Ultrastructure of siliceous spicules and microsclerocytes in the marine sponge Neofibularia irata N. SP.

The marine sponge Neofibularia irata contains four different categories of siliceous spicules. These spicules are evident in the tissues as distinct bundles that act to increase the structural rigidity of the sponge. All spicules have a normal structural morphology with silica deposition around a hexagonal axial canal containing a crystalline axial filament. The megasclere strongyles are secreted in typical megasclerocytes. The sigma and raphid microscleres are secreted in individual microsclerocytes that are grouped together in parallel to form loose bundles. However, the microxea microscleres are apparently secreted in distinct tight bundles (trichodragmas) within a single cell. These cells, containing between 13 and 39 spicules, are grouped to form large packets of bundles of spicules.     

Analysis of the axial filament in spicules of the demosponge Geodia cydonium: different silicatein composition in microscleres (asters) and megascleres (oxeas and triaenes)

The skeleton of the siliceous sponges (Porifera: Hexactinellida and Demospongiae) is supported by spicules composed of bio-silica. In the axial canals of megascleres, harboring the axial filaments, three isoforms of the enzyme silicatein (-alpha, -beta and -gamma) have been identified until now, using the demosponges Tethya aurantium and Suberites domuncula. Here we describe the composition of the proteinaceous components of the axial filament from small spicules, the microscleres, in the demosponge Geodia cydonium that possesses megascleres and microscleres. The morphology of the different spicule types is described. Also in G. cydonium the synthesis of the spicules starts intracellularly and they are subsequently extruded to the extracellular space. In contrast to the composition of the silicateins in the megascleres (isoforms: -alpha, -beta and -gamma), the axial filaments of the microscleres contain only one form of silicatein, termed silicatein-alpha/beta, with a size of 25kDa. Silicatein-alpha/beta undergoes three phosphorylation steps. The gene encoding silicatein-alpha/beta was identified and found to comprise the same characteristic sites, described previously for silicateins-alpha or -beta. It is hypothesized, that the different composition of the axial filaments, with respect to silicateins, contributes to the morphology of the different types of spicules.     

The biology of the marine sponge Microciona prolifera (Ellis and Solander). III. Spicule secretion and the effect of temperature on spicule size

The secretion of siliceous spicules in the marine demosponge Microciona prolifera (Ellis and Solander) is by three different means. Styles are secreted by sclerocytes with archeocyte characteristics (nucleolate nucleus, phagosomes). chelas are formed by small sclerocytes with anucleolate nuclei, and toxas are apparently formed extracellularly within membranous material. Genetically and physiologically equivalent explants of this sponge were grown at 15, 20, and 25 C for four weeks. Analyses of spicule dimensions show little correlation of temperature with spicule length, except in the case of toxas. but a clear inverse relationship of spicule width with temperature. It is suggested that thicker spicules are formed at lower temperatures due to the more efficient entrapment of silicon rather than to effects upon silicon transport. Chela dimensions are very uniform implying an all or none process in their secretion. Differences in spicule dimensions between individual sponges grown at these temperatures may be due to the highly complex pathways of silicon transport and/or to genetic differences.     

Reassessment of homology of morphological characters in tetractinellid sponges based on molecular data

In sponges, as in other taxa with simple organization, the evaluation and use of morphological characters is difficult. Phylogenetic analysis of the first 850 nucleotides from the 5' end of the 28S rRNA gene is used here to assess the homology of spicules used in the classification of the subclass Tetractinellida. A single well-supported MP tree was obtained. The monophyly of the nine Tetractinellida species studied confirms the tetraxon megasclere as a morphological synapomorphy for the Tetractinellida. Two species are reallocated, Penares helleri as a Geodiidae, now thought to have lost sterraster microscleres, and Stryphnus mucronatus to the Streptosclerophorida. SEM micrographs of Stryphnus microscleres show that the morphology of the sanidasters is compatible with the hypothesis that they are homologous with streptoscleres and confirm this reallocation. Two other synapomorphies are confirmed within the tetractinellid clade, the simultaneous presence of tetraxon megasclere and aster-type microsclere (Astrophorida) and the loss of the streptosclere and persistence of the euaster s.s. microscleres (Euastrophorida) evidenced by the reallocation of Stryphnus mucronatus. The streptosclere microscleres cannot be evaluated in terms of homology because Streptosclerophorida may be paraphyletic (although these nodes are not supported by reliable bootstrap proportions) contrary to the currently accepted classification.     

