Elevated levels of nitrogen and phosphorus reduce fertilisation success of gametes from scleractinian reef corals

Spawned gametes were collected from colonies of Acropora longicyathus at One Tree Island and Goniastrea aspera at Magnetic Island, Great Barrier Reef, Australia, for use in fertilisation trials. Mean fertilisation rates were significantly reduced compared with controls (P<0.003), when gametes from the branching coral A. longicyathus were exposed to elevated ammonium concentrations at 1 µM and above in one cross (60-64% reduction), and at 100 µM in another cross (16% reduction). Mean fertilisation success of A. longicyathus gametes was also significantly reduced compared with controls in both crosses (P=0.000) at concentrations of 1 µM phosphate and above (35-75% reduction), and at 1 µM ammonium plus 1 µM phosphate and all higher concentrations (68-74% reduction). Similarly, the mean percentage of regular embryos that were developing normally was significantly reduced in most nutrient treatments compared with controls (P=0.000). Fertilisation trials using gametes from the brain coral G. aspera resulted in a significantly lower percentage of regular embryos (P=0.001) and a significantly higher percentage of deformed embryos (P=0.001) developing after exposure to elevated nutrient treatments compared with controls. Mean fertilisation rates for this species were only significantly reduced (P=0.034) in the 50 µM ammonium plus phosphate treatment in one cross (8% reduction), compared with the control. Therefore, ammonium and phosphate enrichment significantly impairs fertilisation success and embryo development in scleractinian reef corals.     

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     

Environmental levels of sediment-associated tri-n-butyltin chloride (TBTCl) and ionic regulation in flounders during seawater adaptation

The effects of exposure to sediment-associated tri-n-butyltin chloride (TBTCl) were examined in the euryhaline European flounder, Platichthys flesus (L.). The effects were quantified by measuring the changes in sodium efflux; Na⁺/K⁺-ATPase activity; and the numbers, areas, and distribution of chloride cells in the gills of freshwater-adapted fish, following a rapid transfer to seawater. Following the transfer to seawater, the Na⁺/K⁺-ATPase activity and the sodium efflux were significantly increased in the control group but remained unchanged in the TBTCl-exposed group. The normal morphological changes to the gill epithelium associated with seawater adaptation, which involve chloride cell distribution, took place in the control group but were significantly inhibited or delayed in the TBTCl group. The results presented in this study lead to the conclusion that environmental concentrations of tri-n-butyltin chloride in sediments are capable of significantly disrupting both the physiological and the morphological components of iono-regulatory functions of an estuarine flatfish.     

Growth of zooxanthellae in culture with two nitrogen sources

Physiological characteristics of zooxanthellae were examined under nutrient-saturated conditions created by mixing ammonium (¹⁵NH₄) with nitrate (¹⁵NO₃) to give 0.88 mM total nitrogen. Growth rate varied with the form of nitrogen provided. Ammonium alone resulted in the lowest C:N and C:chl-a ratios. Although zooxanthellae took up nitrate in the absence of ammonium, ammonium assimilation was 1.3 times higher than nitrate assimilation. Ammonium strongly inhibited nitrate assimilation. While high-ammonium treatments resulted in the highest ¹⁴C incorporation into intermediate compounds, high nitrate levels resulted in the highest ¹⁴C incorporation into protein, suggesting that the intermediate compounds are produced prior to the subsequent production of protein when ammonium is the dominant N source. The enhanced production of intermediate compounds at the expense of carbon directed to protein synthesis in the presence of ammonium might be analogous to the “host factor” observed in zooxanthellae–host symbioses, since growth rate is depressed due to low production of protein. Received: 16 March 2000 / Accepted: 26 August 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     

Hepatopancreatic endosymbionts in coastal isopods (Crustacea: Isopoda), and their contribution to digestion

