The morphological changes in the gill by oxygen deficiency of common squids Todarodes pacificus

The difficulty associate with maintaining live squids for two days or more severely limits the geographic area and time over which they can be distributed and sold. The cases of death during transportation are not yet clearly understood, but it is thought to be due to the physical damage they inflict on each other, the stress of competition for space, stress due rapid changes of surroundings or by combination of these factors. We investigated the damage to gills due to a shortage in the dissolved oxygen (DO) in water to examine its possible contributions to the cause of the death. Deformations in the protruding quasi-lamellae of the gill were measured for different DO Levels by megascopic and microscopic observations. The experiments were conducted on the Ushio maru training-ship outside Hakodate port, using squids caught by angling. Squids were maintained in boxes (20 cm x 42 cm x 22 cm) with 8 L of water, and aeration was stopped at the beginning of the test. The DO in the water decreased slowly in response to respiration of the squid, which died approximately after three hours by a shortage of oxygen. Behavioral observations showed that squids were initially active in the box. Once oxygen decreased to approximately 6 mg/L, squids remained motionless near the bottom of the box that might be due in response to a shortage of oxygen. Squids kept this behavior for two or three hours depending on the temperature. Although squids did not move in this period, they reacted to external stimulations, such as touch, or changes in light. Then, their body color changed to white and they died. Gill damage was compared in three categories: (1) initial DO, (2) half DO and (3) low DO. Gills were surgically removed and preserved in formalin until such observations in the laboratory. The number of deformation of quasi-lamellae was counted, and the extent of the protrusion was measured using microscope and by viewing. The extent of deformations differed significantly depend on the shortage of DO.