What fraction of the organic carbon in sacoglossans is obtained from photosynthesis by kleptoplastids? An investigation using the natural abundance of stable carbon isotopes

any sacoglossans (opisthobranch gastropods) have the potential for carbon acquisition from photosynthesis by plastids sequestered from their macroalgal food as well as by ingestion, digestion and assimilation of the organic carbon derived from the alga. A new method for obtaining a minimum estimate of the fraction of sacoglossan carbon supplied from photosynthesis by kleptoplastids is suggested, based on the mass balance of stable carbon isotopes at the natural abundance level. The method involves comparison of ¹³C/¹²C ratios in sacoglossans with those of the algae on which they are found. Differences in ratios between alga and sacoglossan are used to give a minimum estimate of carbon acquisition by kleptoplasty, granted assumptions about the range of ¹³C/¹²C fractionation values which can occur for marine photolithotrophs. The new method is applied to several green (ulvophycean) alga–sacoglossan associations from Rottnest Island, Western Australia, and the values compared with those obtained previously by other means. The method suggests values of up to 0.6 of the total carbon input to the sacoglossans from photosynthesis by their kleptoplastids. To improve the estimates of the minimum role of kleptoplastidy in the carbon nutrition of sacoglossans, further information is needed: (1) on the fidelity of a given sacoglossan to a given algal individual (or species), (2) on the ¹³C/¹²C ratio of the part of the alga ingested by the sacoglossan, and (3) on the allocation of dietary organic carbon and of kleptoplastidic photosynthate to carbon lost in respiration, mucopolysaccharide production and gametes (and hence not sampled with the animal). Received: 24 November 1999 / Accepted: 20 October 2000     

Atypical plant–herbivore association of algal food and a kleptoplastic sea slug (Elysia clarki) revealed by DNA barcoding and field surveys

The identity of food sources and feeding preferences of specialist herbivores have been commonly inferred from spatial associations between consumer and food items. However, such basic information for well-known marine herbivores, sacoglossans (sea slugs), and their algal diets remains disappointingly lacking, especially from field studies. The sacoglossan, Elysia clarki (Pierce et al. in Molluscan Res 26:23–38, 2006), is kleptoplastic and sequesters chloroplasts from algal food to photosynthesize, so DNA identification of sequestered chloroplasts was employed to verify the algal species fed upon by the slug across its geographic range. The molecular information on the algae consumed by E. clarki was combined with field surveys of slugs and algae in slug habitats in the Florida Keys in July and August of 2008 in order to evaluate whether the diet of this herbivore could be predicted based on its spatial association with algae in the field. A considerable mismatch between food availability and kleptoplast identity was recorded. E. clarki commonly occupied areas devoid of potential food and often contained symbiotic plastids from algal species different from those most frequently found in the surveyed habitats. In three of the four study sites, algal species present were poor predictors of slug diet. These findings suggest that the photosynthetic capability of E. clarki may release the slug from the constraint of requiring proximity to its food sources and may allow for the potential lack of spatial coupling between this herbivore and its algal food. This combination of field surveys and DNA barcoding provided critical and previously unavailable information on herbivore feeding in this marine system.     

Retention of functional chloroplasts in some sacoglossans from the Indo-Pacific and Mediterranean

Previous studies on kleptoplasty in sacoglossans have used different methodology to investigate how long the sacoglossans are able to keep photosynthetically active (functional) chloroplasts. In this study we have used Pulse Amplitude Modulated Fluorometry to measure the quantum yield of charge separation in photosystem II in dark acclimated cells (Φ_IIe) to detect the status of photosynthetic activity. Seven species of sacoglossa, Plakobranchus ocellatus, Elysia timida, Elysia sp, Elysia tomentosa, Thuridilla carlsoni, T. lineolata and Elysiella pusilla, were investigated regarding their ability to retain functional chloroplasts (RFC). The results show three different levels of RFC’s where P. ocellatus has the longest RFC for more than 11 months, E. timida with a RFC 1/4 than P. ocellatus (almost 3 months) and the rest with RFC’s down to 1/22 of P. ocellatus (up to 15 days). Based on these results, and compared to previous studies, eight different levels of retention abilities of non-functional and functional chloroplasts in sacoglossans are proposed. As far as we know, this is a novel method studying chloroplast functionality in sacoglossans.     

