Allelopathy in the tropical alga Lobophora variegata (Phaeophyceae): mechanistic basis for a phase shift on mesophotic coral reefs?

Macroalgal phase shifts on Caribbean reefs have been reported with increasing frequency, and recent reports of these changes on mesophotic coral reefs have raised questions regarding the mechanistic processes behind algal population expansions to deeper depths. The brown alga Lobophora variegata is a dominant species on many shallow and deep coral reefs of the Caribbean and Pacific, and it increased in percent cover (>50%) up to 61 m on Bahamian reefs following the invasion of the lionfish Pterois volitans. We examined the physiological and ecological constraints contributing to the spread of Lobophora on Bahamian reefs across a mesophotic depth gradient from 30 to 61 m, pre‐ and post‐lionfish invasion. Results indicate that there were no physiological limitations to the depth distribution of Lobophora within this range prior to the lionfish invasion. Herbivory by acanthurids and scarids in algal recruitment plots at mesophotic depths was higher prior to the lionfish invasion, and Lobophora chemical defenses were ineffective against an omnivorous fish species. In contrast, Lobophora exhibited significant allelopathic activity against the coral Montastraea cavernosa and the sponge Agelas clathrodes in laboratory assays. These data indicate that when lionfish predation on herbivorous fish released Lobophora from grazing pressure at depth, Lobophora expanded its benthic cover to a depth of 61 m, where it replaced the dominant coral and sponge species. Our results suggest that this chemically defended alga may out‐compete these species in situ, and that mesophotic reefs may be further impacted in the near future as Lobophora continues to expand to its compensation point.     

Age and growth of early-life-stage Atlantic tarpon (Megalops atlanticus) from the northcentral Gulf of Mexico

Age and growth of early-life-stage Atlantic tarpon Megalops atlanticus collected from Mississippi coastal waters in the northcentral Gulf of Mexico (GOM) are described using otolith microstructure analysis. Tarpon leptocephali (n = 95, 16.0—27.8 mm standard length, L_S) collected from June throughOctober 2013—2018, ranged in age from 22 to 43 days (mean = 30.9 ± 0.5 days). Leptocephalus somatic growth rates ranged 0.46—1.24 mm day⁻¹ (mean = 0.76 ± 0.02 mm day⁻¹), and leptocephalus otolith growth rates ranged 1.78—3.97 μm day⁻¹ (mean = 2.58 ± 0.04 μm day⁻¹). Growth rates were inversely correlated to leptocephalus age, indicating the shrinkage phase associated with leptocephalus metamorphosis. Juvenile tarpon (n = 358, 50—359 mm fork length, L_F) were collected from August through December 2007—2018. Juveniles exhibited a positive allometric relationship (adjusted R² = 0.99, P < 0.001) between length and mass. The age of 100 juveniles (71—277 mm L_F) ranged from 76 to 174 days. Juvenile growth rate was estimated as 1.56 ± 0.11 mm day⁻¹. Significant (P < 0.001) linear relationships were found between juvenile age and otolith metrics, including otolith mass (R² = 0.81) and radius (R² = 0.68). Evaluation of the backcalculated hatch dates suggests that specimens in the collection hatched from late May through mid-September with slight peaks during July and August. A Rao's Spacing Test of Uniformity indicates the presence of significant lunar periodicity in leptocephalus hatch dates (n = 95, U = 250.1, P < 0.05), with 50% of the leptocephali hatched within 5 days (before or after) of the full moon. This study fills critical gaps in the scientific knowledge of tarpon and provides estimates of early-life-history metrics for an iconic game fish at the northernmost extent of its GOM range.     

Carbonate production by scleractinian corals at Aqaba,Gulf of Aqaba,Red Sea

Summary In a fringing reef at Aqaba at the northern end of the Gulf of Aqaba (29°26′N) growth rates, density, and the calcification rate ofPorites were investigated in order to establish calculations of gross carbonate production for the reefs in this area. Colony accretion ofPorites decreases with depth as a function of decreasing growth rates. The calcification rate ofPorites is highest in shallow water (0–5 m depth) with 0.9 g·cm⁻²·yr⁻¹ and falls down to 0.5 g·cm⁻²·yr⁻¹ below 30 m. Scleractinian coral gross production is calculated from potential productivity and coral coverage. It is mainly dependent on living coral cover and to a lesser extent on potential productivity. Total carbonate production on the reef ranged from 0 to 2.7 kg/m² per year, with a reef-wide average of 1.6 kg/m² perycar. Maximum gross carbonate production by corals at Aqaba occurs at the reef crest and in the middle fore-reef from 10 to 15 m water depth. Production is low in sandy reef parts. Below 30 m depth values still reach ca. 50% of shallow water values. Mean potential production of colonies and gross carbonate production of the whole reef community at Aqaba is lower than in tropical reefs. However, carbonate production is higher than in reef areas at the same latitude in the Pacific, indicating a northward shift of reef production in the Red Sea.     

