Variability in grazer-mediated defensive responses of green and red macroalgae on the south coast of South Africa

Variabilities in the responses of several South African red and green macroalgae to direct grazing and the responses of one green alga to cues from grazers were tested. We used two feeding experiments: (1) testing the induced responses of three red and one green algae to direct grazing by mesograzers and (2) a multi-treatment experiment, in which the direct and indirect effects of one macrograzer species on the green alga Codium platylobium were assessed. Consumption rates were assessed in feeding assays with intact algal pieces and with agar pellets containing non-polar extracts of the test algae. Defensive responses were induced for intact pieces of Galaxaura diessingiana, but were not induced in pellets, suggesting either morphological defence or chemical defence using polar compounds other than polyphenols. In contrast, exposure to grazing stimulated consumption of Gracilaria capensis and Hypnea spicifera by another grazing species. In the multi-treatment experiment, waterborne cues from both grazing and non-grazing snails induced defensive algal traits in C. platylobium. We suggest that inducible defences among macroalgae are not restricted to brown algae, but that both the responses of algae to grazers and of grazers to the defences of macroalgae are intrinsically variable and complex.     

Comparing macroalgal food and habitat choice in sympatric, tube-building amphipods, Ampithoe lacertosa and Peramphithoe humeralis

For small tube-building amphipods that live on the algae they consume, food and habitat are tightly linked. This study compared two closely related amphipods to determine whether the species’ algal preferences are based on the food value of the algae or on some other aspect of their algal habitat. Ampithoe lacertosa and Peramphithoe humeralis are both abundant on Shannon Point beach (Anacortes, Washington, USA; 48°30.542′ N, 122°41.070′ W) but specialize on different algae. In observations and laboratory experiments conducted July–September 1997, 2007, and 2008, the two species exhibited markedly different choices of food and habitat when offered six common macroalgae. Ampithoe lacertosa ate all algae offered, but preferentially built tubes on the green alga Ulva lactuca. Survival was relatively low among juveniles maintained on single species diets, except when they were fed Mazzaella splendens. Conversely, P. humeralis consumed primarily the brown kelp Saccharina latissima, Alaria marginata, and Desmarestia ligulata and preferred those species for tube building. Juvenile P. humeralis could not survive on a diet of U. lactuca or M. splendens. While A. lacertosa builds simple, temporary tubes and relocates frequently, P. humeralis is a highly thigmotactic species that builds long-term, complex tubes on the alga it prefers to eat. Feeding and habitat preferences of the two species were not clearly linked to nitrogen content of the algae, C:N ratio, or toughness of the algal tissue. Instead, preferences of the species may be related to their mobility and the permanence of the tubes they build. Ampithoe lacertosa and P. humeralis also use different feeding strategies; the former appears to mix algae to produce a high-quality diet, while the latter is more selective and has a capacity for compensatory feeding. The species are abundant on the same protected rocky shores, but specialize on different algae for habitat and food. Results suggest that the nutritional requirements of these sympatric mesograzers differ considerably and even closely related species can exhibit divergent behavioral strategies.     

Effects on larval crabs of exposure to algal toxin via ingestion of heterotrophic prey

We tested whether ingesting toxic algae by heterotrophic prey affected their nutritional value to crab larval predators, using toxic algal strains that are either ingested directly by larval crabs or rejected by them. Ingestion of toxic strains of the dinoflagellates Alexandrium andersoni and A. fundyense by the rotifer Brachionus plicatilis was confirmed. Rotifers having ingested either algal type for five days were fed to freshly hatched larvae of three crab species, with larval survival and stage durations determined. For both algal/rotifer treatments in all three crab species, larvae fed algae directly died during the first zoeal stage, while those fed rotifers that had been fed either algal strain survived to the experiment’s end (zoeal stage 3). Survival was lower, and stage duration longer, for larvae fed rotifers cultured on toxic algae when compared to those fed non-toxic algae. The role of toxic algae in the planktonic food web may be influenced by its direct or indirect ingestion by larval crabs.     