Obligate mixotrophy inLaboea strobila,a ciliate which retains chloroplasts

The planktonic ciliateLaboea strobila Lohmann sequesters photosynthetically functional chloroplasts derived from ingested algae. The chloroplasts lie free in the cytoplasm and are most abundant just under the pellicle of the ciliate. The maximum rate of photosynthesis (P_max) was 925 pg C ciliate^-1h^-1 (3.7 pg C pg chl.a ^-1h^-1). At saturating irradiance, the amount of carbon fixed h^-1 equaled 12.6% of the body carbon of the ciliate. To grow,L. strobila requires both light and algal food. In the absence of food, survival ofL. strobila is significantly longer in the light than in the dark. Based on ingestion rate and photosynthetic rate, we calculate that photosynthesis can make an important contribution to this ciliate's carbon budget even when algal food is plentiful.     

Photosynthetic activity of the non-dormant <Emphasis Type="Italic">Posidonia oceanica</Emphasis> seed

The photosynthetic adaptive features of non-dormant seeds in Posidonia oceanica were studied in order to evaluate the effects of light on germination success. Transmission electron micrographs showed the presence of chloroplasts in the epidermal cells, close to the nucleus at the periphery of the cytoplasm. The well-developed thylakoid membranes and the presence of starch granules indicated that the chloroplasts were photosynthetically active. The relationship between photosynthesis versus irradiance in P. oceanica seeds incubated at 15 and 21°C was analysed. The net photosynthesis in the non-dormant seed of P. oceanica was positive and compensated its respiration demand (90 μmol quanta m⁻² s⁻¹) at both temperatures. Net photosynthesis was negative at the other irradiance values. To test the effects of light on germination success, seeds were placed both in dark and light conditions. Germination success was significantly higher in light rather than in dark condition. The characteristics observed in the photosynthesis in P. oceanica seed could be a mechanism to guarantee seedling survival in temperate waters, demonstrating though the specialized nature of this species.     

Morphology, molecular phylogeny and photosynthetic activity of the sacoglossan mollusc, Elysia nigrocapitata, from Korea

The morphology, external coloration as well as the life span of a kleptoplastic mollusc, Elysia nigrocapitata, was affected by its algal diet. Among algal diets, Chaetomorpha moniligera was the best for growth but not for animal longevity. TEM studies showed several distinctive layers composed of different cell types in sectioned parapodia. The chloroplast-containing digestive cells were located beneath the layer of vacuolated cells. The digestive cells contained 10–15 chloroplasts, in varying states of intactness, and several nuclei. Chloroplasts were not enclosed by any membranous structure in the host cytosol. Chlorophyll a fluorometry showed that the photosynthetic activity of kleptoplasts in E. nigrocapitata could be maintained for a long time only when animals were kept in the dark. The photosynthetic activity of kleptoplasts lasted 3–4 days when the animals were exposed to continuous illumination of 200 μmol photons m^?2 s^?1. These results suggested that the contribution of kleptoplasts to the survival of the animals might be minimal if the chloroplasts are not sequestered continuously. Cox I, 16S rDNA, and 28S rDNA sequence data have been obtained in order to phylogenetically place the new species of Elysia found in Korea.     

