A female gray nurse shark at Shanghai Ocean Aquarium has been pregnant for 11 months but still shows no sign of labor, experts said.

"The problem is that a due date for this type of shark is unpredictable," said Yang Xiaofang, spokesperson at Shanghai Ocean Aquarium. "No aquarium in the world has successfully bred gray nurse sharks."

The shark is being watched closely by its feeders.

Pregnancy terms for wild nurse sharks vary from nine to 12 months. However, successful births are rare even among nurse sharks in the ocean, said Guan Xuebin, a marine biologist at Shanghai Fisheries University.

A female nurse shark produces eggs that are stored inside its two uteri. The shark can give birth to as many as 30 offspring at one time.

"Usually nurse sharks only have one or two offspring at a time," Guan said.

Guan said nurse sharks do not have an umbilical cord to nourish fetuses. To survive, the tiny sharks eat siblings inside each uterus until there's only one left in each.

"The endangered gray nurse shark is its own worst enemy," Guan said.

According to Guan, female gray nurses usually give birth during the winter at coastal locations where they mass. The marine biologist said the gray nurse shark at the aquarium will likely have a hard time giving birth.

"The environment is too different and it's hard to predict when it will happen," Guan said.

The aquarium said it would continue waiting and watching for another month or two. Spokesman Yang said the aquarium is unsure of what it can do to help the shark.

According to Guan, a caesarean section is possible in theory, but the procedure is very complicated and requires costly technology.

A nurse shark is a large, sluggish, docile marine creature that is generally harmless unless provoked.

In 1984, nurse shark became the first shark to be protected in the world. Its numbers are still critically low due to over hunting and its problems with reproducing.

Nurse sharks range in size from about 0.75 to 4 meters long. They are found in the western Atlantic Ocean and the eastern Pacific Ocean.

(Shanghai Daily January 6, 2006)