Tornado Fish
A schooling type of fish living in 10-20 meter areas of the tidal mudflats of the Skyscraper Reefs. Individually, they are quite normal and edible, but they have adapted the typical 'bait ball' reflect mechanism into a method of foraging for food (buried copeoids and plankton). Each fish is a flattened ovoid, with two large fins along the wide axis with sharp spines protruding from the ends. They have a shorter set of retractable fins along the keel, as well as two mouth slits. Their eyespots and gills are on the dorsal side. Coloration is muddy. Each individual is approximately 10-15cm long, and they typically school in shoals of 50 to 100, but many shoals can gather during high tides to form super-schools of thousands.
Normal behavior for a shoal is to skim the bottom of the tidal mudflats, using their ventral fins like rakes to cut through the mud to loosen copeoids (small krill-like crusteaceans) and mud-algae. The shoal will form a v-headed column when feeding, with frequent rotation of the lead fish toward the back (the lead fish tend not to get as much feed as those who follow due to the fact that the raking fins are behind the mouth). Their ventral spines have chemical receptors sensitive to tell-tale waste products generated by their prey.
During high tides, when water levels in the mudflats are highest and the concentration of their normal prey thickest, multiple shoals will form a `super shoal' in these waters and begin to swim in circles. As they spin faster, their orientation twists 90 degrees and they form a spinning tower (hence the nickname `Tornado Fish'.) The spines of their lateral fins dig into the mud and rasp out a cloud of mud & food. The cyclonic action of the super shoal then carries the plume of goo to the surface, allowing the fish to feed on the effluvia.
The trigger for this super-shoaling is reproductive: the tornado fish will spawn while the water depth is greatest after a double-moon tide to allow for their fry to have maximum nutrients available. Once they have fed themselves, they will then tornado and release eggs and sperm (respectively; the females are identical to the males except for a slight bulge under their tail with twin release ducts for eggs). In these reproductive tornadoes, the males will form themselves toward the bottom and release clouds of sperm, while the females will form near the top and release the slightly-heavier eggs, relying on the cyclonic action to properly mix them for fertilization and then disperse them. The only visible difference between a feeding tornado and a reproductive one is that the reproductive tornado is slightly lighter in color and frothier than a feeding one (which is basically roiled mud).
The fish itself is edible and individually the fish are quite innocuous, though it is dangerous to handle due to the spines on the fins, the spines are sharp enough to penetrate a wetsuit. When a shoal is threatened, they have been seen to form a bait ball with their ventral spines facing outwards to discourage predators, and when in tornado shoal-sized groups they could very easily surround and disorient a swimmer or even capsize a small (canoe-sized) boat.
Wounds from the spines tend to get infected, since the mud they typically feed from is a soup of weird bacteria and fungus.
Contributed on behalf of the author, Mike C, this entry appeared on the Blue Planet Yahoo! Group and is used with kind permission of the author.