Arthropods (Sep 2022)
The potential functions of mechanoreceptors found on trilobite larva of Limulus polyphemus (Linnaeus, 1758)
Abstract
In this study trilobite larvae and early juvenile stages of Limulus polyphemus (Linnaeus, 1758) are examined using light microscopy and scanning electron microscopy to examine the mechanoreceptors present on the cuticle. Three forms of mechanoreceptors were described on the trilobite larvae of L. polyphemus, (1) tactile sensilla, (2) peg sensilla and (3) helical sensilla. Tactile sensilla are adapted to detect touch. Peg sensilla are adapted to detect water currents and near field sound. The helical sensilla are new to science and their function is not known. The genus Limulus Müller 1785 has a fossil record from the Jurassic era, 148 million years. Before the Common Era (BCE) to the present. Having survived through the mass extinction at the end of the Mesozoic era, it is referred to as a "Stabilomorph". As a Stabilomorph the morphology of mechanoreceptors, specifically sensilla capable of sensing particle motions, found on trilobite larva of L. polyphemus may shed some light on the mechanisms of near field sound or particle motion detection in Jurassic seas. To detect particle motion, near-field sound energy, the sensory organ must extend through the boundary layer. The thickness of the boundary layer (δ) for particle motion in water is equal to [kinematic viscosity/sound wave frequency]1/2, less than 200 μm in the range of human hearing. The value of δ in air is 2.54 [kinematic viscosity/sound wave frequency]1/2, about 1 to 2 millimeters in the range of human hearing. Therefore, the mechanoreceptors active in the detection of particle motion are typically shorter in aquatic animals compared to terrestrial animals. Near field sound energy is the dominant sound energy at a distance less than or equal to the wavelength.
