Short story about exobiology during human exploration of an alien planet

This sounds very much like Grandpa by James H Schmitz.

At the beginning of the story, the main character (Cord) discovers a new species of bug:

The thing was, in the free and easy terminology of the Sutang Colonial
Team, a swamp bug. Concealed in the downy fur behind the bug's head
was a second, smaller, semi-parasitical thing, classed as a bug rider.

Cord shoots the bug and the rider tries to attack him.

Later, Cord and 3 others use a large specimen of some sort of "plant animal" as a raft. They call this specimen "Grandpa".

They looked somewhat like exceptionally broad-brimmed, well-worn
sugar-loaf hats floating out there, green and leathery. Or like lily
pads twenty-five feet across, with the upper section of a big,
grey-green pineapple growing from the centre of each.

I have it in the collection Decade the 1950's, edited by Brian W Aldiss and Harry Harrison.

It appears to be available online