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.