Cleaner water helps male fish again look and act like guys | Science News for Students

Cleaner water helps male fish again look and act like guys

Cutting pollution could keep some male fish from becoming feminine
Feb 8, 2017 — 7:10 am EST
rainbow darter

Researchers caught this male rainbow darter fish in the Grand River after improvements to a water treatment plant upstream. This fish now sports its normal rainbow colors.

Courtesy of Mark Servos

Some types of water pollution can make male fish look and act like females. But a new study shows that better water treatment can prevent that. And that could allow  fish populations to thrive.

Water treatment plants are supposed to clean the water from our toilets, showers and sinks before releasing it into rivers, lakes and oceans. They are also supposed to treat water from manufacturing plants. But these water-cleanup plants were never designed to remove all pollutants. Most were built before anyone realized that hormones and hormone-like chemicals could show up in the water. And such chemicals can prove a big problem for fish.

How? They can fake out cells of a male’s body by sending signals telling those cells that this he fish is actually a she. These feminized guy fish may then have little interest in fertilizing a female’s eggs — or he may do so poorly. The result: Fewer baby fish. Or at least that’s the risk.

Male feminization of fish has been showing up in rivers throughout North America.

Mark Servos and his team have been monitoring it in Canada’s Grand River. Servos is an aquatic toxicologist at the University of Waterloo in Ontario. There, he studies the effects of water pollution on rainbow darter fish.

At least once a year from 2007 to 2012, his team caught and examined rainbow darters at a site downstream of a water treatment plant. And depending on the year, between 80 and 100 percent of the males had eggs in them. Egg-making is something that only female fish should do. Moreover, those eggs were in the males’ testes — reproductive organs that normally make sperm (cells used to fertilize a female’s eggs).

grand river
Toxicologist Mark Servos (left) and his team collect rainbow darter fish in Ontario, Canada.
Courtesy of Mark Servos

Many affected males didn’t even look right on the outside. "Male rainbow darter fish are really colorful,” Servos says. Or at least they should be. “This color is important for attracting mates." Yet some local males were becoming drab. That could make it hard for them to find a mate.

Clearly, something was very wrong in these male fish. Until 2013, that is. Suddenly, the number of feminized male fish started to fall. This happened at the same time that the local treatment plant changed how it cleaned the water.

The team’s data now link these two observations in a paper published early online in Environmental Science and Technology.

The problem with feminized males

The bodies of animals — including humans — use hormones to tell their cells when to switch various activities on or off. Those hormones fit like keys into “locks” on the outside of a cell. Scientists call these locks receptors. When hormones connect with their locks, they affect how an animal will develop and act. But certain pollutants act like fake keys. These hormone mimics are known as endocrine (EN-doe-krin) disruptors. They can turn on or off some normal function of an animals’ cells — but at the wrong time. That can make an animal develop or act in a way that isn’t natural.

From 2007 to 2012, Servos’ team found high levels of endocrine disruptors in the water downstream of the water treatment plant. Many of these chemicals mimicked the action of estrogen, a female sex hormone. Some endocrine disruptors came from birth control pills. Their synthetic hormones left a woman’s body in urine. Flushed down the toilet, they ended up at a water-treatment plant. Another common endocrine disruptor is nonylphenol (NON-ul-FEE-nul). It’s a breakdown product of certain surfactants. (Surfactants are chemicals that let liquids mix that would not ordinarily do so.) The problem: Nonylphenol, too, can mimic estrogen.

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feminized fish
When scientists cut open this male rainbow darter, they found it had inappropriately made eggs (round yellow features). Those eggs were growing in its testes.
Courtesy of Mark Servos

Some male rainbow darters exposed to these pollutants produced eggs in their testes. This took a lot of energy. That reduced the energy available for them to make sperm. Affected fish may have made damaged sperm — or no sperm at all. Eggs laid in the water by females won’t mature and hatch unless males release sperm to fertilize those eggs. So egg-making by males could cause fish populations  to shrink. Oh, and the eggs made by those males: They’re worthless. The bodies of males lack the tubes needed to release those eggs. So the eggs just collect and take up space in the males’ bodies.

Bacteria, bubbles and healthier fish

The good news: Changes to the wastewater-treatment plant in 2013 led to changes in those males.

The plant had always used bacteria to break down harmful chemicals in the water. But workers upgraded the system to give the bacteria more time to break down chemicals. The plant now also bubbled oxygen into the wastewater. This extra oxygen helped the hard-working bacteria grow faster.

Servos and his team tested the water and fish for three years after these water-cleaning changes. As expected, levels of various pollutants dropped. These included the estrogen-mimicking chemicals.

blood collection
Graduate student Patricija Marjan collects blood from a rainbow darter to examine its health.
Courtesy of Mark Servos

"We don't know exactly how estrogens are reduced," Servos says of the water treatment plant. "The key seems to be giving bacteria more time to break down harmful compounds and to feed them oxygen to speed the process."

And as levels of these chemicals in the water fell, so did the number of feminized males. Within three years, nearly all male fish appeared normal again, inside and out. The researchers suspect the males’ bodies had re-absorbed the useless eggs. The male darters also regained their rainbow colors.

