Heterozostera tasmanica (Martens ex aschers.) den Hartog in Chile

A 1.2 km² meadow of Heterozostera tasmanica (Martens ex Aschers.) den Hartog was found at Pto. Aldea, Bahia Tongoy, 60 km south of Coquimbo, 30°S on the coast of Chile. This provides documented evidence of seagrass occurrence on the west coast of South America. Since H. tasmanica is widespread throughout southern Australia, the Chilean stock represents a highly disjunct population. It appears from several growth parameters (density, leaf biomass, leaf area index, specific leaf area, δ ¹³C) that the H. tasmanica meadow at Pto. Aldea is very similar to those found in other parts of the world.     

Variation in fish density,assemblage composition and relative rates of predation among mangrove,seagrass and coral reef habitats

We tested the hypothesis for several Caribbean reef fish species that there is no difference in nursery function among mangrove, seagrass and shallow reef habitat as measured by: (a) patterns of juvenile and adult density, (b) assemblage composition, and (c) relative predation rates. Results indicated that although some mangrove and seagrass sites showed characteristics of nursery habitats, this pattern was weak. While almost half of our mangrove and seagrass sites appeared to hold higher proportions of juvenile fish (all species pooled) than did reef sites, this pattern was significant in only two cases. In addition, only four of the six most abundant and commercially important species (Haemulon flavolineatum, Haemulon sciurus, Lutjanus apodus, Lutjanus mahogoni, Scarus iserti, and Sparisoma aurofrenatum) showed patterns of higher proportions of juvenile fish in mangrove and/or seagrass habitat(s) relative to coral reefs, and were limited to four of nine sites. Faunal similarity between reef and either mangrove or seagrass habitats was low, suggesting little, if any exchange between them. Finally, although relative risk of predation was lower in mangrove/seagrass than in reef habitats, variance in rates was substantial suggesting that not all mangrove/seagrass habitats function equivalently. Specifically, relative risk varied between morning and afternoon, and between sites of similar habitat, yet varied little, in some cases, between habitats (mangrove/seagrass vs. coral reefs). Consequently, our results caution against generalizations that all mangrove and seagrass habitats have nursery function.     

Effects of in situ light reduction on density and growth of the seagrass Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Western Port,Victoria, Australia

Neutral density screens were used to reduce the level of irradiance available to an intertidal population of Heterozostera tasmanica (Martens ex Aschers.) den Hartog in Western Port, Victoria, Australia. When irradiance was reduced to 9 and 2% of control (ambient) levels, death of all leaf clusters of H. tasmanica resulted within 2 to 10 months. Reduction of irradiance to 35 and 25% of control levels resulted in a 25–50% decrease in leaf cluster density for the duration of the experiment (14 months). As irradiance level decreased leaf length increased (leaf length at 9% irradiance was twice leaf length in control areas) while leaf growth rate and leaf width remained the same. It is suggested that leaf growth rate per leaf cluster remains the same under reduced irradiance because of decreased likelihood of self-shading by surviving leaf clusters and increased surface area per leaf cluster. Density decreased more rapidly during summer than during winter at reduced light levels. This response may be due to an increase in the plant's light compensation point because of increased respiration at summer temperatures. Information on the lower limits of vertical distribution H. tasmanica in Western Port and Port Phillip Bay, Victoria together with the experimental irradiance reduction data suggests that H. tasmanica requires a minimum of ≈ 5% of surface irradiance for survival.     

The effect of sand and light on predation of juvenile plaice (Pleuronectes platessa) by fishes and crustaceans

Rates of predation on 0-group plaice, Pleuronectes platessa . in aquaria were compared under four different combinations of conditions to test the hypothesis that the presence of sand in which they may bury affords a refuge from predators. The effect of light and darkness on predation rate was also examined, Two crustaceans, the shrimp, Crangon crangon , and the portunid crab, Liocarcinus holatus , and two fishes, cod, Gadus morhua , and pollack, Pollachius pollachius , were used as predators. Predaton rates were significantly higher in the dark for all predators except pollack. Predation rates in the absence of sand were signifcantly greater only for pollack. The results suggest that predation rates on plaice during their juvenile nursery stage on sandy beaches will be significantly greater during darkness than during the day. Burying in sand appears to provide only a partial refuge from predation, perhaps because natural predators have evolved effective methods of foraging for buried prey.     

The rate of predation by fishes on hatchlings of the green turtle (Chelonia mydas)

This study addresses the need for empirical data on the survival of sea turtle hatchlings after entry into the sea by (1) developing a method for measuring marine predation; (2) estimating predation rates while crossing the reef; and (3) investigating the effect of environmental variables on predation rates. Predation rates were quantified by following individual hatchlings, tethered by a 10m monofilament nylon line, as they swam from the water's edge towards the reef crest. Predation rates under particular combinations of environmental variables (tide, time of day, and moon phase) were measured in separate trials. Predation rates varied among trials from 0 to 85% with a mean of 31% (SE=2.5%). The simplest logistic regression model that explained variation in predation contained tide and moon phase as predictor variables. The results suggest that noctural emergence from the nest is a behavioral adaptation to minimize exposure to the heat of the day rather than a predator-escape mechanism. For the green turtle populations breeding in eastern Australia, most first year mortality is caused by predation while crossing the reef within the first hour of entering the sea.     

