Anatomy and functional morphology of the feeding apparatus of the lesser electric ray, Narcine brasiliensis (Elasmobranchii: Batoidea)

Protrusion of the jaws during feeding is common in Batoidea (rays, skates, sawfishes, and guitarfishes), members of which possess a highly modified jaw suspension. The lesser electric ray, Narcine brasiliensis, preys primarily on polychaete annelids using a peculiar and highly derived mechanism for jaw protraction. The ray captures its prey by protruding its jaws beneath the substrate and generating subambient buccal pressure to suck worms into its mouth. Initiation of this protrusion is similar to that proposed for other batoids, in that the swing of the distal ends of the hyomandibulae is transmitted to Meckel's cartilage. A "scissor-jack" model of jaw protrusion is proposed for Narcine, in which the coupling of the upper and lower jaws, and extremely flexible symphyses, allow medial compression of the entire jaw complex. This results in a shortening of the distance between the right and left sides of the jaw arch and ventral extension of the jaws. Motion of the skeletal elements involved in this extreme jaw protrusion is convergent with that described for the wobbegong shark, Orectolobus maculatus. Narcine also exhibits asymmetrical protrusion of the jaws from the midline during processing, accomplished by unequal depression of the hyomandibulae. Lower jaw versatility is a functional motif in the batoid feeding mechanism. The pronounced jaw kinesis of N. brasiliensis is partly a function of common batoid characteristics: euhyostylic jaw suspension (decoupling the jaws from the hyoid arch) and complex and subdivided cranial musculature, affording fine motor control. However, this mechanism would not be possible without the loss of the basihyal in narcinid electric rays. The highly protrusible jaw of N. brasiliensis is a versatile and maneuverable feeding apparatus well-suited for the animal's benthic feeding lifestyle.     

Eocene isopods on electric rays: tracking ancient biological interactions from a complex fossil record

Isopod crustaceans have developed a variety of feeding strategies, which impact on their various life habits over time. Apart from morphological adaptations and the typical secondary deformations that some parasitic isopods inflict on their host's exoskeleton, traces of feeding behaviours involving isopods on their victim/food are seldom fossilized. Many of these rare occurrences consist of cases for which the degree of association between the isopods and their potential food source is unclear, or the interaction only very briefly explained (recently analysed cases excepted). There are two limiting problems in identifying the biological nature of fossilized associations: (1) direct associations of organisms preserved as ‘imprint’ (as opposed to inclusions in cherts or amber) are shaped by several taphonomic events difficult to identify (such as the time of death, burial and fossilization of the organism); (2) even in modern nature, differences within syn vivo interactions (like parasitism and micropredation) are poorly understood in marine systems. We report the occurrence of isopods associated with ancient chondrichthyans which also represent rare cases of the preservation of several fossil isopods on larger organisms. These organisms are adult electric rays of the genus ?Titanonarke Carvalho from the late Ypresian (Eocene) of the Monte Postale site, Bolca Lagerstätte, Italy. By examining: (1) the involved lineages of rays and isopods; (2) the taphonomy of the association; (3) its environmental context; and (4) biological/adaptive features; we identify this association as a selective case, either of ancient scavenging or of micropredation on specific electric rays by isopods.