For this study, Higginson compared features of female reproductive tracts and sperm from 42 species of diving beetles. Close to 4,000 species of diving beetles occur worldwide.
"Diving beetles can be found in just about every puddle of water," she said. "Here in the Arizona desert, most cattle tanks or natural pools will have numerous species of them. They're everywhere, except salt water, which they can't tolerate."
All are voracious predators, such as the Great Diving Beetle, which is common in Europe. Both larva and adult stalk small fish or tadpoles underwater, until they pounce and subdue them with pointy mandibles shaped like curved daggers.
In an attempt to better understand sperm evolution, Higginson and Pitnick came across descriptions of paired sperm in diving beetles dating back more than a hundred years. Female reproductive tracts in these animals were not systematically investigated until ten years ago.
Previous studies on the interaction between sperm and female reproductive organs established simple relationships between the two such as duct length and sperm length, but did not look much farther than that, Higginson said.
She remembered squashing a spermatheca, which is the part of the female tract that stores the sperm, on a microscope slide, and out came a single, enormous mass of sperm, all connected to each other and swirling and wiggling around. Previous studies had reported diving beetle species whose sperm cells join up in pairs, but seeing lots of individual sperm cells aggregating into one single conjugate was something unexpected and new.
"I could see the tails beating and moving the whole conjug
|Contact: Daniel Stolte|
University of Arizona