Thecosomata includes specialized holoplanktonic or pelagic
species that use broad, flat epipodia for swimming. Most are small and
are seldom seen near shore.
However, the shells of shell-bearing species are frequently found in
sand and some may be very
numerous in off-shore sediment samples. They are most active at night
by entangling other plankton in a mucous web. In older
literature, they are often included with the Gymnosomata in the order
Pteropoda (known as "sea butterflies"). At least three families
known from Hawaii. The Limacinidae and Cavolinidae, in the suborder
Euthecosomata, have shells. The Cymbuliidae, in the
suborder Pseudothecosomata, lack shells.
In addition to the families listed below, Peraclis reticulata (d'Orbigny,
1836) in the Peraclididae (a family in the Pseudothecosomata with
sculptured, sinistral shells)
been recorded from the North Pacific near
Hawaii based on the range maps in Pelagic
Molluscs on the Marine
Species Identification Portal.