Investigations on the incidence of cystic calculus in female cobia Rachycentron canadum (Linnaeus, 1766) broodfish leading to spawning failure and mortality

Abstract

The morphological, chemical and high resolution electron microscopic analysis of a cystic calculus (urinary bladder stone) from a ready-to-spawn female broodfish of cobia Rachycentron canadum (Linnaeus, 1766) is reported. The stone was elliptical in shape with 31 mm dia having chalky, amorphous and fragile consistency with several concentric major layers covering the core. Chemically the stone was uniform in composition with 52.63% calcium oxalate monohydrate (COM), 31.58 % uric acid and 15.70% hydroxyapatite (HAP). However, the peripheral layer differed significantly from the middle and the core in elemental composition. Ultrastucturally, each of the individual layers were made of multiple fine
sub-layers. Additionally, all the layers/portions of the stone displayed a hard, coral-like structure entrapping spherical sacs and contained isolated and fused spherules inside and outside the sacs. However, hexagonal forms of COM crystals were unique to the peripheral layers. While the obstruction of the oviduct due to the large sized stone was probably the cause of spawning failure. Continuous but futile contraction of the musculature during the unsuccessful spawning effort might have caused energy exhaustion and ultimate death of the animal.