An adaptive explanation for the horse-like shape of seahorses

Abstract

The body shape of seahorses resembles the head and neck of horses because of their curved trunk, their ventrally bent head and their long snout. seahorses evolved from ancestral, pipefishlike species, which have a straight body. Here, we use a biomechanical analysis and show that the seahorse's peculiar head, neck and trunk posture allows for the capture of small shrimps at larger distances from the eyes compared with pipefish. The results from the mathematical modelling were confirmed by kinematic data of prey-capturing syngnathids: compared with straight-bodied pipefish, all seahorse species studied consistently show an additional forward-reaching component in the path travelled by the mouth during their strikes at prey. This increased strike distance enlarges the volume of water they can probe for food, which is especially useful for tail-attached, sit-and-wait predators like seahorses. The biomechanics of prey capture thus provides a putative selective advantage that may explain the bending of the trunk into a horse-like shape.

Mechanics of snout expansion in suction feeding seahorses: musculoskeletal force transmission

“…This allows for some torsion along the long axis (sensu Aerts et al, 1987). A notable intra-mandibular flexion in the sagittal plane, however, was not observed in syngnathids Roos et al, 2009b;Van Wassenbergh et al, 2011).…”

“…Suction feeding in syngnathiform fishes is generally referred to as pivot feeding to indicate the crucial role of dorsal rotation of the head during feeding (de Lussanet and Muller, 2007;Van Wassenbergh et al, 2011). By rotating the long snout, the distance between the small mouth and the prey is bridged in only a few milliseconds.…”

Why the seahorse tail is square

“…These specialized joints provide the prototypes several internal degrees of rotational and translational freedom, allowing the structures to bend, twist, and compress with multiple modes of deformation. In building the prototypes, we excluded two features of the natural tail morphology: the tapering of the segments toward the distal end of the tail, and the SCIENCE sciencemag.org 3 skewing of the segments. This simplification focuses attention on the cross-sectional structure of the plates.…”

Morphology of seahorse head hydrodynamically aids in capture of evasive prey

“…3a,b), faster than the 2-4 ms response latency of the copepod 5 . However, although the use of pivot feeding to explain the adaptive significance of head morphology in sygnathid fish 19 is important, one cannot ignore the fact that without the ability to approach evasive prey undetected, short range pivot feeding becomes ineffective. The dwarf seahorse, H. zosterae, is found within seagrass beds 34 that dampen water motion and reduces turbulence 25,35 .…”

“…To accomplish this, syngnathid fishes (seahorses, pipefish and sea dragons) are equipped with large tendons that has been proposed to store and release elastic energy to accelerate the head rapidly towards the prey 16 . The S-shaped body of the seahorse also aids in this rapid upward thrust of the mouth 19 . However, this mechanism only works at close range 18,20 , and thus the seahorse must approach very close to the prey in order for a strike to occur.…”