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.     

Characteristics of feeding on dinoflagellates by newly hatched larval crabs

The zoeal larvae of brachyuran crabs must feed soon after hatching on a diet that includes large micro- and mesozooplankton in order to satisfy nutritional requirements. However, newly hatched larvae have been shown to ingest a variety of dinoflagellates, perhaps using microbial carbon sources to sustain them until they encounter more favored prey. Ingestion of dinoflagellates by larval crabs has been documented previously under conditions in which the larvae were exposed to algae provided in monoculture or in defined mixtures of cells. We report here on experiments conducted on the hatching stage of five crab species to determine if ingestion of dinoflagellates occurred when they were provided in combination with Artemia sp. nauplii or after a period of feeding on mesozooplankton. Quantitative measurements of chl a in the larval guts provided evidence of ingestion of algal cells. Active ingestion of the dinoflagellate Prorocentrum micans at specified intervals during an extended feeding period was determined on larvae of two crab species using fluorescently labeled cells provided for brief periods at prescribed time intervals. Stage 1 larvae of four of the five crab species ingested dinoflagellates when they were provided in combination with nauplii and larvae of all five species ingested cells after feeding solely on nauplii for 24 h. Ingestion of algal cells was first evident in the larval guts after 6 h of feeding at both low (200 cell ml⁻¹) and high (1,000 cells ml⁻¹) prey densities. Higher prey densities resulted in higher gut chl a. Larvae continuously exposed to dinoflagellates actively ingested cells at every 3 h interval tested over a 36 h period. Results confirm previous studies that larvae will ingest dinoflagellates even when they are encountered in a mixed prey field or when having previously fed. Ingestion of cells may occur on a continual basis over time.     

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.     

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.     

Respiration,photosynthesis and carbon metabolism in planktonic ciliates

Release of¹⁴C-labelled carbon dioxide from uniformly labelled cells was used to measure respiration by individual ciliates in 2-h incubations in 1989 and 1990. In a strictly heterotrophic ciliate,Strobilidium spiralis (Leegaard, 1915), release of labelled carbon dioxide was equivalent to ca. 2.8% of cell C h^–1 at 20°C, and there was no difference between rates in the dark and light. In the chloroplast-retaining ciliatesLaboea strobila Lohmann, 1908,Strombidium conicum (Lohmann, 1908) Wulff, 1919 andStrombidium capitatum (Leegaard, 1915) Kahl, 1932, release of labelled carbon dioxide was less in the light than in the dark in experiments done at 15°C. InL. strobila release of radiolabel as carbon dioxide was equivalent to ca. 2.4% of cell C h^–1 in the dark but ca. 1% at 50µE m^–2 s^–1, an irradiance limiting to photosynthesis. InS. conicum release of radiolabel as carbon dioxide was equivalent to ca. 4.4% of cell C h^–1 in the dark, but at an irradiance saturating to photosynthesis (250 to 300µE m^–2 s^–1) there was no detectable release of labelled carbon dioxide. InS. capitatum release of radiolabel as carbon dioxide was equivalent to ca. 4.3% of cell C h^–1 in the dark but at an irradiance saturating to photosynthesis was ca. 2.4% of cell C h^–1. These data, combined with data from photosynthetic uptake experiments, indicate that¹⁴C uptake underestimates the total benefit of photosynthesis by 50% or more in chloroplastretaining ciliates.Contribution no. 7510 from the Woods Hole Oceanographic Institution     

Species-specific grazing rates of heterotrophic dinoflagellates in oceanic waters,measured with a dual-label radioisotope technique