Mechanisms of photoadaptation in three strains of the symbiotic dinoflagellate Symbiodinium microadriaticum

Mechanisms of photoadaptation of photosynthesis have been studied in three strains of the symbiotic dinoflagellate Symbiodinium microadriaticum. Algal strains isolated from the clam Tridacna maxima, the sea anemone Aiptasia pulchella, and the scleractinian coral Montipora verrucosa were maintained in the defined medium ASP-8A, and were grown at irradiances ranging from 22 to 248 μE m^-2 s^-1 on a 14 h:10 h (light:dark) photoperiod at 26°C. All algal cultures were analysed during log-phase of growth. At all light levels, rates of cell division and photosynthesis were determined, as were cell volumes, pigmentation (including chlorophyll a, chlorophyll c ₂, peridinin, β-carotene and xanthophylls), and carbon and nitrogen content. Low light-induced changes in pigmentation were evident to varying degrees in all three algal strains, although alterations in the photosynthesis-irradiance relations were distinctly different in each strain. The algae from T. maxima show the least photoadaptive capability, and seem to photoadapt by changing photosynthetic unit (PSU) size. Algae from A. pulchella appear to adapt by changing PSU number, while algae from M. verrucosa appear to photoadapt by changes in the activities of CO₂-fixing enzymes or electron transport systems. These are the first observations that demonstrate functional differences in different strains of S. microadriaticum. The adaptive capabilities of the algae appear to correlate well with the ecological distribution of their respective hosts. The study was made from July 1981 through December 1982.     

Algal chloroplasts in the protoplasm of three species of benthic foraminifera: taxonomic affinity,viability and persistence

The ultrastructure and pigment content of algal chloroplasts (derived from Bacillariophyceae or Chrysophyceae) are described from 3 benthic species of brackish-water foraminiferans.Elphidium williamsoni Haynes contains 4×10⁶ chloroplasts mg^-1, whereas the contents ofNonion germanicum (Ehrenberg) andE. excavatum (Terquem) are about 10% of this value. The two former contain chlorophyllsa andc and fucoxanthin, but these pigments were not detectable in the latter.E. williamsoni andN. germanicum had a net uptake of¹⁴C–HCO ₃ ^- , proportional to their content of chlorophyll and number of chloroplasts, increasing linearly up to approximately 10 Klux. At light saturation the former assimilates 2.3x10^-3 mg C mg^-1 h^-1 and the latter only about 20% of this value. Dark uptake was insignificant in all cases. Uptake could not be demonstrated inE. excavatum. The photosynthesis effected by these species is trivial in terms of the total benthic carbon fixation effected by the microflora. The chloroplasts survived longer in forminiferans kept in the dark than in light/dark adapted individuals. To keep a steady state population of chloroplasts under light/dark conditions,E. williamsoni must eat at least 65 chloroplasts individual^-1 h^-1, whereas the minimum consuption rate inN. germanicum is 20.     

Dynamics of a peculiar plant-herbivore relationship: the photosynthetic ascoglossan Elysia timida and the chlorophycean Acetabularia acetabulum

The ascoglossan mollusc Elysia timida Risso, 1818 retains functional chloroplasts from its algal food, the chlorophycean Acetabularia acetabulum (L.). Photosynthates from the plastids are an important source of organic nutrients for the mollusc. Chloroplast exploitation has an ecological function, allowing the ascoglossan to live entirely on an algal diet which is of limited, seasonal availability to other herbivores. Between October 1987 and July 1988, the annual evolution of the molluscan and algal populations was studied in a cove of Mazarrón Bay, southeast Spain. The population density of the mollusc is highly dependent on its food supply, being controlled by the seasonal life cycle of the algal population. During its life cycle, the degree of grazing by the mollusc decreases with increasing algal calcification, the cell walls of the alga progressively calcify, and the eventually highly calcificied stalks are completely resistant to ascoglossan grazing. In contrast, the exploitation of the algal chloroplasts retained by the molluscs increases during the seasonal cycle. The progressively increasing scarcity of food during the seasonal cycle may have led to the retention of symbiotic chloroplasts by E. timida. The developmental strategy of the ascoglossan also changes during the year: when food is abundant (in November, December, January, February and March) it is direct, with no planktonic larval phase, when food is scarce (in October, April, May and June) it is lecithotrophic, with a short planktonic larval phase. Chloroplast retention acts as a buffer, alleviating the effects of annual changes in density, structure and abundance of the alga on the nutritional state of the molluse.     

