i was really confused by this weird little critter. i have never seen it before - i found it attached to a piece of washed in sargassum- but if i’m right about it being a tanaid, it probably colonised the sargassum when it was inshore, rather than living on the high seas. it was big, like half the length of my finger. it looked like a dead stick from the top, but then i saw it had pale yellow antennae and lots of legs. it could crawl and also could swim! it did this cute twirly kind of swimming, like it was always travelling in a spiral through the water. would appreciate any ideas on an id!

I turned over a few rocks at the edge of the mud flats and these were under all of them.

There are some other photos from nearby here: https://www.inaturalist.org/observations/38409318
and the little greenish hoppers are here: https://www.inaturalist.org/observations/38410395

On moss in a disturbed lowland area. First photos of larvae of this genus perhaps? I haven't seen any others.

ID by John Lawrence: "Your beetle is a member of the Diaperinae, tribe Leiochrinini, probably the genus Derispia or something close.".