Brittle stars, relatives of starfish, have tough exoskeletons and long, slender arms. Most of the 2,000 or so species use the standard, hands-off marine method of reproduction, spewing their eggs and sperm into the water and leaving it at that. But other brittle-star species have evolved a more surprising strategy: They carry their growing young inside themselves, a dozen or more babies packed into sacs inside the parent’s body. Depending on the species, this parent might be a female or a hermaphrodite.
It’s a mystery that still stumps scientists researching brittle stars: How do the animals squeeze all those offspring—and their many arms—into the same tight space that houses all of their vital organs? “The adult basically looks like it’s completely filled with juveniles,” says Jannes Landschoff, a graduate student at the University of Cape Town in South Africa who studied brittle stars for his master’s thesis. “That’s quite a big burden that the mother’s carrying around.”
Landschoff has dissected plenty of brittle stars for his research, but each time he sliced into one of these animals, he created flawed data by disturbing the miraculous packing job inside. So a few months ago, Landschoff decided to explore an alternative: He got an unprecedented glimpse inside intact brittle stars using microCT scanning, which provides detailed 3-D information without the use of a knife.
CT, or computed tomography, uses x-rays to scan a body in a series of virtual slices. Stacked together, these slices produce a 3-D image, similar to an MRI scan. MicroCT, a technique that’s been growing in popularity, takes those pictures at a very high resolution, allowing a detailed look at even a tiny animal. Other researchers have used the technology to look inside various creatures including starfish, but no one had ever used it to peek inside a brooding brittle star.
For their microCT experiment, Landschoff and Charles Griffiths, a zoology professor at the University of Cape Town, chose two brittle-star species native to South Africa. Amphipholis squamata is tiny and hermaphroditic, with a body just a few millimeters wide (not counting its long legs). Ophioderma wahlbergii is a giant by comparison—its body is more than an inch across—and can be either male or female. Although the species are distant relatives, they’ve both independently evolved to brood their young inside their bodies.
Landschoff put deceased brittle stars of each species in the scanner, first removing their legs and securing the bodies to a hard piece of foam. He notes although these brittle stars were dead, living animals can go through a CT scanner; the scan itself won’t harm them. But scans can take a few hours, and any animal that isn’t dead needs to be anesthetized to hold perfectly still. Any wiggling will ruin the images.
The high-resolution slices showed, in black and white, the armored limbs of young brittle stars nestled inside their mothers like a tightly packed sock drawer. These slices were stacked together to create 3-D views of the mothers’ bodies. Some of the images became an animation that virtually unpacks 13 juvenile O. wahlbergii brittle stars from the body of their mother.
“It looks amazing,” Landschoff says. The scans showed that inside the parents, all the babies were positioned upside-down. (For a brittle star, that means the mouth, which is on the underside of the body and doubles as the anus, points up.) The young seemed to have their mouths pressed against the upper walls of their sacs, as if sucking in nutrients from their mothers. This supported a long-standing hypothesis that brittle-star mothers feed their young, in addition to housing them.
The 3-D views of the brittle stars turned out to support Landschoff’s observations from his dissections of the animals. But even when they don’t provide brand-new information, scans like this can be a useful way to generate public interest for scientific research. “I think the really strong benefit of the method is to use the data, for example, in museums,” he says. The images could even be reproduced on a 3-D printer to make a physical model.
MicroCT scanning “is growing rapidly,” says Alexander Ziegler, an evolutionary biologist at the University of Bonn in Germany. Thanks to the high resolution scientists can get from today’s microCT scanners, they’re using the technology to explore the insides of more animals. It’s especially useful for studying the structures inside tiny animals.
Another benefit of using scans to learn about animal anatomy, rather than cutting the animals open, is that scientists can study valuable samples in museum collections without destroying them. Noninvasive scanning can also be a quick and cost-effective way to gather information, Ziegler says, and the digital data can be easily shared.
Landschoff decided to share his full dataset, making all the scans freely available to the public and other scientists. “I only used a very small part of the information that is stored in the images I have,” he says. He hopes other scientists will be able to unlock more of that information in their own research. For example, how much of a brooding brittle star’s body is usually taken up by its young? In the smaller species that Landschoff studied, the six babies filled a remarkable 25 percent of the mother’s body volume. By comparing his images to brittle stars in different parts of the world, other scientists might learn more about how the babies squeeze in among their siblings and the mother’s organs.
Sometimes, when you want to understand exactly what’s happening in an animal’s body, cutting it open just won’t do. But there’s another fascinating element of brittle-star reproduction that scientists have been able to observe without scans or scalpels: No matter how they manage it, brittle-star babies grow and are born without harming their mothers. At the end of the months-long brooding period, the fully formed young simply crawl out of their mothers through flexible slits and walk away.