Fine structure of the aesthetasc hairs ofPagurus hirsutiusculus dana

Summary The thin-walled aesthetasc pegs on the antennules of a small hermit crab,Pagurus hirsutiusculus, were studied by light and electron microscopy. The lumen of each aesthetasc was found to be filled with the dendrites of 300–500 bipolar neurons whose cell bodies lie beneath the base of the aesthetasc. These dendrites are ciliary in nature, having well developed basal bodies and rootlets.Each basal body gives rise to a cilium which divides to form a cluster of slender branches, each of which contains a microtubule running lengthwise. These structures occupy most of the length of the hair.The cuticle of the aesthetasc wall is thin and tenuous. Except for the pore canals in the basal region, we have found no pores at either light or electron microscope level, but as the hair is extremely permeable, we conclude that the cuticle itself may permit the passage of solutions. This permeability of the cuticle and the large numbers of dendrites within support the hypothesis that the aesthetascs are chemoreceptors.     

The innervation and chemical sensitivity of single aesthetasc hairs

Summary Each aesthetasc hair of the lateral antennule of the California spiny lobsterPanulirus interruptus (Randall) is shown by light and scanning electron microscopy to be innervated by a basally situated cluster of sensory neurons encased in a glial sheath which isolates each cluster from those of other hairs (Figs. 1, 3, 4). The dendrites of these neurons penetrate the aesthetasc hairs and their axons extend to the central nervous system. Extracellular recordings with suction electrodes from the axons of single neuronal clusters were used to determine the responsiveness of individual hairs to a spectrum of amino acids, amines, amides, carbohydrates, carboxylic acids, nucleotides, and a tripeptide (Tables 1, 2, Figs. 6, 8). Randomly selected hairs from the antennules of juvenile, and male and female adult lobsters were shown to be broadly sensitive to a variety of stimuli and are homogeneous in their breadth of responsiveness (Figs. 5, 7). Cluster analysis does not reveal distinct chemoreceptive hair types based on their response spectra, suggesting that the receptor populations of single hairs are uniformly competent to respond to diverse chemical stimuli (Figs. 6, 8). Further, the sensitivity profile of aesthetascs to these stimuli correlates well with behavioral responses ofPanulirus interruptus to these same stimuli (Tables 1, 2).Abbreviation ² Chi-squared     

Boldness related to size in the hermit crab Coenobita compressus at undisturbed,but not disturbed beach

There is growing interest in the consequences of consistent individual behavioral differences within and between populations. We compared the magnitude, repeatability, and plasticity of boldness in the terrestrial hermit crab Coenobita compressus in three habitats with different degrees of disturbance in Costa Rica. We encourage researchers to test additional sites to assess the generality of our results. Boldness decreased with crab size at the less disturbed sites, while no relationship between size and boldness was found at the disturbed site. Boldness increased with habitat disturbance, perhaps because bolder crabs succeed in exploiting food around people. Repeatability and plasticity were similar across sites. We hypothesize that population differences may be the result of developmental plasticity when aquatic larvae settle on a beach or selection for boldness at the disturbed site acting on each new generation.     

Desiccation resistance in megalopae of the terrestrial hermit crab Coenobita compressus: water loss and the role of the shell

Abstract. The terrestrial hermit crab Coenobita compressus H. M ilne E dwards undergoes larval development in the sea and then moves to land as a megalopa, where it metamorphoses and remains for the rest of its life. As a small organism (generally <3 mg), in a body adapted for pelagic life, the megalopa must avoid desiccating in air to make a successful sea-to-land transition. In this study, I measured rates of water loss in 1 to 26-day-old megalopae without mollusk shells to determine if there is an improvement in desiccation resistance with age. I also exposed 26-day-old megalopae with or without shells to different relative-humidity conditions for 1 h to determine if shells worn by megalopae allow them to function in air that is not fully saturated. Megalopae without shells did not survive exposure to a relative humidity (RH) of less than 99%, while those with shells survived 52% RH. Older megalopae lost water more slowly than younger ones. However, the amount of body water explained more of the variation in water-loss rate than age; individuals with smaller body-water masses showed lower rates of absolute water loss. Though megalopae of C. compressus become less water permeable as they approach metamorphosis on land, most of their ability to avoid desiccation comes from the shell. Shell-wearing can be considered a pre-adaptation to a terrestrial life-style because shell-wearing behavior predates land invasion in hermit crabs, and desiccation-proofing in air is a novel function for shells.     

Fine structure and molting of aesthetasc sense organs on the antennules of the isopod,Asellus aquaticus (Crustacea)

In Asellus aquaticus certain distal antennular segments bear single sensilla referred to as aesthetascs. These show a proximal stem and a distal bulbous region. Depending on its position, each aesthetasc is innervated by either 50-60 or 70-80 bipolar sensory cells, the perikarya of which are situated within the pedunculus. Within the antennular segment the dendrites develop unbranched cilia (9 X 2 + 0 structure). The sensory cells are unusual in that mono- as well as biciliary dendrites are present within a single aesthetasc, the ratio of both types being correlated with the number of sensory cells. Cilia and receptor lymph cavity are enveloped by a set of 3-4 inner and 13-14 outer sheath cells, which terminate at the base of the sensillum, so that the delicate and poreless cuticle of the bulbous region encloses only outer segments within the receptor lymph fluid. A new molting type in arthropods is described in which the outer sheath cells alone build the new cuticle, whereas the inner sheath cells most probably have a protective function. A definition of aesthetascs is proposed based on fine-structural criteria. Functionally the sensilla are considered to be chemoreceptors. This assumption is confirmed by experiments with diluted vital dye as well as lanthanum showing that dissolved substances penetrate the poreless cuticle instantaneously.     