Effects of germanium on the morphology of silica deposition in a freshwater sponge

The effect of germanium on the secretion of siliceous spicules by the freshwater sponge Spongilla lacustris was investigated by exposing germinating and hatching gemmules to varying concentrations of germanium (Ge) in the presence of silicon (Si). Results were analyzed quantitatively and qualitatively and demonstrate that a [Ge]/[Si] (= molar ratio) of 1.0 completely inhibits silicon deposition. Intermediate ratios (0.5, 0.1, 0.01) which are permissive to spicule appearance result in fewer, shorter, and thinner spicules, in proportionately fewer microscleres, and in short bulbous megascleres. The size of the bulb increases with increasing [Ge]/[Si], while the length of the bulbous megascleres decreases with increasing [Ge]/[Si]. Microscleres do not demonstrate these graded responses suggesting that they are secreted in an all or none manner. Swellings produced in pond water and bulbs produced in germanium appear to decrease in size with time indicating a spreading of the accumulated silica. The effect of germanium on spicule secretion can be partially explained by its ability to uncouple the growth in length of the axial filament from the growth of the surrounding silicalemma. Under these conditions excess silicalemma is produced in which silica accumulates as bulbs in short spicules. Continuous exposure to Ge is necessary to produce this altered morphology. It is conjectured that the bulbs may be retained due to an inhibition of spreading. which in turn may be caused by the incorporation of germanium into the silica.     

The greater seedling high-light tolerance of <Emphasis Type="Italic">Quercus robur</Emphasis> over <Emphasis Type="Italic">Fagus sylvatica </Emphasis>is linked to a greater physiological plasticity

The responses of Quercus robur (oak) and Fagus sylvatica (beech) seedlings to four different light environments (full, 50%, 40% and 15% sunlight) and to a rapid increase in irradiance were explored during the summer, after 2 years of growth in a forest nursery at Nancy (France). Significant differences between the two species were found for most variables. Phenotypic plasticity for morphological variables (root-shoot ratio, leaf size, leaf weight ratio) was higher in beech than in oak, while the reverse was true for anatomical (stomatal density, epidermis thickness, exchange surface area of the palisade parenchyma) and physiological (maximum photosynthetic rate, stomatal conductance, Rubisco activity) variables. Predawn photochemical efficiency (F_v/F_m) was higher in oak than in beech in all light environments except in 15% sunlight. F_v/F_m was significantly lower in 100% sunlight than in the other light environments in beech but not in oak. Maximum photosynthetic rates (A_max) increased with increasing light availability in the two species but they were always higher in oak than in beech. Oak exhibited higher Rubisco activity than beech in full sunlight. The transfer of shade-adapted seedlings to the open caused a decrease of F_v/F_m, which was larger for beech than for oak. Transferred oak but not beech plants recovered gradually to the control F_v/F_m values. The decreased chlorophyll content and the increased non-photochemical quenching observed in high-light beech seedlings were not enough to avoid photoinhibition. The results suggest that a greater tolerance of strong irradiance is linked to an enhanced physiological plasticity (variables related to photosynthesis), while shade tolerance relies on an enhanced plasticity in light-harvesting variables (crown morphology and chlorophyll content).     

