Despite its name, the red-tailed shark (Epalzeorhynchos bicolor) is not a shark. It has scales and lacks teeth. It is more closely related to carps and goldfish. It gets its name from its dorsal fin, which is shaped somewhat like a shark's, and its red tail. Red-tailed sharks were native to Thailand's rivers and streams but are now thought to be extinct in the wild, although they thrive in Thailand's pet-fish farms. The red-tailed shark's breeding involves the spawning process.
Swollen, but Not Exactly Pregnant
Female red-tailed sharks lay eggs that the males fertilise. You cannot correctly call a red-tailed shark pregnant.
You can tell when a female is getting ready to spawn. The female fish acquires a noticeably swollen belly as it fills with eggs.
You will rarely see red-tailed sharks mate, however. The fish are extremely territorial and if paired one will usually harass the other to death. If you're lucky enough to get two red-tailed sharks to mate, the young will hatch in only a day or two.
How to Tell the Sex of Your Fish
Male and female red-tailed sharks both have jet-black bodies and bright red tails. The females are generally larger and thicker than the males but that is difficult to discern unless you have fish to compare. A large adult can measure 5 inches, according to Seriouslyfish.com.
Some sources say a male red-tailed shark has a pointed tip to his dorsal fin while a female's dorsal fin is more square, but other sources say this characteristic is unreliable. Similarly, experts disagree over whether females have greyer bellies.
In their native habitat, red-tailed sharks probably lived solitary lives and only came together to breed. Producers of pet fish don't have the patience to wait for natural matings so they coerce egg and sperm production with injections of a mixture of the pituitary glands of carp and human chorionic gonadotrophic hormone.