It is hard to tell which animals truly have the shortest attention spans, but much research has been done on the attention span of the fruit fly. Fruit flies have one of the shortest attention spans in the animal kingdom.
Scientists study pathways in the brain of the fruit fly in order to better understand the human attention span and causes for disorders like ADHD and autism. After being fed Ritalin, flies were able to pay better attention, suggesting their brains have something in common with those of humans.
It has long been said that goldfish only have a 3-second memory span, but further research has proven that myth to be wrong. One experiment involved placing food around a Lego block in a fish tank for several weeks and then removing the block. The goldfish still swam to the space where the block had been during feeding time, suggesting they could remember where the block was previously placed.