Crustaceans called ostracods face an unusual challenge: Their sperm can be up to 10 times as long as their bodies.
Admittedly, an ostracod’s body fits on the head of a pin, and the longest sperm filaments stretch only about a centimeter. Still, the mismatch intrigues biologists of a species that would have to produce sperm more than 15 meters long to reproduce in ostracod style. Now, two scientists have tackled the question of how ostracods’ internal plumbing handles such extreme sperm.
Part of the ostracod sperm duct has evolved into a segment called a Zenker organ, toughened with crabshell-like chitin in ornate shapes. Detailed microscopy now suggests how Zenker organs work as pumps for giant sperm, says Shinnosuke Yamada of Shizuoka University in Japan.
In the freshwater ostracod Pseudocandona marchica, with sperm about half its body length, the Zenker organ looks like a tree trunk wearing wide ruffs of fringe. A narrow tube running down the center has an opening so narrow that only one sperm can pass through at a time, Yamada and Renate Matzke-Karasz of Ludwig Maximilians University in Munich report in the July Naturwissenschaften. The organ contracts and the tip of a sperm edges through a valve opening into the central tube, then muscles squeeze again and shorten the organ. When the muscles relax, the Zenker organ springs back to length, sliding the open valve a little way along the sperm filament. The whole process works a bit like a mechanical pencil ejecting more lead with each click.
But a Zenker organ is short and an ostracod’s sperm is long, so releasing even one sperm takes multiple contractions. During mating, P. marchica eventually delivers about a dozen sperm.
Giant sperm appear in various other species, including some flatworms, beetles and a fruit fly species, Drosophila bifurca, with sperm nearly 6 centimeters long. It’s not entirely clear how these species evolved outsized sperm, but researchers suspect it can involve both male competition and female favoritism for longer sperm, taken to extremes.