Capability of dynamic photoinhibition in Arctic macroalgae is related to their depth distribution

The capability of several macroalgal species to protect photosynthesis from excessive irradiance by dynamic photoinhibition was investigated relative to their depth distribution in summer 1995 in the Kongsfjord (79°N; 12°E, Ny Ålesund, Spitsbergen, Norway). Photoinhibition of photosynthesis was induced by exposure of algae from different water depths to a high photon fluence rate of 500?μmol?m^?2?s^?1 for 2?h. Changes in optimal quantum yield (F _v/F _m) were measured during the inhibition phase. Recovery of photosynthesis was subsequently induced by dim white light (10?μmol?m^?2?s^?1) and observed as changes in the variable fluorescence. With a newly developed mathematical model different parameters of the response kinetics of inhibition and recovery were calculated and related to the depth distribution of each algal species. It is shown that two components with slow and fast reaction kinetics, respectively, are involved in photoinhibition and recovery of photosynthesis. Their possible molecular bases are discussed. The half-life time (τ) of the inhibition and recovery phases, i.e. the time necessary to reach half maximal response, is clearly related to the depth distribution of the investigated species. Algae collected close to the water surface show a fast reaction of both photoinhibition and recovery and, hence, have a low τ. With increasing depth the reactions become slower and τ increases. τ was highest in deep water algae. Further analysis of the reaction kinetics in Laminaria saccharina shows that the relative proportion of the two kinetics involved change with the collection depth. In contrast, a significant difference in the reaction rates of both kinetics was not observed.     

Dark survival of autotrophic,planktonic marine diatoms

A general ecological problem is considered: how long can a photoautotrophic microalga, incapable of producing a resting spore (stage), retain its viability in the dark following removal from the euphotic zone? Nine coastal diatoms, including some capable of producing resting spores, were kept in the dark for 90 days at 15°C, and their growth (viability) checked at periodic intervals upon reillumination. Seven of the 9 diatoms retained their viability for 90 days; generation time of illuminated cells then ranged from 2.5 to 10 days. Skeletonema costatum survived only 7 weeks of darkness. Based on the present and published observations, dark survival of this species is inversely related to temperature; it survives at least 24 weeks at 2°C, and from 1 to 4 weeks at 20°C. None of the species was observed to grow in the dark. The effects of temperature and light on dark survival, and of darkness on the chemical composition and photosynthesis following reillumination as reported in the scattered literature are evaluated. Together with the present observations, it is suggested that dark survival of photoautotrophic microalgae: (1) varies between species; (2) may be temperature dependent in some species, as in S. costatum; (3) may be prolonged by periodic illumination at subcompensation intensities for photosynthesis, as shown in Dunaliella tertiolecta. The potential ecological significance of these findings is also considered, should these in vitro results apply to natural populations.     

Experimental ecology of the temperate scleractinian coral Astrangia danae

The combined effects of temperature, light and symbiont density on the metabolic rate and calcification of the temperate coral Astrangia danae were studied experimentally using colonies containing different concentrations of zooxanthellae. After acclimation to five temperatures between 6.5° and 27°C, and incubation at three light levels and in darkness, respiration and photosynthesis were measured and corrected for rates due to commensals alone. Calcification rates were regressed on zooxanthellae concentration and production in order to define “symbiotic” and “non-symbiotic” averages, and the enhancement of calcification by symbiotic interactions in the polyps. Respiration by the polyparium varied less with temperature between 11.5° and 23°C than that of the commensals, suggesting physiological acclimation by the coral tissue. In-vivo zooxanthellae photosynthesis increased linearly with temperature and was near its maximum at 400 μEin m^?2 s^?1, but the photosynthesis of the endolithic algae of the corallum varied little between 11.5° and 27°C. Calcification at any given temperature was near its maximum at 40 μEin m^?2 s^?1 in both symbiotic and non-symbiotic corals. CaCO₃ deposition increased linearly with temperature in non-symbiotic colonies and in symbiotic colonies incubated in the dark. In symbiotic colonies, calcification in the light increased above these basic rates as temperature rose above 15°C. Below 15°C, symbiotic interactions failed to stimulate calcification, apparently due both to a lowering of zooxanthellae photosynthesis and to a decrease in the enhancing effect of any given level of primary production.     

