedlings aren't growing much. Growth is very slow, and plants seem to lag behind. However, don't think that just because the rice is not growing, other organisms are also slowed down. I've been hearing of shrimp in several fields, and saw some in a field that was planted two weeks ago. Look at the size of the shrimp and the size of the seedlings in the pictures below.
Tadpole shrimp can grow very fast. A few days with water temperatures between 60 and 84 can promote egg germination and shrimp development. Since the rice is growing slower than normal because of the cold weather, it is under more risk of being injured by shrimp. Monitor your field closely during seedling development, and worry about shrimp only until the rice breaks the water surface. Once the rice is out of the water, shrimp will not cause damage. A lengthy discussion of the biology of the tadpole shrimp was included in the May issue of the Rice Briefs newsletter, available at the Colusa County Rice Program website: http://ucanr.org/sites/colusa/rice/Newsletter/
For monitoring and treatment recommendations, go to the UC IPM website: http://www.ipm.ucdavis.edu/PMG/selectnewpest.rice.html