Marine-Based Cultivation of <Emphasis Type="Italic">Diacarnus</Emphasis> Sponges and the Bacterial Community Composition of Wild and Maricultured Sponges and Their Larvae

Marine organisms including sponges (Porifera) contain many structurally diverse bioactive compounds, frequently in a low concentration that hampers their commercial production. Two solutions to this problem are: culturing sponge explants for harvesting the desired compound and cultivation of sponge-associated bacteria. These bacteria (often considered the source of the desired compounds) include the Actinobacteria, from which many novel drugs were developed. In a long-term experiment (lasting 767 days), we evaluated the culture amenability of the sponge Diacarnus erythraenus in a mariculture system, placed at 10- and 20-m depths. The growth and survival rates of sponge fragments were monitored. Wild and maricultured sponges from both depths and their larvae were sampled at different time intervals for denaturing gradient gel electrophoresis (DGGE) profiling of the bacterial community residing within them. 16S rRNA gene sequences of both cultured bacterial isolates and clone libraries of unculturable bacteria were composed and compared, focusing on Actinobacteria. Sponges from both depths did not differ significantly either in mean growth rates (percent weight change year⁻¹ ± S.E.) (64.5% ± 21% at 10 m and 79.3% ± 19.1% at 20 m) or in seasonal growth rates. Survival was also very similar (72% at 10 m and 70% at 20 m). There were 88 isolates identified from adults and 40 from their larvae. The isolates and clone libraries showed diverse bacterial communities. The DGGE profiles of wild and maricultured sponges differed only slightly, without a significant effect of depths or dates of sampling. This long-term experiment suggests that D. erythraenus probably remained healthy and indicates its mariculture suitability.     

Role of deep sponge grounds in the Mediterranean Sea: a case study in southern Italy

The Mediterranean spongofauna is relatively well-known for habitats shallower than 100 m, but, differently from oceanic basins, information upon diversity and functional role of sponge grounds inhabiting deep environments is much more fragmentary. Aims of this article are to characterize through ROV image analysis the population structure of the sponge assemblages found in two deep habitats of the Mediterranean Sea and to test their structuring role, mainly focusing on the demosponges Pachastrella monilifera Schmidt, 1868 and Poecillastra compressa (Bowerbank, 1866). In both study sites, the two target sponge species constitute a mixed assemblage. In the Amendolara Bank (Ionian Sea), where P. compressa is the most abundant species, sponges extend on a peculiar tabular bedrock between 120 and 180 m depth with an average total abundance of 7.3 ± 1.1 specimens m⁻² (approximately 230 gWW m⁻² of biomass). In contrast, the deeper assemblage of Bari Canyon (average total abundance 10.0 ± 0.7 specimens m⁻², approximately 315 gWW m⁻² of biomass), located in the southwestern Adriatic Sea between 380 and 500 m depth, is dominated by P. monilifera mixed with living colonies of the scleractinian Madrepora oculata Linnaeus, 1758, the latter showing a total biomass comparable to that of sponges (386 gWW m⁻²). Due to their erect growth habit, these sponges contribute to create complex three-dimensional habitats in otherwise homogenous environments exposed to high sedimentation rates and attract numerous species of mobile invertebrates (mainly echinoderms) and fish. Sponges themselves may represent a secondary substrate for a specialized associated fauna, such zoanthids. As demonstrated in oceanic environments sponge beds support also in the Mediterranean Sea locally rich biodiversity levels. Sponges emerge also as important elements of benthic–pelagic coupling in these deep habitats. In fact, while exploiting the suspended organic matter, about 20% of the Bari sponge assemblage is also severely affected by cidarid sea urchin grazing, responsible to cause visible damages to the sponge tissues (an average of 12.1 ± 1.8 gWW of individual biomass removed by grazing). Hence, in deep-sea ecosystems, not only the coral habitats, but also the grounds of massive sponges represent important biodiversity reservoirs and contribute to the trophic recycling of organic matter.     

