Factors limiting rosette recruitment in scarlet gilia, Ipomopsis aggregata: seed and disturbance limitation

We performed a factorial combination of seed additions, surface soil disturbance, and protection from potential seed predation in experimental plots to elucidate the relative importance of each factor in the recruitment of scarlet gilia seedlings. We detected a significant interaction between seed addition and surface soil disturbance on seedling recruitment over 2 years of establishment. Plots that received both supplemental seed addition and disturbance established 10.5 times as many rosettes as control plots. We did not detect main or interactive effects of caging on seedling establishment. We explored the importance of density-dependent recruitment by investigating the number of rosettes per experimental plot through time. In addition, we used planned contrasts to compare the absolute and percent changes in control and treatment plot rosette densities between the study years. We found significant downward concave curvature to the seedling recruitment curve, suggesting negative density-dependent recruitment across the experimental range of rosette densities. We found a significant difference between the control plots and the seed addition-disturbance plots in terms of absolute changes in plot density from 1997 to 1998. Greater net mortality occurred in plots that received both seed addition and disturbance. However, we found no differences among treatments in the percent change in rosette density across the study years. Our study demonstrates that seedling recruitment in natural plant populations may be limited by the interaction of seed and microsite availability, and that seedling recruitment in scarlet gilia may be negatively density dependent. Our results suggest that the fecundity effects of particular plant-animal interactions (e.g., pollination, herbivory) may affect scarlet gilia population dynamics, particularly under conditions of high soil disturbance. Received: 11 June 1999 / Accepted: 24 November 1999     

Reef-based micropredators reduce the growth of post-settlement damselfish in captivity

Despite the ubiquity of micropredators and parasites on coral reefs, their effects on the survival and growth of juvenile fishes are virtually unstudied. Caging and laboratory experiments were used to investigate whether reef based micropredators fed on recently metamorphosed damselfish, the time of day that micropredation occurred, and whether micropredation affected fish growth and survival. Caged juveniles of the damselfish, Pomacentrus moluccensis, were held on the reef over four consecutive time periods. Micropredators (gnathiid and cirolanid isopods) were found associated with caged fish at night only, and cirolanids were observed attacking and killing some caged fish. In order to test the effect of micropredation on growth and survival without the influence of predatory fishes, groups of five P. moluccensis were caged for 2 weeks in one of three treatments: micropredators excluded, mesh control, or micropredators present. There were no significant differences in survival among the treatments, but fish were larger in cages with fewer survivors suggesting that competition for food was intense. Fish exposed to micropredators were larger than fish in the other two treatments, however, micropredator exclusion also excluded plankton; thus, differences in food availability among treatments during crepuscular periods likely confounded the treatment effect on fish growth. A laboratory growth experiment was performed to better control food availability and minimise handling stress, using a validated host-micropredator model. Individual juvenile damselfish, Dischistodus perspicillatus, were exposed to 0, 1 or 2 micropredators (Gnathia falcipenis) each evening and fed equally for 8 days. Mortalities only occurred in fish exposed to micropredators on the first evening of the experiment, and fish exposed to two micropredators each evening were significantly smaller than unexposed fish. These results suggest that repeated gnathiid infections can reduce fish growth in the first week after settlement. Consequently, micropredation may affect the ecology of damselfish after settling on coral reefs.     

How does shoreline development impact the recruitment patterns of coral reef fish juveniles (Moorea Island,French Polynesia)?

The study examined the effects of coastal embankment building on fish recruitment in three habitat types (beach-rock, white sand and muddy sand) in the near shore and fringing reef habitats of Moorea lagoon (French Polynesia). The results showed a positive relationship between the presence of embankments and the density and species richness of juvenile fish along the shoreline (whatever the habitat types). However, embankments deteriorated adjacent fringing reefs (decrease of live coral), which led to a decrease of fish density on beach-rock and white sand sites, and a decrease of fish species richness on muddy sand sites.     

