Observations of ultrasound-induced effects in the fish Xiphophorous maculatus

Tails of the fish Xiphophorous maculatus have been studied by transmission light microscopy during irradiation with continuous wave ultrasound (frequencies 0.78-3 MHz, spatial average intensities 0.01-3 Wcm-2). Blood flow started to increase a few minutes after the start of an irradiation, reaching a maximum after 5-10 min. Periodic variations in blood flow rate were often seen, and the response varied considerably among individual specimens. Acoustic microstreaming, which resulted in rapid rotation of clusters of cells, was observed in blood vessels adjacent to cartilaginous rods in the tail. The threshold average spatial intensities to initiate this were a few hundred mW cm-2 at 0.78, 1.5 and 3 MHz. The microstreaming resembled that occurring around ultrasonically stimulated microbubbles, but no evidence of any association was found. Microbubbles, possibly originating in water, were sometimes seen in the tissue of the fish following treatment with ultrasound.