Effect of temperature on cell growth and production of transparent exopolymer particles by the diatom Coscinodiscus granii isolated from marine mucilage

In the autumn of 2007, marine mucilage caused by the diatom Coscinodiscus granii occurred in the central area of Ariake Sound, Japan, and resulted in damage to fishery. To elucidate the mechanism underlying the outbreak of marine mucilage, we examined the effect of temperature on cell growth and production of transparent exopolymer particles (TEPs) in a culture of this species. Growth and TEP production of C. granii are influenced by temperature. The maximum growth rate (1.63 divisions day⁻¹) and cell yield (1,280 cells mL⁻¹) at all temperatures were obtained at 30°C. Optimal growth rates (>1.15 divisions day⁻¹: ca. 70% of maximum) and cell yield (>900 cells mL⁻¹: ca. 70% of maximum) were observed at temperatures of 25–30°C. TEP production by C. granii depended on whether volume- or cell-related values were considered. The maximum volume-normalized increase rates and concentrations of TEP at all temperatures were observed at 25°C. However, when production rates and concentrations of TEP were normalized to cell numbers, optimal values were measured at 10–15°C. In Ariake Sound, when marine mucilage caused by C. granii occurred, the temperature ranged from 25.0 to 25.4°C. This suggests that growth conditions of C. granii are important factors for production of marine mucilage.     

Sponge biomass and bioerosion rates increase under ocean warming and acidification

The combination of ocean warming and acidification as a result of increasing atmospheric carbon dioxide (CO₂) is considered to be a significant threat to calcifying organisms and their activities on coral reefs. How these global changes impact the important roles of decalcifying organisms (bioeroders) in the regulation of carbonate budgets, however, is less understood. To address this important question, the effects of a range of past, present and future CO₂ emission scenarios (temperature + acidification) on the excavating sponge Cliona orientalis Thiele, 1900 were explored over 12 weeks in early summer on the southern Great Barrier Reef. C. orientalis is a widely distributed bioeroder on many reefs, and hosts symbiotic dinoflagellates of the genus Symbiodinium. Our results showed that biomass production and bioerosion rates of C. orientalis were similar under a pre‐industrial scenario and a present day (control) scenario. Symbiodinium population density in the sponge tissue was the highest under the pre‐industrial scenario, and decreased towards the two future scenarios with sponge replicates under the ‘business‐as‐usual’ CO₂ emission scenario exhibiting strong bleaching. Despite these changes, biomass production and the ability of the sponge to erode coral carbonate materials both increased under the future scenarios. Our study suggests that C. orientalis will likely grow faster and have higher bioerosion rates in a high CO₂ future than at present, even with significant bleaching. Assuming that our findings hold for excavating sponges in general, increased sponge biomass coupled with accelerated bioerosion may push coral reefs towards net erosion and negative carbonate budgets in the future.     

Temperature × light interaction and tolerance of high water temperature in the planktonic freshwater flagellates Cryptomonas (Cryptophyceae) and Dinobryon (Chrysophyceae)

Using microcosm experiments, we investigated the interactive effects of temperature and light on specific growth rates of three species each of the phytoplanktonic genera Cryptomonas and Dinobryon. Several species of these genera play important roles in the food web of lakes and seem to be sensitive to high water temperature. We measured growth rates at three to four photon flux densities ranging from 10 to 240 μmol photon · m^?2 · s^?1 and at 4–5 temperatures ranging from 10°C to 28°C. The temperature × light interaction was generally strong, species specific, and also genus specific. Five of the six species studied tolerated 25°C when light availability was high; however, low light reduced tolerance of high temperatures. Growth rates of all six species were unaffected by temperature in the 10°C–15°C range at light levels ≤50 μmol photon · m^?2 · s^?1. At high light, growth rates of Cryptomonas spp. increased with temperature until the temperature optimum was reached and then declined. The Dinobryon species were less sensitive than Cryptomonas spp. to photon flux densities of 40 μmol photon · m^?2 · s^?1 and 200 μmol photon · m^?2 · s^?1 over the entire temperature range but did not grow under a combination of very low light (10 μmol photon · m^?2 · s^?1) and high temperature (≥20°C). Among the three Cryptomonas species, cell volume declined with temperature and the maximum temperature tolerated was negatively related to cell size. Since Cryptomonas is important food for microzooplankton, these trends may affect the pelagic carbon flow if lake warming continues.     

