The distribution of hydroid sibling species on hermit crabs in estuaries in the Gulf of Maine

Estuaries in the Gulf of Maine contain disjunct, isolated populations of a number of invertebrate taxa typically distributed only sound of Cape Cod. These estuarine populations may either be remnants of a southern fauna that were stranded by Quaternary changes in water temperature, or they may have been established more recently via larvae that entered the estuaries from the open ocean. We tested these hypotheses by examining the distribution of sibling species of hydroids that are symbiotic on paguriid hermit crabs. These crab-hydroid species pairs differ greatly in their ability to colonize new areas through the water column. The hermit crabs possess a planktonic larva that can disperse great distances, while the hydroids have a crawling planula larva that is extremely limited in its dispersal capabilities. Consequently, estuarine hermit crab populations that lack their native hydroid symbiont are likely to have originated by larval colonization. Hydroid sibling species were identified on the basis of MDH electromorphs. Species identifications were confirmed through a limited number of test matings with colonies of known species identity. In three out of the four estuaries that we examined, shells inhabited by the southern hermit crab,Pagurus longicarpus, were occupied only by the local northern hydroid,Hydractinia polyclina. This pattern is consistent with a more recent establishment of the hermit crab populations through larval dispersal or transport. However, in Casco Bay, shells inhabited byP. longicarpus were occupied by a mixture of the northern hydroid,H. polyclina, and the southern hydroid,H. symbiolongicarpus, that is associated withP. longicarpus throughout the southern part of its range. Thus, theP. longicarpus population in Casco Bay may either be a relict of a former, more broadly distributed population of this crab in the Gulf of Maine, or if recently established, is likely to have originated by the introduction of adult crabs with hydroid-covered shells.     

Mg tracer diffusion in synthetic forsterite and San Carlos olivine as a function of P,T and fO2

We present new experimental data on Mg tracer diffusion in oriented single crystals of forsterite (Fo₁₀₀) and San Carlos olivine (Fo₉₂) between 1000–1300° C. The activation energies of diffusion are found to be 400 (±60) kJ/mol (96 kcal/mol) and 275 (±25) kJ/mol (65 kcal/ mol) in forsterite and San Carlos olivine, respectively, along [001] at a fO₂ of 10^–12 bars. There is no change in activation energy of Mg tracer diffusion within this temperature range. Mg tracer diffusion in a nominally pure forsterite is found to be anisotropic (D_c > D_a > D _b) and a function of fO₂. This fO₂ dependence is different from that in olivine containing Fe as a major element, which suggests that the diffusion mechanism of Mg in forsterite is different from that in Fe-bearing olivine at least over some range of fO₂. The diffusion mechanism in nominally pure forsterites may involve impurities present below the limits of detection or alternately, Si or Fe³⁺ interstitial defects, Fe being present as impurity (ppm level) in forsterite. Pressure dependence of Mg tracer diffusivity in forsterite measured to 10 GPa in a multianvil apparatus yields an activation volume of approximately 1–3.5 cm³/ mol. It is found that presence of small amounts of hydrogen bearing species in the atmosphere during diffusion anneal (f_H2 0.2 bars, f_H20 0.24 bars) do not affect Mg tracer diffusion in forsterite within the resolution of our measurement at a total pressure of 1 bar. The observed diffusion process is shown to be extrinsic; hence extrapolation of the diffusion data to lower temperatures should not be plagued by uncertainties related to change of diffusion mechanism from intrinsic to extrinsic.     

Transformation kinetics of polycrystalline aragonite to calcite: new experimental data,modelling, and implications

Experimental data are used to model the transformation rate of polycrystalline aragonite (grain diameter 80 m) to calcite. Optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280° to 380°C and 0.10 MPa to an expression for a grain-boundary-nucleated and interface-controlled transformation. The nucleation rate is 4–5 orders of magnitude faster than for calcite nucleated within aragonite grains, and the growing in rate is slower below 300°C than for calcite growing in aragonite single crystals. The activation enthalpy for growth in polycrystalline aggregate is 247kJ/mol compared to 163 kJ/mol for growth in single crystals. Permanent deformation of the phases limits the elastic strain energy due to the 7% volume change and reduces the coherency of the calcite/aragonite interace. Theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures, and data from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation of the phases, the effective strain energy for nucleation is 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percent transformation for various P-T-t paths; without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235° C, and the kinetic data are also consistent with published P-T-t paths which include heating during uplift. The predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite, but strongly dependent on the effective strain energy.     

The effect of fluid pressure on oxygen isotope exchange between feldspar and water

The rate of oxygen isotope exchange between adularia and 2 M KCl solution has been measured at 650°C at pressures from 125 to 4000 bar. Isotropie diffusion coefficients calculated from these data show a positive dependence on the fluid pressure. This dependence is opposite to the predicted effect of hydrostatic pressure and is attributed to the activity of ‘water’ (H₂O, H⁺ or OH^?) in the feldspar.     

