Exceptionally rare in every means possible is the only way to characterize the recent findings of an international team of palaeontologists. Not only did they uncover a new crustacean species from the Cretaceous period, but the animal’s soft parts were also fossilized. This allowed the researchers to see the animal’s inner organs, including those needed for reproduction—surprising them with the image of giant, fossilized sperm within one of the females.
“When we identified the sperms inside the female, and knowing the 100-million-year-old age of the amber, it was one of the special Eureka moments in a researcher’s life,” said lead author He Wang of the Chinese Academy of Science in Nanjing.
Using cutting-edge X-ray microscopy, Wang’s research team examined the fossils (each less than a millimeter long) trapped in a small piece of amber by virtually cutting them into thousands of slices, and using a computer to reconstruct them three-dimensionally.
The findings in amber provide unprecedented insights into an unexpectedly ancient and advanced instance of evolutionary specialization. In some species, females mate with more than one male. Physical mating rivalries, such as fighting, occur on the outside, but inside sperm also end up competing for fertilization success. Most species, including humans, take the path of quantity: hundreds to thousands of small, fast-swimming sperms. On the other hand, a number of species invest in low numbers of high-quality, extra-large-sized sperms to increase the chances of winning the race to egg fertilization. These sperm, often longer than the animal producing them, is what researchers found inside a female Myanmarcypris hui, as the new species has been named.
“The giant sperm in our amber specimens provides the first direct evidence that this remarkable reproductive mode already existed in the Early Cretaceous,” the researchers write in their paper, published in Proceedings B.
Since the sperm was found inside a female specimen, the researchers say this suggests the animal must have had sex shortly before becoming trapped in amber. When animals produce giant sperms, a lot of energy is not only put into the production of each and every sperm cell, but also in allocating organs for manoeuvring the sperms. For example, octracods, the living ancestors of Myanmarcypris hui, use their sperm pump—called a Zenker organ—to force the sperms through a long sperm duct toward a copulatory appendage and finally into the female’s vagina. Once in the female, the sperms are pushed through another long, spiralled canal into a sac-like organ, or sperm receptacle, where they gradually gain mobility while being stored until eggs are ready to be fertilized.
For almost 15 years, the research team has been trying to determine when giant sperms first appeared on Earth in order to evaluate if the reproduction method is evolutionarily advantageous—or not. In 2014, study co-author Renate Matzke-Karasz led the discovery of 17-million-year-old giant sperms in ostracod fossils in Australia. Those were the oldest examples—until now.
“The fact that animals had already developed giant sperm 100-million-years ago implies that this reproductive strategy can indeed be successful in the long-term,” says Matzke-Karasz. "That's a pretty impressive record for a trait that requires a considerable investment from both the males and females of the species. From an evolutionary point of view, sexual reproduction with the aid of giant sperm must therefore be a thoroughly profitable strategy."
Photo: Tomographic reconstructions of a female of the Cretaceous ostracod crustacean Myanmarcypris hui. Upper left: the tips of both pairs of antennae can be seen extending from the front of the strongly ornamented bivalved shell. Upper right: inside the shell (now transparent) some of the appendages can be seen, as well as eggs (green) and the sperm receptacles (purple). Lower right: reconstruction of the paired sperm receptacles. Lower left: one of the sperm receptacles in more detail, filled with filamentous giant sperms. Credit: He Wang