Invasive alga <Emphasis Type="Italic">Caulerpa racemosa</Emphasis> var. <Emphasis Type="Italic">cylindracea</Emphasis> makes a strong impact on the Mediterranean sponge <Emphasis Type="Italic">Sarcotragus spinosulus</Emphasis>

Here we present the first observation of the impact of the invasive Caulerpa racemosa var. cylindracea on native photophilic sponge species in the Adriatic Sea, with special focus on Sarcotragus spinosulus. Caulerpa racemosa var. cylindracea is able to completely overgrow the sponge, developing an exceptionally thick canopy with a maximum measured density of 1,887 m of stolons m⁻² and 40,561 fronds m⁻². Necrosis of the sponge surface was significantly correlated with the algal dry biomass, frond number and stolon length. Dense algal canopy, penetration of the algal stolon and rhizoids into the sponge oscula and covering of the ostiae probably diminishes the seawater circulation through the sponge and consequently results in its smothering and even death. We suggest that chemotropism is the reason why C. racemosa penetrates the sponge oscula and establishes such dense canopy on the sponge.     

Bioerosion rates of the sponge Cliona orientalis Thiele, 1900: spatial variation over short distances

We studied bioerosion rates and tissue growth of the sponge Cliona orientalis Thiele, 1900. Experimental blocks grafted with sponge tissue were deployed at three sites in Moreton Bay, QLD, Australia, which have different environmental conditions. Bioerosion rates varied between 4, 5, and 10 kg m⁻² year⁻¹ when related to final tissue area and between 4, 7, and 16 kg m⁻² year⁻¹ when related to initial tissue area of the graft, which supports findings of earlier studies. Comparing results between the sites, eutrophication appeared to have the most stimulating effect and is most likely to have caused the measured differences. However, slight differences between shading and current speeds may also have played a role. Variation may have masked spatial differences of sponge growth, which were insignificant between study sites. Growth and bioerosion nevertheless followed the same trend and were weakly correlated. Habitat quality itself had no influence. Overall, the twofold difference in sponge bioerosion over a distance as short as 10 km suggests that when estimating bioerosion rates, subsamples should be tested at different locations.     

Microbially mediated redox transformations of manganese (II) along with some other trace elements: a study from Antarctic lakes

The significance of freshwater systems in global manganese cycles is well appreciated. Yet, the polar systems, which encompass the largest freshwater repository in the world, have been least studied for their role in manganese cycling. Here, we present results from a study that was conducted in the brackish water lakes in the Larsemann Hills region (east Antarctica). The rate of in situ manganese oxidation ranged from 0.04 to 3.96 ppb day⁻¹. These lakes harbor numerous manganese-oxidizing bacteria (10⁵ to 10⁶ CFU l⁻¹), predominantly belonging to genera Shewanella, Pseudomonas and an unclassified genus in the family Oxalobacteriaceae. Experiments were conducted with representatives of predominant genera to understand their contribution to Mn cycling and also to assess their metabolic capabilities in the presence of this metal. In general, the total and respiring cell counts were stimulated to a maximum when the growth medium was amended with 10 mM manganese. The addition of manganese promoted the use of d-mannitol, maltose, etc., but inhibited the use of maltotriose, l-serine and glycyl l-glutamic acid. The bacterial isolates were able to catalyze both the redox reactions in manganese cycling. In vitro manganese oxidation rates ranged from 3 to 147 ppb day⁻¹, while manganese reduction rates ranged from 35 to 213 ppb day⁻¹. It was also observed that the maximum stimulation of manganese oxidation occurred in the presence of cobalt (81 ± 57 ppb day⁻¹), rather than iron (37 ± 16 ppb day⁻¹) and nickel (40 ± 47 ppb day⁻¹). Our studies suggest that cobalt could have a more profound role in manganese oxidation, while nickel promoted manganese reduction in polar aquatic systems.     