Selective consumption and facilitation by mesograzers in adult and colonizing macroalgal assemblages

Grazing pressure on macroalgae in littoral communities may vary with algal species, the age of an algal individual and grazer identity. Previous studies on alga–grazer interactions have shown that grazer preference for an algal species may release another one from interspecific competition. We measured the impacts of four common grazer taxa and the natural grazer guild on macroalgal communities at both their colonization and adult stages, and compared the impacts to grazer exclosures. The grazer effects were stronger on colonizing than on adult macroalgae; grazers did not reduce the total density of adult algae. Grazers both feed on propagules and indirectly facilitate other algae, depending on the grazer or algal species. Hydrobia species increased the settlement of spores of the red alga Ceramium tenuicorne. Similarly, the gastropod Theodoxus fluviatilis tended to facilitate one crustose algal species, but decreased the propagule density of annual filamentous algae, suggesting a preference for one species to the advantage of another. Effects of crustacean mesograzers on the studied macroalgae were weak. These results indicate that northern Baltic macroalgae are limited to grazing mainly during their colonization stage.     

Responses of common SE Australian herbivores to three suspected invasive<Emphasis Type="Italic"> Caulerpa</Emphasis> spp.

We sought to determine whether common intertidal and shallow subtidal zone grazers would consume extracts or fronds of three invasive Caulerpa spp., all of which are now resident in southern New South Wales, Australia. We examined the responses of herbivorous fishes, echinoderms and molluscs to C. filiformis. A subset of these organisms was tested with extracts of C. scalpelliformis and C. taxifolia. Polar (seawater) extracts of C. filiformis deterred a single herbivore, Aplysia sydneyensis, but confirmed that the biological activity reported from some Caulerpa spp. is not restricted to the lipophilic fractions. The large turbinid Turbo torquatus was deterred by an ethanol extract of C. filiformis, while the small congener T. undulatus demonstrated a significant preference for palatable agar discs containing ethanol extracts of C. filiformis. However, when T. undulatus were offered a choice of fronds from five algal species in the laboratory, they readily consumed Ulva spp. and Sargassum sp., showing the lowest preference for C. filiformis. Solvent extracts of C. scalpelliformis and C. taxifolia did not significantly deter any grazers. However, the overall trend was for reduced consumption of discs containing solvent extracts of these seaweeds. Indeed, for the large urchin Centrostephanus rodgersii and in the fish trials these effects were very near significant (P<0.06). We conclude that common herbivores associated with hard substrata are highly unlikely to intercede in the spread or control of these invasive algae.Communicated by M.S. Johnson, Crawley     

Nutrient enrichment of seagrass beds in a rhode island coastal lagoon

Seagrass and algal beds showed a variety of reponses when the water column was treated with low level additions of ammonium, nitrate and phosphate. The nutrients were added separately to 3 uniform seagrass beds of a temperature coastal lagoon during 1979 and 1980. (1) Ammonium caused the production of dense mats of free-floating green algae Enteromorpha plumosa and Ulva lactuca. It also stimulated growth in both the leaf and root-rhizome fractions of Zostera marina. This growth response in Z. marina was greater in the area where current reached 12 cm · s^-1 than in the area with little or no current. The concentration of nitrogen in the tissue did not change. In contrast, where current was lacking, Z. marina growth increase with ammonium was small, but the concentration of nitrogen in the tissue doubled over that in control plots. The growth of Ruppia maritima was inversely related to the growth of green algae in the same plots. The red alga Gracilaria tikvahiae did not grow better in ammonium, but its tissue reddened. (2) Nitrate additions enhanced the growth of the green seaweeds Enteromorpha spp. and U. lactuca, but not Z. marina or R. maritima. G. tikvahiae, when fertilized in isolation from other plants, showed a marginal response to this nutrient, and the tissue always reddened. (3) Phosphate enhanced growth in Z. marina and R. maritima exposed to moderate current. G. tikvahiae growing alone showed a small growth response to phosphate. The phosphate made no difference in the growth of the green seaweeds. (4) None of the nutrient supplements noticeably altered the species composition of either epiphytic or planktonic algae associated with the beds, although we did detect small increases in their numbers. The rapid and dense growth of green algae in nitrogen-enriched water probably limited growth of adjacent seagrasses and red algae. Because these seaweeds did not use the phosphate, it became available to other plant components. The overall floral response to nutrient addition in seagrass communities depends, therefore, upon the particular nutrient supplied, the ability of alternate species in the area to compete for that nutrient and the velocity of current in the specific area.     