Patterns of co-occurrence and interactions between age classes of the common triplefin,Forsterygion lapillum

Many marine organisms have pelagic larval stages that settle into benthic habitats occupied by older individuals; however, a mechanistic understanding of intercohort interactions remains elusive for most species. Patterns of spatial covariation in the densities of juvenile and adult age classes of a small temperate reef fish, the common triplefin (Forsterygion lapillum), were evaluated during the recruitment season (Feb–Mar, 2011) in Wellington, New Zealand (41°17′S, 174°46′E). The relationship between juvenile and adult density among sites was best approximated by a dome-shaped curve, with a negative correlation between densities of juveniles and adults at higher adult densities. The curve shape was temporally variable, but was unaffected by settlement habitat type (algal species). A laboratory experiment using a “multiple-predator effects” design tested the hypothesis that increased settler mortality in the presence of adults (via enhanced predation risk or cannibalism) contributed to the observed negative relationship between juveniles and adults. Settler mortality did not differ between controls and treatments that contained either one (p = 0.08) or two (p = 0.09) adults. However, post hoc analyses revealed a significant positive correlation between the mean length of juveniles used in experimental trials and survival of juveniles in these treatments, suggesting that smaller juveniles may be vulnerable to cannibalism. There was no evidence for risk enhancement or predator interference when adults were present alongside a heterospecific predator (F. varium). These results highlight the complex nature of intercohort relationships in shaping recruitment patterns and add to the growing body of literature recognizing the importance of age class interactions.     

Influence of algal concentration and temperature on the filtration rate of Mytilus edulis

The filtration rates of Mytilus edilis (=galloprovincialis; 40 mm) were determined in relation to food concentration and temperature, using pure suspensions of the unicellular alga Platymonas suecica in concentrations ranging from 3x10⁵ cells/l to 1.5x10⁸ cells/l. The rate of filtration (ml/h/mussel) generally decreased as cell concentrations increased, and dropped to low values when concentrations above 5x10⁷ cells/l were supplied. The amount of water swept clear varied continuously, and noticeable differences in the filtration activity of M. edulis were observed over short time intervals (5 min). Fluctuations of filtered volumes per unit time were greater with lower than with higher concentrations of algae. The influence of temperature on filtration activity was highest between 5°–15°C and 25°–30°C. A temperature increase from 15° to 25°C resulted in only a slight increase in filtration rate. At 5° and 30°C, filtration dropped to very low values, namely 350 and 100 ml/h, respectively. The temperature coefficients for the filtration rates of M. edulis were determined as: Q₁₀ (5° to 15°C)=4.96; Q₁₀ (10° to 20°C)=1.22. The amount of algae cells ingested per mussel per hour is directly related to food concentration. The maximum number of cells filtered/mussel/h in an algal suspension of 70x10⁶ cells/l was 21.5x10⁵ cells/h. Cell concentrations of up to 40x10⁶ cells/l were swept clear without producing pseudofaeces. The critical cell density for M. edulis was reached at algal concentrations of 70 to 80x10⁶ cells/l. Above these concentrations no normal filtration activity was observed.     