Seasonal variation of Sargassum ilicifolium (Phaeophyceae) growth on equatorial coral reefs

Temporal and spatial variations in Sargassum ilicifolium thallus density and length were investigated on equatorial coral reefs in Singapore from November 2011 to October 2012. Thalli density varied little throughout the year, however, we found strong seasonal patterns in thallus length and identified temperature as the significant driver. Sargassum ilicifolium reached maximum length in December (110.39 ± 2.37 cm) during periods of cooler water temperatures, and minimum length in May (9.88 ± 0.48 cm) during periods of warmer water temperatures. Significant spatial variation was also observed for both thallus density and length of S. ilicifolium among reefs. Within reefs, densities of S. ilicifolium were higher on reef flats (20.40 ± 0.40 individuals · 0.25 m^?2) compared to upper reef slopes (5.66 ± 0.23 individuals · 0.25 m^?2). Our findings highlight that marked seasonality in the growth of canopy‐forming macroalgae can occur within equatorial reef systems where temperature ranges are restricted (<3°C).     

Estimating the role of three mesopredatory fishes in coral reef food webs at Ningaloo Reef,Western Australia

Within the complex food webs that occur on coral reefs, mesopredatory fish consume small-bodied prey and transfer accumulated biomass to other trophic levels. We estimated biomass, growth and mortality rates of three common mesopredators from Ningaloo Reef in Western Australia to calculate their annual turnover rates and potential contribution to local trophic dynamics. Biomass estimates of the serranid Epinephelus rivulatus (4.46 ± 0.76 g m⁻²) were an order of magnitude greater than two smaller-bodied mesopredatory fishes, Pseudochromis fuscus (0.10 ± 0.03 g m⁻²) and Parapercis clathrata (0.23 ± 0.31 g m⁻²). Growth parameters generated from a von Bertalanffy growth function fitted to size-at-age data, however, indicated that mortality rates for the three mesopredators were similar and that 32–55 % of fish survived each year. Consequently, interspecific differences in annual turnover rates among E. rivulatus (1.9 g m⁻² yr⁻¹), Pa. clathrata (0.10 g m⁻² yr⁻¹) and Ps. fuscus (0.07 g m⁻² yr⁻¹) were an artefact of differences in local biomass estimates. The rapid turnover estimates for E. rivulatus suggest this species is an important conduit of energy within the isolated patch reef habitat where it is typically found, while Ps. fuscus and Pa. clathrata channel smaller amounts of energy from specific habitats in the Ningaloo lagoon. Apparent differences in habitat, diet and turnover rates of the three species examined provide an insight into the different roles these species play in coral reef food webs and suggest that life-history traits allow for variability in the local and spatial contribution of these species at Ningaloo Reef. Moreover, calculating turnover rates of a broader suite of fish species from a range of trophic groups will help better define the role of fishes in coral reef trophic dynamics.

     

Discovery of a “Living Dinosaur”: Globally unique modern hexactinellid sponge reefs off British Columbia, Canada

Summary Globally unique hexactinellid sponge reefs occur on the continental shelf off British Columbia, Canada. They cover about 425 km² of seafloor on the continental shelf off British Columbia (Canada) in water depths between 165 and 240 metres and occur on a low-angle deep shelf, iceberg scoured seafloor, characterized by very low sedimentation rates and very stable environmental conditions. The sponge bioherms are up to 19 metres high with steep flanks, whereas the biostromes are 2–10 metres thick and many kilometres wide. They all consist of dense populations of only seven hexactinellid species. Three of them, all hexactinosan species (Aphrocallistes vastus, Heterochone calyx, Farrea occa) are the main frambuilders, composing a true rigid framework of sponge skeletons encased in a organic rich matrix of modern clay baffled by the sponges. Growth rates of hexactinosan sponges range in the order of 0–7 centimetres per year. The base of the oldest sponge reefs date from approximately 9000 years b.p. Different invertebrate and fish faunas occupy the reefs than occur on adjacent seafloor areas and some species appear to use the sponge reef complex structures as refugia where they can hide. Sidescan sonar data and direct observation by manned submersible clearly show that large areas of sponge reefs have been heavily damaged by seafloor trawling in the past decade. These unique extant siliceous sponge reefs can be used as a modern analogue for a better understanding and interpretation of fossil siliceous sponge reefs, known from many ages and many locations world wide.     

