[Show abstract][Hide abstract] ABSTRACT: In the aquatic environment, living organisms emit weak dipole electric fields, which spread in the surrounding water. Elasmobranchs detect these dipole electric fields with their highly sensitive electroreceptors, the ampullae of Lorenzini. Freshwater sawfish, Pristis microdon, and two species of shovelnose rays, Glaucostegus typus and Aptychotrema rostrata were tested for their reactions towards weak artificial electric dipole fields. The comparison of sawfishes and shovelnose rays sheds light on the evolution and function of the elongated rostrum ('saw') of sawfish, as both groups evolved from a shovelnose ray-like ancestor. Electric stimuli were presented both on the substrate (to mimic benthic prey) and suspended in the water column (to mimic free-swimming prey). Analysis of around 480 behavioural sequences shows that all three species are highly sensitive towards weak electric dipole fields, and initiate behavioural responses at median field strengths between 5.15 and 79.6 nV cm(-1). The response behaviours used by sawfish and shovelnose rays depended on the location of the dipoles. The elongation of the sawfish's rostrum clearly expanded their electroreceptive search area into the water column and enables them to target free-swimming prey.
[Show abstract][Hide abstract] ABSTRACT: Jawed fishes that possess an elongated rostrum use it to either sense prey or to manipulate it, but not for both. The billfish rostrum, for instance, lacks any sensory function and is used to stun prey , while paddlefishes use their rostrum to detect and orient towards electric fields of plankton . Sturgeons search through the substrate with their electroreceptive rostrum, and engulf prey by oral suction . Here, we show that juvenile freshwater sawfish Pristis microdon are active predators that use their toothed rostrum - the saw - to both sense prey-simulating electric fields and capture prey. Prey encountered in the water column is attacked with lateral swipes of the saw that can stun and/or impale it. We compare sawfish to shovelnose rays, which share a common shovelnose ray-like ancestor  and lack a saw.
Full-text · Article · Mar 2012 · Current biology: CB
[Show abstract][Hide abstract] ABSTRACT: Sclerorhynchids (extinct sawfishes, Batoidea), pristids (extant sawfish, Batoidea) and pristiophorids (sawsharks, Squalomorphi) are the three elasmobranch families that possess an elongated rostrum with lateral teeth. Sclerorhynchids are the extinct sawfishes of the Cretaceous period, which reached maximum total lengths of 100 cm. The morphology of their rostral teeth is highly variable. Pristid sawfish occur circumtropically and can reach maximum total lengths of around 700 cm. All pristid species are globally endangered due to their restricted habitat inshore. Pristiophorid sawsharks are small sharks of maximum total lengths below 150 cm, which occur in depths of 70–900 m. Close examination of the morphology of pectoral fin basals and the internal structure of the rostrum reveals that sclerorhynchids and pristids evolved independently from rhinobatids, whereas pristiophorids are squalomorph sharks. The elongation of the rostrum may be an adaptation for feeding, as all marine vertebrate taxa that possess this structure are said to use it in the context of feeding.
Full-text · Article · May 2009 · Reviews in Fish Biology and Fisheries