Systematic and functional anatomy of seedlings in mangrove Rhizophoraceae: vivipary explained?

The hypocotyl of viviparous seedlings of mangrove Rhizophoraceae (tribe Rhizophoreae) has a common suite of anatomical features but a range of anatomical structures that may be diagnostic at the generic or even specific level. This information is summarized in the form of a dichotomous key. A common and previously unreported anatomical feature is the presence of tension wood fibres developed eccentrically in secondary xylem at the distal pole of seedlings grown in both artificial and natural conditions. These fibres are apparently the source of the mechanism whereby an initially horizontal seedling becomes erected rapidly once it is rooted distally, even though this appears to be achieved at a considerable mechanical disadvantage. The morphological result is a distinct 'hook' at the distal end of the established seedling. This mechanism allows rapid erection of a horizontally stranded seedling, such that the plumule is raised, snorkel-like, above immediate tidal influence. This may be necessary, since gas exchange through the surface of the seedling is not possible via the thick, cutinized and stomata-free epidermis of the hypocotyl. We suggest that the need to produce an elongated propagule, with this self-erecting capability favours the evolution of vivipary in these plants because it permits the earliest development of the plumule in air rather than water. The suitability of mangrove seedlings for further experimental and anatomical study is emphasized. The study also exemplifies how anatomy can be placed in a dynamic context.     

Defective Translation in RC<Superscript>−</Superscript> Cells

Mutation in the RC gene results in the formation of a species of β galactosidase with reduced specific activity and increased thermolability.     

Environmental diversity constrains learning in Drosophila melanogaster

1. Much is known about how enriched environmental diversity affects ability to learn across the months and years that are the developmental periods of large animals. 2. Less is known about how diversity impacts learning across the minutes and hours during which sensory environments of small foraging animals such as insects may vary dramatically. 3. This study shows that Drosophila melanogaster exposed to a diversity of odour–taste associations over a few minutes subsequently learn standard associative learning tasks poorly. 4. This effect is robust to variation in odours used in all parts of experiments. 5. Findings may have an impact on at least three major research areas in ecology: the relationship between biodiversity and ecosystem functioning; the evolution of floral constancy in pollinators; and the pest‐protective effects of mixed species crops.     

Land management affects grass biomass in the Eucalyptus tetrodonta savannas of monsoonal Australia

Abstract We surveyed herbaceous biomass across the range of Eucalyptus tetrodonta savannas in north‐western Australia. Sample sites (n = 211) were stratified within four broad geographical regions characterized by different mixes of land management regimes. Grasses dominated (87% mean) the herbaceous biomass. After controlling for climatic and edaphic gradients, herbaceous biomass was highest in the Greater Darwin region (2.2 t ha⁻¹) which is managed predominantly by Europeans, and least under semi‐traditional Aboriginal management in Arnhem Land region (1.1 t ha⁻¹). In the drier Gulf of Carpentaria and Kimberley regions, where a mix of Aboriginal, conservation and pastoral land uses occurs, fuel loads were higher than in Arnhem Land region but still considerably lower than around Darwin. Sarga was recorded in all regions except the Gulf of Carpentaria and had the highest biomass in Darwin (0.88 t ha⁻¹) and lowest biomass in the Kimberley (0.54 t ha⁻¹). The proportion of herbaceous biomass made up of perennial grasses was least in Darwin (17%) and greatest in the Gulf (77%) regions. We suggest that climate, soils and land management account for differences between the drier pastoral regions of the Gulf of Carpentaria and the Kimberley and the wet Greater Darwin region relative to the Arnhem Land region. The high frequency, and larger spatial scale, of fires in the Greater Darwin region relative to the Arnhem Land region underpins the contrasting trends in total herbaceous biomass and abundance of flammable annual grasses.     

Experimental confirmation of competition between two dominant species of Iridomyrmex (Hymenoptera: Formicidae)

The well-defined ant succession following disturbance of coastal heathland in eastern Australia includes community replacement triggered by one dominant species of Iridomyrmex (species C) being replaced by another (species A). This paper investigates the consequences of experimentally reducing the abundance of species A by injecting petrol into the nest entrances of colonies. The response of species C was monitored at 2 and 10 months after removal and shows highly significant increases in the number of pitfall traps occupied and the area of territory maintained by this species. It is confirmed that interspecific competition plays an important role in maintaining the observed mosaic pattern of ant distribution and is also important in ant succession. The mechanism of competition is interference or, more exactly, territorial competition. The species appear to actively defend territorial space by patrolling territory boundaries that are breached when the defenders are eliminated.     

Temporal variation in gametogenic cycles of vesicomyid clams

Summary

Sulfide-rich cold seeps in Monterey Canyon support dense communities of vesicomyid clams, which harbor chemosynthetic bacterial endosymbionts. Because these animals rely upon non-photosynthetic food sources, their life histories may be decoupled from seasonal phytoplankton productivity. We examined this hypothesis by investigating temporal changes in gametogenesis in two vesicomyid clam species. Clams were collected from two cold seeps (600 m and 900 m depths), and their gonads were removed and processed for histological analyses. Oocyte diameters and relative abundances of tissue types were measured in thin sections taken across the gonad. Mean oocyte diameters of Calyptogena kilmeri increased from August to November, 1994, but did not change by March, 1995. Relative proportions of ovarian tissue types (reproductive vs. somatic) also changed during this period, indicating reproductive seasonality. Analyses of Calyptogena pacifica tissues showed no significant change within a 3-month period. Existence of seasonal cycles for vesicomyid clams suggests that either (1) chemosynthetic production is seasonal or (2) reproductive output is tied to factors other than food production, such as synchrony of spawning to maximize fertilization success, or larval survival.     

Distributions of lizard species across edges delimiting open‐forest and sand‐mined areas

Abstract The distributions of lizards across habitat edges delimiting open‐forest and regenerating sand‐mined areas as a function of distance from the edge were studied at Tomago, New South Wales, Australia. Pitfall‐trapping was used to survey lizards across the northern edges of four forest fragments, to determine if lizards displayed characteristic responses across the edge, and whether these could be explained by the different habitat conditions. At each site, 11 equally spaced drift fences (each parallel to the edge) were arranged in a transect running perpendicular to the edge, and stretching 50 m into each habitat type. Captures of Amphibolurus muricatus (Agamidae) decreased substantially across the edge from the mine‐path to the forest so that it was identified as a mine‐path specialist lizard species. Captures of two skink species decreased across the mine‐path before reaching the edge, and were not caught (Ctenotus taeniolatus) or were seldom caught (Ctenotus robustus) in the forest, so they were identified as mine‐path specialist, edge avoiding, lizard species. Captures of Lampropholis delicata (Scincidae) increased across edges into the forest, consistent with the expectation for a forest specialist. Regression analyses indicated the responses to edges of three lizard species (A. muricatus, C. robustus and C. taeniolatus) were negatively correlated with canopy cover (probably due to its influence on temperature, as captures of A. muricatus and C. robustus were also correlated positively with mean daily temperature). In addition, the response of C. robustus correlated negatively with a vegetation factor (dense, even vegetation in the first 50 cm from ground level). The response of L. delicata correlated positively with understorey height. We have identified edge response strategies for four species of lizards across edges delimiting temperate open‐forest and mined areas, and identified habitat and microclimate variables that may have driven these responses.