Reduction of species in the wild potato Solanum section Petota series Longipedicellata: AFLP,RAPD and chloroplast SSR data

Species boundaries were assessed with three molecular markers [AFLPs, RAPDs and chloroplast simple sequence repeats (cpSSRs)] for all six species of wild potatoes (Solanum section Petota) assigned to ser. Longipedicellata: Solanum fendleri, S. hjertingii, S. matehualae, S. papita, S. polytrichon and S. stoloniferum. These tetraploid (2n = 4x = 48) species grow in the southeastern United States (S. fendleri) and Mexico (all six species), and a recent morphological analysis supported only three species: (1) S. polytrichon, (2) S. hjertingii (including S. matehualae) and (3) S. stoloniferum (including S. fendleri and S. papita). We analyzed all six species of ser. Longipedicellata (tetraploid) and also analyzed diploids in ser. Bulbocastana, ser. Pinnatisecta, ser. Polyadenia and ser. Tuberosa; tetraploids in ser. Acaulia and hexaploids in ser. Demissa. Concordant with morphological data, AFLP and RAPD results support the synonymy of S. hjertingii and S. matehualae, and completely intermix S. papita and S. fendleri. However, accessions of S. stoloniferum have a tendency to cluster but with exceptions, and S. polytrichon is completely intermixed with S. fendleri and S. papita. The cpSSRs fail to distinguish any of the species in ser. Longipedicellata. Combined morphological and molecular data support only two species in ser. Longipedicellata: S. hjertingii and S. stoloniferum.     

Epifaunal community structure in Acropora spp. (Scleractinia) on the Great Barrier Reef: implications of coral morphology and habitat complexity

The role of microhabitat in structuring epifaunal communities on four corals of varying morphology in the genus Acropora (A. millepora, A. hyacinthus, A. pulchra, A. formosa) was determined on two fringing reefs in the central Great Barrier Reef. Greater abundance and species richness of epifauna on tightly branched coral species in comparison to their rarity or absence on open-branched species suggests that protection afforded by complex habitats is important in structuring coral epifaunal communities. Within species, neither total colony space nor live surface area of corals was correlated with either the abundance or species richness of associated epifauna. However, space between branches significantly affected the size of Tetralia crabs associated with different coral species. Patterns in the size distribution of Tetralia on two species of Acropora suggest that crabs select coral hosts according to branch spacing, changing host species as they grow larger.     

Burning bridges: priority effects and the persistence of a competitively subordinate acacia-ant in Laikipia, Kenya

Theoretical models suggest that subordinate competitors may rely on strong colonization ability and/or high persistence (e.g., the ability to resist invasion) as a strategy to coexist with competitively dominant species. While strong colonization ability among subordinate competitors has been widely documented, we know less about the role of persistence in facilitating species coexistence. In upland East Africa, four species of acacia-ants (Crematogaster sjostedti, C. mimosae, C. nigriceps, Tetraponera penzigi) compete for possession of Acacia drepanolobium host trees. Despite a strong dominance hierarchy, the four acacia-ant species coexist at fine spatial scales. Here we present evidence that T. penzigi, the least aggressive competitor, modifies host trees in two ways that reduce the probability of aggressive takeover by neighboring colonies. First, T. penzigi workers destroy virtually all leaf nectaries on their host trees. Second, T. penzigi workers create and maintain entryways into their swollen thorn domiciles that are too small to allow entry by their Crematogaster competitors. In a 2Ƕ factorial experiment, we manipulated nectar availability and swollen-thorn entryway size to determine the influence of these factors on the probability of aggressive displacement by a dominant competitor (C. mimosae) in staged conflicts. Addition of artificial nectaries and enlargement of swollen-thorn entryways on T. penzigi-occupied trees increased the probability of aggressive displacement of T. penzigi by C. mimosae from swollen thorns 14-fold and 8-fold, respectively. Further, empty saplings with nectaries destroyed by T. penzigi workers were colonized by half as many C. mimosae workers as saplings where nectaries were left intact. Our results demonstrate that T. penzigi's unusual strategy of nectary destruction and the maintenance of small entryways in swollen thorns produce priority effects, effectively reducing the probability that T. penzigi colonies will be displaced from host trees by more dominant competitors.     

