Electroreceptive and mechanoreceptive anatomical specialisations in the epaulette Shark (Hemiscyllium ocellatum)

The arrangement of the electroreceptive ampullary system and closely related mechanoreceptive lateral line canal system was investigated in the epaulette shark, Hemiscyllium ocellatum. The lateral line canals form an elaborate network across the head and are continuously punctuated by pores. Ampullary pores are distributed in eleven distinct pore fields, and associated ampullary bulbs are aggregated in five independent ampullary clusters on either side of the head. Pores are primarily concentrated around the mouth and across the snout of the animal. We provide the anatomical basis for future behavioural studies on electroreception and mechanoreception in epaulette sharks, as well as supporting evidence that the electroreceptive ampullary system is specialised to provide behaviourally relevant stimuli. In addition, we describe ampullary pores distributed as far posteriorly as the dorsal fin and thus reject the assumption that ampullary pores are restricted to the cephalic region in sharks.6 page(s

The function of the sawfish's saw

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.2 page(s

Study species: <i>Hemiscyllium ocellatum</i>.

<p><b>A</b>) View of the head of <i>Hemiscyllium ocellatum</i> divided into dorsal (D), lateral (L), and ventral (V) planes. <b>B</b>) Ventral view of the head of <i>H. ocellatum</i> showing mouthparts specialised for benthic suction-feeding. <b>C</b>) The close physical association between electroreceptive (AOL) and mechanoreceptive (LL) pores in the skin.</p

Summary of the mean number of ampullary pores in <i>H. ocellatum</i>.

<p>Pore counts are presented per pore field on one body half, according to their affiliation and location. Pores were counted in n = 3–8 pore fields each.</p

Description of the mechanoreceptive lateral line and electroreceptive ampullary systems in the freshwater whipray, Himantura dalyensis

Mechanoreceptive and electroreceptive anatomical specialisations in freshwater elasmobranch fishes are largely unknown. The freshwater whipray, Himantura dalyensis, is one of a few Australian elasmobranch species that occur in low salinity (oligohaline) environments. The distribution and morphology of the mechanoreceptive lateral line and the electroreceptive ampullae of Lorenzini were investigated by dissection and compared with previous studies on related species. The distribution of the pit organs resembles that of a marine ray, Dasyatis sabina, although their orientation differs. The lateral line canals of H. dalyensis are distributed similarly compared with two marine relatives, H. gerrardi and D. sabina. However, convolutions of the ventral canals and proliferations of the infraorbital canal are more extensive in H. dalyensis than H. gerrardi. The intricate nature of the ventral, non-pored canals suggests a mechanotactile function, as previously demonstrated in D. sabina. The ampullary system of H. dalyensis is not typical of an obligate freshwater elasmobranch (i.e. H. signifer), and its morphology and pore distribution resembles those of marine dasyatids. These results suggest that H. dalyensis is euryhaline, with sensory systems adapted similarly to those described in marine and estuarine species.9 page(s

Summary of the length of ampullary canals in <i>H. ocellatum</i>.

<p>Canal lengths are presented as a percentage of total body length (mean and standard deviation) per pore field. Total body lengths are as follows; specimen 1: 84.0 cm, specimen 2: 75.7 cm, specimen 3: 82.8 cm, specimen 4: 65.0 cm. Calculations are based on measurements from each pore field of the left lateral half of a specimen (n = 4).</p

Electric field detection in sawfish and shovelnose rays

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 nVcm(-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

Scatterplot of the orientation distance [cm] plotted against the orientation angle [°] at PIR towards dipole electric fields.

<p>(a) Values measured for reactions by sawfish <i>Pristis microdon</i> and shovelnose rays <i>Aptychotrema rostrata</i> and <i>Glaucostegus typus</i> towards electric dipoles presented on the bottom, and (b) for <i>P. microdon</i> towards dipoles located on the bottom and in the water column. The orientation distance decreases slightly at higher angles, but this relationship is not significant (see text for statistical analysis).</p

Reaction sequence of juvenile sawfish towards electric dipoles.

<p>Dipoles (indicated with two red dots) are presented (1) on the substrate and (2) in the water column. The sawfish swims past the dipole and turns towards it (1a–b), wiggles the rostrum over the dipole centre (1c–d), approaches further (1e) and bites the dipole centre (1f). When reacting to the dipole suspended in the water, the sawfish almost passes the dipole (2a–b), turns towards it (2c) and produces ‘saw in water, (2d–f).</p