Picture of the Week: Horn Shark Egg Case

You won’t find this egg in any typical Easter hunt. Instead, you’ll have to venture into rocky reefs or kelp forests off the California coast from Santa Barbara southward, where California horn sharks generally lay the corkscrewed packages, also called egg cases.

About 43 percent of sharks and rays—including skates, most cat sharks, and the nine species of horn shark—lay eggs rather than give birth to live young. California horn sharks lay about two every 10-14 days or so during the spring and summer (the stat is based primarily on aquarium research), and each contains one little embryo. 

The auger bit-like case is typically “a little pliable when it first comes out [of the mother], but it usually firms up relatively quickly,” says David Ebert, the program director for California’s Pacific Shark Research Center at Moss Landing Marine Laboratories.

The packaging consists of a material similar to fingernails or hair and forms around a fertilized egg in the mother in an organ called the oviducal gland (sometimes referred to as the shell gland). “It’s unclear how that shape actually develops,” says Christopher Lowe, a marine biology professor who runs the Shark Lab at California State University, Long Beach, but he guesses that the gland itself is twisted.

After she lays the egg, the female doesn’t keep vigil. “She takes off,” says Lowe. The baby is left to develop on its own, feeding off a yolk sac inside. “The mom basically packs a little lunchbox full of nutrients that the developing embryo will grow off of,” says Lowe.

As the theory goes, the egg case’s spiral ridges enable it a good grip among rocky nooks and crannies where the mother usually lays it. “It wedges into a rock really well, so that if there’s surge and waves, it won’t move the egg, and it makes it harder for predators to get it,” says Lowe.

As egg cases are exposed to the sea environment for a time, “they’ll start to get like little coralline algae and things growing on them, which actually help camouflage them a little bit,” adds Ebert. 

Yet, some predators do find a way in, such as sea snails called whelks. Using their proboscis—a kind of mouth-snout—“they can drill a hole into the egg case,” says Ebert. “They’re going for the yolk.” 

Once the embryo has wiped out the contents of its lunchbox—which takes about seven to 10 months—it hatches. But newbie horn sharks don’t break out on a completely empty stomach. “They have a bit of food reserve internally,” says Ebert, which sustains them for a little while until they prowl for prey.

Fortunately for pups, their maritime nursery tends to brim with shark baby food, such as small shrimp and crabs. “So, Mom’s not being totally careless when she just ditches ’em and runs,” adds Lowe. “She tries to put them in a place where she thinks they’ll be successful.”