Even within the first day after being expelled from the male's brood pouch, H. reidi
are capable of performing an extremely fast and powerful pivot-feeding strike (). Newborn H. reidi
reach velocities of head rotation that are more than three times higher than that observed for adults (Roos et al. 2009
). Furthermore, the small hyoid reaches an angular velocity that is even higher than the peak angular velocity of the swing of a mantis shrimp when breaking a snail (less than 57
; Patek et al. 2004
), and is probably the fastest rotation ever observed in a prey-capture system. While mantis shrimp and seahorses are both equipped with an elastic recoil system to power their prey-capture movements, the smaller size (and thus lower rotational inertia) of the seahorse hyoid probably allows it to reach this exceptionally high rotational velocity.
These remarkably high speeds suggest that an elastic recoil mechanism of head rotation must be functional in newborn H. reidi
. Estimates of muscle-mass-specific power requirement of head rotation can address this issue as the mechanical power output of muscle is limited (approx. 1100
; Curtin et al. 2005
). Our calculation showed peak instantaneous power requirements exceeding this value considerably, which suggests that a catapult-like mechanism, as described for adult pipefish, is already operational in newborn H. reidi
Previous workers have suggested that the high rotational accelerations, torques and fluid pressures involved during pivot feeding demand a relatively advanced degree of ossification of the cranial skeletal components (Osse & Muller 1980
). However, one-day-old H. reidi
have a predominantly cartilaginous cranial skeleton (), which may have important consequences for their ability to withstand tensile and compressive stresses during feeding. Interestingly, CFD showed relatively low hydrodynamic pressures on the dorsal surface of the snout (650
) compared with adult pipefish (more than 2000
Pa; Van Wassenbergh & Aerts 2008
). This indicates that, owing to their small size, the early developmental stages of syngnathids do not necessarily require the strong degree of ossification observed in adults because of a reduced hydrodynamic stress.
The observed doubling of the volume of the mouth cavity is significantly higher than what is observed for adults (less than 50% increase; G. Roos 2008, personal observation). Since this increase in volume is related to the amount of water being sucked into the mouth, newborn seahorses are capable of transporting a relatively large volume of water during feeding. Apparently, the cartilaginous head skeleton also manages to withstand the stresses occurring during such powerful suction. In this respect, the relatively short length of the snout in newborn seahorses compared with adults (Choo & Liew 2006
) may be important to prevent buckling when high sub-ambient pressures are generated inside the snout during suction.
Our data show that whereas other fishes require a period of gradually improving their feeding performance after hatching, seahorses give birth to young with a fully functional prey-capture system. The relatively late developmental stage of seahorses at ‘birth’ is reflected in a matured functionality of the feeding system compared with other fishes and highlights the crucial role of the brood pouch in male seahorses.
The feeding apparatus of fishes is potentially one of the most complex, integrated musculoskeletal systems in vertebrates, and consequently provides a unique opportunity to study the evolution and development of biomechanical design. In this respect, feeding in newborn H. reidi illustrates that increasing anatomical complexity during the pouch phase, resulting in an immediate high-performance system at birth, may help to overcome critical periods during early ontogeny.