WASHINGTON: When it comes to sex, the little sea squirt can teach males a thing or two about how to retain the upper hand in competing for females.

The marine organisms regulate their reproductivity depending on the level of competition, Angela Crean of Queensland University School of Integrative Biology said.

"When there are lots of competing males trying to fertilise the eggs of females, males produce larger, more competitive sperm that live for longer," said Crean, the lead researcher in a new study.

The study was published in the Proceedings of the National Academy of Sciences Monday.

"Similarly, when females detect that there are too many males competing for her eggs (too many sperms can kill the eggs of some organisms) the females 'play hard to get' producing smaller eggs that are harder for searching sperm to find," Crean said.

"These changes make sense if you're stuck to a rock like sea squirts are. If you can't run away from a highly competitive environment, you must therefore make your gametes more competitive in order to get a higher chance of successfully reproducing."

The fact that sea squirts don't move made it possible for the majority of the investigation to be conducted in the Moreton Bay field environment.

The primary experiment involved placing a large number of sea squirts in a confined space for an extended period.

"We manipulated adult densities in the field by placing either one (low density) or 15 (high density) sea squirts in a cage for one month," Crean said.

"This research tells us a little as to how sex evolved in the first place.

"Specifically, it gives us some insight into why sperm are so tiny and males make millions of them whereas female eggs are much larger and are produced in fewer numbers.

"Because these organisms reproduce by the ancestral mode of reproduction, it gives us an indication as to how competition between males led to all sorts of reproductive strategies including the one used by humans: internal fertilisation."