Predation risk and competition effects on the life-history characteristics of larval Oregon spotted frog and larval red-legged frog

We conducted an artificial pond experiment to test hypotheses about the effects of competition and non-lethal predator cues on metamorphic characteristics of sympatric Oregon spotted frogs (Rana pretiosa) and red-legged frogs (Rana aurora) in southwestern British Columbia. Tadpoles were exposed to the presence or absence of one another, two density levels and to the presence or absence of predacious odonate larvae (Aeshna palmata) isolated in enclosures. In the artificial pond study, R. aurora were significantly larger at metamorphosis (12%) and exhibited only slightly longer larval periods when exposed to Aeshna. In the presence of R. pretiosa, they significantly decreased time to metamorphosis, and were significantly larger at metamorphosis (12%) than those reared alone. Rana pretiosa in treatments with R. aurora were somewhat larger at metamorphosis when a non-lethal predator was present, and in treatments where R. pretiosa were alone with a predator tadpole mass at metamorphosis was smaller than those in the absence of Aeshna, but these results were not statistically significant. Both species reduced activity and moved away from the predator in the presence of an enclosed dragonfly larva in the laboratory. Most tadpole mesocosm experiments have found that the trade-off between size and timing of metamorphosis is extremely important to amphibians, but we suggest that the trade-off discussed in traditional amphibian models may not apply to species like R. pretiosa that are exposed to the same gape-limited predators upon reaching metamorphosis.     

Photosynthetic performance of Arctic macroalgae after transplantation from deep to shallow waters

Transplantation experiments conducted in the Arctic Kongsfjord (Spitsbergen) in summer 1997 investigated the effects of various types of filtered natural radiation (solar, solar without UV-B, solar without UV-A/B) on photosynthesis of various macroalgae. Two brown algal species (Laminaria solidungula, Saccorhiza dermatodea) and four red algal species (Palmaria palmata, Phycodrys rubens, Phyllophora truncata, Ptilota plumosa) were collected from deeper waters, kept in UV-transparent plexiglass tubes wrapped with different spectral cut-off filter foils and positioned at fixed depths in shallow waters for 7-9 days. At regular intervals, chlorophyll fluorescence of photosystem II (optimum quantum yield, F_v/F_m) was determined, as an indicator of photosynthetic performance. The data demonstrate that shallow-water species such as P. palmata are much less affected by natural photosynthetically active radiation (PAR) and UV radiation near the surface than extremely sensitive deep-water species such as Phyc. rubens which exhibited strong decreases in photosynthetic performance, as well as photobleaching of part of the thallus. The other species showed intermediate response patterns. In most species investigated inhibition of photosynthesis was mainly caused by the UV-B wavelengths. Interpretation of the data clearly indicates species-specific tolerances of photosynthesis to ambient solar radiation which can be explained by broad physiological acclimation potentials and/or genetic adaptation to certain (low or high) irradiances. The species-specific photosynthetic performance under radiation stress is in good accordance with the vertical distribution of the macroalgae on the shore.     

Soldier behaviour and division of labour in the aphid genus Pemphigus (Hemiptera, Aphididae)

Summary. We devised a test for quantitatively measuring the defensive behaviour of individual aphids of the gall generations of the genus Pemphigus. Five species (P. spyrothecae, P. gairi, P. bursarius, P. phenax, and P. protospirae) were shown to have soldiers, but the early instars of Pemphigus populi showed no aggressive behaviour. A higher proportion of the first instars of P. spyrothecae showed aggressive behaviour, compared with the other four soldier-producing species. Although the first instars were always the most likely to be aggressive, later instars also showed significant levels of aggression in all the soldier-producing species. Soldier behaviour was elicited by a range of stimuli. A live, restrained, predator was the most effective stimulus tested, but physical stimulation (with a paintbrush) enhanced the attack rate. The aphids could distinguish between lures soaked in water and those soaked in aphid haemolymph, which elicited a higher level of response. First instars of the early and late gall generations of P. spyrothecae were equally likely to attack predators. Although the species with the most specialized soldiers (P. spyrothecae) also had the most prolonged gall phase, this is clearly not the only factor that is important in the evolution of soldiers in this genus.     

