Bothragonus swani Steindachner, 1877, previously called Bothragonus swanii, Port Townsend, Puget Sound, Washington, United States.
OTHER COMMON NAMES
English: Pithead poacher, pit-headed poacher, pitted poacher, deep-pitted poacher, deep-pitted sea-poacher.
Short, stout body of 3.5 in (9 cm), covered with armor plates and with small fins other than the pectorals. Like other poachers (Agonidae), all fin rays are unbranched, and tail bones are fused. The rockhead, or pithead poacher, is most obvious for its wide, flattened head with a deep indentation in the top. Eyes and mouth are small. As a larva, the pectoral fins become enlarged like butterfly wings, and the body is covered with fine spines. The pelagic juvenile develops heavier, recurved body spines that hook toward the tail. These sharp hooks flatten into armor plates after settlement to the bottom.
Northern California to Kodiak Island, Alaska.
Rocky shorelines exposed to waves, down to 66 ft (20 m) depth. Found under rocks, in crevices, or among kelp holdfasts.
Very little is known of this elusive fish. When first settled, the hooked body plates serve to prevent the fish from being washed backward by wave surge. It is not known whether the adult braces itself against overhead rock protrusions with the pit in its head. This poacher relies on close hiding quarters and cryptic appearance rather than flight, for predator evasion. If picked up, it vibrates.
FEEDING ECOLOGY AND DIET
Larval rockheads tend to feed on copepods or fish larvae. Adult rockheads feed on small crustaceans.
Orange eggs are deposited within a kelp holdfast. It is not known whether copulation and internal gametic association are required to permit the female to extrude eggs into such interstitial spaces.
Not listed by the IUCN. This poacher is too rarely encountered to permit evaluation, but the exposed, rocky shorelines it inhabits tend not to face human development.
SIGNIFICANCE TO HUMANS
The rockhead is not sought for any purpose and is only rarely displayed in aquariums, since it tends to hide from view. It is poorly studied.
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