Shell Apex Research Articles

Metabolic and behavioural effects of hermit crab shell removal techniques: Is heating less invasive than cracking?

Hermit crabs (Paguroidea; Latreille 1802) offer great opportunities to study animal behaviour and physiology. However, the animals' size and sex cannot be determined when they are inside their shell; information crucial to many experimental designs. Here, we tested the effects of the two most common procedures used to make crabs leave their shells: heating the shell apex and cracking the shell with a bench press. We compared the effects of each of the two procedures on the metabolic rate, hiding time, and duration of the recovery time relative to unmanipulated hermit crabs. The hermit crabs forced to abandon their shell through heating increased their respiratory rate shortly after the manipulation (1 h) and recovered their metabolic rate in less than 24 h, as occurs in individuals suddenly exposed to high temperatures in the upper-intertidal zone. Hermit crabs removed from their shells via cracking spent more time hiding in their new shells; this effect was evident immediately after the manipulation and lasted more than 24 h, similar to responses exhibited after a life-threatening predator attack. Both methods are expected to be stressful, harmful, or fear-inducing; however, the temperature required to force the crabs to abandon the shell is below the critical thermal maxima of most inhabitants of tropical tide pools. The wide thermal windows of intertidal crustaceans and the shorter duration of consequences of shell heating compared to cracking suggest heating to be a less harmful procedure for removing tropical hermit crabs from their shells.

Morphometric analysis of stem-group mollusks from the northern Yangtze Craton, China

AbstractCap-shaped skeletal fossils are the earliest undisputed body fossils of mollusks appearing in the basal Cambrian. A study on the morphometry of cap-shaped fossils from the Nanjiang area (North Sichuan, China) is undertaken to understand the origin and evolution of the early mollusks. The distribution of these fossil cap-shaped mollusks indicates a stepwise increase in their diversity during the early Cambrian. Maikhanella Zhegallo in Voronin et al., 1982 co-occuring with the spinose sclerites of siphogonuchitids, is regarded as the earliest scleritized mollusk. It is followed by other maikhanellids, e.g., Purella Missarzhevsky, 1974 and Yunnanopleura Yu, 1987, which co-occur with the earliest univalved helcionellids, e.g., Igorella Missarzhevsky in Rozanov et al., 1969. Cluster analysis of their morphometric characteristics shows that the Maikhanella group is similar to the Purella and Yunnanopleura groups, but is less comparable with univalved helcionellids. The maikhanellids are interpreted as representatives of the stem group Aculifera, although it remains uncertain if one or two larger cap-shaped shell plates were present on the elongate slug-like body, comparable to those of Halkieria Poulsen, 1967 or Orthrozanclus Conway Morris and Caron, 2007. Maikhanellids are characterized by the prominent protrusions or scales on the cap-shaped shell plates arranged in a concentric pattern around the shell apex. Evolutionarily, the protrusions or scales are reduced in younger strata, whereas subsequently a typically concentric ornament developed, the cap-shaped shell plates developed higher profiles, and the apical region became increasingly bare of scales. Meanwhile, the cap-shaped shell plates gradually evolved into a helcionellid-like appearance with an anteroposteriorly inclined apex. The morphological evolution of the earliest sclerotized mollusks reflects biotic evolution and environmental adaption among the stem-group mollusks during the early Cambrian.

A semi-analytical method for free vibration of semi-closed shells of revolution with variable curvature

This study is concerned with the free vibration analysis of semi-closed shells of revolution with variable curvature, with a focus on the analytical solution to the natural frequency of such shells. A semi-closed shell of revolution with variable curvature is firstly divided into some narrow small shell segments, then the first small shell segment with a closed shell apex is approximated with a semi-closed conical shell, while the remaining small shell segments are approximated with a series of progressively larger truncated conical shells. Since each small conical shell is part of the large semi-closed shell of revolution with variable curvature and has to vibrate in sync with the large shell, the analytical solution to the axisymmetric free vibration of the large shell can be transformed into the analytical solutions to the synchronous free vibration of these small conical shells. The general solutions for the natural frequencies of all small conical shells are analytically derived under the condition that conical shells have zero meridional curvature. The undetermined constants in the general solutions are determined by letting all natural frequencies of these small conical shells be equal. Finally, the analytical solution for the axisymmetric free vibration of semi-closed shells of revolution with variable curvature is presented and is proved to be basically reliable by conducting a confirmatory experiment based on polymer 3D-printing technique.