Relationships between sexual dimorphism and niche partitioning within a clade of sea-snakes (Laticaudinae)

Previous studies in Fiji have shown that females of the amphibious sea-krait Laticauda colubrina are much larger than males, and have larger heads relative to body size. The dimorphism has been interpreted in terms of adaptation to a sex divergence in prey-size: females primarily eat large (conger) eels rather than smaller (moray) eels. The hypothesis that dimorphism is affected by niche divergence predicts that the degree of sex dimorphism will shift when such a species invades a habitat with a different range of potential prey sizes. On the island of Efate in Vanuatu, L. colubrina and a regionally endemic sibling species (L. frontalis) both consume smaller eels (in absolute terms, and relative to the snake's body size) than do the previously-studied Fijian snakes. Patterns of morphology and sexual dimorphism have shifted also. Both Vanuatu taxa are slender-bodied, and frontalis is smaller and less dimorphic than L. colubrina. Females grow larger than males in all taxa, and have larger heads (relative to body length), but the degree of sexual divergence is lower in Vanuatu (especially in frontalis). Dietary overlap (in prey species as well as size) is high between adult frontalis and juvenile colubrina, but the two taxa differ in prey size/predator size relationships. In particular, male frontalis eat very small prey and have very short heads. Our results are consistent with the hypothesis that sex differences in the mean adult body sizes and relative head sizes of laticaudine snakes are linked to sex differences in feeding biology.     

Observations on population structure and reproductive features of <Emphasis Type="Italic">Laetmonice producta</Emphasis> Grube (Polychaeta,Aphroditidae) in Antarctic waters

The large scale-worm Laetmonice producta Grube 1877 is the most abundant aphroditid polychaete in Antarctic coastal waters. We investigated the demographic structure and some reproductive features of different L. producta populations from high-Antarctic (Weddell Sea) and Antarctic Peninsula (King George Island) shelf bottoms, collected in summer 1996 (ANT-XIII/3, EASIZ I cruise) and autumn 2000 (ANT-XVII/3, EASIZ III cruise). L. producta in the studied geographic areas showed a wide bathymetric range (200-850 m depth), and a different size distribution pattern with depth, characterised by a reduction of large specimens in the deepest stations. The species is gonochoric, with females more abundant in specimens of larger sizes. Eggs at different stages of maturation (ranging from 40 to 320 wm in diameter) were examined in 270 individuals from different stations and size classes. Egg size showed a slightly bimodal trend, with largely overlapping egg cohorts, suggesting a continuous reproduction, and a long-lasting gametogenesis. Significant differences (Kolmogorov-Smirnov test, P<0.05) in egg-size frequency distribution were detected only when data of the two geographic areas were compared (Weddell Sea vs King George Island), and not according to stations within each area, and females' size. The two sets of geographic samples were collected in different seasons and therefore it was not possible to assess if differences observed are due to sampling time or to geographic factors. Mature spermatozoa were recognisable only in autumn male specimens from King George Island, and showed a rounded nucleus and a short conical acrosome. Occurrence of an endosymbiont polychaete, Veneriserva pygoclava meridionalis (new sub-species of Dorvilleidae), was recorded in the coelomic cavity of 163 specimens of L. producta, 125 of which were from the deepest station of the Weddell Sea (stn. 14, 850 m depth). L. producta females with and without the endosymbiont did not show differences in egg-size distribution. The reproductive features of L. producta, together with its large size and slow growth, seem typical of a long-living predator species, and uncoupled from the typical summer environmental conditions and the pulsating system of coastal Antarctic waters.     

An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers

The uniform stress hypothesis of stem formation was evaluated by comparing stem taper of Abies balsamea, Abies lasiocarpa, Picea rubens, Pinus contorta, Pinus elliottii, Pinus palustris, Pinus ponderosa, Pinus taeda, and Pseudotsuga menziesii to the taper expected if stems develop to uniformly distribute bending stress. The comparison was conducted by regressing stem diameter at height h (D_h) against bending moment at h (M_h) using the model D_h=J (M_h)^' where J and ' are fitted coefficients, and testing for '=0.333, the hypothesized value. Twelve curves were fitted with the model. Seven of the fitted values of ' were significantly different from 0.333, but eight of the values were within ᆞ% of 0.333 and eleven values were within ᆣ% of 0.333. Where the fitted value of ' was >15% of 0.333, residuals were biased with height. Fit by relative height, values of ' were within ᆞ% of 0.333 for large portions of these stems. While most of the fitted values of ' support the uniform-stress hypothesis, the values of ' for Pseudotsuga menziesii trees clearly did not. Many of the fitted values of J were inversely related to the modulus of elasticity (E) of green wood reported for these species. With the exception of Pseudotsuga menziesii, growing conditions appeared to account for extraordinary values of J. Increases in J with stem height corresponded with reported decreases in E with height. The covariance between J and E suggests some regulation of bending curvature by adjustments in cross-sectional area. These results suggest that stems taper to maintain a uniform bending curvature and that when E is relatively constant within and among stems, diameter along the stem or across stems can be predicted from bending moment using a simple power function.     

