The heterocercal tail of a shark is a caudal fin with unequal lobes.
A heterocercal tail is a caudal fin composed of two asymmetrical lobes. Often, such as the case in many sharks, the vertebral column passes through the upper lobe, making it the larger of the two lobes. A heterocercal tail is contrasted with a homocercal tail which has equal lobes.
The ampullae of Lorenzini on this chimaera are the two rows of dots on its snout (NOAA)
Ampullae of Lorenzini are a network of electroreceptors, sensory organs that detect electric fields in water, found in chondrichthyes (sharks, rays, and chimaeras). The ampullae are a series of symmetrical pores, concentrated around the snout and nose, connected by gel-filled canals. They can conduct electrical impulses so small, that chondrichthyes are likely to be more sensitive to electric fields than any other group of animals. Because all muscle contractions produce a weak electrical field, these electroreceptors make sharks, rays, and chimaeras highly capable of detecting other organisms, such as prey, nearby in water.
Ampullae of Lorenzini, shown in red, are a electroreceptors that can detect electrical impulses from other organisms in water.
Spotted Eagle Rays are a member of the cartilaginous subclass Elasmobranchii along with other rays, sharks, and sawfishes (NOAA).
Elasmobranchs, including sharks, rays, and sawfishes, belong to the taxonomic subclass of cartilaginous fish Elasmobranchii. Like most chondrichthyes, they have exposed gills, no swim bladder, internal fertilization, and placoid denticles. They differ from the other subclass, chimaera (subclass: Holocephali), in that they have rigid dorsal fins, placoid denticles cover most of their bodies, and they usually have spiracles (modified gill slits directly behind the eye).
Chimaeras are a predominately deep sea branch of chondrichthyes with most found deeper than 200m (NOAA).
Chondrichthyes are a taxonomic class of cartilaginous fishes that encompass sharks and rays (elasmobranchs) and chimaera. Though there are exceptions, in general, Chondrichthyes have exposed gills, no swim bladder, internal fertilization, and placoid denticles. These characteristics differentiate them from the more evolutionarily derived branch of fishes, bony fish (Osteichthyes).
While these Great White placoid denticles may look like scales, they are actually modified teeth (Trevor Sewell/Electron Microscope Unit, University of Cape Town).
Placoid denticles are found on sharks, rays, and chimaeras. Not really a true “scale,” like ctenoid or cycloid scales, placoid denticles are actually modified teeth. They have an inner tissue component, which contains both blood vessels and nerves, that is covered by a layer of dentin and an outer enamel. They form a tough protective skin layer for sharks, rays, and chimaeras and also have shown to reduce friction and drag so that these fish can swim more efficiently through water.