A physical model of the extreme mantis shrimp strike: kinematics and cavitation of Ninjabot

Abstract

To study the mechanical principles and fluid dynamics of ultrafast power-amplified systems, we built Ninjabot, a physical model of the extremely fast mantis shrimp (Stomatopoda). Ninjabot rotates a to-scale appendage within the environmental conditions and close to the kinematic range of mantis shrimp's rotating strike. Ninjabot is an adjustable mechanism that can repeatedly vary independent properties relevant to fast aquatic motions to help isolate their individual effects. Despite exceeding the kinematics of previously published biomimetic jumpers and reaching speeds in excess of 25 m s(-1) at accelerations of 3.2 × 10(4) m s(-2), Ninjabot can still be outstripped by the fastest mantis shrimp, Gonodactylus smithii, measured for the first time in this study. G. smithii reached 30 m s(-1) at accelerations of 1.5 × 10(5) m s(-2). While mantis shrimp produce cavitation upon impact with their prey, they do not cavitate during the forward portion of their strike despite their extreme speeds. In order to determine how closely to match Ninjabot and mantis shrimp kinematics to capture this cavitation behavior, we used Ninjabot to produce strikes of varying kinematics and to measure cavitation presence or absence. Using Akaike Information Criterion to compare statistical models that correlated cavitation with a variety of kinematic properties, we found that in rotating and accelerating biological conditions, cavitation inception is best explained only by maximum linear velocity.

The comparative hydrodynamics of rapid rotation by predatory appendages

“…Rates of change were calculated from the first and second derivatives of a least-squares fit to a 10th-order polynomial determined to positional data (Cox et al, 2014;deVries et al, 2012). If maximum speed occurred at the beginning or end of the digitized portion of the strike, then the strike was eliminated from the dataset, because the true peak speed could have occurred outside the digitized segment of the strike.…”

“…We incorporated kinematics from Lysiosquillina maculata and Alachosquilla vicina (deVries et al, 2012), Gonodactylus smithii (Cox et al, 2014;Patek et al, 2007) and Neogonodactylus bredini (Kagaya and Patek, 2016) in a form consistent with present measurements. We measured the propodus length (length of the propodus between the propodus-dactyl joint to the proximal-most point visible on the propodus) and the striking body length (length from the propodus-dactyl joint to the insertion point of the lateral extensor muscle on the dorsal surface of the carpus) from archived digital images (Fig.…”

“…'Smasher' mantis shrimp evolved from 'spearers' (Porter et al, 2010) and, during this transition, the dactyl-open evasive prey capture motion characteristic of spearers switched to a primarily closed-dactyl, hammering motion directed at hard-shelled prey . Along with this shift in the orientation and target of the raptorial strike, smashers dramatically increased in speeds and accelerations (Table 1) (Cox et al, 2014;deVries et al, 2012;Kagaya and Patek, 2016), evolved enhanced elastic energy storage capabilities Rosario and Patek, 2015), yet also experienced a substantial decrease in range of body size and appendage size compared with their ancestral spearers (Anderson et al, 2014;Blanco and Patek, 2014). Thus, the evolutionary shift to smashing encapsulates the major axes of drag: shape, speed and size.…”

Mechanical sensitivity reveals evolutionary dynamics of mechanical systems

“…) [28,29]. Raptorial appendages are morphologically diverse, and are used for a variety of functions ranging from spearing to smashing (figure 2b,c) [30,31].…”

“…Raptorial appendages are morphologically diverse, and are used for a variety of functions ranging from spearing to smashing (figure 2b,c) [30,31]. Energy released by the spring mechanism is delivered to the strike via a series of linked mechanisms, including an exoskeletal, four-bar linkage [28][29][30][31][32].…”