Horseshoe crabs are marine animals that are more closely related to spiders than crabs. These animals have been around for more than 450 million years and are sometimes referred to as “living fossils” because they appear to have changed little throughout their evolutionary history. However, because the fossil record of horseshoe crabs is poor, this lack of apparent variation may also be due to a lack of evidence, especially regarding reproduction. What scientists want to know: Are horseshoe crabs really living fossils?
Modern horseshoe crabs reproduce by releasing hundreds of eggs and sperm into saltwater through a process called fertilization. in vitro fertilization. Until now, scientists have thought that horseshoe crabs have always reproduced this way. However, this assumption is largely based on a lack of evidence to the contrary, as paleontologists have only discovered three fossils of horseshoe crabs from the past 252 million years with preserved reproductive systems.
Scientists in the past interpreted the horseshoe crab’s long history of in vitro fertilization as an indication of its evolutionary consistency. But James Ramsdell and colleagues recently discovered a new species of horseshoe crab fossil in Michigan’s 450-million-year-old Big Hill Formation that challenges this assumption.
To study the specimens, scientists took pictures of them with two types of microscopes. The first types of microscopes used polarized light to observe the structures of fossils that may have been fused into rocks, similar to the way special glasses are fitted to see subtle textures. The second type of microscope used fluorescence to reveal soft tissues within fossils. Because only some tissues, such as ovarian tissue and preserved eggs, glow under fluorescence, this method helped the team rule out non-biological structures such as bubbles, cracks, and minerals.
Researchers have discovered that their analytical techniques have revealed surprising details about ancient horseshoe crabs. One of the unique features of this fossil was the shape of its exoskeleton. This fossil’s exoskeleton was more shovel-like in appearance than the exoskeletons of modern horseshoe crabs, which are round rather than elongated. It was also about 11 times smaller than modern adult horseshoe crabs.
The researchers also found that the fossil preserved a complex network of ovaries containing unfertilized eggs. This ovarian network lines the inner surface of the shell with a band of interconnected tubes and stores both unfertilized and developing eggs. They noted that the fossil eggs were all about 0.1 to 0.15 millimeters in diameter, about the width of a human hair. This consistency in size suggests that all eggs developed at the same time through a process called. synchronous development.
Because the fossil crab’s oviduct contained a developing egg, it was an adult, meaning that early adult horseshoe crabs were much smaller and differently shaped than modern adult horseshoe crabs. Despite the differences in size and shape, the reproductive system of fossil horseshoe crabs is very similar to modern reproductive systems in that the ovarian system and eggs co-occur. Similar to previous studies, no evidence of internal fertilization was shown.
The researchers concluded that early horseshoe crabs also reproduced through in vitro fertilization. This horseshoe crab fossil is more than 215 million years older than existing fossils that preserve evidence of external fertilization, suggesting that the horseshoe crab’s reproductive mode has been preserved for almost 500 million years.
However, the team’s findings reveal that horseshoe crabs’ evolutionary history is more nuanced than previously thought. Although they have maintained the same reproductive system for almost 500 million years, the size and shape of their exoskeletons have changed. The discovery revealed that different parts of the horseshoe crab evolved at different rates. It appears that the horseshoe crab is not the living fossil that scientists once thought…its body has evolved, but its reproduction has endured.
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Source: sciworthy.com