Volume and grain boundary diffusion of calcium in natural and hot-pressed calcite aggregates

 Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa in a stream of CO₂. Diffusion coefficients (D) were determined from ⁴²Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol and pre-exponential factor (D₀)=0.13 m²/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural (Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary diffusion width (δ) were determined from ⁴²Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa is described by the Arrhenius parameters D^′ ₀δ=1.5×10⁻⁹ m³/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks. Received: 19 December 1994/Accepted: 30 June 1995     

Dislocation-assisted diffusion of oxygen in albite

Oxygen diffusion in albite has been determined by the integrating (bulk ¹⁸O) method between 750° and 450° C, for a P _H2O of 2 kb. The original material has a low dislocation density (<10⁶ cm^?2), and its lattice diffusion coefficient (D ₁), given below, agrees well with previous determinations. A sample was deformed at high temperature and pressure to produce a uniform dislocation density of 5 × 10⁹ cm^?2. The diffusion coefficient (D _a) for this deformed material, given below, is about 0.5 and 0.7 orders of magnitude larger than D ₁ at 700° and 450° C, respectively. This enhancement is believed due to faster diffusion along the cores of dislocations. Assuming a dislocation core radius of 4 Å, the calculated pipe diffusion coefficient (D _p), given below, is about 5 orders of magnitude larger than D ₁. These results suggest that volume diffusion at metamorphic conditions may be only slightly enhanced by the presence of dislocations. $$\begin{gathered} D_1 = 9.8 \pm 6.9 \times 10^{ - 6} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 33.4 \pm 0.6(kcal/mole)/RT] \hfill \\ \end{gathered} $$ $$\begin{gathered} D_a = 7.6 \pm 4.0 \times 10^{ - 6} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 30.9 \pm 1.1(kcal/mole)/RT] \hfill \\ \end{gathered} $$ $$\begin{gathered} D_p \approx 1.2 \times 10^{ - 1} (cm^2 /\sec ) \hfill \\ {\text{ }} \cdot \exp [ - 29.8(kcal/mole)/RT]. \hfill \\ \end{gathered} $$      

Potassium and sodium self-diffusion in alkali feldspar

The rate of potassium self-diffusion in pure microcline was measured between 600° and 800° C using K⁴⁰ as a tracer. Transport of K⁴⁰ by processes other than volume diffusion was insignificant or minimal. Isotropic diffusion coefficients were calculated assuming spherical grains. The data are well fit by the Arrhenius relation and yield a pre-exponential factor (D₀) of 133.8 cm²/sec and an activation energy (Q) of 70 kcal/mole. Similar experiments on the self-diffusion of Na²² in a pure low-albite (exchanged microcline) yield D₀ of 2.31×10^–6 cm²/sec and Q of approximately 19 kcal/mole for the temperature interval from 200° to 600° C. The large difference in these activation energies suggests that the atomic mechanisms for sodium and potassium diffusion are different.     

Siphonostomatoid copepods infecting Squalus acutipinnis Regan, 1908 off South Africa

Squalus acutipinnis (previously included in S. megalops) individuals caught off the east, south and west coasts of South Africa were examined for siphonostomatoid copepod infection. Collected siphonostomatoids were fixed and preserved in 70% ethanol and studied under microscopes using both transmitted and incidental light. The copepods were identified and the host-parasite associations estimated by calculating their prevalence, mean intensity and mean abundance. The siphonostomatoids represented four families and six species, and included new host records for Nemesis sp., Lernaeopoda sp. and Eudactylina acanthii, and a new geographical record for E. acanthii. Squalus acutipinnis individuals were infected by relatively low loads of siphonostomatoids. The most commonly found species was Achtheinus pinguis, the only species found off all three coasts. There was no difference in the prevalence of A. pinguis in sharks sampled off the three coasts but the highest mean intensity was found on the East Coast followed by the West Coast. The overall prevalence of A. pinguis was 33.6%, and was an order of magnitude higher than that of the other siphonostomatoids recorded. At such low levels of infection it will thus be necessary to examine a large number of hosts to determine the biodiversity of siphonostomatoids and geographical differences in prevalence in South African waters.     

Hydrolytic weakening of experimentally deformed Westerly granite and Hale albite rock

In order to determine the effect of water on deformation in the brittle-ductile transition region of crustal rocks, experiments have been conducted on Westerly granite and a polycrystalline albite rock, comparing samples dried at 160°C for 12 h (‘dry’) and samples with about 0.2 wt% water added (‘wet’). The deformation mechanisms and style of deformation of the wet and dry samples, determined using optical and transmission electron microscopy, have been found to depend on temperature, pressure, strain rate, and strain. At 15 kb and 10⁻⁶, the added water reduces the temperature of the transition between microcracking and dislocation glide and climb by about 150–200°C for both quartz and feldspar. However, the penetration of ‘water’ into the grains is slow compared with the time of the experiments and many of the wet samples show evidence of initial microcracking and later dislocation creep. Wet samples deformed at 10 kb show less hydrolytic weakening than wet samples deformed at 15 kb. Because the deformation mechanism and strength of silicates depend so sensitively on trace amounts of water, and because the water content of experimental samples varies with temperature and pressure and thus with time, flow laws for any samples are only meaningful if the water content has been carefully controlled or characterized.