Corallina and Ellisolandia (Corallinales,Rhodophyta) photophysiology over daylight tidal emersion: interactions with irradiance,temperature and carbonate chemistry

The photophysiology of three geniculate coralline algal species (Corallina officinalis, C. caespitosa and Ellisolandia elongata) was determined in intertidal rock pools in the south-west UK at Combe Martin (51°12′31N 4°2′19W) and Heybrook Bay (50°31′66N 4°11′41W), at the start, middle and end of summer (September 1 and 2) and winter (February 9 and 10) daylight tidal emersion periods, in relation to prevailing irradiance, temperature and carbonate chemistry conditions. Algal photophysiology was assessed from rapid light curves performed using pulse amplitude modulation fluorometry. Corallina and Ellisolandia experienced significant fluctuations in irradiance, temperature and carbonate chemistry over seasonal and tidal cycles. Rock pool carbonate chemistry was predictable (R ² = 0.82, P < 0.0001) by photodose (summed irradiance) plus water temperature, but not significantly related to photophysiology. In contrast, Corallina and Ellisolandia relative maximum electron transfer rate showed a significant negative relationship (R ² = 0.65, P < 0.0001) with irradiance plus water temperature. At a seasonal resolution, photoacclimation to maximize both light harvesting during winter months and photoprotection during summer months was observed for all species. Dynamic photoinhibition was apparent over both summer and winter tidal emersion, in relation to irradiance fluctuations. More effective photoinhibition was apparent during summer months, with greater sensitivity to irradiance and slower recovery in F _v/F _m, observed during winter. With sustained high irradiance over tidal emersion, the establishment of high pH/low inorganic carbon conditions may impact photochemistry. This study represents the first assessment of C. officinalis, C. caespitosa and E. elongata photophysiology underpinned by clear species concepts and highlights their ability to adapt to the dramatically fluctuating conditions experienced in intertidal rock pools.     

Within- and transgenerational effects of ocean acidification on life history of marine three-spined stickleback (Gasterosteus aculeatus)

Some studies have demonstrated that elevated CO₂ concentrations in ocean waters negatively impact metabolism and development of marine fish. Particularly, early developmental stages are probably more susceptible to ocean acidification due to insufficient regulations of their acid-base balance. Transgenerational acclimation can be an important mechanism to mediate impacts of increased CO₂ on marine species, yet very little is known about the potential of parental effects in teleosts. Therefore, transgenerational effects were investigated on life history in juvenile three-spined sticklebacks Gasterosteus aculeatus by acclimating parents (collected in April 2012, 55°03′N, 8°44′E) and offspring to ambient (~400 µatm) and elevated (~1,000 µatm) CO₂ levels and measured parental fecundity as well as offspring survival, growth and otolith characteristics. Exposure to elevated CO₂ concentrations led to an increase in clutch size in adults as well as increased juvenile survival and growth rates between 60 and 90 days post-hatch and enlarged otolith areas compared with fish from ambient CO₂ concentrations. Moreover, transgenerational effects were observed in reduced survival and body size 30 days post-hatch as well as in enlarged otoliths at the end of the experiment, when fathers or both parents were acclimated to the high-CO₂ environment. These results may suggest that elevated CO₂ concentrations had rather positive effects on life-history traits of three-spined sticklebacks, but that parental acclimation can modify these effects without improving offspring fitness. Although the mechanistic basis of such transgenerational acclimation remains unclear, selective gradients within generations seem to determine the direction of transgenerational effects.     

Diffusive boundary layers and photosynthesis of the epilithic algal community of coral reefs

