RESUMO

The feeding anatomy, behavior and diet of the whale shark Rhincodon typus were studied off Cabo Catoche, Yucatan Peninsula, Mexico. The filtering apparatus is composed of 20 unique filtering pads that completely occlude the pharyngeal cavity. A reticulated mesh lies on the proximal surface of the pads, with openings averaging 1.2mm in diameter. Superficial to this, a series of primary and secondary cartilaginous vanes support the pads and direct the water across the primary gill filaments. During surface ram filter feeding, sharks swam at an average velocity of 1.1m/s with 85% of the open mouth below the water's surface. Sharks on average spent approximately 7.5h/day feeding at the surface on dense plankton dominated by sergestids, calanoid copepods, chaetognaths and fish larvae. Based on calculated flow speed and underwater mouth area, it was estimated that a whale shark of 443 cm total length (TL) filters 326 m(3)/h, and a 622 cm TL shark 614 m(3)/h. With an average plankton biomass of 4.5 g/m(3) at the feeding site, the two sizes of sharks on average would ingest 1467 and 2763 g of plankton per hour, and their daily ration would be approximately 14,931 and 28,121 kJ, respectively. These values are consistent with independently derived feeding rations of captive, growing whale sharks in an aquarium. A feeding mechanism utilizing cross-flow filtration of plankton is described, allowing the sharks to ingest plankton that is smaller than the mesh while reducing clogging of the filtering apparatus.

Assuntos

RESUMO

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.

Assuntos

RESUMO

Jaw protrusion is a major functional motif in fish feeding and can occur during mouth opening or closing. This temporal variation impacts the role that jaw protrusion plays in prey apprehension and processing. The lesser electric ray Narcine brasiliensis is a benthic elasmobranch (Batoidea: Torpediniformes) with an extreme and unique method of prey capture. The feeding kinematics of this species were investigated using high-speed videography and pressure transduction. The ray captures its food by protruding its jaws up to 100% of head length (approximately 20% of disc width) beneath the substrate and generating negative oral pressures (< or = 31 kPa) to suck worms into its mouth. Food is further winnowed from ingested sediment by repeated, often asymmetrical protrusions of the jaws (> 70 degrees deviation from the midline) while sand is expelled from the spiracles, gills and mouth. The pronounced ram contribution of capture (jaw protrusion) brings the mouth close enough to the food to allow suction feeding. Due to the anatomical coupling of the jaws, upper jaw protrusion occurs in the expansive phase (unlike most elasmobranchs and similar to bony fishes), and also exhibits a biphasic (slow-open, fast-open) movement similar to tetrapod feeding. The morphological restrictions that permit this unique protrusion mechanism, including coupled jaws and a narrow gape, may increase suction performance, but also likely strongly constrain dietary breadth.