Sensor Fleet
The original idea for the Sensor Fleet came in thinking about how to get measurements inside of the vents. In vent fields, cold sea water seeps in cracks on the ocean floor, heats under the surface and bursts back into the ocean in dramatic plumes called black smokers or white smokers because of the dissolved minerals they carry. Thinking about such a process, especially in a scientifically oriented mission like this one, one wonders how to see what actually happens along the water's path. The fundamental science is still mostly theory there.
Hence! Make a fleet of tiny sensor bundles to drop into the cracks. They could resemble marbles, but have a chip recording measurements along the trip, and when the smokers spew them out, they could be equipped with a sonar beacon so that our vehicles could find them again and pick up the data. Of course the majority of them would probably be lost along the way, but if even a couple survive it would really enhance our understanding of the vent processes. The Sensor Fleet would be little more than its sensors. Robots with brains but no limbs.
Once the initial idea for the Sensor fleet appeared, other uses in experiments that might be more realistic became apparent. The same idea of a sensors and homing beacon assembly deployed and picked up by another vehicle can be used just outside of the vent. One potential project involves deploying the sensors in the plume of a smoker to track chemical composition and deposition.
The scientists will have the freedom to design a fleet with the parameters they suspect to be most useful after doing their initial experiments. In preparation for what they come up with, the other robots will be capable of deploying this type of fleet.