Grammistin
The frothing skin toxin of soapfishes, released as a lathering mucus when the fish is stressed - and the origin of their name.
Grammistin is the skin toxin of the soapfishes, and it is where their name comes from. Stressed, netted or handled, the fish releases a mucus that foams like lather in the water - the toxin is a peptide with the same soap-like structure that keeps turning up in fish defenses, and it froths for the same reason.
It is aimed at other fish, not at people: bitter enough that a predator spits the soapfish out immediately, and destructive to gill tissue in a small volume of water. Divers can handle a soapfish with no consequences beyond an unpleasant film on the fingers. A soapfish in a bucket with other fish is a different story, and the same is true of a soapfish caught for the table - the flesh is treated as inedible across its range.
The trait runs through the whole subfamily Grammistinae, which is why a Goldenstriped Soapfish, a Red Sea Soapfish and an Arrowhead Soapfish all behave the same way in a net.
First aid
Nothing to do for a person. It leaves a bitter film on the fingers, washes off with water, and does nothing through skin.
The rule that matters is the same as for boxfish: never put a soapfish in a bucket, bag or tank with other fish, and do not eat one.
