Here's an interesting article passed on by Greg Herbert of UC Davis relating
to the relationship between certain gastropods and crustaceans (platonic not
planktonic).

David Kirsh
Durham, NC

Cake, E W Jr.  Symbiotic associations involving the southern oyster drill
Thais haemastoma floridana and macrocrustaceans in Mississippi waters [USA].
Journal of Shellfish Research, v.3, n.2,:117-128
Abstract: The symbiotic relationships between the southern oyster drill T.
h.
floridana (Conrad) and 2 spp. of crabs, the blue crab Callinectes sapidus
Rathbun and the striped hermit crab Clibanarius vittatus (Bosc.), were
investigated in Mississippi. The crabs provided passive transport and food
(attached fouling organisms) for the attached drills; 99 blue crabs carried
203
drills (.high vinculum.X = 2.0 .plus-minus. 2.1 drills crab-1, range = 1-17;
mode = 1, n = 55 crabs); 233 hermit crabs carried 299 drills (.high
vinculum.X =
1.3 .plus-minus. 0.8 drills crab-1, range = 1-6; mode = 1,n = 194 crabs).
Drills
attached to blue crabs were twice the mean height and 6 times the mean
weight of
those attached to hermit crabs (36.8 mm and 8.9 g vs. 18.5 mm and 1.4
g,respectively). During one survey period 30 of 423 blue crabs (7.1%) and 97
of
1360 hermit crabs (7.1%) carried drills. The oyster drill/blue crab
symbiosis
persisted while spawning female crabs congregated around Mississippi's
offshore
barrier islands during the early fall of 1980 and ceased when the crabs died
or
migrated to deeper water during late fall. The oyster drill/hermit crab
symbiosis was continuous. Drills attached while the crabs were buried at the
seawater/substrate interface, resting under peat outcroppings, or while
scavenging among grass roots and jetsam. Once mounted, the drills were not
readily dislodged by movement of the crabs. In the laboratory drills more
readily mounted hermit crabs with attached drills and/or acorn barnacles
than
hermit crabs without these organisms. A typical mounting took only seconds
to
complete; drills readily attached to moving hermit crabs. Drills dismounted
from
hermit crab shells when in the immediate vicinity of live oysters. The
drills
preyed on acorn barnacles (Chelonibia patula [Ranzani], Balanus spp.,),
oysters
(Crassostrea virginica [Gmelin], Ostrea equestris Say), and slipper shells
(Crepidula spp.) that fouled the blue crab carapaces and hermit crab shells.
Two
other gastropods (Cantharus cancellarius [Conrad] and Odostomia impressa
[Say])
were occasionally attached to blue and hermit crabs that carried oyster
drills.