Feeding Biology, Distribution, and Ecology of Two Species of Benthic Polychaetes: Paraonis fulgens and Paraonis pygoenigmatica (Polychaeta: Paraonidae)

Paraonis fulgens and Paraonis pygoenigmatica inhabit sandy littoral and sublittoral sediments of the northem Gulf of Mexico and U.S. East Coast, but seldom overlap in distribution. The purpose of this study was to compare the feeding ecology and distribution of these species. We analyzed distributions and gut contents of Gulf of Mexico specimens and found that P. fulgens inhabited substrates with slightly more silt and clay than those inhabited by P. pygoenigmatica. Although Paraonis fulgens ingested more diatoms than P. pygoenigmatica, this distinction likely resulted from habitat differences, not selective feeding. Previous studies suggested that P. fulgens fed selectively on diatoms only

Crangonid shrimps (Crustacea: Caridea), with a description of a new species of Pontocaris

(39pp

Mobile work station concept for assembly of large space structures (zero gravity simulation tests)

The concept presented is intended to enhance astronaut assembly of truss structure that is either too large or complex to fold for efficient Shuttle delivery to orbit. The potential of augmented astronaut assembly is illustrated by applying the result of the tests to a barebones assembly of a truss structure. If this structure were assembled from the same nestable struts that were used in the Mobile Work Station assembly tests, the spacecraft would be 55 meters in diameter and consist of about 500 struts. The struts could be packaged in less than 1/2% of the Shuttle cargo bay volume and would take up approximately 3% of the mass lift capability. They could be assembled in approximately four hours. This assembly concept for erectable structures is not only feasible, but could be used to significant economic advantage by permitting the superior packaging feature of erectable structures to be exploited and thereby reduce expensive Shuttle delivery flights

New Langevin and Gradient Thermostats for Rigid Body Dynamics

We introduce two new thermostats, one of Langevin type and one of gradient (Brownian) type, for rigid body dynamics. We formulate rotation using the quaternion representation of angular coordinates; both thermostats preserve the unit length of quaternions. The Langevin thermostat also ensures that the conjugate angular momenta stay within the tangent space of the quaternion coordinates, as required by the Hamiltonian dynamics of rigid bodies. We have constructed three geometric numerical integrators for the Langevin thermostat and one for the gradient thermostat. The numerical integrators reflect key properties of the thermostats themselves. Namely, they all preserve the unit length of quaternions, automatically, without the need of a projection onto the unit sphere. The Langevin integrators also ensure that the angular momenta remain within the tangent space of the quaternion coordinates. The Langevin integrators are quasi-symplectic and of weak order two. The numerical method for the gradient thermostat is of weak order one. Its construction exploits ideas of Lie-group type integrators for differential equations on manifolds. We numerically compare the discretization errors of the Langevin integrators, as well as the efficiency of the gradient integrator compared to the Langevin ones when used in the simulation of rigid TIP4P water model with smoothly truncated electrostatic interactions. We observe that the gradient integrator is computationally less efficient than the Langevin integrators. We also compare the relative accuracy of the Langevin integrators in evaluating various static quantities and give recommendations as to the choice of an appropriate integrator.Comment: 16 pages, 4 figure

Pattern and Coloration of Periclimenes rathbunae from the Turks and Caicos Islands, with Comments on Host Associations in Other Anemone Shrimps of the West Indies and Bermuda

The commensal shrimp Periclimenes rathbunae Schmitt, 1924, lives in association with the sea anemones Condylactis gigantea Weinland, 1860, and Stichodactyla (= Stoichactis) helianthus Ellis, 1767, in the Turks and Caicos Islands, British West Indies. We describe its pattem and coloration in life. Published reports of distribution and host acceptance by P. rathbunae and three of its congeners (P. anthophilus Holthuis and Eibl-Eibesfeldt, 1964; P. pedersoni Chace, 1958; and P. yucatanicus Ives, 1891) are reviewed. The last two species are recorded for the first time from the Turks and Caicos, and the association of P. yucatanicus with Stichodactyla helianthus on the Caicos Bank is a new host record. Finally, we offer the first direct evidence that P. yucatanicus leaves the host anemone temporarily to clean fishes, discounting a previous suggestion of its role as a noncleaning mimic of P. pedersoni

Pattern and Coloration of Periclimenes rathbunae from the Turks and Caicos Islands, with Comments on Host Associations in Other Anemone Shrimps of the West Indies and Bermuda

The commensal shrimp Periclimenes rathbunae Schmitt, 1924, lives in association with the sea anemones Condylactis gigantea Weinland, 1860, and Stichodactyla (= Stoichactis) helianthus Ellis, 1767, in the Turks and Caicos Islands, British West Indies. We describe its pattem and coloration in life. Published reports of distribution and host acceptance by P. rathbunae and three of its congeners (P. anthophilus Holthuis and Eibl-Eibesfeldt, 1964; P. pedersoni Chace, 1958; and P. yucatanicus Ives, 1891) are reviewed. The last two species are recorded for the first time from the Turks and Caicos, and the association of P. yucatanicus with Stichodactyla helianthus on the Caicos Bank is a new host record. Finally, we offer the first direct evidence that P. yucatanicus leaves the host anemone temporarily to clean fishes, discounting a previous suggestion of its role as a noncleaning mimic of P. pedersoni

Finding the Center of Mass of a Soft Spring

This article shows how to use calculus to find the center of mass position of a soft cylindrical helical spring that is suspended vertically. The spring is non-uniformly stretched by the action of gravity. A general expression for the vertical position of the center of mass is obtained.Comment: LaTeX, 7 pages, 2 figures. Minor changes to agree with published versio

A mobile work station concept for mechanically aided astronaut assembly of large space trusses

This report presents results of a series of truss assembly tests conducted to evaluate a mobile work station concept intended to mechanically assist astronaut manual assembly of erectable space trusses. The tests involved assembly of a tetrahedral truss beam by a pair of test subjects with and without pressure (space) suits, both in Earth gravity and in simulated zero gravity (neutral buoyancy in water). The beam was assembled from 38 identical graphite-epoxy nestable struts, 5.4 m in length with aluminum quick-attachment structural joints. Struts and joints were designed to closely simulate flight hardware. The assembled beam was approximately 16.5 m long and 4.5 m on each of the four sides of its diamond-shaped cross section. The results show that average in-space assembly rates of approximately 38 seconds per strut can be expected for struts of comparable size. This result is virtually independent of the overall size of the structure being assembled. The mobile work station concept would improve astronaut efficiency for on-orbit manual assembly of truss structures, and also this assembly-line method is highly competitive with other construction methods being considered for large space structures

Trophic Structure of Macrobenthic Communities in Northern Gulf of Mexico Estuaries

Trophic structure of estuarine benthic communities in the northern Gulf of Mexico was characterized according to the functional roles and geographic distributions of the macrobenthos. Macrobenthic organisms collected during two years of study were assigned to trophic groups to assess the relative utilization of detritus and other resources. Three groups of detritivores (surface-deposit feeders, subsurface-deposit feeders, and filter feeders) were numerically dominant among the benthos, each of which accounted for 25-30% of total abundance across regions. Carnivorous macrobenthos also comprised an appreciable portion (12%), while omnivores (\u3c3%) and other groups (\u3c4%) were poorly represented. Dominance by detritivores is consistent with current concepts regarding the role of macrobenthos in processing detritus of Gulf of Mexico estuaries