Toadfish, Opsanus tau, were wild captured from Chesapeake Bay by otter trawl, and transported to a fish health research laboratory. They were acclimated to laboratory conditions (16 hours light : 8 hours dark, 20 degrees Celcius, 15 ppt salinity) in a 450-gallon recirculating system with biofiltration. They readily accepted a gelfood made from trout chow and seaweed (nori).

Case History

After approximately 5 weeks, several of the animals began floating near the surface, head down. However,they responded to gentle prodding with a net, and maintained a healthy appetite. This air bubble - floatation problem dissapated in these animals after approxiamtley 5 days, but the problem resurfaced in other animals the next week.

Observations

Note the numerous air bubbles on the dorsal fin of this only moderately affected toadfish. Careful observation reveals several dark, 3-5 mm organisms on and around the bubbles.

More Observations

These small, dark organisms were present over the entire body surface, although they were most notable on the dorsal fish and peduncle region. How many of these organisms can you count? Roll the mouse cursor over the chin of the toadfish to reveal the parasites. Perhaps you can understand how these parasites were missed when the fish were first examined (and the parasite number was fewer).

Argulus shown to scale

Argulus: a parasitic branchiuran

Branchiuran
The majority of branchiurans are adapted for living between intertidal sand grains. Some species, such as Argulus are parasitic on fish.

Bacterial Culture

When the inside of the bubble lesions of the toadfish were cultured on several bacteriological media, growth only occured in aneraobic media. Note the air gap between the cooked meat medium (below) and the wax plug (above). The wax plug was pushed upward by an anerobic, gas produced from a gas-forming bacillus.

Diagnosis

The air bubbles most resulted from bacterial infection which was most likely secondary to parasitic infection. The parasite itself may have been a carrier of the bacterium. Once the parasites were removed (by hand) from the fish, the bacterial problems no longer persisted.