Prickleback locomotion

Not all fishes are confined to the briny waters of the sea. A surprising number of fishes make terrestrial excursions, walking around on land like they own the place. There are gobies that climb waterfalls, fish that hop around mudflats, catfish that walk from pond to pond, and a whole bunch of fishes that can flop around on land in an effort to return to water. The reasons that fish move around on land are numerous and varied. They do it to look for food, escape conditions where the temperature, salinity, oxygen, or moisture content is hazardous to their health, move between locations, or return to water if they get stranded high and dry.

There is a lot of interest in how fish move on land. For some fishes, the ability comes from modifications of the skeleton. Strengthened pectoral- or pelvic-fin girdles. Fin spines or fin rays modified in some unique way. The exact modifications are of keen interest to researchers who study biomechanics, evolutionary biology, and ecology. Other fishes have no apparent modifications and yet still perform well on land.

Welcome to my second published research project: aquatic vs terrestrial locomotion in the rock prickleback - Clardy 2012!

The rock prickleback, Xiphister mucosus, is an intertidal fish that lives high up in the intertidal zone. Unfortunately for the poor rock prickleback, they occasionally get stranded at high tide in pools that dry out. They sometimes need to search for pools of water further down the shore slope. They do not have any obvious, weird adaptations that might help them move on land. Instead, they have elongate bodies and slither, snake-like, across the ground.

This project, part of the Functional Morphology and Ecology of Marine Fishes course at the Friday Harbor Labs in Washington, compared how rock prickleback swam in water and crawled on land. The goal of the project was to compare their locomotion to see if they do anything different between aquatic and terrestrial environments. To test this, I used high-speed video to film rock prickleback swimming in a tank of water and crawling over a bed of gravel. I measured a couple of locomotion parameters to see how efficiently they move.

It turns out that rock prickleback do not use any crazy tricks when they crawl on land. Fundamentally, they use the same mechanics they use in water. The difference is that their movements on land are exaggerated and their overall movement is slower. This works because their elongate bodies are efficient at generating propulsion in water, and they also are efficient at generating force on land. Basically, they coopt their already efficient swimming locomotion onto land.

​Frankly, when I finished the project, the results did not feel all that groundbreaking or earth shattering. Rock prickleback don’t have fancy, modified fins, weird pectoral girdles, or other strange features that stand out. They just kinda make do with what they have. It turns out, though, that this is an important observation, and documenting this behavior is useful for researchers studying fish locomotion. I have been quite surprised by how many citations this little study has gotten in other peer-reviewed publications. The interest in this paper from the broader scientific community has far exceeded my expectations. You never know when the broader scientific community may value your small, simple study that produces clear, easy-to-understand results.

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