The function of placoid scales (dermal denticles) is considered along three perspectives. Firstly, in a historical context, denticles have been modified along several functional lines, including protection from predators and ectoparasites, reduction of mechanical abrasion, accommodation of bioluminescent and sensory organs, and reduction of frictional drag. Each function selects for a unique morphology described herein. Secondly, denticles characteristic of benthic and/or sedentary sharks are re-examined through an analysis of 15 genera. Generalizations are difficult due to the wide range of variation in crown morphology and overall scale weight, but individual denticle types do reflect combined optimal characteristics for both hydrodynamics and protection, tending toward the latter. Lastly, the relationship between slight variations in regional flow dynamics over the body, and the potential impact of this relationship on the previously defined hydrodynamic scale characteristics, is examined in two pelagic carcharhinids. These findings suggest that along the anterior portion of the body, strong indications of reduced ridge dimensions (heights and spacings) on the dorsum are present and may be related to water velocity.

A comparison of the ampullae of Lorenzini among 40 species of skates (Rajoidei) demonstrates a close relationship between inferred electroreceptive capabilities and feeding mechanisms. Three general lines of morphological modifications are noted. (1) Whereas the majority of ampullary pores are located on the ventral surface of the dorsoventrally flattened body, the relative proportion of ventral pores is significantly lower on species inhabiting aphotic waters. (2) The ventral pores on more piscivorous species are distributed over a larger portion of body surface than they are on those species that feed primarily on invertebrates. Ventral pores in this latter group are more noticeably concentrated around the mouth and their densities on the adult are inversely related to the overall mobility of preferred prey species. (3) The size of each ampulla and the number of alveoli associated with it are directly related to the habitat depth occupied by each species. Shallow-water species have smaller ampullae with fewer alveoli than deeper-dwelling (> 1,000 m) species. The general distribution of ampullary pores on deep dwelling rajoids appears to compensate for reduced visual input, whereas their relative densities are a measure of the system's resolution and reflect major differences in feeding strategies. The increased ampullary size and complexity observed in deep-sea rajoids provides mechanisms to increase both the sensitivity and signal-to-noise ratios.

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The peripheral electroreceptive system of the African freshwater stingray, Dasyatis garouaensis, consists of numerous tubules, each originating as a minute cutaneous pore and terminating within the dermis as a slightly expanded bulb. These "mini-ampullae'' range in size from ca. 0.75 to 2.00 mm long in adults and appear to increase in both size and number ontogenetically. They are heavily concentrated on the ventrum, particularly around the mouth. Each canal wall consists of several (2-6) layers of stratified epithelium that varies from columnar to squamous with the proximity to the pore. The corresponding ampulla is innervated and contains a patch of relatively small sensory cells interspersed among large, vacuolated cells. Both ampulla and canal are surrounded by a thick connective tissue coat.The electroreceptive system of this exclusively freshwater, African species is similar to that of the South American Potamotrygonidae. Both taxa possess shorter, narrower canals with smaller, much simpler (less alveolate) ampullae than the marine dasyatids. The arrangement of the sensory epithelium also appears similar. However, D. garouaensis differs from Potamotrygon in the histological nature of the canal wall, the complexity of the cutaneous pore and the relative thickness of the connective tissue tunic. The functional significance of these similarities and differences reflects the constraints of the freshwater environment on electrosensitivity.

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