Epifluorescence microscopy,a technique for the study of feeding in Crassostrea virginica veliger larvae

Epifluorescence microscopy was applied as a new technique to observe the response of Crassostrea virginica veliger larvae to the algal foods Monochrysis lutheri or Chlorella autotrophica. Uptake, lysis, and digestion or rejection of these algae by oyster larvae were observed microscopically through the autofluorescence of chlorophyll a and its derivatives. Both species of algae were taken up by large numbers of larvae within 5 min after being added to larval cultures. The good growth of larvae obtained with M. lutheri as a food source was related to rapid uptake, lysis, and digestion of this alga. Larvae fed with C. autotrophica did not grow. Althogh C. autotrophica was also taken up rapidly by the larvae, there was no lysis or digestion of these algae and cells were eventually voided.     

Ontogenetic change in the diet ofAplodactylus punctatus (Pisces: Aplodactylidae): an ecophysiological explanation

Aplodactylus punctatus is a temperate berbivorous fish that changes from an omnivorous to a herbivorous diet and increases its ability ot assimilate algae as it grows. To investigate whether this dietary shift is related to size-specific differences in energetic demands imposed by metabolism and the amount of assimilated energy, oxygen consumption ( ) was determined experimentally in 12 specimens ranging in size from 62 to 545 g. increased allometrically with body size from 8.41 to 55.95 mg O₂ individual^-1 h^-1. Individual energetic requirements were 2.8 to 33.7 kJ d^-1. The assimilated energy was estimated, taking into consideration: (1) the energetic value of the most important alga in the diet (Lessonia trabeculata); (2) size-specific differences in assimilation rates for fish fed on this alga; (3) size-specific differences in throughput time and in the amount of food in a full gut. Comparison of the energy required and the assimilated energy revealed that fishes of < 22 to 29 cm total length had a negative energetic balance when consuming algae exclusively. This may explain the reliance of smallA. punctatus on more easily-digested invertebrates. The largest individuals can meet their energetic demands by consuming algae alone, apparently because of their higher assimilation capability. InA. punctatus, changing energetic requirements and capacities for algal assimilation may be responsible for the observed ontogenetic change in diet.     

Growth and metamorphosis of Aplysia oculifera larvae in laboratory culture

Eggs of Aplysia oculifera (Adams and Reeve, 1850) were incubated in the laboratory. They hatched 8 to 9 d after spawning. Shell length (SL) of the hatched larvae was 102±2 m. Larvae were fed on the unicellular algae Isochrysis galbana in a concentration of 10⁴ cell ml^-1, and after 45 to 60 d grew to a maximum SL of 385±11 m. Larvae survived up to 330 d. A total of 12 species of algae from the natural habitat of A. oculifera were examined as metamorphosis inducers. Red algae Dasia sp., Jania sp., Hypnea sp. and Liagora sp. induced metamorphosis in 66.7±21.2, 28.3±17.7, 26.0±18.5 and 4.0±8.0% of the larvae, respectively. Green algae Enteromorpha intestinalis and Ulva sp. induced metamorphosis in 37.0±11.0 and 9.0±10.4% of the larvae, respectively. Cladophora sp. and Codium dichotomum, and the brown algae Padina pavonia, Colpomenia sinuosa, Hydroclathrus clathratus and Cystoseira sp. did not induce metamorphosis. There was no significant difference in the rate of metamorphosis between young (2 to 4 mo) and old (6 to 8 mo) larvae. Postmetamorphic juveniles grew and developed only when fed with E. intestinalis. They grew to a body length of>8 mm in 50 d. Postmetamorphic juveniles did not survive on other algae. The longevity of the planktonic A. oculifera larvae supports the hypothesis that the larvae can exist in the plankton and survive for several months until the next recruitment. The advantage of non-specificity in metamorphosis induction is discussed.     

Herbivory in the gammarid amphipod Aora typica: relationships between consumption rates, performance and abundance across ten seaweed species

