Developing elastic mechanisms: ultrafast motion and cavitation emerge at the millimeter scale in juvenile snapping shrimp.

Organisms such as jumping froghopper insects and punching mantis shrimp use spring-based propulsion to achieve fast motion. Studies of elastic mechanisms have primarily focused on fully developed and functional mechanisms in adult organisms. However, the ontogeny and development of these mechanisms can provide important insights into the lower size limits of spring-based propulsion, the ecological or behavioral relevance of ultrafast movement, and the scaling of ultrafast movement. Here, we examined the development of the spring-latch mechanism in the bigclaw snapping shrimp, Alpheus heterochaelis (Alpheidae). Adult snapping shrimp use an enlarged claw to produce high-speed strikes that generate cavitation bubbles. However, until now, it was unclear when the elastic mechanism emerges during development and whether juvenile snapping shrimp can generate cavitation at this size. We reared A. heterochaelis from eggs, through their larval and postlarval stages. Starting 1 month after hatching, the snapping shrimp snapping claw gradually developed a spring-actuated mechanism and began snapping. We used high-speed videography (300,000 frames s-1) to measure juvenile snaps. We discovered that juvenile snapping shrimp generate the highest recorded accelerations (5.8×105±3.3×105 m s-2) for repeated-use, underwater motion and are capable of producing cavitation at the millimeter scale. The angular velocity of snaps did not change as juveniles grew; however, juvenile snapping shrimp with larger claws produced faster linear speeds and generated larger, longer-lasting cavitation bubbles. These findings establish the development of the elastic mechanism and cavitation in snapping shrimp and provide insights into early life-history transitions in spring-actuated mechanisms.

Scaling and development of elastic mechanisms: the tiny strikes of larval mantis shrimp.

Latch-mediated spring actuation (LaMSA) is used by small organisms to produce high acceleration movements. Mathematical models predict that acceleration increases as LaMSA systems decrease in size. Adult mantis shrimp use a LaMSA mechanism in their raptorial appendages to produce extremely fast strikes. Until now, however, it was unclear whether mantis shrimp at earlier life-history stages also strike using elastic recoil and latch mediation. We tested whether larval mantis shrimp (Gonodactylaceus falcatus) use LaMSA and, because of their smaller size, achieve higher strike accelerations than adults of other mantis shrimp species. Based on microscopy and kinematic analyses, we discovered that larval G. falcatus possess the components of, and actively use, LaMSA during their fourth larval stage, which is the stage of development when larvae begin feeding. Larvae performed strikes at high acceleration and speed (mean: 4.133×105 rad s-2, 292.7 rad s-1; 12 individuals, 25 strikes), which are of the same order of magnitude as for adults - even though adult appendages are up to two orders of magnitude longer. Larval strike speed (mean: 0.385 m s-1) exceeded the maximum swimming speed of similarly sized organisms from other species by several orders of magnitude. These findings establish the developmental timing and scaling of the mantis shrimp LaMSA mechanism and provide insights into the kinematic consequences of scaling limits in tiny elastic mechanisms.

Scaling of dentition and prey size in the California moray (Gymnothorax mordax).

Scaling patterns of tooth morphology can provide insights on prey capture strategy and dietary patterns as species grow through ontogeny. We report the scaling of dentition and diet and how it relates to body size in the California moray, Gymnothorax mordax. We sampled lengths, widths, and curvature for teeth lining five distinct regions of the oral jaws across 21 G. mordax individuals ranging from 383 to 1110mm total length. Absolute tooth length in relation to moray size shows positive allometry only for the outer maxillary teeth, while teeth lining the inner maxilla display positive allometry in tooth base width. All other regions exhibit isometric growth in both length and width relative to moray size. Similar to previous descriptions of other moray species, the longest teeth in the oral jaws are the median intermaxillary teeth. This series of three teeth are depressible and rooted in the center of the ethmovomer, the bone that forms the roof of the rostrum. We hypothesize that caudal mobility of the median intermaxillary teeth aids in prey transport by enabling the pharyngeal jaws to remove pierced prey without requiring full abduction of the oral jaws. The predominantly isometric tooth growth in G. mordax suggests that the oral teeth grow proportionately as individuals increase in size. Stomach contents from the field suggest that G. mordax is highly piscivorous. While a strong positively allometric relationship between vertical gape and head length supports the expectation that moray increase relative prey size over ontogeny, we found no relationship between prey standard length and moray size. This suggests that while larger individuals are capable of consuming larger prey, individual G. mordax are opportunistic predators that do not specialize on prey of a specific size over ontogeny.