Assessment of intramuscular activation patterns using ultrasound M-mode strain

The intramuscular activation pattern can be connected to the motor unit recruitment strategy of force generation and fatigue resistance. Electromyography has earlier been used in several studies to quantify the spatial inhomogeneity of the muscle activation. We applied ultrasound M-mode strain to study the activation pattern through the tissue deformation. Correlation values of the strain at different force levels were used to quantify the spatial changes in the activation. The assessment was done including the biceps brachii muscle of 8 healthy subjects performing isometric elbow flexion contractions ranging from 0% to 80% of maximum voluntary contraction. The obtained results were repeatable and demonstrated consistent changes of the correlation values during force regulation, in agreement with previously presented EMG-results. Both intra-subject and inter-subject activation patterns of strain were considered along and transverse the fiber direction. The results suggest that ultrasound M-mode strain can be used as a complementary method to study intramuscular activation patterns with high spatial resolution.     

Two levels of spacing and limits to local population density for settled larvae of the ascidian Clavelina moluccensis: a nearest-neighbour analysis

Among sessile marine invertebrates, the dispersion patterns of settling larvae are likely to strongly affect the nature and strength of subsequent intraspecific interactions. We use a recently developed graphical analysis technique to examine in situ spacing among settled larvae of the colonial ascidian Clavelina moluccensis. Larvae were mapped at settlement each day for 1 month at the time of their release from adults. Experimental plots measuring 90×90 mm were located on naturally occurring bare patches of wood on pier pilings which were extensively encrusted with sponges and ascidians. An inclusive one-tailed nearest-neighbour analysis applied at the time of the maximum number of individuals per plot revealed consistent fine-scale regular spacing between settled larvae. Furthermore, a two-tailed goodness-of-fit analysis indicated two modes of spacing: one at about 4.5 mm and the other at around 9 mm. No contact between larvae was observed at settlement and the spatial regularity is interpreted as a response of settling larvae to water-borne cues released from settled individuals. The two modes of spacing indicate the possibility of kin recognition as has been demonstrated for bryozoans. Water-borne cues may also limit local population density, as those plots with a high density of settlers showed a much greater decrease in settlement rate over time than low-density plots. Our findings are consistent with the hypothesis that intraspecific competitive interactions shape the patterns of dispersion of these invertebrates on hard substrata in the field.     

Fish reproduction and the impact of acidification in the Kyrönjoki River estuary in the Baltic Sea

Synopsis Several freshwater species use the Kyrönjoki River estuary as a spawning and nursery area. The main reasons for this seem to be the morphology of the estuary, the abundance of shelter provided by aquatic macrophytes, high food production and favourable temperature conditions. Acidification of the estuary due to drainage from acidic soils has made part of the estuary unsuitable for fish reproduction. In addition, year to year fluctuations in the acidity of the estuarine water have affected the reproductive success of several species. The severity of the effects of the acidification at the population level is determined by the spatial and temporal distribution of the larvae and juveniles.     

Development of a Marine Subtidal Epibiotic Community in Hong Kong: Implications for Deployment of Artificial Reefs

A 2-year study was conducted in Hong Kong to examine the effects of substratum, season and length of submersion on the development of a subtidal epibiotic community using four types of settlement panels (concrete, steel, wood and tyre). The season and length of submersion had a strong influence on the total biomass and on community structure while the type of substratum had very little impact on the total biomass or the structure of the epibiotic community. The season of submersion determined the species composition of the newly submerged surfaces. In the spring and summer, tubeworms were the most abundant. In the autumn and winter, barnacles and tunicates dominated. Community succession was not obvious in the first year of submersion as it was intermingled with strong seasonal settlement, growth and death of barnacles and tunicates. In the second year of submersion, green mussels and tunicates settled and grew to occupy most of the panel surfaces, forming an assemblage that was characteristic of climax communities in the local subtidal waters. The results suggest that the type of construction material has limited impact on the development of epibiotic communities on artificial reefs deployed in Hong Kong; the season of submersion may affect community structure in the early successional stage, but not the characteristics of the climax communities. This study indicates that the type of substratum should not be of concern when deploying artificial reefs in the subtidal waters in this region. The design of artificial reefs should focus more on other physical and economical aspects such as durability, flow dynamics, stability, cost, and effects on the ambient environment.     

