Semester of Graduation

Fall 2025

Degree

Master of Science (MS)

Department

Biological Sciences

Document Type

Thesis

Abstract

Fishes are the most morphologically diverse vertebrates. Habitat structure, ecological niche, and evolutionary history are prime determinants of their anatomy. However, even bizarre features are rarely restricted to a single fish species. This is a consequence of convergent evolution, interrelatedness, and the sheer volume of extant fish diversity. Truly exceptional anatomy offers a chance to explore fringe cases in vertebrate morphology. In the shallow intertidal of the northeastern Pacific, a small, armored benthic fish bears such an oddity atop its head. The Rockhead Poacher, Bothragonus swanii, has a skull dominated by a deep, bowl-shaped cavity, henceforth referred to as the cranial pit. Relatively speaking, the pit is enormous—occupying a volume comparable to B. swanii’s brain. Its interior contains complex microstructures with unique compositions and orientations.

The Rockhead’s pit has inspired multiple functional hypotheses. Early naturalists observed its utility in camouflage, but this seemed inadequate given the pit’s complexity. The Rockhead, like many fishes, produces sound. Subsequent observers suggested the pit could aid in vocalization. It has also inspired sensory hypotheses—particularly that the dish-like structure concentrates and amplifies sound. The mystery remains unresolved. With modern micro-computed tomography and supplemental histology, we examined B. swanii, and its congener Bothragonus occidentalis, in detail. We found strong evidence that the pit is multifunctional. The first ribs are free-moving, enlarged, and flattened against the pit—ideal for stridulatory vocalization. The pit’s shape and differential mineralization are well-suited for substrate-based communication in the noisy intertidal. The sensory hypothesis is likewise promising. The pit is innervated by a branch of the posterior lateral line nerve and bears surface microstructures suggesting directionally sensitive mechanoreception. We discuss these functions in the context of B. swanii’s ecology and evolutionary background.

Date

11-3-2025

Committee Chair

Prosanta Chakrabarty

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Available for download on Tuesday, November 03, 2026

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