Habitat-Related Divergence among Tailfan Sensory Systems in Reptantian Decapod Crustaceans

Abstract

Squat lobsters (Galatheidae) and mole sand crabs (Hippidae) differ in posture and locomotion from each other and from crayfish, their surrogate ancestor for neurobehavioral features. Galatheids resemble crayfish more closely in general behavior and niche, but are intermediate between crayfish and hippids with respect to morphology and neuromusculature. The tailfan is inverted under the abdomen in both, due to the flexed abdominal posture, but its morphology has diverged considerably. Nothing is known about adaptations of the tailfan exteroceptors to the new sensory world of either group. We used SEM, vital staining, and extracellular electrophysiological techniques to survey the sensory structures on the telsons of the galatheid <i>Munida quadrispina</i> and the hippid <i>Emerita analoga</i> for comparison with published data on the homologous mechanosensory system in crayfish. Both telsons bear plumose, peg, and non-annulate (natatory or guard) setae. In addition, <i>M. quadrispina</i> has singly-innervated smooth setae and <i>E. analoga</i> a previously undescribed type of small seta the outer face of which is covered by transversely-oriented, thin setules that are much broader than they are long and angled outward toward the seta’s distal end, overlapping loosely. The ‘stack-of-scales’ appearance of its distal portion viewed from the side engendered the name: scaly seta. Some shared features with other small setae that are chemo- and mechanoreceptive suggest scaly setae might be bimodal sensilla. The telson of <i>M. quadrispina</i> is very flexible. Plumose setae on its dorsal surface are arranged into hemi-circlets and most, if not all, appear not to be innervated. They may contribute to adjacent smooth setae’s mechanosensitivity via mechanical coupling through adjacent cuticle, as occurs between feathered and smooth setae on crayfish antennae. Sensory nerve recordings show many afferents to have low thresholds to mechanical disturbance, suggesting they are hydrodynamic receptors. The telson of <i>E. analoga</i> is rigid, and all dorsal setae are relegated to the margins. Patches of scaly setae on the anterior lateral dorsal telson are strategically located to sense the substrate when the crabs are in sand. Scaly and peg setae are arrayed along shallow grooves, one along each side, that are flanked laterally by a fringe of plumose and pappose setae. Substantial deflection from resting position of the latter was required to reliably elicit afferent activity, suggesting most function as touch receptors. The different, non-random distributions of tailfan setae match these animals’ divergent sensory worlds and might have engendered species-specific alterations in their central sensory systems.