Effects of elevated CO<Subscript>2</Subscript> on foliar chemistry of saplings of nine species of tropical tree

This study examined the effects of elevated CO₂ on secondary metabolites for saplings of tropical trees. In the first experiment, nine species of trees were grown in the ground in open-top chambers in central Panama at ambient and elevated CO₂ (about twice ambient). On average, leaf phenolic contents were 48% higher under elevated CO₂. Biomass accumulation was not affected by CO₂, but starch, total non-structural carbohydrates and C/N ratios all increased. In a second experiment with Ficus, an early successional species, and Virola, a late successional species, treatments were enriched for both CO₂ and nutrients. For both species, nutrient fertilization increased plant growth and decreased leaf carbohydrates, C/N ratios and phenolic contents, as predicted by the carbon/nutrient balance hypothesis. Changes in leaf C/N levels were correlated with changes in phenolic contents for Virola (r=0.95, P<0.05), but not for Ficus. Thus, elevated CO₂, particularly under conditions of low soil fertility, significantly increased phenolic content as well as the C/N ratio of leaves. The magnitude of the changes is sufficient to negatively affect herbivore growth, survival and fecundity, which should have impacts on plant/herbivore interactions.     

Induced colonization of corals by a clionid bioeroding sponge

Colonization abilities of the bioeroding sponge Cliona orientalis were studied in a field experiment conducted at Orpheus Island, on the central Great Barrier Reef. Live grafts of sponge tissue were fixed onto nine coral species. The sponge was able to invade seven of these nine coral species: Porites australiensis, Porites cylindrica, Porites rus, Acropora formosa, Astreopora myriophtalma, Favites abdita and Montastrea curta. No sponge tissue was observed in Lobophyllia hemprichii and Pachyseris speciosa. While colonization of dead substrates can take place within a few weeks, invasion through live coral tissue occurred after 2-3 months. The frequency and area of sponge tissue in coral tissue were statistically independent of host coral species. The coral species affected sponge survival and health, presumably due to coral chemical defense. We ranked coral defense abilities against the sponge in the order: L. hemprichii > P. cylindrica = P. rus = F. abdita > A. formosa = M. curta (= P. speciosa) > A. myriophtalma = massive Porites. Overall, sponge fragments had a considerable capacity to survive on live coral and to recover from injury, handling and the initial stress caused by contact with corals (96% survived for 3 months). The ability of the sponge to resist coral defense on direct contact may offer it an alternative to sexual reproduction - by propagation through fragments - and may enable the sponge to invade various coral species laterally.     

Mesozooplankton community structure in the Southern Ocean upstream of the Prince Edward Islands

