Horseshoe crabs are marine animals more closely related to spiders than crabs. These animals have existed for more than 450 million years, and are sometimes called “living fossils” because they appear to have changed very little throughout their evolutionary history. However, horseshoe crabs have a poor fossil record, so this apparent lack of change could also be due to a lack of evidence, particularly when it comes to their reproduction. Scientists want to know: Are horseshoe crabs truly living fossils?

Modern horseshoe crabs reproduce by releasing hundreds of eggs and sperm into tidal waters, via a process called external fertilization. Until now, scientists have assumed that horseshoe crabs have always reproduced this way. But paleontologists have only found 3 fossil horseshoe crabs from the past 252 million years with their reproductive systems preserved, so this assumption is mainly based on an absence of evidence to the contrary. 

Scientists in the past interpreted the horseshoe crab’s long history of external fertilization as indicative of its evolutionary consistency. However, James Lamsdell and colleagues recently discovered a new fossil species of horseshoe crab in the 450 million-year-old Big Hill Formation in Michigan that challenges this assumption. 

To study the specimen, the scientists photographed it under 2 types of microscopes. The first type of microscope used polarized light to see structures in the fossil that may have blended into the rock, like putting on special glasses to see subtle textures. The second type of microscope used fluorescent light to reveal soft tissue in the fossil. Only some tissues, like ovarian tissue and preserved eggs, glow under fluorescence, so this method helped the team rule out non-biological structures, like bubbles, cracks, or minerals. 

The researchers found that their analytical techniques revealed remarkable details about the ancient horseshoe crab. One unique feature of the fossil was the shape of its exoskeleton. The fossil’s exoskeleton had a more shovel-like appearance than that of modern horseshoe crabs, which are less elongated and more round. It was also about 11 times smaller than a modern adult horseshoe crab. 

The researchers also discovered that the fossil preserved an intricate network of ovaries containing unfertilized eggs. This ovarian network lined the interior surface of the shell in a band of interconnected tubes, preserving both unfertilized eggs and developing eggs. They noted that the fossil eggs were all about 0.1 to 0.15 millimeters in diameter, about the same width as a human hair. This size consistency suggested that all the eggs developed at the same time, via a process called synchronous development. 

Since the fossil crab’s ovarian tubes contained developing eggs, it was an adult, meaning that the earliest adult horseshoe crabs were significantly smaller and shaped differently than modern adult horseshoe crabs. Despite their differences in size and shape, the fossil horseshoe crab’s reproductive system closely mirrored its modern one, in terms of its ovarian system and synchronous development of eggs. As in previous studies, it showed no evidence of internal fertilization.

The researchers concluded that the earliest horseshoe crabs also reproduced through external fertilization. This fossil horseshoe crab is more than 215 million years older than preexisting fossils preserving evidence of external fertilization, and suggests that the horseshoe crab’s mode of reproduction has been conserved for almost half a billion years!

However, the team’s findings reveal a more nuanced evolutionary history for the horseshoe crab than previously assumed. Although they’ve maintained the same reproductive system for almost half a billion years, the size and shape of their exoskeleton have changed. This discovery reveals that different parts of the horseshoe crab evolved at different rates. It seems that horseshoe crabs are not entirely the living fossils scientists once thought… while their bodies have evolved, their reproduction has endured.

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Humans own the stove, but nature has many ways to prepare a meal.

An oceanic mystery has unfolded with the discovery of a large crab inside a plastic bottle, with an opening far too small for the crustacean to have fit through.

The seemingly impossible discovery, made by researchers with Hiroshima University, has now revealed a new dimension to the effects of pollution in our environment, and how plastic objects can have unexpected impacts on ocean creatures.

The researchers made the curious discovery off the coast of Okinawa, Japan. Upon discovering a polyethylene Shaoxing wine bottle drifting along on the ocean surface, they soon realized that the discarded plastic container was thriving with signs of life.

Most peculiar of all was the presence of a large swimming crab within the bottle, which was obviously too have made its way inside the container through its narrow opening.

“During juvenile fish surveys in offshore waters, we happened to encounter a floating plastic bottle approximately 500 meters (about 550 yards) off Sesoko Island, Okinawa, Japan, with many juvenile fish associated with it,” said Hajime Sato and Yoichi Sakai, the authors of a new study describing the discovery.

According to Sato, “a large live swimming crab, Portunus sanguinolentus, was trapped inside the bottle.”

“The crab was clearly larger than the opening of the bottle!” Sato said.

The Marine Plastic Problem

It is no secret that plastic has been a longtime contributor to pollution in Earth’s oceans, with wide-ranging impacts on oceanic environments that include ocean creatures becoming ensnared in discarded plastic bags, dishing nets, or other plastic refuse.

Sato and Sakai say the plastic bottle they collected in July 2022 had been manufactured the previous November, and had been left uncapped, allowing seawater to enter and leave. However, it was clear that the crab they discovered within it could not have made its way in as easily as the ocean water it contained.

Unraveling this oceanic mystery required an array of biological analyses, which ultimately led them to a striking conclusion: the crab had entered the bottle when it was far smaller, and remained there, subsisting on organisms and other food sources making their way into the small plastic vessel, allowing it to grow as the bottle drifted along.

Based on DNA analysis of the crab’s stomach contents, the crustacean had recently consumed juvenile fish that entered the bottle, including specimens of the rough triggerfish Canthidermis maculate. Additional analysis also revealed the presence of the sergeant major Abudefduf vaigiensis.

Evidence of algae within the bottle indicated another likely food source for the crab, while on the bottle’s exterior, goose barnacle growth revealed the likely time the bottle must have spent drifting for these oceanic hitchhikers to become attached and grow.

The most likely scenario, according to Sato and Sakai, is that the young crab entered the bottle, where it fed on juvenile fish and algae over the course of two months, eventually growing large enough that it could no longer escape.

“Plastic bottles discarded by humans can trap crabs and prevent their escape,” the authors conclude, adding that “Similar cases have already been reported from waters around Japan, suggesting that this was not an isolated accident.”

“Through this striking example, we would like readers to recognize that objects that make our lives more convenient can sometimes have unexpected effects on small marine animals,” Sato and Sakai add, “while also appreciating the remarkable vitality of the swimming crab,” the authors conclude.

The unusual discovery was detailed in a recent study, “Swimming crab in a bottle: A two‐month drift on the ocean surface while entrapped,” which appeared in the journal Ecosphere.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.