Akera is reported to be herbivorous, feeding on algae.

Bulla are generally carnivorous, engulfing their prey (largely other
mollusks) whole. One notable claim to the contrary is Kay's Hawaiian
Marine Shells, in which the author states that Bulla in Hawaii is
herbivorous - and even names the type of algae it feeds on!  Can anyone
confirm this??

Abbott (1974) said that Umbraculum umbraculum feeds on sponges.  Other
sources say that it grazes on microscopic algae that live on sponges and
other surfaces.

Siphonaria, like most mollusks of a "limpet form", are grazers on
microscopic algae.

Spirula is a cephalopod, and is therefore a carnivore.  The animal is
squid-like in appearance, and catches small animals with its tentacles,
but I don't know just what is its usual prey.

Paul M.