What can a platypus bill read from a shrimp's muscles rather than from water motion?

Answer: Tiny prey electric fields

Scent trails in water — Scent feels like a useful underwater clue, but it is not the electric channel. A diving platypus seals its nostrils, and odor trails would not give the same instant location of a hidden shrimp twitch. The electric clue is produced by living tissue itself, while smell is a chemical cue.

Tiny prey electric fields ✓ — Correct. A platypus bill is packed with electroreceptors and mechanoreceptors, so it can combine faint prey electricity with pressure and motion cues. The surprising scale is the hardware: about 40,000 electroreceptors plus about 60,000 mechanoreceptors have been described on the bill. That turns a duck-like snout into a living underwater scanner.

Water-pressure ripples — Pressure ripples are real, but they belong to the bill's touch side, not the electric side. That is why the platypus story is richer than a simple vibration detector: it combines mechanical movement with bioelectric fields. A shrimp can leave both a water-motion clue and an electrical clue, and the question asks for the latter.