A meso-scale oceanographic grid survey was conducted during the first cruise of the Marion Offshore Ecosystem Variability Study in the upstream region of the Prince Edward Islands in austral autumn (April/May) 2001. Mesozooplankton samples, collected using a Bongo net (fitted with 200-µm and 300-µm mesh nets), were separated into three size fractions, 200-500 µm, 500-1,000 µm, 1,000-2,000 µm, by reverse filtration. Total surface (depth<5 m) chlorophyll-a concentration during the study ranged between 0.11 and 0.34 µg l^-1 and was always dominated by picophytoplankton (0.45-2.0 µm). Total mesozooplankton abundance and biomass during the survey ranged between 49 and 1,512 ind. m^-3 and between 0.7 and 25 mg Dwt. m^-3, respectively. Throughout the survey, the 200 to 500 µm class numerically dominated the mesozooplankton community, with an average of ~69% (SD=ᆠ.3%). The dominant species in the 200- to 500-µm size fraction were the copepods, Oithona similis, Calanus simillimus and Metridia lucens, and the pteropod, Limacina retroversa. However, in terms of biomass, the 1,000- to 2,000-µm group was predominant, with dry weight values constituting an average of ~66% (SD=ᆞ.2%). The most well-represented species in this group were the carnivorous Euphausia vallentini, Thysanoessa vicina, Sagitta gazellae and Eukrohnia hamata. Three distinct groupings of stations were identified by numerical analysis. The different station groupings identified reflect changes in the relative contributions of the dominant species, as opposed to the presence/absence of species.     

Pruning of host plant neighbours as defence against enemy ant invasions: <Emphasis Type="Italic">Crematogaster </Emphasis>ant partners of <Emphasis Type="Italic">Macaranga</Emphasis> protected by "wax barriers" prune less than their congeners

Many ant partners of tropical ant-plants prune the leaves and shoot tips of other plants growing around their hosts. According to the hypothesis proposed by Davidson et al. (Ecology 69:801-808), this specialized behaviour not only protects the host plants against overgrowth, but it also conveys a direct benefit to the ant colony as it removes contact points to the neighbouring vegetation where invasions of enemy ants could occur. Here we test this hypothesis by comparing pruning intensity in five closely related Crematogaster (subgenus Decacrema) plant-ant species (and one species of Technomyrmex) that differ in their exposure to competition by other ants. Pruning intensity was quantified by measuring the area loss of paper tape pieces wrapped around the stems of Macaranga host plants. All Crematogaster (Decacrema) ants tested but not Technomyrmex sp. pruned, but the intensity of the behaviour varied strongly between and within species. Pruning was significantly weaker in the three tested Crematogaster species inhabiting Macaranga host plants with a slippery, waxy stem surface, which functions as a mechanical barrier protecting the specific ant partners against generalist competitors. Pruning was generally stronger on more densely ant-populated trees. Even though the number of ants per twig length was lower in associations of ants with glaucous Macaranga hosts, only part of the variation of pruning activity could be explained by "ant density". When corrected for ant density, "wax-running" Crematogaster (Decacrema) ants still pruned more weakly than their congeners inhabiting non-glaucous Macaranga hosts. Pruning is obviously most important when an ant-plant is potentially accessible to intruders, but less necessary when the ant colony is isolated by a protective wax barrier. Our results support the hypothesis that "selfish" defence against invasions is the major selective pressure that has led to the development and maintenance of pruning behaviour in weakly competitive plant-ants.     

Silica spicules and axial filaments of the marine sponge Stelletta grubii (Porifera,Demospongiae)

Summary In all cases an organic axial filament within the silica spicules of Stelletta grubii forms the core of the major axes of the glass. In the small, star-shaped silica spicules (asters) the filament is shown for the first time to be radial with an enlarged center; in the large four-rayed spicules (triaenes) it is four-rayed; and in the large single-rayed spicules (oxeas) the filament is single-rayed. In situ, the filament is not dissolved by boiling nitric acid and thus is apparently protected by encasement within the glass which can also be stratified. The small silica asters are formed by single cells which resemble the so-called spherulous cells of other sponges. The very large size of triaenes and oxeas suggests that they may possibly be formed by more than one cell. The diameter of the filament in the much smaller asters is much narrower than the filament in the larger spicules, indicating a possible relationship between filament diameter and spicule diameter. While the axial filament in larger spicules frequently has a triangular cross-section it can also be hexaognal. Some aster filaments also retain a close to hexagonal cross-section. Filaments freed from large spicules by hydrofluoric acid display a complex morphology; possibly there is an internal silicified core. Some reported aspects of filament morphology are, however, probably artefacts of desilicification with hydrofluoric acid. Offprint requests to: T.L. Simpson, Department of Biology, University of Hartford, West Harford, Connecticut 06117, USA (Permanent affiliation)