Biomass production and nutrient uptake by <Emphasis Type="Italic">Neochloris oleoabundans</Emphasis> in an open trough system

The purpose of this paper is to present biomass and nutrient uptake data from Neochloris oleoabundans production in an open trough system. The growth medium used was BG11, temperature ranged from 16.7 °C to 25.3 °C, and pH ranged from 5.52 to 9.94 because the customary pH increase during algal biomass production was moderated by incoming CO₂ gas streams (atmospheric, 2%, 4%, and 6% CO₂). Peak concentrations of algal biomass ranged from 643 to 970 mg L⁻¹, specific growth rates ranged from 0.15 to 0.37 day⁻¹, and doubling times ranged from 4.8 to 1.9 days. Carbon, nitrogen, and phosphorus were incorporated into the biomass at 0.05%, 8.3%, and 54% of supplied amounts. Open growth systems supplemented with CO₂ should be designed to regulate medium pH within the range of 6.3 to 7.1. Future research should examine various media and alternative carbon sources to decrease doubling times, increase peak concentrations, and optimize nutrient uptake.     

Growth of dinoflagellates, Ceratium furca and Ceratium fusus in Sagami Bay, Japan: The role of temperature, light intensity and photoperiod

Seasonal changes of field populations and growth rates of two dinoflagellates, Ceratium furca and Ceratium fusus, were examined in the temperate coastal water of Sagami Bay, Japan. Weekly field sampling was conducted from August 2002 to August 2003, and laboratory experiments were also carried out to investigate effects of temperature, irradiance and photoperiod on the growth rates of these two Ceratium species. In the field, the abundances of both species increased significantly from April to August 2003, were gradually decreased from November 2002 and were not observed in January 2003. C. fusus was able to increase at lower temperatures in February 2003 compared to C. furca. In the laboratory, the two species did not grow at <10 °C or >32 °C. The highest specific growth rate of C. furca was 0.72 d⁻¹ at 24 °C and 600 μmol m⁻² s⁻¹. Optimum growth rates (>0.4 d⁻¹) of C. furca were observed at temperatures from 18 to 28 °C and at irradiances from 216 to 796 μmol m⁻² s⁻¹. The highest growth rate of C. fusus was 0.56 d⁻¹ at 26 °C and 216 μmol m⁻² s⁻¹. Optimum growth rates of C. fusus were observed at the same irradiance rage of C. furca, whereas optimum temperature range was narrower (26–28 °C). The growth curves of both species indicated saturation of the growth rates when light intensity was above 216 μmol m⁻² s⁻¹, and did not show photoinhibition at irradiances up to 796 μmol m⁻² s⁻¹. The specific growth rates of both Ceratium species were clearly decreased at L:D = 10:14 relative to those at L:D = 14:10 and L:D = 12:12. The present study indicates the two Ceratium species can adapt to a wide range of temperature and irradiance.     

Fragments or propagules? Reproductive tradeoffs among Callyspongia spp. from Florida coral reefs

Fragmentation and propagule formation are alternative reproductive strategies found in both plants and animals, with the latter generally providing greater dispersal capability. When both strategies occur, life history theory predicts that resources should be divided between the two. On coral reefs, both strategies are exhibited by branching corals and sponges, which are broken‐up after storm events and rapidly recolonize. In this study, we compared two congeneric Caribbean reef sponges, Callyspongia armigera, which is branched and easily fragmented, and C. vaginalis, which is not, to test whether there is a tradeoff in growth and propagule formation for C. armigera relative to C. vaginalis. Both species were equally abundant on coral reefs off Key Largo, Florida (10.1 ± 3.7 vs 11.9 ± 3.0 per 100 m², respectively), suggesting that they are equally successful relative to two other non‐fragmenting congeneric species (C. fallax and C. plicifera) that are much less common. The number of substratum attachment points per sponge was significantly higher for C. armigera compared to C. vaginalis (2.31 ± 1.47 vs 1.03 ± 0.18 sponge^?1), providing further evidence of the reliance of C. armigera on fragmentation, and of C. vaginalis on recruitment from larval settlement and subsequent growth. Growth rates in predator‐exclusion experiments were ～4‐fold higher for C. armigera compared to C. vaginalis (0.36 ± 0.31 vs 0.08 ± 0.11 % initial mass day^?1), but C. armigera produced ～13‐fold fewer propagules than C. vaginalis (0.04 ± 0.22 vs 0.53 ± 1.08 % tissue area). Our results support a tradeoff between growth and propagule output for C. armigera relative to C. vaginalis, suggesting that these closely related sponge species took different evolutionary trajectories in reconciling their resource constraints.     

