Pollution and global warming combine to devastate
reproductive ability of marine species

Reduced oxygen in the world’s oceans is causing sex changes and deformities in fish and other marine life, and is leading to a potential ecological disaster, researchers at the City University of Hong Kong warn.
Within a few years, says Principal Investigator Prof Rudolph Wu, some species could be virtually wiped out.

Prof Wu with fish being prepared for his hypoxia research
Whelks, already a gender imbalance

Oxygen starvation in water, or hypoxia, is caused by both human activities and natural phenomenon.
Pollution from sewage and agricultural activities can lead to hypoxia in coastal waters. But added to the problem of coastal pollution are the results of global warming which are affecting vast areas of oceans.
“With the ice caps melting, there are large inputs of fresh water into the sea which, because it’s not as dense as salt water, sits on top and prevents oxygen exchange,” said Prof Wu. “The fresh water also adds nutrients which lead to algal blooms and an increased demand for oxygen,” he added.
In recent years, scientists have been concerned that chemicals known as endocrine disruptors, although occurring in concentrations as low as parts in a trillion, can be enough to disturb the normal balance and change the sex status of a marine creature, said Prof Wu.
Prof Wu’s research has shown, for the first time in science, that hypoxia is also an endocrine disruptor, meaning that it interferes with functions of the glands that secrete sex hormones. For example, it inhibits the production of the female hormone oestrogen which stimulates the development of female reproductive organs and promotes female sexual characteristics.
In male fish, he showed that hypoxia led to the depletion of the male hormone testosterone which caused a decline in fertilisation capabilities.
Prof Wu demonstrated the changes using carp and zebra fish and simulating hypoxic conditions by introducing nitrogen into fish tank water, depleting the level of oxygen from a normal 7 parts per million to 1 part per million.
After six to eight weeks, levels of the sex hormones oestradiol and testosterone in both female and male fish were only 20 percent of those in normal fish.
The reproductive processes and output of the fish were seriously affected; whereas an average of 92 percent of eggs from normal fish successfully hatched, the survival of eggs from oxygen-deficient fish dropped dramatically to 4 per cent.
Prof Wu also demonstrated that hypoxia is a teratogen, causing stunted growth and malformation during embryonic stages.
Malformations included the lack of a vascular system, changes in apoptosis (the programmed death of cells), and an altering of the expression of genes controlling sex differentiation and development.
The change of sex in marine species as diverse as crocodiles and sea snails by chemical endocrine disruptors has been noted by scientists for some time, with dominant gender orientation being male. Said Prof Wu: “Reproductive impairments could lead to population decline and extinction of species in the long run. In the coming years, hypoxia is going to get worse because of the effects of global warming. What we are tackling is a global problem that is highly relevant to Hong Kong. Globally, it could be a real disaster.”

Principal Investigator
Prof Rudolf S S Wu : bhrswu@cityu.edu.hk