Unprecedented bleaching-induced mortality in Porites spp. at Rangiroa Atoll, French Polynesia

In April-May 1998, mass coral bleaching was observed in the lagoon of Rangiroa Atoll, Tuamotu Archipelago, French Polynesia. Six months later, the extent of bleaching-induced coral mortality was assessed at three sites. Corals in the fast-growing genus Pocillopora had experienced >99% mortality. Many large colonies of the slow-growing genus Porites (mean horizontal cross-sectional area 5.8 m²) had also died - a phenomenon not previously observed in French Polynesia and virtually unprecedented world-wide. At one site, 25% of colonies, or 44% of the pre-bleaching cover of living Porites, experienced whole-colony mortality. At the two other sites, recently dead Porites accounted for 41% and 82% of the pre-bleaching live cover. Mortality in Porites was negatively correlated with depth between 1.5 and 5 m. Using a 50-year dataset of mean monthly sea surface temperature (SST), derived from ship- and satellite-borne instruments, we show that bleaching occurred during a period of exceptionally high summer SST. 1998 was the first year in which mean monthly SSTs exceeded the 1961-1990 upper 95% confidence limit (29.4°C) for a period of three consecutive months. We suggest that the sustained 3-month anomaly in local summer SST was a major cause of coral mortality, but do not discount the synergistic effect of solar radiation. Recovery of the size-frequency distribution of Porites colonies to pre-bleaching levels may take at least 100 years.     

Life on glass houses: sponge stalk communities in the deep sea

Photographs of the deep-sea floor often show organisms attached to biogenic structures that protrude from the soft bottom. In particular, the stalks of glass sponges (hexactinellids) provide hard substrata and act as habitat islands for deep-sea fauna. The primary objectives of this study were to determine the abundance of glass sponge “stalks” at an abyssal station in the NE Pacific, to identify the fauna associated with stalks, and to compare the distribution patterns of epifaunal taxa both horizontally and vertically. Densities of stalks and large epifauna were estimated from analysis of ∼9 km of photographic transects taken in 1994–1995 at station M (34°45′N; 123°00′W; 4,100 m depth) off California, USA. At least 87% of the stalks were the spicule columns of live or dead hexactinellids in the genus Hyalonema (Gray, 1832). Stalks appeared to be distributed randomly across the sea floor (density: 0.13 stalks m⁻²). A colonial zoanthid, Epizoanthus stellaris (Hertwig, 1888), inhabited 20% of the stalks and was the most commonly observed epifaunal organism, followed by other suspension feeders that generally were situated at the top of the structures. Thirty-five stalk communities were collected in tube cores in 1994–1995 using the submersible “Alvin”. A total of 139 taxa was associated with these hard-substratum habitats (another five species were observed only in photographs). Although taxon richness was high, the species diversity of these communities was relatively low due to the dominance in percentage abundance of a foraminiferan, Cibicides lobatulus (Walker and Jakob, 1798), and a serpulid polychaete, Bathyvermilia sp. (Zibrowius, 1973). The relationship between number of taxa and surface area of the stalks yielded a slope (z-value) typical of islands with a low rate of immigration. Three-dimensional complexity created by branching epifauna on the stalks provided more surface area and a variety of cryptic microhabitats. Vertical zonation on the stalks appeared to be controlled by biological interactions among species, with solitary fauna and certain functional groups of colonial organisms restricted by sheet-like colonial organisms that appeared to be dominant space competitors. Received: 13 April 2000 / Accepted: 9 November 2000     

Lignoid chemical defenses in the freshwater macrophyte Saururus cernuus

Summary. Chemical defense against herbivores has rarely been investigated for freshwater plants, possibly due to the common misconception that herbivory on aquatic macrophytes is low and would not select for chemical defenses. In previous work, the freshwater angiosperm Saururus cernuus was shown to be a low preference food for omnivorous crayfish despite its high nutrient value and relatively soft texture. We used feeding by the crayfish Procambarus clarkii to guide fractionation of the deterrent lipid-soluble extract of this plant, leading to the identification of seven deterrent lignoid metabolites, (–)-licarin A, (+)-saucernetin, (–)-dihydroguaiaretic acid, (–)-sauriols A and B, (–)-saucerneol, and (–)-saucerneol methyl ether. Lignans have been implicated in terrestrial plant chemical defenses as insect growth inhibitors, insect toxins, nematocides, antibacterial, and antifungal agents. However, these activities have rarely been demonstrated using ecologically relevant methodologies in terrestrial systems, and never before in freshwater systems. The widespread nature of lignans amongst very distantly related plants, along with their rich diversity of molecular structure, suggests that they could play a large role in mediating plant-herbivore interactions. In addition to the lignoid compounds we identified, there were other compounds present in low concentration or unstable compounds that were deterrent, that did not appear to be lignans, but that we were unable to identify. This plant thus appears to be defended by a complex mixture of natural products. Received 6 June 2000; revised 23 August 2000; accepted 2 September 2000     