This paper reveals a substantial capacity for herbivory of seaweeds in the gammarid amphipod Aora typica, adults eating seven of ten taxonomically and morphologically diverse seaweed species offered to them in a no-choice assay. The green algae Ulva spathulata and Enteromorpha intestinalis were consumed at the highest rates in both no-choice (2.3–2.5 mg blotted weight individual⁻¹ day⁻¹) and multiple-choice assays (0.5–1.3 mg blotted weight individual⁻¹ day⁻¹). Adult A. typica collected from two different species of brown seaweeds had very similar feeding preferences to each other. Juvenile A. typica grew to reproductive maturity on the green algae E. intestinalis and U. spathulata, and the brown algae Carpophyllum maschalocarpum and Ecklonia radiata. In common with previous studies on members of other amphipod families, survivorship of juvenile amphipods was positively correlated with feeding preferences of adults across seaweed species (r ²=0.43, P=0.04). However, densities of A. typica on seaweeds in the field (excluding the intertidal E. intestinalis and U. spathulata) were not significantly correlated with feeding preferences of adults (r ²=0.07, P=0.5) or survivorship of juveniles (r ²=0.17, P=0.31). This suggests that either host seaweeds are not a major dietary component of these amphipods in nature, or that the host’s value as a food source is overridden by other properties such as the degree of shelter it affords from larger consumers. This study provides the first demonstration that a member of the cosmopolitan amphipod family Aoridae is capable of consuming a diverse range of seaweeds.     

Trophic modification of essential fatty acids by heterotrophic protists and its effects on the fatty acid composition of the copepod Acartia tonsa

To test whether heterotrophic protists modify precursors of long chain n−3 polyunsaturated fatty acids (LCn−3PUFAs) present in the algae they eat, two algae with different fatty acid contents (Rhodomonas salina and Dunaliella tertiolecta) were fed to the heterotrophic protists Oxyrrhis marina Dujardin and Gyrodinium dominans Hulbert. These experiments were conducted in August 2004. Both predators and prey were analyzed for fatty acid composition. To further test the effects of trophic upgrading, the calanoid copepod Acartia tonsa Dana was fed R. salina, D. tertiolecta, or O. marina that had been growing on D. tertiolecta (OM-DT) in March 2005. Our results show that trophic upgrading was species-specific. The presence of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the heterotrophic protists despite the lack of these fatty acids in the algal prey suggests that protists have the ability to elongate and desaturate 18:3 (n−3), a precursor of LCn−3PUFAs, to EPA and/or DHA. A lower content of these fatty acids was detected in protists that were fed good-quality algae. Feeding experiments with A. tonsa showed that copepods fed D. tertiolecta had a significantly lower content of EPA and DHA than those fed OM-DT. The concentration of EPA was low on both diets, while DHA content was highest in A. tonsa fed R. salina and OM-DT. These results suggest that O. marina was able to trophically upgrade the nutritional quality of the poor-quality alga, and efficiently supplied DHA to the next trophic level. The low amount of EPA in A. tonsa suggests EPA may be catabolized by the copepod.     

Dietary-induced responses in the phagotrophic flagellate Oxyrrhis marina

Primary producers may be limited by different nutrients as well as by light availability, which in turn affects their quality as food for higher trophic levels. Typically, algae with high C:N and/or C:P ratios are low-quality food for consumers. Heterotrophic protists are important grazers on these autotrophes, but despite their importance as grazers, knowledge of food quality effects on heterotrophic protists is sparse. In the present study, we examined how differently grown Rhodomonas salina (nutrient replete, N-limited and P-limited) affected the phagotrophic flagellate Oxyrrhis marina. The functional response of O. marina (based on ingested biovolume) did not show significant differences between food sources, thus food uptake was independent of food quality. O. marina was weakly homoeostatic which means that its C:N:P ratio still reflected the elemental composition of its food to some extent. Food quality had a significantly negative effect on the numerical response of O. marina. Whereas N-limited R. salina and nutrient replete R. salina resulted in similar growth rates, P-limited algae had a significantly negative effect on the specific growth rate of O. marina. Hence, the lack of elemental phosphorus of O. marina feeding on P-limited algae caused a reduction in growth. Thus, despite their weaker homoeostasis, heterotrophic protists are also affected by high C:P food in a similar way to crustacean zooplankton.     

