Robert/Fred:
Sonobuoys were developed independently by US and British scientists (I have never heard of any German work, but it is possible).

In the US, the National Research Laboratory began work on an anchored version in June 1940, which became operational in the autumn of 1941. It used an FM transmitter and a suspended hydrophone. Development continued and in January of 1942 NRL began work on the ‘bomb-buoy’, a parachute dropped hydrophone and transmitter intended for use in the area around convoys. NRL were instructed by the USN to concentrate on radar so RCA took over the project shortly afterwards, on a US Army contract, and then other companies took over from them. Ultimately, two working models were produced and made in several marks. The main difference between the two was that one was nondirctional and the other could give the bearing of a contact. Deployment and use was basically the same for both types: on hitting the water, a hydrophone was lowered about 20 feet and the transmitter, housed in the floating main body, would broadcast HF signals to a listening aircraft. The maximum range of the signal was 35 miles, and up to six buoys could be watched at the same time, with the aircraft at up to 5,000 feet. Battery life was about four hours, then the whole buoy would automatically sink. These were in use widely by the end of 1942, despite the ‘hot potato’ treatment of the contract, and then the USN took an interest in this USAAC development. Figures are vague, but 160,000+ of the omnidirectional and 7,000 of the directional buoys were manufactured during the war. Some of that number were supplied to Britain, but I do not know how many.

The variants used by blimps were towed, housed in streamlined casings.

Hope this helps,
Bruce