The impact of predation on the viburnum whitefly, (Aleurotrachelus jelinekii)

Summary In association with a detailed study of the population dynamics of the viburnum whitefly, Aleurotrachelus jelinekii, observations were made on the predators including spiders and five species of insect, notably the mirid Campyloneura virgula and the neuropteran Conwentzia psociformis. Although the species richness of the predator guild was positively correlated with prey population density, at the individual level there was little evidence over twelve years of a density dependent relationship. In these populations natural enemies do not seem to play an important role in the dynamics.     

Susceptibility of Prussian carp infected by metacercariae of Posthodiplostomum cuticola (v. Nordmann, 1832) to fish predation

The susceptibility of prey fish infected by metacercariae of Posthodiplostomum cuticola (Digenea: Diplostomatidae) to the predation of non-host predators under experimental conditions was investigated. Parasitized young-of-the-year Prussian carp Carassius auratus were consumed significantly more often by perch Perca fluviatilis compared to non-parasitized individuals, independent of Prussian carp density. The proportion of parasitized and non-parasitized fish consumed by the predator remained stable at four different prey densities. The probability of predation did not increase with the intensity of parasite infection. The effect of P. cuticola on the host seems to result mostly from pathological changes (poor condition, black spots). However, our results provide evidence of higher probability of parasitized Prussian carp being consumed by fish predators under experimental conditions.     

Skull structure of the flying fish Cheilopogon pinnatibarbatus (Beloniformes, Exocoetidae)

The skull of the flying fish Cheilopogon pinnatibarbatus is first described in comparison with literature data on other four-winged flying fishes (subfamily Cypselurinae). The skull of Ch. pinnatibarbatus is characterized by the following features: (1) a multitude (about 40) of minute pores in the canal of the lateral line of the nasale; (2) a big opening in the roof of the myodome; (3) the lateral line canal of dentale by far does not reach the symphysis; (4) the tip of praeoperculum does not reach the upper edge of the hyomandibulare; (5) the posterior part of the angulare is weakly covered by the articulare and almost reaches from the outside the joint for articulation with the quadratum. Ppublished in Voprosy Ikhtiologii, 2007, Vol. 47, No. 3, pp. 284–291. This article was submitted by the authors in English.     

Colonization under threat of predation: avoidance of fish by an aquatic beetle, Tropisternus lateralis (Coleoptera: Hydrophilidae)

Documenting the role of past interactions in the assembly of present communities has proven problematic. Colonization is a key process in community assembly that is both potentially driven by past interactions and amenable to experimental approaches. Colonization and oviposition by an aquatic beetle (Tropisternus lateralis) was assayed in the presence and absence of both 'harmless' and tactilely/visually isolated predatory fish (Lepomis gibbosus and L. macrochirus). Beetles avoided each treatment with fish when compared to fish-free experimental pools. Activity levels after colonization also differed significantly between adults in fish and fish-free tanks. Predator effects on species composition are typically ascribed to contemporary predation events; the presence of a strong avoidance response demonstrates that past species interactions affect present distributions and may play an important role in the ongoing assembly of contemporary communities. Documentation of such avoidance behavior in a growing number of species fundamentally alters our view of the processes affecting species distributions and the process of community assembly.     

A comparison of mosquito predation by the fish Pseudomugil signifier Kner and Gambusia holbrooki (Girard) in laboratory trials.

Using larval Cx. annulirostris mosquitoes, this study compared the predation rate of Pseudomugil signifer and Gambusia holbrooki for four larval instars, three prey densities, and three vegetation densities. There was no significant difference in the quantity of first, second, and third instar larvae consumed by the two fish species after 24 h. There was a significant negative relationship between the predation rate and both larval instar and prey density for both fish species. In simulated vegetation trials, P. signifer performed marginally better than G. holbrooki in medium to high density vegetation (0.3 stems/cm 2 and 0.6 stems/cm2, respectively).     

Effect of nutrient availability, grazer assemblage and seagrass source population on the interaction between Thalassia testudinum (turtle grass) and its algal epiphytes

