Date of Award
Doctor of Philosophy (PhD)
Ocean Engineering and Marine Sciences
Robert van Woesik
Despite the prevalence of fishes that express cranial spines, quantitative investigations into the function of these spines are rare. Function is often assumed with little or no evidence. Commonly, spine-like structures are considered to be used for defense despite spines potentially being used for other purposes. Here, I took a high-level approach to investigate the morphology and potential functions of cranial spines in over 2000 fishes. Identifying the function of cranial spines in fishes not only informs us as to how these fishes interact with conspecifics, with other species, and with their environment, but also may provide advances in human technology mimicking biological design. To assess the potential functions of cranial spines, I began by analyzing the morphology of the spines to identify trends or patterns consistent among the thousands of fishes that express cranial spines. Specifically, I investigated (i) spine expression, (ii) the spine-group expression (i.e., which spines are expressed), (iii) the number of spines expressed, and (iv) the morphology of the spines, focusing on the length (relative to the body length in the orientation of the spine), the angle, and the aspect ratio of the spines. I then identified patterns of cranial spine expression within and between taxa at the order, family, and genus level to determine the similarities of spine expression of closely related species. Then, I took a multi-faceted approach to assess the common functions of spines. I investigated the association of cranial spine expression with environmental factors to determine how spines interact with the environment and to assess the viability of functions associated with environmental gradients. Then, I compared the morphology of cranial spine with that of known defensive spines to determine whether cranial spines have effective defensive morphologies. Finally, I attempted to directly test how the spines interact with water by visualizing fouling patterns of the bodies of 3D-printed models with various cranial spine expressions. I found that an estimated 6−8% of ray-fined fishes around the word express cranial spines. Cranial spine expression (i.e., spine group, number, morphology, etc.) is highly variable, even within closely related species. However, cranial spines were consistently small (i.e., an average of 2.5% of the size of the body in the direction that the spine is directed), and significantly smaller than known defensive spines. Even after accounting for differences in habitat and body size, most cranial spines did not conform to a morphology that would be effective at fending off a predatory attack. In addition, several environmental factors were associated with cranial spine expression with the best performing model indicating sea-surface temperature and salinity were the most important variables in predicting cranial spine expression whereas chlorophyll-a and current velocity were least important. The attempt to evaluate the effect of spines on water flow around the body of the fishes was unsuccessful. Fouling was used as a proxy to visualize the waterflow; however, despite supposedly optimal conditions for fouling, minimal fouling occurred preventing interpretation. Through this large-scale investigation into the function of cranial spines in adult fishes, I identified trends in cranial spines and assessed potential functions. Several species have cranial spines likely used for resource acquisition whereas the function of most spines remains unknown. However, it appears most cranial spines do not have a morphology that would be effective for defense. This dissertation will assist in future investigations into cranial spines of specific species or into other potential functions of cranial spines. Further investigations into how cranial spines interact with their environment is necessary to fully understand the function of these spines and to determine whether the function of cranial spines may provide any technological advances for humans.
Penrod, Louis Michael, "The Form and Function of Cranial Spines in Fishes" (2022). Theses and Dissertations. 1343.
Available for download on Tuesday, May 07, 2024