The study shows that although fish may be exposed to endocrine disruptors early in life, those changes may not harm them forever. "That was part of the surprise — [that] adult fish could recover," says Servos. He doesn’t know whether other species of fish would respond the same way. He suspects many would. 

Chris Metcalfe is an environmental toxicologist at Canada's Trent University in Peterborough, Ontario. He studies materials that can act as poisons in the environment. Metcalfe cautions that not all endocrine disruptors behave the same way. Just because one type can be removed from wastewater doesn’t mean all others will, too.

He also points out that not all urban areas have good water treatment. Some have none; they just spew polluted wastes directly into rivers. And outside of cities, many people rely on underground septic tanks to store water from toilets and showers. Septic tanks filter and capture many pollutants. If people don’t take good care of these systems,  wastes can leak from these tanks into groundwater. From there, pollutants can enter downstream rivers, lakes or the ocean. 

What the new work shows is that hormone mimics can hurt fish populations, but that good water-cleansing techniques can limit the risk that this happens.

Power Words

(more about Power Words)

aquatic     An adjective that refers to water.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

downstream     Further on in the direction in which a stream is flowing or the path at which stream water will flow in its trek to towards the oceans.

endocrine       Relating to hormones (chemicals secreted by the body) and the tissues in which they turn on (or off) cellular action. Medical doctors who study the role of hormones in health and disease are known as endocrinologists. So are the biologists who study hormone systems in non-human animals.

endocrine disruptor     A substance that mimics the action (sometimes well, sometimes poorly) of one of the body’s natural hormones. By doing this, the fake hormone can inappropriately turn on, speed up or shut down important cellular processes.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.

environmental science     The study of ecosystems to help identify environmental problems and possible solutions. Environmental science can bring together many fields including physics, chemistry, biology and oceanography to understand how ecosystems function and how humans can coexist with them in harmony. People who work in this field are known as environmental scientists.

estrogen     The primary female sex hormone in most higher vertebrates, including mammals and birds. Early in development, it helps an organism develop the features typical of a female. Later, it helps a female’s body prepare to mate and reproduce.

feminize        (in biology) For a male animal to take on physical, behavioral or physiological traits typical of females. It usually results from exposure to an abnormal amount of female sex hormones — or pollutants that mimic these hormones. Feminizing is sometimes used as a synonym for demasculinizing. In fact, they can be different. A demasculinized male may appear more feminine. But that will be largely because it had too little exposure to male hormones, not an excess of female hormones.

fertilize     (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism. (in agriculture and horticulture) To provide basic chemical nutrients for growth.

filter     (in chemistry and environmental science) A device which allows some materials to pass through but not others, based on their size or some other feature. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

groundwater     Water that is held underground in the soil or in pores and crevices in rock.

hormone     (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body.

mature     (noun) An adult individual. (verb) The process of growth and development that occurs as an individual moves toward adulthood.

nonylphenol     The name for a family of chemicals that can survive in the aquatic environment for a long time. These persistent chemicals are used primarily to make NPE surfactants and to strengthen certain plastics. Studies have shown these chemicals can mimic the action of estrogen, a female sex hormone. Animals can accumulate these from the environment. Nonylphenols can be extremely toxic to aquatic organisms.

online     A term that refers to things that can be found or done on the internet.

organ     (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that interprets nerve signals and a plant’s roots are organs that take in nutrients and moisture.

oxygen     A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

pollutant     A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

population     (in biology) A group of individuals from the same species that lives in the same area.

receptor     (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.

reproductive organs     The organs in a creature’s body that allows it to make and deliver eggs or sperm, and where appropriate, to nurture developing eggs and fetuses.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.

septic tank     An underground tank in which sewage is stored and treated. The sewage decomposes in the tank and drains out into a drain field.

sex     An animal’s biological status, typically male or female. There are a number of indicators of biological sex, including sex chromosomes, gonads, internal reproductive organs, and external genitals.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

sperm     The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.

surfactant     A chemical that decreases the attraction between water molecules. Manufacturers use such compounds to make it easier for water to spread on surfaces and to mix with other substances (such as oil).

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

testes     (singular: testis) Organs in the males of many animal species that makes sperm, the reproductive cells that fertilize eggs. This organ also is the primary site that makes testosterone, the primary male sex hormone.

urban     Of or related to cities, especially densely populated ones or regions where lots of traffic and industrial activity occurs. The development or buildup of urban areas is a phenomenon known as urbanization.

waste     Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.

wastewater     Any water that has been used for some purpose (such as cleaning) and no longer is clean or safe enough for use without some type of treatment. Examples include the water that goes down the kitchen sink or bathtub or water that has been used in manufacturing some product, such as a dyed fabric.


Journal:​ ​​K. A. Hicks​ ​et​ ​al.​ ​ Reduction of intersex in a wild fish population in response to major municipal wastewater treatment plant upgrades. Environmental Science and Technology. Published online December 27, 2016. doi: 10.1021/acs.est.6b05370.