Kinetic measurements of metal accumulation in two marine macroalgae

 We measured the uptake kinetics of four metals (Cd, Cr, Se and Zn) in two marine macroalgae (the green alga Ulva lactuca and the red alga Gracilaria blodgettii). Metal uptake generally displayed a linear pattern with increasing exposure time. With the exception of Cr, which exhibited comparable uptake rate constants at different concentrations, uptake rate constants of Cd, Se and Zn decreased with increasing metal concentration, indicating that the seaweeds had a higher relative uptake at lower metal concentration. Uptake of Cd and Zn was higher in U. lactuca than in G. blodgettii, whereas uptake of Cr and Se was comparable between the two species. Only Cd and Zn uptake in U. lactuca was significantly inhibited by dark exposure. A decrease in salinity from 28 to 10‰ enhanced the uptake of Cd, Cr, Se and Zn in U. lactuca 1.9-, 3.0-, 3.6-, and 1.9-fold, respectively. In G. blodgettii, Cd uptake increased twofold when salinity was decreased from 28 to 10‰, whereas uptake of Cr and Zn was not significantly affected by salinity change. The calculated depuration rate constants of metals in U. lactuca were 0.01 d⁻¹ for Cd, 0.05 to 0.08 d⁻¹ for Cr, 0.14 to 0.16 d⁻¹ for Se, and 0.12 to 0.15 d⁻¹ for Zn, and were relatively independent of the metal body burden in the algae. The predicted bioconcentration factor was 3 × 10⁴ for Cd, 2 × 10³ for Cr, 40 to 150 for Se, and 1 to 2 × 10⁴ for Zn in U. lactuca. Our kinetic study suggested that U. lactuca would be a good biomonitor of Cr and Zn contamination in coastal waters. Received: 14 September 1998 / Accepted: 29 May 1999     

Food and habitat choice in floating seaweed clumps: the obligate opportunistic nature of the associated macrofauna

The species composition of macrofauna associated with floating seaweed rafts is highly variable and influenced by many factors like spatial and temporal variation, period since detachment and probably also the seaweed species. The presence of seaweed preferences was assessed by a combination of in situ seaweed samplings and multiple-choice aquarium experiments in a controlled environment, using the seaweed-associated grazing organisms Idotea baltica and Gammarus crinicornis. Results from the sampling data confirm that the seaweed composition influences macrofaunal species composition and abundance: samples dominated by Sargassum muticum displayed higher densities but lower diversities compared to samples dominated by Ascophyllum nodosum and Fucus vesiculosus. Seaweed preference was also apparent from the multiple-choice experiments, but did not exactly match the results of the community analysis: (1) I. baltica had high densities in seaweed samples (SWS) dominated by F. vesiculosus and A. nodosum, while in the experiments, this isopod was most frequently associated with Enteromorpha sp. and F. vesiculosus, and fed mostly on S. muticum, A. nodosum and Enteromorpha sp.; (2) G. crinicornis had high densities in SWS dominated by F. vesiculosus, while in the experiments, this amphipod was most frequently associated with S. muticum, but fed most on A. nodosum and F. vesiculosus. It is clear from the laboratory experiments that preference for habitat (shelter) and food can differ among seaweed species. However, food and habitat preferences are hard to assess because grazer preference may change if choices are increased or decreased, if different sizes of grazers are used, or if predators or other grazers are added to the experiments. The effects of seaweed composition may also be blurred due to the obligate opportunistic nature of a lot of the associated macrofaunal species.     

Chemoautotrophic symbiosis in the tropical clamSolemya occidentalis (Bivalvia: Protobranchia): ultrastructural and phylogenetic analysis

Patterns of algal seasonality, and their effect on the diet and feeding preferences of the herbivorous crab Grapsus albolineatus were investigated over an 18-mo period from March 1993 to August 1994 on an exposed tropical rocky shore (Hok Tsui Peninsula at Cape d' Aguilar, Hong Kong). Algal cover was greatest in the winter months, and lowest in the summer. Foliose algae such as Ulva fasciata, Porphyra suborbiculata, and Dermonema frappieri were dominant in the winter, but died off in the summer. During the hot summer months, perennial encrusting algae e.g., Ralfsia expansa, Hildenbrandia rubra, H. occidentalis, coralline crusts and the encrusting cyanobacteria Kyrtuthrix maculans, were the dominant algal species. Seasonal variation in algal abundance influenced the dietary selectivity of the herbivorous crab G. albolineatus. In the winter, the crab fed selectively on filamentous algae (e.g. Hincksia spp., Cladophora spp., Enteromorpha spp., and the cyanobacteria Lyngbya sp.). Foliose algae (e.g. U. fasciata, P. suborbiculata, Pterocladia tenuis) formed a small part of the diet, despite being the dominant species on the shore. Foliose and filamentous algae were virtually absent from the shore in the summer, and the crabs switched to feeding solely on encrusting algae. Electivity indices revealed preferences for green and brown turf species, and avoidance of foliose algae. Faecal analysis revealed that a greater proportion of the food is digested in the winter, suggesting that G. albolineatus is able to digest filamentous algae more efficiently than encrusting algae. Feeding preferences of G. albolineatus appear to be influenced by a number of factors, including the availability, digestibility and morphology of algae. The foraging behaviour and cheliped morphology of the crab also affect food choice. The monsoonal nature of Hong Kong's climate controls the diversity and abundance of intertidal algae and, therefore, indirectly influences the diet and subsequent growth and reproductive success of the herbivorous crab G. albolineatus.     