Lipid metabolism of the Antarctic krill Euphausia superba and its ecological implications

 Various developmental stages (early larvae to adults) of Euphausia superba have been collected in different seasons in the Weddell Sea, the Lazarev Sea and off the Antarctic Peninsula to investigate the role of lipids and fatty acids in the life cycle of the Antarctic krill. The total-lipid data for E. superba exhibited seasonal variations, with low lipid levels in late winter/early spring and the highest levels in autumn. Seasonal changes were most pronounced in the immature and adult specimens, increasing from about 10% lipid of dry mass to more than 40%. The fatty-acid compositions of the younger stages were dominated by 20:5(n-3), 22:6(n-3) and 16:0. These are typical phospholipid fatty acids, which are major biomembrane constituents. The phospholipid composition was similar in the older stages. With increasing storage of triacylglycerols in the lipid-rich immature and adult stages, the fatty acids 14:0, 16:0 and 18:1(n-9) prevailed, comprising about 70% of total triacylglycerol fatty acids. The trophic-marker fatty acids 16:1(n-7) and 18:4(n-3), indicating phytoplankton ingestion, were less abundant. They reflected, however, the dependence of the larvae on phytoplankton as well as the seasonal changes in algal composition. The generally close linear relationships between fatty acids and lipid suggest that the fatty-acid compositions of the collected specimens were largely independent of the respective developmental stage, season and region. The linear fit indicates that triacylglycerol accumulation started at a level of about 5% of total lipid. Considering the various overwintering scenarios under discussion, the life cycle and reproductive strategies of krill are discussed in the context of the lipid metabolism and fatty-acid composition of E. superba. Lipid production is effective enough to accumulate large energy reserves for the dark season, but E. superba does not exhibit the sophisticated biosynthetic pathways known from other Antarctic euphausiids and copepods. Although important, lipid utilisation appears to be just one of several strategies of E. superba to thrive under the extreme Antarctic conditions, and this pronounced versatility may explain the success of this species in the Southern Ocean. Received: 16 June 2000 / Accepted: 18 December 2000     

Chromosomal evolution in Mugilidae: karyotype characterization of Liza saliens and comparative localization of major and minor ribosomal genes in the six Mediterranean mullets

This study continues a comparative cytogenetic analysis of the fish family Mugilidae, reporting the karyotype characterization of the leaping mullet, Liza saliens, by C-banding, Ag- and fluorochrome-staining, and completing the fluorescence in situ hybridization (FISH)-mapping of the 18S and 5S rRNA genes (rDNA) to the chromosomes of the six Mediterranean mullets, namely L. saliens, L. ramada, L. aurata, Mugil cephalus, Chelon labrosus and Oedalechilus labeo. In all species, except M. cephalus, the 5S rDNA sites were localized on a medium-sized acrocentric chromosome pair, which was considered homeologous in all of them. In L. saliens, an additional 5S rDNA site was detected in a location close to the one shown by major ribosomal genes in M. cephalus, i.e. the subtelomeric region of chromosome pair 1. The 5S rDNA site in M. cephalus is located on the smallest chromosome pair of the complement, which, on the other hand, though on a different position, bears 18S rDNA in all the species of Liza and Chelon examined. The heterochromatin composition and the major and minor ribosomal gene locations suggest that the karyotype of L. saliens (subgenus Protomugil) can be considered intermediate between the karyotype of the more primitive M. cephalus and those of the other Liza (subgenus Liza) species and of the representatives of the more derived genera Chelon and Oedalechilus.     