A dual-isotope method was developed to measure grazing rates and food preferences of individual species of heterotrophic dinoflagellates from natural populations, collected from the Slope, Gulf Stream, and Sargasso Sea and from a transect from Iceland to New England, in 1983. The isotope method measures the grazing rates of microzooplankton which cannot be separated in natural populations on the basis of size. Tritiated-thymidine and ¹⁴C-bicarbonate were used to label natural heterotrophic and autotrophic food, respectively. Nine oceanic dinoflagellate species in the genera Protoperidinium, Podolampas, and Diplopsalis fed on both heterotrophic and autotrophic food particles with clearance rates of 0.4 to 8.0 l cell^-1 h^-1, based on ³H incorporation, and 0.0 to 28.3 l cell^-1 h^-1, based on ¹⁴C incorporation. Two dinoflagellate species, Protoperidinium ovatum and Podolampas palmipes, fed only on ³H-labelled food particles. Several species of dinoflagellates fed on bacteria (<1 m) which had been prelabelled with ³H-thymidine. The clearance rates of heterotrophic dinoflagellates and ciliates were similar and within the range of tintinnid ciliate clearance rates reported in the literature. As heterotrophic dinoflagellates and ciliates can have comparable abundances in oceanic waters, we conclude that heterotrophic dinoflagellates may have an equally important impact as microheterotrophic grazers of phytoplankton and bacteria in oceanic waters.Partially supported by a grant from the National Science Foundation, OCE-81-17744     

Food-particle size and selection by bivalve larvae in a temperate embayment

Epifluorescence microscopy was used to analyze the stomach contents of bivalve larvae collected in the Baie des Chaleurs (western Gulf of St. Lawrence, Canada) in order to document food-particle sizes, compare feeding among taxa, and compare the diet with the in situ phytoplankton community. Stomach contents were mainly composed of small autotrophic flagellates (<5 μm) and cyanobacteria (<2 μm), reflecting the microbial food web which characterizes these waters. More than half (55%) of all veligers examined contained algal cells of 5 to 15 μm, whereas only 3% had cells of 15 to 25 μm. Differences in the size ranges of ingested algal cells among similar-sized larvae of different species suggests that veligers actively selected food particles. Among the smallest veligers (185 to 260 μm), scallops (Placopecten magellicanus) and mussels (Mytilus edulis) ingested more <5 μm and 5 to 15 μm algae than clams (Mya arenaria). Among larger veligers (261 to 405 μm), clams contained significantly more <5 μm cells than mussels, whereas mussels contained significantly more 5 to 15 μm algae than clams. Algal cells of 15 to 25 μm were preferentially ingested by mussel veligers. Feeding also differed between different-sized veligers within taxa, i.e. the smallest clam veligers ingested fewer of 5 to 15 μm algae than the larger size classes. Mussel veligers ingested significantly more 15 to 25 μm and fewer <5 μm cells as their size increased. The dominance of ultraplankton in the nearshore waters of Baie des Chaleurs and in the stomach contents suggests that veliger larvae may be an important export path for carbon produced by small phytoplankton. Received: 17 July 1996 / Accepted: 20 September 1996     

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.     

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.     

Relation of food preference to fitness for the green sea urchin,Strongylocentrotus droebachiensis

We maintained 30 to 35 mm sea urchins (Strongylocentrotus droebachiensis) on 17 different macroalgal diets in the field over a 130-d period and correlated the resulting changes in somatic and gonadal tissues with measures of food preferences. The varions algae fell into there distinct categories according to their ability to support growth. The most striking effect of diet was on gonadal mass, there being a 14-fold increase for the best algal diet, compared to initial gonadal mass, and nearly a threefold decrease for the worst diet. The relation of food preference to somatic growth was nonlinear. A sharp increase in the growth of test diameter, total mass and test mass occurred between the nonpreferred and intermediately preferred algae, but growth rates were similar for intermediate and preferred algae. Gonadal growth tended to increase exponentially with an important measure of food preference, feeding rate (g ingested d^–1). These observations suggested that urchins only channel food resources into somatic growth up to a given point, additional reserves being used for producing gametes. The positive relationship of food preference to growth, and especially the strong correlation with gonadal production, indicates that the highly selective feeding of the urchin contributes to ils fitness.     

