Bothragonus swanii

Bothragonus swanii, the rockhead, deep-pitted poacher or deep-pitted sea-poacher, is a species of fish in the family Agonidae. It was described by Franz Steindachner in 1876, originally in the genus Hypsagonus. It is found in the eastern Pacific Ocean from Alaska to Carmel Bay, California down to depths of 18 metres. It can also inhabit the intertidal zone. Males can reach a maximum total length of 8.9 centimetres.

Cranial pit and hypothesized sound production

Bothragonus swanii has a prominent bowl-shaped cranial pit on the top of its skull, roughly the size of its brain.
Using high-resolution micro-CT scanning, LSU researcher Daniel Geldof created 3D models of preserved specimens, revealing dense, flattened, mobile anterior ribs connected to powerful muscles beneath the pit.
Geldof hypothesizes that these ribs strike the pit's underside to produce low-frequency vibrations via stridulation, potentially for communication in the species' noisy intertidal habitat, similar to substrate-transmitted sounds in some sculpins. Biomechanist Adam Summers called the findings "stunning."
The hypothesis is unconfirmed, with no direct observations or sound recordings from live fish. Functional morphologist Eric Parmentier has questioned whether bone-on-bone contact could produce the proposed low frequencies.
The species epithet "swanii" refers to James G. Swan of Port Townsend, Washington. The rockhead spawns nearshore, during the months of January–May in the California Current region. Its diet consists of benthic shrimp and crabs.