Researchers are Telling 'Best Fish Story on Campus'

When certain species of prickleback fish get hungry, they may either dine on the “catch of the day” – smaller fish – as carnivores or marine plants, as herbivores.

Unlike humans, who are meat and plant eaters, some prickleback are carnivores that later switch to a plant diet. “It’s a complex process because it’s not just a matter of dining on smaller fish one day and gulping algae the next,” said Michael H. Horn, professor of biological science. “It’s a matter of the fish’s digestive system and if their digestive enzymes are adapted to a permanent change in diet.”

What certain species of prickleback eat has been the subject of research by Horn, a former campus Outstanding Professor, and Anna Gawlicka, fish nutrition expert and an adjunct faculty member.

The researchers, with the support of $460,000 in National Science Foundation grants, have investigated the eating habits of four prickleback species found in the intertidal waters on California’s central coast – off the shores of San Simeon.

With the notable exception of frogs, all vertebrate creatures – those with a backbone – begin life as carnivores. As they grow, some vertebrates, such as cows and horses, become herbivores and eat hay or grass, while others, such as humans, become omnivores. Fish also fall into the carnivore/herbivore pattern.

Horn and Gawlicka’s research has delved into why two species of pricklebacks switch from being carnivores to herbivores, even though all four species live in the same intertidal environment and ecosystem.

For their studies, the researchers capture the fish when very young – when they are about the length of a photographic slide. The prickleback are placed in a cooler and kept moist for the journey to Fullerton, where they are put in aquariums, according to Horn.

“They are not kept in water for the trip because as air breathers, they have access to a larger supply of oxygen in the air than in seawater,” Horn said, noting that the pricklebacks, like other intertidal fish, have the capacity to breathe air for several hours during periods of low tide.

“Some biologists once believed that plant-eating fish were merely eating the plants to ingest any small creatures trapped or growing in the plant material,” added Horn. “They thought the plants simply passed through the fish’s digestive system without providing any nutrients."

Horn, who earned his doctorate at Harvard and studied fish digestive systems for 25 years, disproved that notion. Fish that are herbivores eat marine plants because they have evolved the capacity to digest a plant diet in areas where seaweeds grow in abundance.

“We hope to provide a greater understanding of how the process works, how it has evolved, how it changes with the fish’s age and the genetic programming involved,” said Horn. “It’s the best fish story on campus.”

The researchers, assisted by graduate and undergraduate students, believe the study will shed more light on energy transfer. The research methodology and procedures could serve as a model for scientists working on similar projects in other marine or freshwater ecosystems, Horn noted.

Gawlicka, a native of Poland who earned her doctorate in nutrition with an emphasis in fish digestive physiology at Université Laval in Canada, will present the prickleback research findings at an international conference in Manaus, Brazil, at the end of this month. She will be accompanied by Horn – who will deliver research findings on Costa Rican freshwater fish that eat leaves and berries dropped from trees – and graduate student Kelly Boyle, who will discuss the differences between fish that live in North and South American intertidal waters.