Feeding behavior and acquisition of zooxanthellae by planula larvae of the sea anemone Anthopleura elegantissima

Symbiotic associations between cnidarians and photosynthetic dinoflagellates (i.e., zooxanthellae) are common in the marine environment. Many symbiotic cnidarians produce offspring that are initially nonsymbiotic. These new hosts must acquire symbiotic algae from environmental sources. We examined zooxanthella acquisition by laboratory-reared planula larvae of the temperate sea anemone Anthopleura elegantissima. Larvae ingested zooxanthellae while they were feeding. However, the signal that prompted larval feeding behavior did not originate from the symbiotic algae; the addition of algal cells to larval cultures never elicited a feeding response. In contrast, the addition of macerated animal tissue from several sources invariably generated a strong feeding response, which resulted in the larvae indiscriminately ingesting any particulate matter that was present, including zooxanthellae or other unicellular algae. Ingested zooxanthellae were incorporated into endodermal cells, where they remained undigested, while all other ingested material was digested or expelled within 24 h. Our results provide evidence that one source of zooxanthellae likely to serve as a route of infection in the natural environment is zooxanthella-laden mucus egested by anemones. This egested material fulfilled both of the criteria necessary for successful infection: it prompted larvae to begin feeding and provided an abundant supply of zooxanthellae that were ingested and taken up into endodermal cells of the new host.     

A continuous recirculating culture system for planktonic copepods

The calanoid copepods, Acartia clausi Giesbrecht and Acartia tonsa Dana, are maintained at high densities in continuous culture at 15°C. Synthetic sea-water medium is recirculated through filters and a foam tower which limits accumulation of dissolved wastes and various metabolites. The ciliate Euplotes vannus Müller is associated in culture with the copepods, and effectively controls bacterial population and accumulation of algal debris. The copepods graze upon the ciliates as well as upon the phytoflagellates Isochrysis galbana Parke and Rhodomonas baltica Korsten.Contribution No. 119 from the Institute of Marine and Atmospheric Sciences, University of Miami.     

Food availability and diet composition of three coexisting Mediterranean limpets (Patella spp.)

The gut contents of three intertidal patellid limpets were analysed by collecting foraging specimens on a breakwater on the Tyrrhenian coast (central Italy) between May 1988 and October 1989. The three species coexist there showing a different, but partially overlapping zonation: Patella aspera dominates the infralittoral fringe; the majority of P. caerulea inhabits the lower midlittoral, while P. rustica is most abundant in the upper midlittoral. The algae present on slivers of substratum over which each limpet collected was moving were identified. Moreover, floristic surveys were made along the shore in order to characterize the algal cover of the different zonal belts. The floristic study revealed that the basic elements of algal communities typical of western Mediterranean rocky shores are present in the study area. The algae found on the slivers under the foraging limpets were generally representative of the algal community typical of the same zone. There was a marked difference between the diets of P. rustica and P. aspera due to the fact that the first species forages on a few low lying epilithic and endolithic Cyanophyceae, while P. aspera feeds on a large number of species belonging to all the main algal classes and life forms considered, including frondose epilithics and epiphytics. The diet of P. caerulea resembles that of P. aspera in algal heterogeneity, but is dominated by Cyanophyceae as in P. rustica. A detailed analysis of the differences between gut contents of each limpet species and the relative slivers showed an obvious general correspondence, but revealed also that the diets of the three species do not completely reflect the availability of algae. These findings suggest that the basic diet segregation mechanism between the three populations is their zonal separation. However, the difference in gut contents of heterospecific limpets foraging in the same zone suggests the existence of supplementary morphy-functional or behavioural mechanisms for diet segregation between the three species.     

Aspects of 14CO2-fixation by endosymbiotic rhodoplasts in the marine opisthobranchiate Hermaea bifida

When the marine opisthobranchiate slug Hermaea bifida Mont. is incubated in a H¹⁴CO ₃ ^- -seawater medium in the light, a considerable net gain of ¹⁴C-assimilates is observed. Electron microscopic control provided evidence that this ¹⁴C-fixation is due to photosynthesis by chloroplasts (=rhodoplasts) endosymbiotic in the cells of the digestive gland of the slug. After thin-layer chromatographic analysis various ¹⁴C-labelled photosynthates could be traced. The assimilate pattern of the rhodoplasts is compared with that of the marine red alga Griffithsia flosculosa (Ellis) Batt., from which the plastids are acquired by feeding. The nutritional relationship of this endosymbiosis is discussed, with emphasis on the occurrence of functional chloroplast endosymbioses among the eolidiform species of the Sacoglossa.     

