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Cover
The Caribbean spiny lobster (Panulirus argus) shown on the cover in its experimental tank (upper left image) is displaying its typical food-search behavior. Its array of anterior thoracic limbs probes the substrate surface, and when a piece of food is contacted, it is picked up and transferred to the mouthparts, which, in crustaceans, comprise the anteriormost segmental appendages. A close-up image (lower left) shows the mouthparts manipulating a shucked mussel (the dark bushy structure is the byssus). The anteriormost of the mouthparts (the third maxillipeds) have a firm grip on the mussel, dimpling the yellow, pillowy surface of its exposed mantle; many of the setae at the tip of the appendages are in intimate contact with the prey's soft tissues. The scanning electron micrograph on the cover illustrates the clusters of simple setae found on the tip of the third maxilliped. The setae (diameter, 30-45 m) are long, hollow cones of exoskeleton enclosing mechanosensory neurons.

In this issue of The Biological Bulletin (p. 195), Anders Garm, Charles Derby, and Jens Hoeg report on a morphological and physiological study of the mechanosensory neurons in the mouthpart setae of P. argus. Most important, they describe their test of the hypothesis that these sensory structures provide tactile information about the texture, shape, and movement of food items-Minformation critical to the control of feeding behavior. The electrophysiological results show that the simple setae, which constitute the vast majority of the mouthpart setae, contain only one of two different types of mechanoreceptor neurons: displacement-sensitive neurons that respond to deflection of the entire shaft at its base, and bend-sensitive neurons that respond to bending of the distal portion of the setal shaft. This is the first experimental demonstration of bend-sensitive neurons in any arthropod sensory seta.

Together the two types of mechanosensory neurons seem to constitute a form of somatosensory system. This would suggest that the mechanosensory input to the central nervous system during feeding is more complex than previously thought and might underlie the flexibility of crustacean feeding behavior. Presumably, displacement-sensitivity provides information about the size, shape, and movement of prey and thus contributes to the manipulation of such food items. Input from bend-sensitive neurons also contributes to these perceptions; but in addition, it indicates the relative hardness of the prey, which will influence the decision about whether the food should be crushed or bitten by the mandibles.

The images of the spiny lobster and its mouthparts were taken from macro-video recordings. (The lobster is about 25 cm in length; its maxillipeds are 4 mm in diameter; and the gray tube behind it serves as a shelter.) The video images and the scanning electron micrograph were both made by Anders Garm (Lund University, Sweden), who also drafted the cover. The final cover design was prepared by Beth Liles (Marine Biological Laboratory, Woods Hole, Massachusetts).


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