Hi, Trevor,
A little googling turned up this article which suggests to me that perhaps they are feasting on microorganisms that convert the vent effluent into food?
"The Microbes That Keep Hydrothermal Vents Pumping"
I think reading and thinking about that article may give you some answers to your question... ;)
😄😇😄
wow! that could be it. It seems such a weird thing for them to do. Sandflies usually suck blood from poor hapless humans with no socks. I'd never thought they would be eating microorganisms.
Thanks for that ;-)