Barnacles latched onto a special place in Charles Darwin’s heart. By the time On the Origin of Species was published in 1859, the father of natural selection had long since become a self-taught expert on these fascinating arthropods.

Why’d he start studying barnacles? As with many interests in Darwin’s career, it all began aboard the HMS Beagle. During the early 1830s, he served as her designated naturalist on a fateful, five-year voyage that took him around the world. One day, while exploring an archipelago near Chile, Darwin happened upon a conch shell riddled with holes. Back in his quarters, he placed the prize under a microscope and thus became acquainted with a curious little creature. Seated inside one of these holes was a tiny barnacle—invisible to the naked eye—whom Darwin nicknamed “Mr. Arthrobalanus.”

In those days, very little was known about Mr. Arthrobalanus’ spineless kin. As recently as 1832, barnacles had been widely misidentified as mollusks by those in academia. That they’re actually crustaceans was a fact scientists wouldn’t recognize until after the Beagle had already set sail.

Through focusing on barnacle studies, Darwin figured an up-and-coming naturalist could really make a name for himself. Besides, he felt that their classification was painfully unorganized. In his words, “literally not one species is properly defined… the subject is heart-breaking.” Obviously, someone needed to clear things up.

By 1846, Darwin had married, started raising a family, and taken up residence in the house he’d call “home” for four decades. He proceeded to spend the next eight years on what became a barnacle project of epic proportions. Day after day, Darwin would toil away in his study, dissecting and classifying his subjects. Soon, the room was overflowing with box upon box of barnacle specimens from all over the globe, delivered to his door by mail. As one might expect, this was hard, monotonous work. “I hate a barnacle as no man ever did before,” Darwin vented, “not even a Sailor in a slow-moving ship.”

Still, he managed to discover much about them. For example, Darwin refuted the notion that all barnacles were hermaphrodites—in some species, it turns out, males bury themselves within the shells of much larger females and essentially become sperm-spewing lumps. Also, by writing several much-needed compendiums on barnacle biology, Darwin established himself as a highly-respected naturalist. When the time came to finally publish his ground-breaking views on evolution, that credibility went a long way.

So, how did all this affect his private life? Well, a weaker marriage might’ve been sorely tested by the stockpiling of lord knows how many barnacles inside the house. But Darwin’s wife and children quickly got used to being around scores of dead crustaceans. Case in point, while visiting a friend’s home, Darwin’s son George was shocked to learn that the boy’s father didn’t have a study. Bewildered, he asked, “But where does he do his barnacles?”