Evaluation of the synergistic effects of climate change on estuarine ecosystems at temperate and tropical latitudes using Littorinids (Mollusca: Gastropoda) as indicators / Avaliação dos efeitos sinérgicos de mudanças climáticas em ecossistemas estuarinos de latitudes temperadas e tropicais utilizando Littorinídeos (Mollusca: Gastropoda) como indicadores

Climate change has been considered as the most serious threat to biodiversity, and therefore, it has been receiving increasing attention regarding its potential impacts on the coastal zone. According to IPCC forecasts, increase in CO2, sea and air temperature, sea level rise, and ocean acidification are expected. Thus, this study evaluated the synergistic effect of future climate changes in Littorinids from temperate and tropical estuarine environments through experimental studies. Mortality, changes in size, weight loss, and specie’s behaviors of the species Littorina fabalis and Littorina littorea in Spain and Littoraria angulifera and Litteraria flava in Brazil were evaluated in the laboratory considering estuarine acidification, temperature increase, and salinity variations due to sea level rise and/or rainfall reduction. The data presented indicate that estuarine Littorinids will respond to climate change as (i) with increased mortality rates, weight loss, and changes in behavior as a function of increased temperatures, and, (ii) with decreased shell sizes resulting from the dissolution of the shell apex due to estuarine acidification. These results suggest changes in these estuarine ecosystems resulting in effects that lead to serious endangerment to animal biodiversity and ecological balance.

Parallel shape divergence between ecotypes of the limpet Nacella concinna along the Antarctic Peninsula: a new model species for parallel evolution?

Parallel phenotypic divergence is the independent differentiation between phenotypes of the same lineage or species occupying ecologically similar environments in different populations. We tested in the Antarctic limpet Nacella concinna the extent of parallel morphological divergence in littoral and sublittoral ecotypes throughout its distribution range. These ecotypes differ in morphological, behavioural and physiological characteristics. We studied the lateral and dorsal outlines of shells and the genetic variation of the mitochondrial gene Cytochrome Oxidase subunit I from both ecotypes in 17 sample sites along more than 2,000 km. The genetic data indicate that both ecotypes belong to a single evolutionary lineage. The magnitude and direction of phenotypic variation differ between ecotypes across sample sites; completely parallel ecotype-pairs (i.e., they diverge in the same magnitude and in the same direction) were detected in 84.85% of lateral and 65.15% in dorsal view comparisons. Besides, specific traits (relative shell height, position of shell apex, and elliptical/pear-shape outline variation) showed high parallelism. We observed weak morphological covariation between the two shape shell views, indicating that distinct evolutionary forces and environmental pressures could be acting on this limpet shell shape. Our results demonstrate there is a strong parallel morphological divergence pattern in N. concinna along its distribution, making this Antarctic species a suitable model for the study of different evolutionary forces shaping the shell evolution of this limpet.

Shell density of planktonic foraminifera and pteropod species Limacina helicina in the Barents Sea: Relation to ontogeny and water chemistry.

Planktonic calcifiers, the foraminiferal species Neogloboquadrina pachyderma and Turborotalita quinqueloba, and the thecosome pteropod Limacina helicina from plankton tows and surface sediments from the northern Barents Sea were studied to assess how shell density varies with depth habitat and ontogenetic processes. The shells were measured using X-ray microcomputed tomography (XMCT) scanning and compared to the physical and chemical properties of the water column including the carbonate chemistry and calcium carbonate saturation of calcite and aragonite. Both living L. helicina and N. pachyderma increased in shell density from the surface to 300 m water depth. Turborotalita quinqueloba increased in shell density to 150-200 m water depth. Deeper than 150 m, T. quinqueloba experienced a loss of density due to internal dissolution, possibly related to gametogenesis. The shell density of recently settled (dead) specimens of planktonic foraminifera from surface sediment samples was compared to the living fauna and showed a large range of dissolution states. This dissolution was not apparent from shell-surface texture, especially for N. pachyderma, which tended to be both thicker and denser than T. quinqueloba. Dissolution lowered the shell density while the thickness of the shell remained intact. Limacina helicina also increase in shell size with water depth and thicken the shell apex with growth. This study demonstrates that the living fauna in this specific area from the Barents Sea did not suffer from dissolution effects. Dissolution occurred after death and after settling on the sea floor. The study also shows that biomonitoring is important for the understanding of the natural variability in shell density of calcifying zooplankton.

