More than one way to be an herbivore

Lay summary by Cassandra Donatelli

Stichaeidae is a family of fishes characterized by an elongate body and long, spiny dorsal fins, giving the family the nickname “Pricklebacks”. Most species in this family live in the northern Pacific near the shore and even in tidepools. Because so many Pricklebacks occupy the same area and eat similar foods, the family is perfect for studying resource partitioning in relation to digestion strategies.

Donovan German and his colleagues set out to determine how Pricklebacks can have such similar diets and still occupy the same environmental niche. German suspected that they may have evolved different strategies for processing the same type of food.

Most fishes use one of two different strategies to digest food. The first is a “ratemaximizing” strategy. A fish using this strategy passes food through the intestine quickly and relies mostly on it’s own enzymes to break down the food. The second strategy is a “yieldmaximizing” strategy. This strategy is characterized by a slower movement of food and more reliance on microbes to break down parts of the food the fish’s own enzymes cannot.

The researchers chose an extensive set of procedures to test this hypothesis. First, they examined the stomach contents of five species of Prickleback. Then, they compared gut length, gut mass, and gut content mass. Next, they examined patterns of digestive enzymes to determine what material was being digested in different parts of the gut. Lastly, they looked at short chain fatty acid (SCFA) content of the back end of the intestine. SCFA’s result from microbial fermentation and the researchers used the SCFA content to determine how much the
fish relied on microbes during digestion.

One of the most interesting results was the difference between the Monkeyface Prickleback and the Rock Prickleback. These fish are distantly related within the Prickleback family, but both species independently evolved herbivory. The Monkeyface and Rock Pricklebacks also occupy the same area and have similar diets. The results of this study suggest that they do indeed have two different strategies for processing food. The Monkeyface Prickleback is a yieldmaximizer, relying on microbial assistance to process its food, while the Rock Prickleback is a ratemaximizer, relying on its own enzymes alone.

This difference in strategies may explain how each species can live in the same area and eat the same foods without being driven out by the other. Knowing the different digestive strategies of these closely related fishes will help researchers to understand the role each has in their shared ecosystem.

For further information

Read the Zoology original research article which this summary is based on More than one way to be an herbivore: convergent evolution of herbivory using different digestive strategies in prickleback fishes (Stichaeidae) (September 2015).

Visit the profile of the research ambassador, Cassandra Donatelli, who wrote this summary.

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