Plasma membrane association of p63 Rho guanine nucleotide exchange factor (p63RhoGEF) is mediated by palmitoylation and is required for basal activity in cells

Activation of G protein-coupled receptors at the cell surface leads to the activation or inhibition of intracellular effector enzymes, which include various Rho guanine nucleotide exchange factors (RhoGEFs). RhoGEFs activate small molecular weight GTPases at the plasma membrane (PM). Many of the known G protein-coupled receptor-regulated RhoGEFs are found in the cytoplasm of unstimulated cells, and PM recruitment is a critical aspect of their regulation. In contrast, p63RhoGEF, a Gα(q)-regulated RhoGEF, appears to be constitutively localized to the PM. The objective of this study was to determine the molecular basis for the localization of p63RhoGEF and the impact of its subcellular localization on its regulation by Gα(q). Herein, we show that the pleckstrin homology domain of p63RhoGEF is not involved in its PM targeting. Instead, a conserved string of cysteines (Cys-23/25/26) at the N terminus of the enzyme is palmitoylated and required for membrane localization and full basal activity in cells. Conversion of these residues to serine relocates p63RhoGEF from the PM to the cytoplasm, diminishes its basal activity, and eliminates palmitoylation. The activity of palmitoylation-deficient p63RhoGEF can be rescued by targeting to the PM by fusion with tandem phospholipase C-δ1 pleckstrin homology domains or by co-expression with wild-type Gα(q) but not with palmitoylation-deficient Gα(q). Our data suggest that p63RhoGEF is regulated chiefly through allosteric control by Gα(q), as opposed to other known Gα-regulated RhoGEFs, which are instead sequestered in the cytoplasm, perhaps because of their high basal activity.     

Regulation of membrane cholecystokinin-2 receptor by agonists enables classification of partial agonists as biased agonists

Given the importance of G-protein-coupled receptors as pharmacological targets in medicine, efforts directed at understanding the molecular mechanism by which pharmacological compounds regulate their presence at the cell surface is of paramount importance. In this context, using confocal microscopy and bioluminescence resonance energy transfer, we have investigated internalization and intracellular trafficking of the cholecystokinin-2 receptor (CCK2R) in response to both natural and synthetic ligands with different pharmacological features. We found that CCK and gastrin, which are full agonists on CCK2R-induced inositol phosphate production, rapidly and abundantly stimulate internalization. Internalized CCK2R did not rapidly recycle to plasma membrane but instead was directed to late endosomes/lysosomes. CCK2R endocytosis involves clathrin-coated pits and dynamin and high affinity and prolonged binding of β-arrestin1 or -2. Partial agonists and antagonists on CCK2R-induced inositol phosphate formation and ERK1/2 phosphorylation did not stimulate CCK2R internalization or β-arrestin recruitment to the CCK2R but blocked full agonist-induced internalization and β-arrestin recruitment. The extreme C-terminal region of the CCK2R (and more precisely phosphorylatable residues Ser(437)-Xaa(438)-Thr(439)-Thr(440)-Xaa(441)-Ser(442)-Thr(443)) were critical for β-arrestin recruitment. However, this region and β-arrestins were dispensable for CCK2R internalization. In conclusion, this study allowed us to classify the human CCK2R as a member of class B G-protein-coupled receptors with regard to its endocytosis features and identified biased agonists of the CCK2R. These new important insights will allow us to investigate the role of internalized CCK2R·β-arrestin complexes in cancers expressing this receptor and to develop new diagnosis and therapeutic strategies targeting this receptor.     

Size-structured demographic models of coral populations

The demographic processes of growth, mortality, and the recruitment of young individuals, are the major organizing forces regulating communities in open systems. Here we present a size-structured (rather than age-structured) population model to examine the role of these different processes in space-limited open systems, taking coral reefs as an example. In this flux-diffusion model the growth rate of corals depends both on the available free-space (i.e. density-dependence) and on the particular size of the coral. In our analysis we progressively study several different forms of growth rate functions to disentangle the effects of free space and size-dependence on the model's stability. Unlike Roughgarden et al. [1985. Demographic theory for an open marine population space-limited recruitment. Ecology 66(1), 54-67], whose principal result is that the growth of settled organisms is destabilizing, we find that size-dependent growth rate often has the potential to endow stability. This is particularly true, if the growth rate is dependent on available free space (i.e. density dependent), but examples are given for growth rates that even lack this property. Further insights into reef system fragility are found through studying the sensitivity of the model steady state to changes in recruitment.