Prevalence of Egg Shell Apex Abnormalities in Commercial Layer Chicken of Namakkal Region of Tamil Nadu

Prevalence of Egg Shell Apex Abnormalities in Commercial Layer Chicken of Namakkal Region of Tamil Nadu

Smashing mantis shrimp strategically impact shells.

Many predators fracture strong mollusk shells, requiring specialized weaponry and behaviors. The current shell fracture paradigm is based on jaw- and claw-based predators that slowly apply forces (high impulse, low peak force). However, predators also strike shells with transient intense impacts (low impulse, high peak force). Toward the goal of incorporating impact fracture strategies into the prevailing paradigm, we measured how mantis shrimp (Neogonodactylus bredini) impact snail shells, tested whether they strike shells in different locations depending on prey shape (Nerita spp., Cenchritis muricatus, Cerithium spp.) and deployed a physical model (Ninjabot) to test the effectiveness of strike locations. We found that, contrary to their formidable reputation, mantis shrimp struck shells tens to hundreds of times while targeting distinct shell locations. They consistently struck the aperture of globular shells and changed from the aperture to the apex of high-spired shells. Ninjabot tests revealed that mantis shrimp avoid strike locations that cause little damage and that reaching the threshold for eating soft tissue is increasingly difficult as fracture progresses. Their ballistic strategy requires feed-forward control, relying on extensive pre-strike set-up, unlike jaw- and claw-based strategies that can use real-time neural feedback when crushing. However, alongside this pre-processing cost to impact fracture comes the ability to circumvent gape limits and thus process larger prey. In sum, mantis shrimp target specific shell regions, alter their strategy depending on shell shape, and present a model system for studying the physics and materials of impact fracture in the context of the rich evolutionary history of predator-prey interactions.

Antibiotic susceptibility profiles of Mycoplasma synoviae strains originating from Central and Eastern Europe

BackgroundMycoplasma synoviae causes infectious synovitis and respiratory diseases in chickens and turkeys and may lead to egg shell apex abnormalities in chickens; hence possesses high economic impact on the poultry industry. Control of the disease consists of eradication, vaccination or medication. The aim of the present study was to determine the in vitro susceptibility to 14 different antibiotics and an antibiotic combination of M. synoviae strains originating from Hungary and other countries of Central and Eastern Europe.ResultsMinimal inhibitory concentration (MIC) values of a total of 41 M. synoviae strains were determined by the microbroth dilution method. The strains were collected between 2002 and 2016 and originated from Hungary (n = 26), Austria (n = 3), the Czech Republic (n = 3), Slovenia (n = 3), Ukraine (n = 3), Russia (n = 2) and Serbia (n = 1). Tetracyclines (with MIC50 values of 0.078 μg/ml, ≤0.25 μg/ml and 0.5 μg/ml for doxycycline, oxytetracycline and chlortetracycline, respectively), macrolides (with MIC50 values of ≤0.25 μg/ml for tylvalosin, tylosin and tilmicosin), pleuromutilins (with MIC50 values of 0.078 μg/ml and ≤0.039 μg/ml for tiamulin and valnemulin) and the combination of lincomycin and spectinomycin (MIC50 1 μg/ml (0.333/0.667 μg/ml)) were found to be the most effective antibiotic agents against M. synoviae in vitro. High MIC values were detected in numerous strains for fluoroquinolones (with MIC50 values of 1.25 μg/ml and 2.5 μg/ml for enrofloxacin and difloxacin), neomycin (MIC50 32 μg/ml), spectinomycin (MIC50 2 μg/ml), lincomycin (MIC50 0.5 μg/ml) and florfenicol (MIC50 4 μg/ml). Nevertheless, strains with elevated MIC values were detected for most of the applied antibiotics.ConclusionsIn the medical control of M. synoviae infections the preliminary in vitro antibiotic susceptibility testing and the careful evaluation of the data are crucial. Based on the in vitro examinations doxycycline, oxytetracycline, tylvalosin, tylosin and pleuromutilins could be recommended for the therapy of M. synoviae infections in the region.