Phytoplankton growth and microzooplankton grazing rates in a restricted Mediterranean lagoon (Bizerte Lagoon,Tunisia)

Phytoplankton growth and microzooplankton grazing were investigated in the restricted Bizerte Lagoon in 2002 and 2004. The 2002 study, carried out at one station from January to October, showed significant seasonal variations in phytoplankton dynamics. High growth rates (0.9–1.04 day⁻¹), chlorophyll a (Chl a) concentrations (6.6–6.8 μg l⁻¹) and carbon biomass (392–398 μg C l⁻¹) were recorded in summer (July), when several chain-forming diatoms had intensively proliferated and dominated the carbon biomass (74%). In 2004, four stations were studied during July, a period also characterized by the high proliferation of several diatoms that made up 70% of the algal carbon biomass. In 2004, growth rates (0.34–0.45 day⁻¹) and biomass of algae (2.9–5.4 μg Chl a l⁻¹ and 209–260 μg C l⁻¹) were low, which may be related to the lower nutrient concentrations recorded in 2004. Microzooplankton >5 μm were mainly composed of heterotrophic dinoflagellates and ciliates. Microzooplankton biomass peaked during summer (2002 320–329, 2004 246–361 μg C l⁻¹), in response to the enhanced phytoplankton biomass and production. The grazer biomass was dominated by ciliates (71–76%) in July 2002 and by heterotrophic dinoflagellates (52–67%) in July 2004. Throughout the year and at different stations, microzooplankton grazed actively on phytoplankton, removing 26–58% of the Chl a and 57-84% of the primary production. In 2002, the highest grazing impact was observed on the large algae (>10 μm) during the period of diatom dominance. These results have a significant implication for carbon export to depth. Indeed, the recycling of most of the diatom production by the microbial food web in the upper water column would reduce the flux of material to the seafloor. This should be considered when modeling the carbon cycling in coastal environments and under conditions of diatom dominance. During both studies, ciliates had higher growth rates (0.5–1.5 day⁻¹) and a higher carbon demand (165–470 μg C l⁻¹ day⁻¹) than dinoflagellates (0.1–0.5 day⁻¹, 33–290 μg C l⁻¹ day⁻¹). Moreover, when grazer biomass was dominated by ciliates (in July 2002), herbivory accounted for 71–80% of the C ingested by microzooplankton while it accounted only for 14–23% when dinoflagellates dominated the grazer biomass (in July 2004). These results suggest that, in contrast to findings from open coastal waters, ciliate species of the restricted Bizerte Lagoon were more vigorous grazers of the large algae (diatoms) than were dinoflagellates.     

A paradox of the ciliates? High ciliate diversity in a resource-poor environment

Ecological theory predicts that low productivity systems should have low biodiversity. However, despite the oligotrophic status of the Gulf of Aqaba (Northern Red Sea) ciliate species richness was unexpectedly high. In addition, phytoplankton, as main ciliate prey, was made up by only few genera, indicating a significant niche overlap among the grazers. Up to 97% of the ciliates were from the same taxonomic group and of the same size range, implying very similar food niches. Ciliate diversity was highest at times of lowest chlorophyll concentrations, during the period of stable abiotic conditions, but relatively high genetic diversity within the ciliate prey, notably among the cyanobacteria Synechococcus and Prochlorococcus. In the absence of disturbance and with little predation pressure, the alternate explanations for the observed ciliate diversity are either very fine niche partitioning by the ciliates, or their competitive equivalence resulting in a random assortment of species immigrating from a larger metacommunity, in accordance with Hubbell’s, (The unified neutral theory of biodiversity and biogeography. Princeton University Press, Princeton, 2001) neutral model. While the use of species abundance distributions (SAD’s) is far from definitive, the theoretical SAD’s that best fit the Gulf of Aqaba ciliate data was most often not that expected by neutral theory.     

