Changing Planet

Round Goby Fish Have Two Kinds of Males, Scientists Discover

This post is part of a special National Geographic news series on global water issues.

In the world of round goby fish there are females and males … and males.

Scientists have found the existence of two types of males of the fiercely invasive fish spreading through the Great Lakes, which may provide answers as to how they rapidly reproduce, McMaster University announced.

“Researchers … discovered evidence that in addition to round goby males which guard the nest from predators and look after their offspring, there exists what scientists call ‘sneaker’ males–little males that look like females and sneak into the nests of the larger males,” the Canadian university said in a statement.


The parental male is the big fish in this picture and the sneaker male is the small fish.

Photo courtesy McMaster University

The study, published in the Journal of Great Lakes Research, looks at the aggressive round goby, a bottom-dwelling fish which infested the Great Lakes watersheds around 1990.”Presently, they are working their way inland through rivers and canal systems and can lead to the decline of native species through competition and predation,” McMaster said.

“The existence of these two kinds of males will help scientists understand how round gobies reproduce, how quickly their populations grow, and track how these populations change over the course of invasion,” said Julie Marentette, lead author and a Ph.D. student in the department of Psychology, Neuroscience & Behaviour at McMaster University. “This has the potential to have a significant impact on how researchers tackle what has become a very difficult problem in the Great Lakes.”

Sneakier Way to Mate

Because males expend lots of energy or eat less while guarding their nests, and attracting females while providing care can be difficult, males in some species have found a sneakier way to mate, Marentette explained. “Instead of courting females and protecting the young, some males will parasitize the courtship–and sometimes the parenting duties–of conventional males. They do this by sneaking into the nests of big males or pretending to be females.”

“Prior to our findings, only one type of male reproductive behaviour would have been incorporated into projections and modeling analyses of the population dynamics of round goby invasive capacities”, said Sigal Balshine, associate professor in the department of Psychology, Neuroscience & Behaviour and academic advisor on the study. “Our results will shed light on how populations of this invasive species are likely to grow and spread through time and space.”

Sneakers produced more sperm … and had bigger testes

The McMaster scientists compared the physical, hormonal and sperm traits of hundreds of males, and found that the nest-guarding, parental males were big, black and had wide heads. The small female-like sneaker males were tiny, mottled brown and had narrow heads.

Both types of males produced sperm, but sneakers produced more sperm than the parental males, and had bigger testes.

By contrast, parental males have bigger glands used to produce pheromones that attract females.

Funding for the research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Fund for Innovation, the Ministry of Research and Innovation and the Department of Fisheries and Oceans.

Forty years in U.S., UK, and South African media gives David Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. He has 120,000 followers on social media. David Braun edits the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directs the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. Follow David on Facebook  Twitter  LinkedIn
  • maryk01

    Hi, Mary here, I live in saginaw , michigan.I often fish the saginaw and tittabawasee river and its tributaries and you have answered my question for me ,while fishing I caught the smaller motteled brown goby with the black spot on the dorsal fin and also caught one that was also a velvet black with eyes that reflected a blue color it was kind of pretty I thought but the people here hate them and throw them up on the shore to die because they are a nuisance .

  • Emily

    I’m up here visting my family and we go fishing everyday I catch these stupid little things like crazy they still my bait. I can’t stand them there really nasty. Don’t ever use them for bait there fillled with toxins.

  • Kevin

    Dear Emily,
    please note that these fish are neither ‘stupid’ nor ‘nasty’, these are very much human characteristics. And as for them being full of toxins….well, it is interesting to see urban myths in action!
    I have been studying these invasive species in Europe for 10 years now, and we have yet to see the ‘terrible’ impacts and ‘nuisance’ predicted by American researchers and anglers.

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