Impact damage and repair in shells of the limpet Patella vulgata.

Experiments and observations were carried out to investigate the response of the Patella vulgata limpet shell to impact. Dropped-weight impact tests created damage that usually took the form of a hole in the shell's apex. Similar damage was found to occur naturally, presumably as a result of stones propelled by the sea during storms. Apex holes were usually fatal, but small holes were sometimes repaired, and the repaired shell was as strong as the original, undamaged shell. The impact strength (energy to failure) of shells tested in situ was found to be 3.4-times higher than that of empty shells found on the beach. Surprisingly, strength was not affected by removing the shell from its home location, or by removing the limpet from the shell and allowing the shell to dry out. Sand abrasion, which removes material from the apex, was found to have a strong effect. Shells were also subjected to repeated impacts, which caused failure after 2-120 repetitions. In situ shells performed poorly in this test. It is proposed that the apex acts as a kind of sacrificial feature, which confers increased resistance but only for a small number of impacts. Microscopy showed that damage initiates internally as delamination cracks on low-energy interfaces, leading to loss of material by spalling. This mode of failure is a consequence of the layered structure of the shell, which makes it vulnerable to the tensile and shear stresses in the impact shock wave.

Deposition of annual growth lines in the apex of the common limpet (Patella vulgata) from Shetland Islands, UK and Norway: Evidence from field marking and shell mineral content of annual line deposition

The abundance, accessibility and value of limpets as a source of food and bait for coastal peoples have resulted in their high frequency in shell middens worldwide. The limpet Patella vulgata is found in middens from the Mediterranean to Norway, and morphometric and sclerochronological analyses of its shell can provide insight into harvesting patterns and paleoenvironmental variables valuable in reconstructing climate. Previous work with P. vulgata has relied on lines on the exterior of the shell, or on lines exiting on the shell surface in shell cross-section, as annual or sub-annual markers. Shell damage may compromise these lines and limit the use of some shells, but growth lines are also found in the better-preserved shell apex. We investigated whether the growth lines in the apex of P. vulgata from two locations in Northern Europe are annual using calcein-marking and recapture. Investigations were performed at one site in the Shetland Islands (UK) and at one site at the northern limit of P. vulgata's range in Northern Norway. We also used laser ablation to measure the concentration of minerals in the shells of two individuals from Shetland to determine if patterns of minerals suggested as bioproxies for temperature and productivity varied annually. All individuals deposited one growth line in the apex during their year in the field, and the lowest ratios of Sr/Ca and Ba/Ca and to a lesser extent Mg/Ca were coincident with annual lines on the shells from Shetland. Growth at both sites was modelled using the von Bertalanffy growth function, and apex growth was nearly five times faster in Shetland than in Norway, probably a result of differences in temperature between the two locations.

Geographical variation in shell morphology of juvenile snails (Concholepas concholepas) along the physical–chemical gradient of the Chilean coast

Changes in phenotypic traits, such as mollusc shells, are indicative of variations in selective pressure along environmental gradients. Recently, increased sea surface temperature (SST) and ocean acidification (OA) due to increased levels of carbon dioxide in the seawater have been described as selective agents that may affect the biological processes underlying shell formation in calcifying marine organisms. The benthic snailConcholepas concholepas(Muricidae) is widely distributed along the Chilean coast, and so is naturally exposed to a strong physical–chemical latitudinal gradient. In this study, based on elliptical Fourier analysis, we assess changes in shell morphology (outlines analysis) in juvenileC. concholepascollected at northern (23°S), central (33°S) and southern (39°S) locations off the Chilean coast. Shell morphology of individuals collected in northern and central regions correspond to extreme morphotypes, which is in agreement with both the observed regional differences in the shell apex outlines, the high reclassification success of individuals (discriminant function analysis) collected in these regions, and the scaling relationship in shell weight variability among regions. However, these extreme morphotypes showed similar patterns of mineralization of calcium carbonate forms (calcite and aragonite). Geographical variability in shell shape ofC. concholepasdescribed by discriminant functions was partially explained by environmental variables (pCO2, SST). This suggests the influence of corrosive waters, such as upwelling and freshwaters penetrating into the coastal ocean, upon spatial variation in shell morphology. Changes in the proportion of calcium carbonate forms precipitated byC. concholepasacross their shells and its susceptibility to corrosive coastal waters are discussed.