The effects of mass transfer resistance due to the presence of a diffusive boundary layer on the photosynthesis of the epilithic algal community (EAC) of a coral reef were studied. Photosynthesis and respiration of the EAC of dead coral surfaces were investigated for samples from two locations: the Gulf of Aqaba, Eilat (Israel), and One Tree Reef on the Great Barrier Reef (Australia). Microsensors were used to measure O₂ and pH at the EAC surface and above. Oxygen profiles in the light and dark indicated a diffusive boundary layer (DBL) thickness of 180–590 μm under moderate flow (~0.08 m s^?1) and >2,000 μm under quasi-stagnant conditions. Under light saturation the oxygen concentration at the EAC surface rose within a few minutes to 200–550% air saturation levels under moderate flow and to 600–700% under quasi-stagnant conditions. High maximal rates of net photosynthesis of 8–25 mmol O₂ m^?2 h^?1 were calculated from measured O₂ concentration gradients, and dark respiration was 1.3–3.3 mmol O₂ m^?2 h^?1. From light–dark shifts, the maximal rates of gross photosynthesis at the EAC surface were calculated to be 16.5 nmol O₂ cm^?3 s^?1. Irradiance at the onset of saturation of photosynthesis, E _k, was <100 µmol photons m^?2 s^?1, indicating that the EAC is a shade-adapted community. The pH increased from 8.2 in the bulk seawater to 8.9 at the EAC surface, suggesting that very little carbon in the form of CO₂ occurs at the EAC surface. Thus the major source of dissolved inorganic carbon (DIC) must be in the form of HCO₃ ^?. Estimates of DIC fluxes across the DBL indicate that, throughout most of the daytime under in situ conditions, DIC is likely to be a major limiting factor for photosynthesis and therefore also for primary production and growth of the EAC.     

Bacterivory of a mudflat nematode community under different environmental conditions

The fate of the benthic bacterial biomass is a topic of major importance in understanding how soft-bottom environments function. Because of their high abundance, production and nutritional value, benthic bacteria may constitute an important food resource for benthic fauna. The trophic role of bacteria for a nematode community on the Brouage mudflat (Marennes-Oléron-France), dominated by three species: Chromadora macrolaima (64% of the abundance), Daptonema oxycerca (15%) and Ptycholaimellus jacobi (8%), was determined in grazing experiments using ¹⁵N pre-enriched bacteria. On intertidal flats, seasonal, tidal and circadian cycles induce strong variations in environmental conditions. Grazing experiments were performed in order to measure the effects of abiotic (temperature, salinity and luminosity) and biotic (bacterial and algal abundances) factors on assimilation rates of bacteria by nematodes. In order to assess simultaneously bacteria and algal assimilation rates, algal abundances were modified adding ¹³C pre-enriched Navicula phyllepta. Assimilation rate was significantly lower at 5°C; moreover, general trend shows a prominent temperature effect with an optimum around 30°C. Assimilation at salinity 18 was not significantly different from the assimilation at salinity 31. Assimilation was higher under light conditions than in the dark. Above 10⁹ bacteria ml⁻¹, assimilation of bacteria remained unaffected by bacterial abundance. However, assimilation of algae increased with the algal concentration. Nematode kept feeding under conditions of stress, which are typical of the surficial sediment habitat and they appeared to be principally dependent on the algal resource.     

In vivo and in vitro differences in chloroplast functionality in the two north Atlantic sacoglossans (Gastropoda,Opisthobranchia) <Emphasis Type="Italic">Placida dendritica</Emphasis> and <Emphasis Type="Italic">Elysia viridis</Emphasis>

The photosynthetic functionality in chloroplasts in the two sacoglossan molluscs Placida dendritica and Elysia viridis from the Trondheim fjord in Norway was studied. P. dendritica and E. viridis with no functional chloroplasts in their digestive system were introduced to the green macroalgae Codium fragile. Our results showed that P. dendritica was not able to retain functional (photosynthetic) chloroplasts. Transmission electron microscopy (TEM) showed that chloroplasts were directly digested when phagocytosed into the digestive cells. Four stages of chloroplast degradation were observed. A corresponding operational quantum yield of chl a fluorescence (Φ_PSII ~ 0) indicated autofluorescence, and the presence of highly degraded chl a supported these observations. In contrast, E. viridis was able to retain functional chloroplasts. For this species it took only 1 week for the chloroplasts inside the digestive cells to acquire the same Φ_PSII and light utilisation coefficient (α) as C. fragile kept under the same light conditions. Data for 8 days showed a 2–6-fold increase in the maximum photosynthetic rate (P _max) and light saturation index (E _k) relative to C. fragile. This increase in available light was probably caused by a reduced package effect in the digestive gland of E. viridis relative to C. fragile, resulting in a partial photoacclimation response by reducing the turnover time of electrons (τ). Isolated pigments from C. fragile compared to E. viridis showed the same levels of photosynthetic pigments (chl a and b, neoxanthin, violaxanthin, siphonaxanthin, siphonein and β,ε-carotene) relative to μg chl a (w:w), indicating that the chloroplasts in E. viridis did not synthesise any new pigments. After 73 days of starvation, it was estimated that chloroplasts in E. viridis were able to stay photosynthetic 5–9 months relative to the size of the slugs, corresponding to an RFC of level 8 (a retention ability to retain functional chloroplasts (RFC) for more than 3 months). The reduction in Φ_PSII, P _max and α as a function of time was caused by a reduction in chloroplast health and number (chloroplast thylakoid membranes and PSII are degraded). These observations therefore conclude that chloroplasts from C. fragile cannot divide or synthesise new pigments when retained by E. viridis, but are able to partially photoacclimate by decreasing τ as a response to more light. This study also points to the importance of siphonaxanthin and siphonein as chemotaxonomic markers for the identification of algal sources of functional chloroplasts.     

