Does the common topknot Zeugopterus punctatus (Teleosteii: Pleuronectiformes: Scophthalmidae) use a novel Venturi-effect attachment mechanism? A testable hypothesis.

Common topknots (Zeugopterus punctatus) attach to vertical rock surfaces and overhangs. It has been speculated that attachment is by a suction cup, with the median (anal, dorsal) fins providing a peripheral seal. Here the authors propose that the attachment is actually based on a Venturi effect. The rear portions of the median fins continually move in a fan-like fashion (at c. 4 cycles per second). This movement produces a tailward fluid flow that ventilates the shallow underbody space between the fish and its rocky substratum. The anterior portions of the median fins seal the space laterally, but the space is open anterior (beneath the raised head) and posterior to the sea. The mid-underbody space likely has a lower cross-sectional area than does the front intake or rear exit, so flow should be faster (and pressure lower) within it than outside, thus providing pressure gradient suction. Topknots attach to rough and heavily biofouled surfaces, presumably because the high numbers of fin rays and their associated membranes plus fine muscle control allow effective sealing. The attachment ability is shared by all members of the flatfish tribe Phrynorhombini; it can be related to anatomical peculiarities and constitutes a probable synapomorphy for this clade.