New land mollusk fauna from Serra da Capivara, Piaui, Brazil, with a new genus and five new species (Gastropoda: Orthalicoidea, Streptaxidae, Subulinidae).

Five new species and one new genus are described from the Serra da Capivara, Piaui, Brazil, a region of semi-dry, Caatinga environment. The described taxa are: Odontostomidae: 1) Clinispira insolita, n. gen. n. sp., possessing strange spire inclination and aperture approaching to the shell apex, it looks closely related to Anostoma, Tomigerus and Biotocus; 2) Cyclodontina capivara n. sp., with well-developed axial ribs and 6 teeth at peristome; 3) Anctusprolatus n. sp., very elongated, with aperture lacking teeth; Simpulopsidae: 4) Rhinus gilbertus n. sp., somewhat elongated and relatively well-sculptured; Streptaxidae: 5) Streptartemon molaris n. sp., possessing a very large basalis tooth at peristome. Two subulinids are also reported from the region: 6) Beckianum beckianum (Pfeiffer, 1846), a population with peculiar pointed shell apex and well-developed axial sculpture; 7) Lamellaxis cf. gracilis (Hutton, 1834), a wide-ranged species that merits further investigation. These descriptions show how scanty is our knowledge on northeast Brazilian malacofauna and may raise efforts for its preservation.

Unrecognized biodiversity in an old lake: a new species ofAcroloxusBeck, 1837 (Pulmonata, Hygrophila, Acroloxidae) from Lake Eğirdir, Turkey

Acroloxus egirdirensis, a new species (Pulmonata: Hygrophila: Acroloxidae) is described based on material collected in the littoral zone of Lake Egirdir, Turkey. The limpet has been found only at the type locality. The shell morphology and body shape of A. egirdirensis are similar to the widespread A. lacustris (Linnaeus, 1758), A. oblongus (Lightfoot, 1786) and A. shadini Kruglov et Starobogatov, 1991. Compared to the first two species, A. egirdirensis has a higher teleoconch, the nuclei in the pigment cells of the mantle are large, and the preputium length is almost identical to that of the penis sheath. The new species differs from A. shadini in the shape of the left shell slope. The shell apex in A. egirdirensis is closer to the posterior apertural edge than in any other European species. The species status is corroborated by DNA data of the mitochondrial gene fragment cytochrome c oxidase subunit I (COI) (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

B-type Protoconchs and All Three Modes of Larval Development in Eastern North AmericanBoonea(Pyramidellidae)