Effects of temperature and delayed feeding on growth and survival of larvae of three species of subtropical marine fishes

In larvae of the bay anchovy Anchoa mitchilli (Valenciennes), the sea bream Archosargus rhomboidalis (Linnaeus), and the lined sole Achirus lineatus (Linnaeus), growth, survival, and starvation times were investigated at temperatures of 22° to 32°C. The rate at which hours after hatching until starvation decreased in relation to temperature for unfed larvae did not differ significantly among the 3 species, ranging from-5.4 to-6.3 h per degree increase in temperature. The total number of hours until starvation did differ for all 3 species: lined soles survived longest, bay anchovies were intermediate, and sea bream survived the least time. At 28°C, unfed sea bream could survive 90.1 h, bay anchovy 102.3 h, and lined sole 119.8 h. The eyes pigmented at nearly the same time after hatching for sea bream and bay anchovy, but took about 20 h longer at all temperatures for lined sole. Quadratic equations best described the relationship between hours after hatching when the eyes became pigmented and temperature. Eye-pigmentation times became nearly constant for all 3 species at temperatures above 28°C. At 28°C, eyes pigmented about 27 h after hatching for bay anchovy and sea bream but not until 47 h for lined sole. Hours after eye pigmentation when unfed larvae starved was a measure of the effective time that larvae had to commence feeding. Bay anchovies and lined soles were nearly alike in this respect, but sea bream starved at tewer hours after eye pigmentation. Slopes of regressions representing decrease in times to staration for increasing temperatures ranged from-3.7 to-4.4 h per degree increase in temperature, and were not significantly different among the 3 species. At 28°C, unfed lined soles starved at 70 a after eye pigmentation, bay anchovies starved at 72.5 h, and sea bream at only 62 h. Yolk absorption was most rapid for all species during the first 20 h after hatching, and was faster at higher temperatures. Amounts of yolk remaining at the time eyes became pigmented were less at higher temperatures for bay anchovy and lined sole, but were greater for sea bream, suggesting that sea bream used yolk more efficiently at higher temperatures. Either no yolk or small traces (>0.20%) remained at 24 h after eye pigmentation in all 3 species. Feeding was delayed for periods of 8, 16, 24, 32, 40 and 48 h after eye pigmentation for all species at a series of experimental temperatures from 24° to 32°C. Growth and survival were affected when food was withheld for more than 24 h at 28°C, but survival did not decrease markedly until food was withheld at least 8 h longer. At lower temperatures food could be withheld longer and at higher temperatures for less time. Feeding can be initiated by most larvae for several hours after all visible yolk reserves have been exhausted. All species tested can survive for 24 to 40 h after eye pigmentation at 24° to 28°C without food and still have relatively good growth and survival when food is offered. If the “critical period” is considered relative to time of hatching, lined soles need not find food for 3 to 3.5 days after hatching, but bay anchovy and sea bream must feed within 2.5 days of hatching.