Newly metamorphosed <Emphasis Type="Italic">Elysia clarki</Emphasis> juveniles feed on and sequester chloroplasts from algal species different from those utilized by adult slugs

The adult, sacoglossan sea slug, Elysia clarki (Pierce et al. in Molluscan Res 26, 2006), sequesters functional chloroplasts in cells of the digestive diverticula from four species of macroalgae in the order Bryopsidales (Penicillus capitatus, Penicillus lamourouxii, Halimeda incrassata, and Halimeda monile). Feeding experiments were conducted in December, 2003, using individuals raised in the laboratory from egg masses laid by E. clarki adults which had been collected from Grassy Key, Florida, USA, and 29 species of macroalgae collected from the Florida Keys, Tampa Bay, Tarpon Springs, Florida, or Woods Hole, Massachusetts or obtained from the Culture Collection of Algae at the University of Texas at Austin, Texas. For the first 14-day post-metamorphosis, juveniles ate only the thin filamentous species, Bryopsis plumosa or Derbesia tenuissima. Transmission electron microscopy showed that the chloroplasts from both algae were sequestered intracellularly in juvenile slugs. Individuals offered any other macroalga, including the four species fed on by adults, did not feed on or incorporate any chloroplasts, and soon died. Juveniles switched from B. plumosa to P. capitatus at a length of ∼ 1.0 cm, and fixed for microscopy 14 days later had intact intracellular chloroplasts from both algae. Nucleotide sequences of the chloroplast gene, rbcL, from DNA extracted from E. clarki, collected from Vaca Key, Florida, were aligned with 22 other available macroalgal rbcL sequences indicating that these E. clarki adults had fed on three species of algae in the Bryopsidales including Bryopsis pennata, and TEM results partially confirmed this conclusion.     

Effects of dietary fatty acids on the reproductive success of the calanoid copepod<Emphasis Type="Italic"> Temora longicornis</Emphasis>

Egg production and hatching success of the copepod Temora longicornis were measured in laboratory experiments and in the field (North Sea). In the laboratory, ingestion of four algal species (Thalassiosira weissflogii, Phaeocystis globosa, Isochrysis sp. and Dunaliella tertiolecta) was followed and the content of fatty acids in the algae was determined. The two food types (T. weissflogii and Isochrysis sp.) that provided the highest ingestion of carbon and long chain fatty acids also resulted in the highest egg production rate (E_r) and hatching success (H%). In contrast, D. tertiolecta led to both low ingestion of carbon and long chain fatty acids, resulting in low reproductive success. There was a positive relationship between the amount of eicosapentaenoic fatty acid [20:5(n-3), EPA] ingested and E_r and H%, and of the ratio between docosahexaenoic and eicosapentaenoic fatty acid [22:6(n-3)/20:5(n-3), DHA/EPA] in the ingested food and H%. In the field, chlorophyll a and specific fatty acids were measured and protists were enumerated, in order to investigate the link between these factors and the reproductive success of T. longicornis. Hatching time was found to be related to temperature and exceeded 120 h at 6°C. No relationship was found between chlorophyll a and reproductive success (E_r or H%). E_r correlated with the concentration of diatoms and ciliates, which were the dominating protists in early spring, indicating that food quantity was the limiting factor for E_r. As in the laboratory experiments, H% was dependent on the fatty acid DHA and the ratio of DHA/EPA, which indicates that the quality of eggs (H%) is linked to the quality of food.Communicated by M. Kühl, Helsingør     

Symbiosis in sacoglossan opisthobranchs: functional capacity of symbiotic chloroplasts

It has been previously reported that many species of the order Sacoglossa (Mollusca: Opisthobranchia) contain algal chloroplasts within the cells of their digestive gland and maintain them in a symbiotic condition. In the present study, two species, Elysia hedgpethi Marcus and Placobranchus ianthobapsus Gould, were compared as to their abilities to retain functional chloroplasts in their tissues. Animals were starved for varying lengths of time, and the functional capacity of the plastids was ascertained at intervals. The chlorophyll content of whole animals, and the ability to incorporate ¹⁴CO₂ were used as the assay for functional capacity. E. hedgpethi decreased in chlorophyll content during starvation until the tenth day, when no chlorophyll was detectable spectrophoto-metrically. Incorporation of ¹⁴CO₂ paralleled the decline in chlorophyll, and was at control levels by the tenth day. P. ianthobapsus showed no decline in chlorophyll content over 27 days starvation, although the ability to incorporate ¹⁴CO₂ showed a decrease.     

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.