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The incredible (relatively) giant sperm of the shrimplike ostracod

Where size definitely matters

From shiny convertibles to cheesy pick-up lines, males have long used various tools to compete for females. But what if that competition happened after mating, rather than before? What if sperm from different males duked it out inside the female's body?

Scientists think that's what happens in a few insects and other creatures, and that it can lead to a startling development: sperm longer than the creatures themselves.

A study published last week in Science suggests that giant sperm have persisted in tiny shrimp-like organisms called ostracods since the time of the dinosaurs.

Millimeter-long ostracods, which look like lima beans with antennae, evolved before life moved onto dry land. Many species still exist.

Adam Bjork, a University of Arizona biologist who has done similar research with fruit flies, said the work showed that "giant sperm are not a fluke."

Bigger sperm may reach the female's egg more quickly or prevent smaller sperm from "scoring." But Renate Matzke-Karasz of Ludwig-Maximilians University in Germany, the paper's lead author, said scientists were still unsure what benefits big sperm confer.

A unique confluence of events allowed the ostracods' giant sperm to be preserved for 100 million years. Before females mate, the tubes they use to receive and store sperm are formless, like an empty plastic bag. Only after mating do these organs take on their characteristic sausage shape.

Amazingly, the ostracods were buried in fine sediment right after mating, but before the females deposited their eggs - capturing that special moment of "fossil impregnation," Matzke-Karasz said.

This is the first time scientists have been able to look inside a tiny and complex fossil with such high precision and contrast, added coauthor Paul Tafforeau of the European Synchrotron Radiation Facility.

With science advancing rapidly, it pays to be careful. Even the love lives of extinct microscopic creatures aren't private anymore.

- Karen Knee