Evaluation of chlorophyll fluorescence, in vivo spectrophotometric pigment absorption and ion leakage as biomarkers of UV-B exposure in marine macroalgae

The photosynthetic fluorescence ratio F_v:F_m, in vivo absorption spectra and ion leakage were evaluated as biomarkers of ambient and elevated UV-B (280 to 320 nm) exposure of the intertidal alga Enteromorpha intestinalis (Chlorophyta) and the sublittoral alga Palmaria palmata (Rhodophyta). Measurements of thallus growth were also used to assess adverse biological effects. Ambient and elevated UV-B significantly inhibited photosynthesis in both species. It was shown that the F_v:F_m ratio is a sensitive, non-specific general biomarker of UV-B exposure in both species. Moreover, the in vivo absorption of what was tentatively identified as chlorophylls a and b as well as phycoerythrin and/or carotenoids, phycoerythrobilin and phycocyanin decreased in a dose-response dependent manner and was associated with a decrease in growth rate in P. palmata. The intertidal alga E. intestinalis showed a greater degree of tolerance to UV-B exposure. These results indicate that changes in the F_v:F_m ratio together with reductions in in vivo pigment absorption could provide an early quantitative warning of the detrimental effects of UV-B in marine macroalgae. Received: 16 May 1997 / Accepted: 16 July 1997     

Lethal Effects of Water Quality on Threatened California Salamanders but Not on Co‐Occurring Hybrid Salamanders

Biological invasions and habitat alteration are often detrimental to native species, but their interactions are difficult to predict. Interbreeding between native and introduced species generates novel genotypes and phenotypes, and human land use alters habitat structure and chemistry. Both invasions and habitat alteration create new biological challenges and opportunities. In the intensively farmed Salinas Valley, California (U.S.A.), threatened California tiger salamanders (Ambystoma californiense) have been replaced by hybrids between California tiger salamander and introduced barred tiger salamanders (Ambystoma tigrinum mavortium). We conducted an enclosure experiment to examine the effects habitat modification and relative frequency of hybrid and native California tiger salamanders have on recruitment of salamanders and their prey, Pacific chorus frogs (Pseudacris regilla). We tested whether recruitment differed among genetic classes of tiger salamanders (hybrid or native) and pond hydroperiod (seasonal or perennial). Roughly 6 weeks into the experiment, 70% (of 378 total) of salamander larvae died in 4 out of 6 ponds. Native salamanders survived (n = 12) in these ponds only if they had metamorphosed prior to the die‐offs. During die‐offs, all larvae of native salamanders died, whereas 56% of hybrid larvae died. We necropsied native and hybrid salamanders, tested water quality, and queried the California Department of Pesticide Regulation database to investigate possible causes of the die‐offs. Salamander die‐offs, changes in the abundance of other community members (invertebrates, algae, and cyanobacteria), shifts in salamander sex ratio, and patterns of pesticide application in adjacent fields suggest that pesticide use may have contributed to die‐offs. That all survivors were hybrids suggests that environmental stress may promote rapid displacement of native genotypes. Efectos Letales de la Calidad del Agua sobre Salamandras de California Amenazadas pero no sobre Salamandras Híbridas Concurrentes     