Phylogenetic relationships within the Octocorallia (Cnidaria: Anthozoa) based on nuclear 18S rRNA sequences

We determined the nuclear 18S rRNA sequences for 41 species of octocorals and used these to address the validity of the historical ordinal divisions and the current subordinal divisions within the subclass Octocorallia. We also explored the phylogenetic affinities of the species Dendrobrachia paucispina, which was originally classified in the order Antipatharia (subclass Ceriantipatharia) although polyp structure indicates it belongs in the subclass Octocorallia. Trees constructed using maximum likelihood techniques are incongruent with the current and historical taxonomy of the Octocorallia. There appeared to be three major clades of octocorals. The first clade included most, but not all, pennatulaceans as a monophyletic group. The second clade contained 21 species, representing all major octocoral groups other than pennatulaceans. The third clade contained members from three suborders of the Alcyonacea and one member of the Pennatulacea. These data could not be used to distinguish the branching order of the three major clades. The species D. paucispina had a close affinity with the genera Corallium and Paragorgia (Alcyonacea: Scleraxonia), although its morphology suggests it is more similar to the genus Chrysogorgia (Alcyonacea: Calcaxonia). The morphological character of dimorphism (the presence of both autozooids and siphonozooids within a single colony) corresponded loosely with the topology of the most likely trees, and a single origin of dimorphism could not be rejected. Despite sampling from the majority of families within the Octocorallia, many of the relationships within this group remain ambiguous. Received: 16 June 2000 / Accepted: 14 September 2000     

Synergistic Interactions between Habitat Fragmentation and Fire in Evergreen Tropical Forests

Abstract: The growing prevalence of fragmentation and fire in tropical forests makes it imperative to quantify changes in these disturbances and to understand the ways in which they interact across the landscape. I used a multitemporal series of Landsat images to study the incidence and coincidence of fire and fragmentation in two areas of Pará state in the eastern Brazilian Amazon: Tailândia and Paragominase. In both areas, deforestation and forest fires were quantified for time series of 6–10 years. The Tailândia study area typifies a landscape with the herringbone pattern of government-settled colonists, and the Paragominas area is dominated by large cattle ranches. In both areas, over 90% of the forests affected by fire were associated with forest edges. Although most burned forest occurred within 500 m of forest edges, some fires occurred in deep forest, several kilometers from any edge. The obvious synergism between forest fragmentation and fire poses serious risks to tropical ecosystems and has important implications for land management.     

Ectomised conductors in the golden orb-web spider, Nephila plumipes (Araneoidea): a male adaptation to sexual conflict?

Male genitalia may facilitate sperm protection by acting as a plug that prevents or hinders future matings. The pedipalps (intromittant organs) of males of the orb-web spider, Nephila plumipes, have a conductor with a peculiarly curved ending and a triangular process near the terminal end. The tip of the conductor, including the process, breaks during most matings and remains inside the female genital tract. We explored the possible function of the conductor as a mating plug using the double-mating sterile-male technique. Our data are not consistent with a plug function because males use only one pedipalp in each mating, thus leaving an unobstructed insemination duct available for future matings; conductors of males mating with virgin females are not more likely to break than those of males mating with mated females, and second males show no preference for used or unused spermathecae. In addition, males that inserted their palp in the insemination duct that contained a tip of the conductor from a previous male obtained a share in the paternity of the female's clutch of eggs. Interestingly, the conductor is more likely to break if it is inserted in an unused spermatheca. We argue that several lines of evidence suggest that the conductor breaks as a result of intersexual conflict over the duration of copulation.     

Predicting the Risk of Extinction through Hybridization

Abstract: Natural hybridization threatens a substantial number of plant and animal species with extinction, but extinction risk has been difficult to evaluate in the absence of a quantitative assessment of risk factors. We investigated a number of ecological parameters likely to affect extinction risk, through an individual-based model simulating the life cycle of two hybridizing annual plant species. All parameters tested, ranging from population size to variance in pollen-tube growth rates, affected extinction risk. The sensitivity of each parameter varied dramatically across parameter sets, but, overall, the competitive ability, initial frequency, and selfing rate of the native taxon had the strongest effect on extinction. In addition, prezygotic reproductive barriers had a stronger influence on extinction rates than did postzygotic barriers. A stable hybrid zone was possible only when habitat differentiation was included in the model. When there was no habitat differentiation, either one of the parental species or the hybrids eventually displaced the other two taxa. The simulations demonstrated that hybridization is perhaps the most rapidly acting genetic threat to endangered species, with extinction often taking place in less than five generations. The simulation model was also applied to naturally hybridizing species pairs for which considerable genetic and ecological information is available. The predictions from these "worked examples" are in close agreement with observed outcomes and further suggest that an endemic cordgrass species is threatened by hybridization. These simulations provide guidance concerning the kinds of data required to evaluate extinction risk and possible conservation strategies.