Morphological Variability and Function of Labial Cartilages in Sharks (Chondrichthyes, Elasmobranchii).

(1) Background: Labial cartilages (LCs), as their name suggests, lie in the folds of the connective tissue, the lips, framing the gape of elasmobranch chondrichthyans. As such, these cartilages lie laterally to the jaws and marginal teeth. They are considered to influence the ability of creating suction during the feeding process. As past studies have shown, LCs in sharks are as diverse as their varied feeding techniques and differ between species in number, size, shape, and position. This allows establishing parameters for inferring the feeding and hunting behaviors in these ecologically important fishes. (2) Methods: We present a study of LCs based on the CT scans of more than 100 extant shark species and, therefore, represent at least one member of every living family within the Euselachii, excluding batoids. (3) Results: Accordingly, sharks without labial cartilages or that have only small remnants are ram feeders or use pure biting and mainly occupy higher trophic levels (tertiary and quaternary consumers), whereas suction-feeding sharks have higher numbers (up to five pairs) of well-developed LCs and occupy slightly lower trophic levels (mainly secondary consumers). Species with unique feeding strategies, like the cookie-cutter shark (Isistius brasiliensis, an ectoparasite), display distinct shapes of LCs, while generalist species, conversely, exhibit a simpler arrangement of LCs. (4) Conclusions: We propose a dichotomous identification key to classify single LCs into different morphotypes and propose combinations of morphotypes that result in suction feeding differing in strength and, therefore, different hunting and feeding strategies. The conclusions of this study allow to infer information about feeding strategies not only in extant less-known sharks but also extinct sharks.

Larval fish counteract ram and suction to capture evasive prey.

A simple hydrodynamic model of predator-prey interactions between larval clownfish and copepod prey is used to elucidate how larval fish capture highly evasive copepods. Fish larvae are considered to be suction feeders; however, video observations revealed that successful captures by clownfish larvae were preceded by rapidly accelerating lunges (ram), while the role of suction to draw prey into the fish's mouth was less clear. Simulations were made of the fish's strike, varying strengths of ram and suction to characterize optimal strategies for copepod capture given known evasive capabilities. Our results suggest that, contrary to expectations, suction feeding is dominant only in older larvae, whereas ram feeding is the dominant mode for early larvae. Despite the relatively weak suction produced by smaller larvae, it still plays a crucial role in prey capture through hydrodynamic stealth. Escape-triggering water deformations from the strike can be cancelled through controlled suction. Experimental data obtained from larval clownfish agree with model results, suggesting that the primary role of suction in early larvae is providing hydrodynamic stealth rather than capture.