Cu和Pb对赤潮异弯藻(Heterosigma akashiwo)生长的影响

以添加了不同浓度的CuSO_4和Pb(NO_3)_2混合液的f/2培养基培养赤潮异弯藻,通过分析藻的最大生长密度、生长周期及比生长率这3个参数与Cu和Pb之间的相关关系,讨论2种重金属对赤潮异弯藻生长的影响以及2种重金属之间的互作效应。结果表明,Cu为0～0.02 mg·L~(-1)或Pb为0～0.32 mg·L~(-1)时,对赤潮异弯藻生长有促进作用;当Pb浓度不同的条件下,Cu浓度达到0.5～2.5 mg·L~(-1)时,赤潮异弯藻生长受到明显抑制,甚至无法生长;在Cu浓度不同的条件下,Pb在1～9 mg·L~(-1)范围内,随着浓度的增加,对赤潮异弯藻生长抑制作用逐步增强。在Cu浓度为0～2.5 mg·L~(-1)或Pb浓度为1～9 mg·L~(-1)及两者互作条件下,赤潮异弯藻最大生长密度和生长周期都受到显著影响(P0.01)。另外,Cu和Pb对赤潮异弯藻比生长率无显著影响,两者相互作用不明显。结合湛江海域已报道的这2种重金属实际含量,进一步评估了海区中重金属的潜在生态效应。     

Species-specific defense strategies of vegetative versus reproductive blades of the Pacific kelps <Emphasis Type="Italic">Lessonia nigrescens</Emphasis> and <Emphasis Type="Italic">Macrocystis integrifolia</Emphasis>

Chemical defense is assumed to be costly and therefore algae should allocate defense investments in a way to reduce costs and optimize their overall fitness. Thus, lifetime expectation of particular tissues and their contribution to the fitness of the alga may affect defense allocation. Two brown algae common to the SE Pacific coasts, Lessonia nigrescens Bory and Macrocystis integrifolia Bory, feature important ontogenetic differences in the development of reproductive structures; in L. nigrescens blade tissues pass from a vegetative stage to a reproductive stage, while in M. integrifolia reproductive and vegetative functions are spatially separated on different blades. We hypothesized that vegetative blades of L. nigrescens with important future functions are more (or equally) defended than reproductive blades, whereas in M. integrifolia defense should be mainly allocated to reproductive blades (sporophylls), which are considered to make a higher contribution to fitness. Herein, within-plant variation in susceptibility of reproductive and vegetative tissues to herbivory and in allocation of phlorotannins (phenolics) and N-compounds was compared. The results show that phlorotannin and N-concentrations were higher in reproductive blade tissues for both investigated algae. However, preferences by amphipod grazers (Parhyalella penai) for either tissue type differed between the two algal species. Fresh reproductive tissue of L. nigrescens was more consumed than vegetative tissue, while the reverse was found in M. integrifolia, thus confirming the original hypothesis. This suggests that future fitness function might indeed be a useful predictor of anti-herbivore defense in large, perennial kelps. Results from feeding assays with artificial pellets that were made with air-dried material and extract-treated Ulva powder indicated that defenses in live algae are probably not based on chemicals that can be extracted or remain intact after air-drying and grinding up algal tissues. Instead, anti-herbivore defense against amphipod mesograzers seems to depend on structural traits of living algae.     

Ethologie alimentaire d'invertébrés benthiques filtreurs (ascidies). II. Variations des taux de filtration et de digestion en fonction de l'espèce

Rates of filtration and digestion of 4 species of ascidians (Clavelina lepadiformis), Müller, Ciona intestinalis (Linné, 1767), Halocynthia papillosa (Linné, 1767) and Microcosmus sabatieri (Roule, 1885) from a rocky shore at banyuls-sur-Mer, France have been studied using the unicellular algae Monochrysis lutheri as food and the methods previously applied to Phallusia mammillata (Fiala-Médioni, 1973). Eleven experiments of 24 h each were performed under constant conditions of temperature, pH, salinity, oxygen and food concentration. No recognizable feeding rhythm emerged; filtration is irregular, varying around a mean value. Observed filtration rates averaged 2489 ml/h/g organ dry weight in Clavelina lepadiformis, 3515 in Ciona intestinalis, 6349 in Halocynthia papillosa and 6909 in Microcosmus sabatieri. The higher rates are related to larger size and higher complexity of the gills. No pseudo-faeces are formed; a very small part of the faecal material is discharged within 24 h. The mean rates of digestion, in mg albumin equivalent/24 h/g organ dry weight, are: Clavelina lepadiformis, 3.05 mg; Ciona intestinalis, 4.74 mg; Halocynthia papillosa, 9.25 mg; Microcosmus sabatieri, 10.41 mg. The amount of digested algae corresponds to 83–92% (mean=85%) of algae filtered. This high percentage indicates good assimilation of Monochrysis lutheri by ascidians.

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Ethologie alimentaire d'invertébrés benthiques filtreurs (ascidies). II. Variations des taux de filtration et de digestion en fonction de l'espèce