The genus Boonea Robertson (1978), ectoparasitic on mollusks, consistently has a B-type protoconch morphology thought by Thorson (1946) to indicate planktotrophy in other pyramidellid genera. Studies in the laboratory of the eggs and larval development of Boonea seminuda (C. B. Adams, 1839), the type species, B. bisuturalis (Say, 1822), and B. impressa (Say, 1822) on the eastern North American seaboard confirmed planktotrophy. The egg diameter range there was 68 to 80 µm. Boonea bisuturalis had been reared through metamorphosis using algal cultures (Robertson 1967; Leibowitz 1979 [unpublished]). The developmental stages are newly illustrated here. Boonea seminuda and B. bisuturalis have different teleoconch sculptures, similar but different spermatophores, and are closely related. Northern B. bisuturalis and southern B. impressa are allopatric and hybridize or intergrade conchologically in a cline between Long Island Sound and Chesapeake Bay. One population of B. “impressa” had been found to be lecithotrophic on the Texas coast, at Port Aransas (White et al. 1985). A second Texan population, at Galveston, is shown here consistently to undergo intracapsular metamorphosis. The eggs were reared to hatching crawlers in the laboratory. The Galveston protoconch sizes and morphologies are the same as at North Carolina, but eggs from Galveston were much larger (212 to 236 µm in diameter). Durations of intracapsular development through metamorphosis and the pelagic larval stage in laboratory B. bisuturalis were virtually the same (respectively 15 to 20 and 16 to 20 days). In the three species, egg sizes greatly affect oviposition to hatching times, hatching sizes, and whether or not there is planktotrophy. Egg sizes do not affect oviposition to metamorphosis times, or protoconch sizes. North Carolinian and Texan B. impressa teleoconch sculptures and spermatophore sizes differ slightly. There could be a wide and continuous intraspecific range of conchological and/or reproductive variation, or allopatric poecilogony, or a chemically-induced ecotype, or (as seems most likely) one or more as-yet unnamed sibling species in the Gulf of Mexico. Regardless, this appears to be the first known exception to the “shell apex theory” (Thorson 1950). Everywhere, B. “impressa” is a serious pest of Crassostrea virginica (Gmelin, 1791). Some new data on Boonea systematics, adult ecology, and life history are presented. “Boonea” scymnocelata Pimenta et al. (2009), from Brazil, is not a Boonea because it has a C-type protoconch; its teleoconch is convergent with that of B. seminuda.

Epistasis, phenotypic disequilibrium and contrasting associations with climate in the land snail Theba pisana.

Hotter conditions favour effectively unbanded (EUB) shells in the snail Theba pisana. T. pisana is also polymorphic for colour of the shell's apex, determined by a pair of alleles at a locus linked to the banding locus. Apex colour is epistatic to shell banding, such that banded snails with a dark apex have darker bands. Annual censuses over 22 years across an ecotone between a sheltered Acacia thicket and open dune vegetation showed a persistent association of both EUB shells and pale apex with the Open habitat. The parallel variation was due partly to strong phenotypic disequilibrium, as the combination of EUB with dark apex was rare. Nevertheless, in fully banded shells the frequency of pale apex was also higher in the Open habitat, confirming independent, parallel associations of the two contributors to paleness. Within the Acacia habitat, temporal variation of the frequencies of banding morphs was much greater than for apex colour, and EUB shells were associated with hotter summers. Consistent with its primary effect only on the very small snails, apex colour did not vary with summer conditions, but instead, higher frequencies of pale apices were associated with sunnier winters. The intensity of selection was lower on apex colour than shell banding, due partly to the constraint of phenotypic disequilibrium. The shell traits in T. pisana are an example of complex responses to climatic variation, in which phenotypic disequilibrium constrains evolution of apex colour, but separate mechanisms of selection are evident.

Growth, reproductive cycle and penis shedding of Littorina littorea in the Ria de Aveiro (north-west Portugal)

The growth and the reproductive cycle of Littorina littorea were investigated in the Ria de Aveiro, Portugal, a country cited as the southern limit of the species distribution in Europe. Littorina littorea growth is well represented by the von Bertalanffy equation SH=24.6[1-e-0.69(t-0.70)] where SH is the shell height (distance from shell apex to the basis of the aperture measured in mm) and t is the number of winters: after periwinkles have reached a mean SH of 4.6 mm in their first winter they grow progressively to about 14.6 mm, 19.6 mm, 22.1 mm and 23.4 mm in the following four years and they attain the asymptotic mean maximum SH of 24.6 mm at the tenth year if they survive. Ripening of the gonads occurred from October to May but was more intense from December to February, whereas spawning extended from January to May in females and November to May in males. There was a seasonal shedding of the male penis at the end of the breeding season (summer) accompanying regression of the testis.

