Last May 2017, the aquarists who tend a Vancouver Aquarium exhibit with C-O sole (Pleuronichthys coenosus) inside noticed that a female flatfish was gravid (full of eggs). They brought the female, along with two males, behind the scenes and tried to artificially inseminate the eggs at the Fish Research Lab. The lab has raised many species of fish and invertebrate over the years, but never have they raised a species of flatfish — until now.
Did you know the C-O sole name comes from the English letters “C” and “O” found on the adult’s tail?
Timing is crucial when it comes to fertilizing eggs. Fish eggs are only receptive to sperm for a brief period of time. Likewise, the sperm themselves are only active for a few minutes once they come in contact with sea water. The researchers attempted to strip milt (seminal fluid) from the males and obtain sperm, but they were unable to. Instead, the soles were placed together in an isolated tank in the hope that they would spawn naturally. A few mornings later, hundreds of sole eggs were floating at the surface of the tank. The fish had done the deed themselves.
While some fish lay demersal eggs (eggs that either lay on or are attached to the bottom), flatfish lay pelagic eggs, which means they float in the water column while the embryo inside develops. A look at these floating eggs under the microscope showed cell division and development, revealing that they were fertilized. The waiting game for the larvae to hatch began.
Around two weeks later, the first baby sole, less than four millimetres long, emerged. When the sole first hatches, the larva swims upright like an ordinary fish with one eye on each side of its body. As it ages and grows larger, the sole starts changing shape and its left eye begins to migrate to the right side.
The genus of the C-O sole’s scientific name, Pleuronichthys, is actually a reference to the right-eyed flounder family (Pleuronectidae). Once transformed, the sole lies horizontally with its left side on the bottom. The left-eyed family of flatfish (Bothidae) does the opposite migration, with the right eye migrating to the left side of the body and the right side of the body facing the bottom.
The sole changes colour as it grows, too: from translucent to pale to dark brown-black to a stark white. At that point, it starts getting more pigment again until it develops adult colouration, such as the pattern on its tails that looks like the two English letters “C” and “O” that gives this species its common name.
Today, in 2018, around 60 little C-O sole are feeding and growing up behind the scenes at Ocean Wise’s Vancouver Aquarium. In the meantime, you can visit three C-O sole living in the sea star habitat in the Canaccord Financial Exploration Gallery at Vancouver Aquarium. Try to find them all on your next visit!