In cartilaginous fish, holocephalians and elasmobranchs, the endolymphatic system of the inner ear generally communicates with the exterior via the endolymphatic ducts. The openings of the ducts on the dorsal side of the head were known already by eighteenth-century anatomists (Retzius, 1881). In certain elasmobranchs mineral particles from the environment pass into the inner ear through the ducts. Stewart (1906) described presence of sand grains in the membranous labyrinth of the shark, Squatina. Nishio (1926) found black volcanic particles of varying sizes in the endolymphatic duct, the sacculus and the lagena of Squatina angelus and the ray, Torpedo ocellata, from the gulf of Naples.

I have studied the inner ear of two species of electric rays, Torpedo nobiliana and T. marmorata. Considerable amounts of fine sand grains are found in the endolymphatic system of both species. The mineral particles are kept together by a gelatinous material or occur free in the endolymph. The size of the sand particles varies between 10 and 400μm. The grains are white or transparent, yellow, brownish or black. Mineralogical analysis with a polarizing microscope reveals that 85–90% of the particles consist of quartz, 5–10% of calcium carbonate and less than 5% of an unidentified opaque mineral, possibly corroded felspar. The grains are irregular with rounded or sharp edges. They differ in appearance from the round or lens-shaped concentrically layered aragonite octoconia found in the labyrinth of most elasmobranchs (Retzius, 1881; Nishio, 1926). The present findings seem to be consistent with the results by Stewart (1906) and Nishio (1926) suggesting that certain psammophilous elasmobranchs lack the ability to produce calcium carbonate otoliths and compensate for this by uptake of sand grains through their endolymphatic ducts.