Soft-surface grasping: radular opening in Aplysia californica

Abstract

Grasping soft, irregular material is challenging both for animals and robots. The feeding systems of many animals have adapted to this challenge. In particular, the feeding system of the marine mollusk Aplysia californica, a generalist herbivore, allows it to grasp and ingest seaweeds of varying shape, texture and toughness. On the surface of the grasper of A. californica is a structure known as the radula, a thin flexible cartilaginous sheet with fine teeth. Previous in vitro studies suggested that intrinsic muscles, I7, are responsible for opening the radula. Lesioning I7 in vivo does not prevent animals from grasping and ingesting food. New in vitro studies demonstrate that a set of fine muscle fibers on the ventral surface of the radula – the sub-radular fibers (SRFs) – mediate opening movements even if the I7 muscles are absent. Both in vitro and in vivo lesions demonstrate that removing the SRFs leads to profound deficits in radular opening, and significantly reduces feeding efficiency. A theoretical biomechanical analysis of the actions of the SRFs suggests that they induce the radular surface to open around a central crease in the radular surface and to arch the radular surface, allowing it to softly conform to irregular material. A three-dimensional model of the radular surface, based on in vivo observations and magnetic resonance imaging of intact animals, provides support for the biomechanical analysis. These results suggest how a soft grasper can work during feeding, and suggest novel designs for artificial soft graspers.

Not just scratching the surface: Distinct radular motion patterns in Mollusca

“…Within Heterobranchia, we observed motion pattern V in most species (also described in e.g. in Eigenbrodt, 1941 ; Wägele, 1983 ; Mackenstedt and Märkel, 2001 ; Neustadter et al, 2002a , b ; Montroni et al, 2019 ; Kehl et al, 2019 ; Krings et al, 2019a ), only S. marmorata was distinct (motion pattern VI; see also Eigenbrodt, 1941 ). We would, hence, propose that the licking movement is plesiomorphic in Heterobranchia and was modified within the Physidae.…”

“…Usually, the feeding motion described in textbooks is the feeding pattern documented for Heterobranchia, which can be described as a licking movement involving the protruding of the radula enabling the teeth to loosen the ingesta (for details see, e.g. Mackenstedt and Märkel, 2001 ; Neustadter et al, 2002a , b ; Montroni et al, 2019 ; Kehl et al, 2019 ; Krings et al, 2019a ). However, studies on the musculature of the buccal mass (e.g.…”

Rapid Adaptation to Changing Mechanical Load by Ordered Recruitment of Identified Motor Neurons

“…The much larger forces animals generate on their own when swallowing tough foods in their natural habitat have not been measured, nor have the neural correlates of adaptation to high loads. Recent studies of the biomechanics of feeding (Neustadter et al, 2002(Neustadter et al, , 2007Kehl et al, 2019), modeling (Neustadter et al, 2002;Sutton et al, 2004a), and neural control (McManus et al, 2014;Cullins et al, 2015b;Lu et al, 2015) have provided further insights into the behavioral roles of individual neurons, suggesting that it might now be possible to relate behavior to neural activity despite the high variability.…”

Radular force performance of stylommatophoran gastropods (Mollusca) with distinct body masses

“…However, for precise interpretation of the relationship between buccal mass size and the mass of the radular muscles (buccal mass musculature) knowledge about the functional role of each feeding muscle must be available, which is not the case for the taxa studied here. There are some detailed studies analyzing the in vivo buccal mass movement and buccal mass muscle function in Aplysia 89 , 90 , 92 , 108 . But since Aplysia belongs to the Opisthobranchia and is not closely related to the taxa involved in this experiment the function of the buccal mass muscles for these stylommatophoran taxa cannot be assigned yet.…”