Usefulness of the opercular nucleus for inferring early development in neritimorph gastropods

The early ontogeny of gastropods (i.e., planktotrophic vs. nonplanktotrophic) may be inferable from the morphology of the protoconch in adult shells. The protoconch consists of both embryonic and larval shells in species with planktotrophic development; the embryonic shell forms in the intracapsular period and the succeeding larval shell gradually develops during the larval period. In nonplanktotrophic species, on the other hand, there is no additional growth of the larval shell and the protoconch consists exclusively of a relatively large embryonic shell formed prior to hatching. This "shell apex theory" has been applied to many species of shell-bearing gastropods, but biotic and abiotic erosion of the apex often prevents detailed examination of the protoconch and subsequent inferences about ontogeny. I examined the gastropod operculum to test its utility for predicting developmental mode, drawing on the Neritimorpha as model taxa. Most aquatic members of Neritimorpha were found to bear an operculum with a clearly demarcated nucleus; SEM observations reveal four types of nuclei, which correspond to different types of protoconch morphologies and observed ontogenies for the study species. The nucleus is secreted before metamorphosis, fits into the shell aperture of the larva, and reflects early ontogeny as morphology, as does the protoconch. Moreover, the apparently organic (rather than calcareous) composition of the nucleus makes it nearly invulnerable to erosion and very advantageous, compared to the protoconch, in this ecologically diverse group, whose habitats range from freshwater streams and mangrove swamps to rocky shores and deep-sea hydrothermal vents. The measurements of the nucleus are also valuable for taxonomic purposes, especially in the species identification of veliger larvae and juvenile snails. On the other hand, the opercular nuclei of the Caenogastropoda and Heterobranchia are often eroded away in adult individuals; even if present, the morphology of the nuclei does not seem to clearly reflect early ontogeny in those groups.

Observations and modeling of the coupled latitude‐altitude patterns of equatorial plasma depletions

The equatorial ionosphere is host to the most dramatic and enigmatic plasma instability mechanism in the geospace environment. Equatorial spread F (ESF) was discovered in early ionosonde measurements and interpreted theoretically using Rayleigh‐Taylor theory. Subsequent diagnostic and modeling advances have improved substantially our understanding of ESF onset and evolution and its associated effects on the ionosphere throughout the low‐latitude domain. The degree to which ESF mechanisms penetrate into the lower midlatitudes is a topic of current study, a reverse of the familiar concept of high‐to‐low latitude coupling for space weather phenomena. Optical diagnostic systems, first ground based and now space based, reveal the presence of ESF structures via images of airglow depletions that are aligned in the approximately north‐south direction spanning the geomagnetic equator. Ground‐based all‐sky camera systems used to capture the two‐dimensional horizontal patterns of airglow depletions are the main source of observations showing that ESF processes intrude to midlatitudes in the L ∼ 1.5 domain. In this paper we review the process of mapping airglow depletions along geomagnetic field lines to the equatorial plane, hence defining the maximum apex heights achieved. A case study comparison of simultaneous radar backscatter data from Kwajalein with optical data from Wake Island, sites that share common magnetic meridians in the Pacific section, confirms the utility of the approach and its applicability to sites at other longitudes. Modeling studies based on buoyancy arguments using flux tube–integrated mean density values versus L shell apex heights show that instability‐induced plasma depletions starting at F layer bottomside heights easily reach altitudes above 2000 km in the equatorial plane, implying that ESF intrusions to lower midlatitudes should be a relatively frequent occurrence.

Axisymmetric acoustic radiation from submerged prolate spheroidal shells

The equations of motion for nonaxisymmetric vibration of prolate spheroidal shells of constant thickness were derived using Hamiltons principle [S. I. Hayek and J. E. Boisvert, J. Acoust. Soc. Am. 114, 2799–2811 (2003)]. The shell theory used in this derivation includes shear deformations and rotatory inertias. The shell displacements and rotations were expanded in infinite series of comparison functions. These include associated Legendre functions in terms of the prolate spheroidal angular coordinate and circular functions in the azimuthal angle coordinate. For axisymmetric vibration of a submerged shell, the external (heavy) fluid loading impedance was computed using expansions of prolate spheroidal wavefunctions. The shell was excited by axisymmetric normal surface forces, including a point load at the shell apex and ring load at other locations. Far-field radiated pressure spectra are presented for several shell thickness-to-half-length ratios ranging from 0.005 to 0.1, and for various shape parameters, a, ranging from an elongated spheroidal shell (a=1.01) to a spherical shell (a∼100). The far-field directivity of acoustic radiation is presented at selected frequencies. [Work supported by the NUWC ILIR Program and the ONR/ASEE Summer Faculty Research Program.]