Strain-related differences in bacterivory and demography of Diaphanosoma mongolianum (Cladocera) in relation to diet and previous exposure to cyanobacteria in nature

We compared the demographic variables and bacterivory of two strains of Diaphanosoma mongolianum from two water bodies in Spain, one without Microcystis (Maidevera in Zaragoza) and the other with dense Microcystis (La Albufera of Valencia). We hypothesized that the strain rarely exposed to Microcystis would be unable to grow on this cyanobacterial diet. We fed both strains Monoraphidium caribeum and Microcystis aeruginosa, together and separately, and compared their demographic variables. Monoraphidium caribeum was cultured in the laboratory on a defined medium, while the cyanobacteria were collected from La Albufera and sonicated before feeding the cladocerans (at 0.5?×?10⁶ cells ml^?1). We also tested the growth of D. mongolianum on bacterial diets by using seston (0–15 µm), bacterioplankton (0–3 µm) and mixed fractions (3–15 µm), from sieving Lake Albufera. We conducted population growth and life table demography experiments at 25 °C, using the two strains of D. mongolianum. Both strains had r (population growth rate) ranging from 0.05 to 0.3 d^?1, on all diets. The r was higher (0.18 d^?1) on the 0–15 µm seston compared to the mixed fraction (0.12 d^?1) although D. mongolianum also grew well on bacterioplankton (0.16 d^?1) alone. The response of the strains collected from two different water bodies was different to the test diets. We found that both strains of D. mongolianum could effectively utilize Microcystis for survival and growth, regardless of previous exposure to the cyanobacteria. The tested cladocerans could also grow well on small sized food particles (0–3 µm and 0–15 µm). Our results explain why D. mongolianum is common in eutrophic water bodies

     

Cross-shelf variation in browsing intensity on the Great Barrier Reef

Herbivory is widely accepted as a key process determining the structure and resilience of coral reefs, with regional reductions in herbivores often being related to shifts from dominance by coral to leathery macroalgae. The removal of leathery macroalgae may therefore be viewed as a critical process on coral reefs. However, few studies have examined this process beyond a within-reef scale. Here, browsing activity was examined across the entire Great Barrier Reef shelf using bioassays of the leathery macroalga Sargassum to directly quantify algal removal. The assays revealed marked cross-shelf variation in browsing intensity, with the highest rates recorded on mid-shelf reefs (55.2–79.9% day⁻¹) and decreasing significantly on inner- (10.8–17.0% day⁻¹) and outer-shelf (10.1–10.4% day⁻¹) reefs. Surprisingly, the variation in browsing intensity was not directly related to estimates of macroalgal browser biomass; rather, it appears to be shaped primarily by the local environment and behaviour of the component species. Removal rates across the inner- and mid-shelf reefs appear to be related to the attractiveness of the assays relative to the resident algal communities. Controlling for the influence of the resident algal communities revealed a positive relationship between removal rates and the biomass of a single macroalgal browsing species, Naso unicornis. In contrast, the low removal rates on the outer-shelf reefs displayed no relationship to algal or herbivore communities and appeared to reflect a negative behavioural response by the resident fishes to a novel, or unfamiliar, alga. These findings not only highlight the complexities of the relationship between fish presence and ecological function, but also the value of examining ecological processes across broader spatial scales.     

Effects of light and temperature on the growth of Takayama helix (Dinophyceae): mixotrophy as a survival strategy against photoinhibition

Takayama helix is a mixotrophic dinoflagellate that can feed on diverse algal prey. We explored the effects of light intensity and water temperature, two important physical factors, on its autotrophic and mixotrophic growth rates when fed on Alexandrium minutum CCMP1888. Both the autotrophic and mixotrophic growth rates and ingestion rates of T. helix on A. minutum were significantly affected by photon flux density. Positive growth rates of T. helix at 6–58 μmol photons · m^?2 · s^?1 were observed in both the autotrophic (maximum rate = 0.2 · d^?1) and mixotrophic modes (0.4 · d^?1). Of course, it did not grow both autotrophically and mixotrophically in complete darkness. At ≥247 μmol photons · m^?2 · s^?1, the autotrophic growth rates were negative (i.e., photoinhibition), but mixotrophy turned these negative rates to positive. Both autotrophic and mixotrophic growth and ingestion rates were significantly affected by water temperature. Under both autotrophic and mixotrophic conditions, it grew at 15–28°C, but not at ≤10 or 30°C. Therefore, both light intensity and temperature are critical factors affecting the survival and growth of T. helix.