Platypuses and electroreceptive dolphins are passive electroreceptors. What are they reading?

Answer: Other bodies' fields

Other bodies' fields ✓ — Correct. Passive electroreception means the animal reads weak fields already present in the environment, often from other living bodies. Prey do not need to be electric eels; ordinary nerves, muscles, gills, and cells can create tiny fields in water. The counterintuitive part is that a hidden shrimp or fish can become visible electrically without making a deliberate signal.

Their own shock pulses — Their own shock pulses describe active electrolocation or strong electric discharge, not the platypus-dolphin case. Electric fish can generate fields and read distortions, and electric eels can deliver strong shocks, but that is a different strategy. Platypuses and dolphins are listening for existing weak fields rather than broadcasting an electric ping.

Reflected sound clicks — Reflected sound clicks are echolocation, the famous dolphin sense, but they are not electroreception. This is the easy mix-up: one channel is acoustic reflection, the other is weak electric fields in water. A dolphin can be excellent at sonar and still have a separate close-range electric sense.