Elastic energy storage in seahorses leads to a unique suction flow dynamics compared to other actinopterygian

Suction feeding is a dominant prey-capture strategy across actinopterygians, consisting of a rapid expansion of the mouth cavity that drives a flow of water containing the prey into the mouth. Suction feeding is a power-hungry behavior, involving the actuation of cranial muscles as well as the anterior third of the fish's swimming muscles. Seahorses, which have reduced swimming muscles, evolved a unique mechanism for elastic energy storage that powers their suction flows. This mechanism allows seahorses to achieve head rotation speeds that are 50 times faster than fish lacking such a mechanism. However, it is unclear how the dynamics of suction flows in seahorses differ from the conserved pattern observed across other actinopterygians, nor how differenced in snout length across seahorses affect these flows. Using flow visualization experiments, we show that seahorses generate suction flows that are 8 times faster than similar-sized fish, and that the temporal patterns of cranial kinematics and suction flows in seahorses differs from the conserved pattern observed across other actinopterygians. However, the spatial patterns retain the conserved actinopterygian characteristics, where suction flows impact a radially symmetric region of ∼1 gape diameter outside the mouth. Within seahorses, increases in snout length were associated with slower suction flows and faster head rotation speeds, resulting in a trade-off between pivot feeding and suction feeding. Overall, this study shows how the unique cranial kinematics in seahorses are manifested in their suction feeding performance, and highlights the trade-offs associated with their unique morphology and mechanics.

Methods
Data set includes measurements of: 

Max Gape (mm)    

time to peak gape (TTPG; seconds)    

Gape opening speed (mm/s)    

Max Flow Speed (mm/s)    

Time to peak flow speed (TTPF; seconds)    

Max suction flux (m^3/s)    

Max Jaw Protrusion (mm)    

time to peak jaw protrusion (TTJP; seconds)    

Jaw Protrusion Speed (mm/s)    

Max Hyoid Depression (mm)    

time to peak hyoid depression (TTHD; seconds)    

Ram Speed (mm/s)    

Head rotation (degrees)    

time to peak head rotation (TTPHR; seconds)    

HR speed (deg/s)

Data was collected for the following 17 species of aquatic vertabrates:

Ambystoma tigrinum

Apteronotus albifrons

Astronotus ocellatus

Carassius auratus

Chromis pelloura

Chromis viridis

Danio rerio

Dascyllus marginatus

Hemigrammus pulcher

Hippocampus fuscus

Hippocampus hippocampus

Hippocampus jayakari (adults and fry)

Lepomis macrochirus

Nimbochromis venustrus

Pimelodus pictus

Poecilia sphenops

Polypterus endlicheri