Karel Frederik Liem, 73; was biologist, Harvard professor

“I just could not figure out what they were,’’ he said in an interview published in the book “Biology,’’ penned by Neil Campbell. Dr. Liem collected a bag filled with the mysterious creatures, and when his adviser also failed to recognize them, he went to work finding the answers.

He eventually learned they were air-breathing fish, migrating by land.

“I wondered, what made them get out of the water to find another pond? How do they survive on land? How do they find their way? Surely a fish in tall grass can’t see ahead. They have very tiny eyes,’’ he said.

The biodiversity of Indonesia, where he was raised, provided a breeding ground for his intellectual curiosity.

“I would look at the geckos crawling around on the ceiling upside down and try to figure out why they didn’t fall off,’’ he said in that interview. “Or flying lizards. I was always fascinated by the incredible adaptations that the animals had, even though I didn’t know anything about adaptations. I also was interested in the behavior of animals.’’

Fish, particularly their evolution and connection to humans, gave him scores of questions on which to base his research and teachings at Harvard University, where he offered witty analogies for more than three decades. One of his more popular courses, which many nonbiology students signed up for, was “Biology of Fishes.’’

Dr. Liem, who made a living of studying the bony elements of fish at Harvard’s Museum of Comparative Zoology, died Sept. 3 at Massachusetts General Hospital from complications of pancreatic cancer. He was 73.

Human bone structure looks simple compared with that of fishes, he often explained. While humans have at least 22 bones in their skulls, a fish has about 140. And much of the human anatomy structure is rooted in our evolutionary ties to fish, he said, such as the voice box, which comprises modified gill arches, and our limbs, which are evolved from fins.

Dr. Liem taught for many years at Harvard University Extension School, usually offering a course titled “Comparative Anatomy of the Vertebrates.’’

On one occasion, he was to teach an evening course, but had day surgery that morning. He left the hospital, hopped on the subway, and arrived in time to teach.

“And that was Karel,’’ said Cheryl Souza, his longtime faculty and curatorial assistant.

Students often appreciated the clever, funny analogies he used to help them remember material, Souza said. “He always made you laugh; there wasn’t a day I didn’t laugh.’’

Practical jokes also were a routine part of the experience in Dr. Liem’s lab.

“He was very social, extremely jovial,’’ said his son, Karel Jr., of New York City. “He was very fun. He was a storyteller, he was a joker, and he loved sharing his experiences.’’

Dr. Liem also studied sex reversal, where some fish species switch genders, and explained that there are different reasons for the change.

“For example,’’ he said in the book interview, “some small sea basses live in schools with only one male and about 24 females. Such a reproductive school remains stable. But if you take the male out, one of the females will become a male almost overnight. Really amazing!’’

And through a number of surgical experiments and cinematic analysis, “He demonstrated the performance and interactions of bone and muscle units more precisely than has previously been done for any fish group,’’ according to an article in a 1972 Museum of Comparative Zoology newsletter.

Dr. Liem was born in Jogjakarta and grew up during the Japanese occupation, attending Dutch schools. His mother was Dutch and his father Chinese.

He earned his bachelor’s and master’s degrees from the University of Indonesia in 1957 and 1958, respectively. When his family moved to Holland, he went to America and earned a doctorate in zoology at the University of Illinois in 1961.

He taught at Leiden University in the Netherlands as an assistant professor of zoology in the mid-1960s and then at the University of Illinois College of Medicine as an assistant professor of anatomy. He was named associate professor in 1967, and a year later, became associate curator in charge of vertebrate anatomy at the Field Museum of Natural History in Chicago.

He arrived at Harvard as the Henry Bryant Bigelow professor of ichthyology, a curator of ichthyology, and a professor of biology in 1972, and had that role at the time of his death. He also served for 12 years as a master of Dunster House.

Alfred Crompton, a Harvard professor emeritus, said a large part of why he hired Dr. Liem was his focus on experimental fieldwork.

“Karel was the one who said, ‘Hey, if this pulls this, and that pulls that, and this is the way it evolved . . .’ Well, he put a new perspective onto it,’’ Crompton said.

Crompton said the ubiquitous sound in the classrooms of Dr. Liem was the click-clicking of multicolored pens as students copied from his multicolored presentations.

“He was one of the rare people who was not only a great scientist but also a really good teacher. Not many people can do that,’’ Crompton said.

Colleagues said he introduced techniques that allowed scientists to look at the mechanics of feeding in a number of fish varieties, particularly in cichlids, and wrote much about his research in two textbooks that will be published later.

And for students like himself who came from afar to study in America, he had simple advice, as told to the magazine Asian Boston: “In our efforts to adapt to our environments, we cannot change who we are.’’

In addition to his son, Dr. Liem leaves his wife, Hetty; a daughter, Erika Liem Palidar of Germany; two brothers, Harry and Liem Kiem Guan, both of Holland; and two sisters, Ireen Nijhuis-Liem and Lan, both of Holland.

A memorial celebration is being planned for later this fall, and a symposium honoring his life will be held in July.