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Goby dick

black goby fish

Is it worth my trouble? The male goby fish releases less sperm when there two or more other males are around. (Pic: Marco Missironi)

Italian researchers have found that when it comes to producing sperm, goby fish know when to hold back.

In an experiment based at the University of Padova, male goby fish proved they can size up the competition from other males and then adjust how much sperm they release.

"People have generally thought that sperm is really cheap to produce, that males have an oversupply of it, and that it's not really something they have to worry about economising on," commented Dr Philip Munday, from the School of Marine Biology and Aquaculture at James Cook University in Townsville, Queensland.

"But that paradigm has really started to change in the last decade or so. We're now starting to realise that especially in fish, males can economise in various ways on their production of sperm. This study is further proof of this."

Traditionally, 'sperm competition theory' predicts that when there is a lot of competition, such as group spawning of fishes, fish release more sperm.

"This means that a species where males compete on average with many other males have larger testes and larger ejaculates than species where the number of males competing for the fertilisation of the same batch of eggs is smaller," said Dr Andrea Pilastro from the Italian research team.

Dr Pilastro and her colleagues wanted to test a counter-intuitive theory. This holds that as the number of competitors rises above two, males actually release less sperm, because there are diminishing returns for providing more sperm.

Male goby fish comprise two types: large territorial males, which guard nests where females lay their eggs, and smaller 'sneaker' males, which attempt to release sperm in or near the nest.

Up to six sneakers can be involved in a spawning. This can last up to 10 hours, giving sneakers a reason and a chance to figure out what they're up against.

The researchers found that sneakers release more sperm when there was a single competitor, but less when there were two or more.

In the experiments, transparent plastic dividers isolated single sneaker males and their sperm without blocking their view of competitors, the nest, the territorial male, or the spawning female.

Dr Munday, who has discovered that some species of goby fish are able to change sex back-and-forth throughout their lives, said that testing sperm competition theory is difficult because there are so many variables affecting sperm production, not just the number of other males competing.

In some cases, males allocate sperm in relation to the size of the female, size being a good indicator of the number of eggs she will produce. In others, males may produce more sperm as they get closer to the nest.

"The neat part of this study is they've managed to actually hold all these other things constant so that the only thing they were varying was competing males," Dr Munday said.