Bite force and feeding kinematics in the eastern North Pacific Kyphosidae

Area of Science:

• * Functional morphology
• * Biomechanics
• * Ichthyology

Background:

• * The intramandibular joint (IMJ) in some fish is hypothesized to increase gape and substrate contact for feeding on attached food items.
• * The mechanical implications of IMJ function in removing attached food remain unclear.
• * Kyphosid fishes, including Girella nigricans, Medialuna californiensis, and Hermosilla azurea, inhabit similar ecological niches and possess IMJs.

Purpose of the Study:

• * To investigate the feeding mechanics of Girella nigricans, focusing on the role of the IMJ.
• * To compare feeding behaviors on different food types (gelatin block vs. Ulva sp.).
• * To determine if the IMJ is actively or passively involved in feeding.

Main Methods:

• * Analysis of feeding mechanics in Girella nigricans during scraping (gelatin) and picking (Ulva sp.) behaviors.
• * Measurement of kinematic variables: cranial elevation, lower jaw rotation, premaxillary protrusion and rotation, gape, and intramandibular rotation.
• * Theoretical bite force calculation per unit length for G. nigricans and related species.

Main Results:

• * Girella nigricans exhibited the highest theoretical bite force per unit length among the studied kyphosids.
• * Scraping on gelatin elicited greater lower jaw rotation and gape compared to picking Ulva sp.
• * Intramandibular rotation was observed only during direct contact with the gelatin substrate, suggesting a passive IMJ.
• * Suction feeding was not evident in G. nigricans.

Conclusions:

• * The intramandibular joint in Girella nigricans appears to be a passive structure, not actively controlled by musculature.
• * The feeding mechanics suggest specialization for maximizing jaw force during scraping, rather than solely increasing gape.
• * The absence of suction feeding further supports a jaw-force-based feeding strategy in this species.