Seagrass leaves are often densely covered by epiphytic algae which can suppress seagrass productivity and has been implicated in declines of seagrass meadows worldwide. The net effect of epiphytes on seagrass growth and morphology depends on the independent and interactive effects of a variety of factors, including nutrient availability and the intensity of grazing on epiphytes. Here I report the results of a mesocosm experiment designed to test the effects of nutrient addition and within-functional group variation (grazer species composition and the source population of seagrass) on the strength of the interactions among grazers, epiphytes, and turtle grass (Thalassia testudinum). Turtle grass ramets from two sites in the northern Gulf of Mexico were cleared of epiphytes and transplanted into common-garden mesocosms. Replicate ramets were grown in a split-split plot design with two levels of dissolved nutrients and four different grazer species combinations (Tozeuma carolinense alone, Pagurus maclaughlinae alone, both species together, and no grazers present). As expected, grazers had a significant negative effect on epiphyte biomass/leaf area and a significant positive effect on turtle grass growth in the mesocosms. The two species were more similar in their direct effects on epiphyte biomass than in their indirect effects on turtle grass growth; this may reflect differences in epiphyte community composition under different grazer treatments. The effect of nutrient addition on turtle grass growth depended critically on the intensity of grazing: in the presence of grazers, turtle grass tended to produce a greater biomass of new leaf tissue in the tanks with nutrients added than in the control tanks. However, when grazers were absent, the direction of the effect was reversed, and plants with nutrients added grew less than the control plants. The two source populations of turtle grass differed significantly in epiphyte biomass/leaf area accrued in the mesocosms as well as in the strength of the effect of grazers on turtle grass growth. This suggests that population differentiation in seagrass interactions with epiphytes, as well as spatial and temporal variation in resources and grazer community composition, can greatly effect the role of epiphytes in limiting seagrass productivity.     

Population genetic structure of Earth's largest fish, the whale shark (Rhincodon typus)

Large pelagic vertebrates pose special conservation challenges because their movements generally exceed the boundaries of any single jurisdiction. To assess the population structure of whale sharks (Rhincodon typus), we sequenced complete mitochondrial DNA control regions from individuals collected across a global distribution. We observed 51 single site polymorphisms and 8 regions with indels comprising 44 haplotypes in 70 individuals, with high haplotype (h = 0.974 +/- 0.008) and nucleotide diversity (pi = 0.011 +/- 0.006). The control region has the largest length variation yet reported for an elasmobranch (1143-1332 bp). Phylogenetic analyses reveal no geographical clustering of lineages and the most common haplotype was distributed globally. The absence of population structure across the Indian and Pacific basins indicates that oceanic expanses and land barriers in Southeast Asia are not impediments to whale shark dispersal. We did, however, find significant haplotype frequency differences (AMOVA, Phi(ST) = 0.107, P < 0.001) principally between the Atlantic and Indo-Pacific populations. In contrast to other recent surveys of globally distributed sharks, we find much less population subdivision and no evidence for cryptic evolutionary partitions. Discovery of the mating and pupping areas of whale sharks is key to further population genetic studies. The global pattern of shared haplotypes in whale sharks provides a compelling argument for development of broad international approaches for management and conservation of Earth's largest fish.     

Short-term effects of inshore restoration measures on early stages,benthic species,and the sublittoral fish assemblage in a large river (Danube,Austria)

The rip-rap along an approx. 3-km-long shore in the main channel of the River Danube in Austria was completely removed to initiate the formation of a river bank through natural erosional and depositional processes. This study aimed to determine the short-term effect of this restoration measure on species composition and abundance of different developmental stages of fish at two spatial scales (micro- and mesohabitat). For this purpose, changes in abundance of early stages, as well as assemblage structure and species diversity of the benthic and the sub-littoral fish community were studied before and after restoration. No significant effects of restoration measures on the benthic fish assemblage were found. Significant and contradictory effects of the measures on the early stages and on the sub-littoral assemblage were observed. The abundance of fish larvae decreased after restoration, and this change could be attributed to the generally higher flow velocities in the new inshore microhabitats. Simultaneously, the species number and the abundance of fishes from the sub-littoral assemblage increased after restoration. This study has found that the main channel of the Danube still contain a high fish species diversity, and potentially plays a crucial role in recruitment of characteristic fluvial fishes.     

Fine structure of the thymus in the adult cling fish Sicyases sanguineus (Pisces, Gobiesocidae)

The structure of the thumus in adult specimens of a marine teleost, the cling fish Sicyases sanguineus, has been studied by light and transmission electron microscopy. Most cling fishes have an outer thymus located beneath the opercular epithelium. A few of them, however, have a large inner thymus besides a poorly developed outer thymus. In the well-developed outer thymus of cling fish there are three different zones: outer cortex, inner cortex, and medulla. The inner cortex is similar to the cortical region of the thumus in other vertebrates, whereas the outer cortex is a specialized lympho-epithelial zone containing cystic cells (also present in medullary region) and true Hassall's corpuscles. In accordance with the development of the thymic parenchyma, the medullary or basal region may appear either like a true thymic medulla or like a subcapsular region. In the inner thymus, a subcapsular or peripheral "medullary" region and a central area (inverted cortex) show structural features like those of the medullary (basal) and deep cortical regions of the outer thymus, respectively. In addition to the above regions, sometimes there is a lymphomyeloid perithymic infiltration that often extends along connective tissue septa into the perivascular spaces of the gland. Reticuloepithelial, mesenchymal, and unidentified types of stromal cells within the thymus are described. Some erythrocytes, granulocytes, and monocytoid cells are found, but no plasma cells nor erythropoietic foci are evident. The probable significance of these findings is discussed.