In the recently discovered Archivolva cheripodia
Lorenz, 2024 from the Maldives, an organ at the tip of the foot was
observed, which aids in navigating across the branches of the host.
It had never been recognized in other species of the family, but closer
scrutiny revealed that it does exist also in other genera and species.
Sequences and movies showing the organ:
Extracts from the description of the new species and the cheripodium:
The foot is
translucent and pale
reddish brown, with darker brown stripes above the edge in the anterior
The posterior half shows fainter and finer branching hairlines. When
its host, the foot is visible only as a narrow band encircling the
shell. While crawling below the surface of water in a container, it can
be extended to almost twice the length of the shell. It is
elongated and not split in a propodial and metapodial section. A
at the anterior end of the foot is well-discernible. It is
embedded in the tissue of the foot, above the pedal fold running across
crawling surface in the frontsection.
When kept in a
were observed to lift their shell from the ground, balancing on the
portion of their foot. A white structure protruded from the pedal
fold, extending to a bell-shaped, muscular, tube with six dark colored
dorsally, and a contractible canal basally, which
could be extended beyond the area bearing dark striped ridges.
The white structure was joined to the darker striped foot by a narrow
transparent flange, and at first sight, it reminded of a gigantic
When fully extended, the upper part appeared somewhat detached from the
foot. It could be extended to an length of more than 3 mm, rapidly
moved up, down, and sideways, with its canal contracting and expanding,
the animal itself was remaining in the same spot, balancing on the
half of its foot. The movements and contractions of the tube reminded
of a hand reaching about in search of a holdfast. When partly
dorsal ridges became visible as short digits along the frame of that
when viewing the animal through a glass. When the animal got
disturbed, or returned to resting on all of its foot, the structure
into the pedal fold within seconds and was then only visible through
of the foot as the whitish area described above.
This remarkable hand-like structure
emerging from above the pedal fold has never been described before. Its
function apparently is that of a grasping hand reaching out for holfast when
the animal is moving between branches of the host. The foot of an ovulid itself is not split in a propodium and
metapodium as in other gastropods. As the cheripodium is not homologous to a
propodium, it deserves recognition as a distinct organ.