Fine structure of marsupial hairs, with emphasis on trichohyalin and the structure of the inner root sheath

The fine structure and cornification of marsupial hairs are unknown. The distribution of keratins, trichohyalin, and transglutaminase in marsupial hairs was studied here for the first time by electron microscopy and immunocytochemistry. The localization of acidic and basic keratins in marsupial hairs is similar to that of hairs in placental mammals, and the keratins are mainly localized in the outer root sheath and surrounding epidermis. Marsupial trichohyalin in both medulla and inner root sheath (IRS) cross-reacts with a trichohyalin antibody that recognizes trichohyalin across placental species, indicating a common epitope(s) among mammalian trichohyalin. Roundish to irregular trichohyalin granules are composed of a network of immunolabeled 10-15-nm-thick coarse filaments within an amorphous matrix in which a weak labeling for transglutaminases is present. This suggests that the enzyme, and its substrate trichohyalin, are associated in mature granules. Transglutaminase labeling mainly occurs in condensing chromatin of mature cells of the outer and inner root sheaths, suggesting formation of the nuclear envelope connected with terminal differentiation of these cells. In mature Huxley or Henle layers the filaments lose the immunolabeling for trichohyalin when they are reoriented into parallel rows linked by short bridges, thus suggesting that the filaments with their reactive epitopes are chemically modified during cornification, as seen in the IRS of hairs of placental mammals. The Huxley layer probably acts as a cushion, absorbing the tensions connected with the distalward movement of the growing hair fiber. Variations in stratification of the Huxley layer are probably related to the diameter of the hair shaft. The cytoplasmic and junctional connections between cells of the Huxley layer and the companion layer and the outer root sheath enhance the grip of the IRS and hair fiber within the follicle. The role of cells of the IRS in sculpturing the fiber cuticle and in the mechanism of shedding that allows the exit of hair on the epidermal surface in mammals are discussed.     

Spermiogenesis in Coenobita clypeatus,I. Sperm structure

The sperm of the tropical land hermit crab, C. clypeatus, has an elongate acrosome anterior to a lamellar region of cytoplasm. Mitochondria near the lamellar region are associated with microtubules. These microtubules project into the 3 cytoplasmic arms. The nucleus occupies the posterior-most position in the sperm. The chromatin is not condensed and numerous projections of nuclear materials are seen. It is not known how the various organelles of the sperm function during fertilization.     

Fine structure and immunocytochemistry of monotreme hairs, with emphasis on the inner root sheath and trichohyalin-based cornification during hair evolution

The fine structure of hairs in the most ancient extant mammals, the monotremes, is not known. The present study analyzes the ultrastructure and immunocytochemistry for keratins, trichohyalin, and transglutaminase in monotreme hairs and compares their distribution with that present in hairs of the other mammals. The overall ultrastructure of the hair and the distribution of keratins is similar to that of marsupial and placental hairs. Acidic and basic keratins mostly localize in the outer root sheath. The inner root sheath (IRS) comprises 4-8 cell layers in most hairs and forms a tile-like sheath around the hair shaft. No cytological distinction between the Henle and Huxley layers is seen as cells become cornified about at the same time. Externally to the last cornified IRS cells (homologous to the Henle layer), the companion layer contains numerous bundles of keratin. Occasionally, some granules in the companion layer show immunoreactivity for the trichohyalin antibody. This further suggests that the IRS in monotremes is ill-defined, as the companion layer of placental hairs studied so far does not express trichohyalin. A cross-reactivity with an antibody against sheep trichohyalin is present in the IRS of monotremes, suggesting conserved epitopes across mammalian trichohyalin. Trichohyalin granules in the IRS consist of a framework of immunolabeled coarse filaments of 10-12 nm. The latter assume a parallel orientation and lose the immunoreactivity in fully cornified cells. Transglutaminase immunolabeling is diffuse among trichohyalin granules and among the parallel 10-12 nm filaments of maturing inner root cells. Transglutaminase is present where its substrate, trichohyalin, is modified as matrix protein. Cornification of IRS is different from that of hair fiber cuticle and from that of the cornified layer of the epidermis above the follicle. The different consistency among cuticle, IRS, and corneous layer of the epidermis determines separation between hair fiber, IRS, and epidermis. This allows the hair to exit on the epidermal surface after shedding from the IRS and epidermis. Based on comparative studies of reptilian and mammalian skin, a speculative hypothesis on the evolution of the IRS and hairs from the skin of synapsid reptiles is presented.     

Fine structure of the honeybee Z-disc

Z-discs from the dorsal longitudinal indirect flight muscles of the honeybee (Apis mellifera) are perforated with hundreds of triangular-shaped tubes ordered into an hexagonal array. Each tube is surrounded by 80 Å thick rims which incorporate six thin filaments, three from each bordering sarcomere. Although the triangular rims of the tubes are oriented identically in any plane perpendicular to the fibril axis, this orientation changes as the tubes cross the Z-line. The tubes rotate approximately 60 ° about an axis parallel to that of the fibril in passing from one I-Z junction to another.On the basis of filament counting in the A (overlap zone) and I bands of stretched myofibrils, it is concluded that the primary filaments are physically continuous with the Z-lines by material which appears to participate both in the formation of Z-rim substance and the surrounding matrix.Finally, evidence is presented to support the view that filament lattices of adjacent sarcomeres are displaced from one another, so that each thick filament faces the trigonal position of three thick filaments on the other side of the Z-disc.