I mean could a race of aquatic arthropods ever organize in scale enough to be a species that can travel light years and invade others? Wouldn't they just eat each other and never have any need to build a community or to colonize space?

We need to have some baseline for what we consider as "intelligent species" in this context and most intelligent species will need to have a system where they can pool their resources together towards something in a sustained social manner. This rules out a lot of social patterns such as beings that are solitary, canibalisitc and hell bent of eating each other.