Three isopod species (Crustacea: Isopoda), commonly found in the intertidal and supratidal zones of the North American Pacific coast, were studied with respect to symbiotic microbiota in their midgut glands (hepatopancreas). Ligia pallasii (Oniscidea: Ligiidae) contained high numbers of microbial symbionts in its hepatopancreatic caeca. Numbers of endosymbionts were strongly reduced by ingestion of antibiotics. By contrast, the hepatopancreas of Idotea wosnesenskii (Valvifera: Idoteidae) and Gnorimosphaeroma oregonense (Sphaeromatidea: Sphaeromatidae) did not contain any microbiota. Results of feeding experiments suggest that microbial endosymbionts contribute to digestive processes in L. pallasii, the most terrestrial of the three isopods that we studied. The acquisition of digestion-enhancing endosymbionts may have been an important evolutionary step allowing isopods to colonize terrestrial habitats where relatively indigestible leaf litter is the primary food source. By contrast, the ability to digest phenolic compounds was most developed in one of the more marine species, suggesting that this trait may have evolved independently in isopod species that consume a phenolic-rich diet, whether in marine habitats or on land. Received: 28 August 2000 / Accepted: 8 December 2000     

Population structure of yellowfin tuna (Thunnus albacares) in the western Pacific Ocean, inferred from microsatellite loci

Five polymorphic microsatellite loci were examined in 1391 yellowfin tuna (Thunnus albacares) from eight regions of the western (Coral Sea, eastern Australia, Fiji, Indonesia, Philippines and Solomon Islands) and eastern (California and Mexico ) Pacific Ocean. Across all samples, numbers of alleles per locus ranged from 7 to 30 (mean: 17.0), and observed heterozygosities per locus ranged from 0.223 to 0.955 (mean: 0.593). Temporal collections were available for three areas: no significant temporal heterogeneity was observed for the Coral Sea (1991/1992 and 1995/1996 collections) or eastern Australia (1994/1995, 1995/1996, 1996/1997 and 1997/1998), but there was slight but significant heterogeneity at one locus (cmrTa-161) between the two Philippines collections (1994/1995 and 1996/1997). Genotypes generally showed a good fit to Hardy-Weinberg expectations within populations; only cmrTa-208 in the pooled Coral Sea population gave a significant deviation after Bonferroni correction for 40 tests, with a small but significant excess of homozygotes. Four loci showed no evidence of population differentiation following contingency Chi-squared and F_ST analyses. The fifth locus, cmrTa-161, showed small but significant differentiation (F_ST=0.002, P<0.001). This heterogeneity was largely a result of the Philippines 1994/1995 and Fiji collections; there was no correlation with geographic distance. The average F_ST across all five loci was very low (F_ST=0.002), but it was significant (P<0.001). It is unclear whether this low but significant differentiation reflects noise in the dataset, perhaps arising from experimental error, or real population differentiation. The finding of very limited population heterogeneity accords with most of the earlier allozyme and mitochondrial DNA studies of yellowfin tuna in the Pacific Ocean.     

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.     

Feeding, growth and reproduction of sea urchins (Strongylocentrotus droebachiensis) on single and mixed diets of kelp (Laminaria spp.) and the invasive alga Codium fragile ssp. tomentosoides

Since its introduction to Nova Scotia in the early 1990s, the invasive green alga Codium fragile ssp. tomentosoides has spread rapidly to become a dominant and persistent component of seaweed assemblages in the rocky subtidal zone. To examine the potential of sea urchins (Strongylocentrotus droebachiensis) to regulate Codium, and the potential of the alga to support urchin growth and reproduction, we fed urchins Codium and/or kelp (Laminaria spp., a high quality and preferred native food) in the laboratory for 11 months. Urchins showed a strong and active preference for kelp: they consumed more Laminaria than Codium (as dry weight) in single and mixed diet treatments. Urchins fed only Codium consumed 3.5 times more of the alga than those on a mixed diet, but did not increase their feeding rate in winter when kelp consumption was greatest. Laminaria was consumed at the same rate in single and mixed diets, indicating that the presence of an alternative food had no effect on kelp consumption. Survival and growth (change in test diameter) were lowest on the Codium diet, although the differences among diets were not statistically significant. Urchins on the Codium diet showed no gonadal production over the annual reproductive cycle, compared to a marked rise in gonad index on the Laminaria and mixed diets. Our results indicate that Codium is an unattractive, unpalatable and poor quality food, which is not readily consumed by urchins. Thus we predict that urchins at moderate densities will be much more likely to graze kelp than Codium in mixed stands, creating a mosaic of Codium patches and barren areas. At high densities, however, urchins are likely to destructively graze all seaweeds, although dense stands of the invasive alga may prolong the widespread formation of barrens.