Introduction
Feeding structures can be highly adapted to the preferred ingesta (i.e.,
food, particles on the food, substrate that the food is attached to). In
molluscs, the radula is the structure that processes food/ingesta
intake. It is a unique molluscan development and consists of a chitin
membrane with embedded rows of teeth. The tooth morphology and the
arrangement of teeth on the membrane can relate to the ingesta source
(e.g., Solem, 1972, 1974; Steneck & Watling, 1982; Hawkins et al.,
1989; Padilla, 2003; Ukmar-Godec et al., 2015; Krings et al., 2021b,
2021c, 2021d).
Even though the radula is constantly renewed by secretion in the
posterior radular region (the “radular sac”) by over- and underlain
epithelia (Runham, 1963; Runham & Isarankura, 1966; Mackenstedt &
Märkel, 1987; Vortsepneva et al., 2022), the radular material properties
are adapted during formation to reduce wear and/or structural failure
induced by the specific ingesta source.
Structural failure can be reduced by the presence of mechanical property
gradients (i.e., of the Young’s modulus) along each tooth. In
polyplacophorans, limpets, or some gastropod taxa (e.g., some members of
the Paludomidae and Nudibranchia), the radula needs to transfer high
forces to solid surfaces (e.g., rocks) by scratching action or to hard
structures of the prey (e.g., sponge spiculae) by piercing action. Here,
each tooth shows pronounced gradients with the cusp as the hardest and
stiffest region, followed by the stylus and finally the basis, as the
softest and most flexible region (van der Wal et al., 1999; Weaver at
al., 2010; Lu & Barber, 2012; Herrera et al., 2015; Krings et al.,
2019, 2022c, 2022d, 2023; Pohl et al., 2020; Gorb & Krings, 2021). This
allows teeth to bend and to either gain support from the next row of
teeth, which redistributes the stress, or to deform and adjust to the
prey item to avoid structural damage. These mechanical property
gradients have their origin in the degree of tanning, the content of
inorganics, the regional water content or the chitin fiber arrangement
(e.g., Brooker & Shaw, 2012; Faivre & Ukmar-Godec, 2015; Joester &
Brooker, 2016; Krings et al., 2021b, 2022d).
With regard to abrasion resistance, some taxa, like Polyplacophora and
Patellogastropoda, incorporate high proportions of iron and silicon into
their very thick tooth leading edge (i.e., the surface of the tooth that
interacts directly with the ingesta) resulting in hard tooth cusps as
adaptation to feeding on algae growing on stone (e.g. van der Wal et
al., 1999; Wealthall et al., 2005; Shaw et al., 2009a, 2009b, 2010;
Weaver at al., 2010; Saunders et al., 2011; Han et al., 2011; Lu &
Barber, 2012; Wang et al., 2014; Barber et al., 2015; Krings et al.,
2022c). High inorganic contents such as Ca or Si were also found on the
leading edges (“leading surfaces”) of other gastropod taxa (e.g., some
Paludomidae foraging on algae also growing on rock, and Nudibranchia
foraging on Porifera as well. However, the coating was very thin in
comparison to the leading edge of Polyplacophora, suggesting that these
teeth resemble highly functional lightweight structures (Krings et al.,
2022a, 2023). Teeth that come in contact with abrasive particles, like
sand, were found to contain a thin layer with high content of Ca on all
tooth surfaces, presumably to prevent high wear (Krings & Gorb, 2023a).
In this context, we here aim at unravelling the functional principles
that reflect adaptation to the ingesta source in the radular teeth of
the gastropod Gastropteron rubrum (Rafinesque, 1814)
(Heterobranchia, Euopisthobranchia), which forages on Foraminifera
(DeLaHoz et al., 2018). Members of these single celled protists have a
cacliumcarbonate skeleton and thus represent a hard and abrasive
ingesta. This is the first tooth analysis of a gastropod that mainly
preys on this kind of food. First, the stomach content of the sea slug
was documented under scanning-electron-microscopy (SEM) to gain insight
into the food composition in detail. Then, the teeth were carefully
documented unravelling the wear at the tooth cusps. The mechanical
properties of the teeth were tested using nanoindentation technique to
get insights on the functional adaptations of the radular apparatus. In
the search for the origins of the mechanical properties in the teeth, we
also investigated the material composition. The degree of tanning of the
teeth was visualized using confocal laser scanning microscopy (CLSM),
and the elemental composition were analyzed with energy-dispersive X-ray
spectroscopy (EDS, EDX).