Spores of the Zygomycete species Cunninghamella echinulata
Fungi produce spores in staggering numbers. A single fruitbody of a mushroom can produce 100 million spores an hour. A mouldy orange will be covered with billions of spores. A giant puffball produces a trillion spores or more. Almost all of these spores perish and are a major food source for other microorganisms such as amoebae, nematodes, rotifers, tardigrades etc. Fungi have developed a number of ways to protect the spores from predation. This extends the life of the spore and improves its chances for successfull reestablishment of the species. Sometimes sheer numbers alone will do the trick. Sometimes thick walls develop or walls impregnated with phenolic compounds or are roughened. Spores may be released in a slimy matrix of unpleasant chemicals.
Cunninghamella produces spiny spores. Reports indicate that the spines are made of calcium oxalate. This is an unpleasant, toxic chemical found in rhubarb leaves and other plants as an antifeedent and very commonly excreted by fungal hyphae to protect them. Oxalates are often produced on mushroom fruitbodies by strategically located secretory cells.
IMPORTANT: if your spores are well defended you don't need to produce as many!
So! An amoeba attempting to ingest a spore of Cunninhamella is going to get something like a hot needle up his pseudopodium!