On the ultrastructure of Gephyrocapsa oceanica (Haptophyta) life stages.

Gephyrocapsa oceanica is a cosmopolitan bloom-forming coccolithophore species belonging to the haptophyte order Isochrysidales and family Noëlaerhabdaceae. Exclusively pelagic, G. oceanica is commonly found in modern oceans and in fossil assemblages. Its sister species Emiliania huxleyi is known to possess a haplo-diplontic life cycle, the non-motile diploid coccolith-bearing cells alternating with haploid cells that are motile and covered by non-mineralized organic scales. Since the cytology and ultrastructure of other members of the Noëlaerhabdaceae has never been reported, it is not clear whether these features are common to the family. Here, we report on the ultrastructure of both the non-motile calcifying stage and the non-calcifying motile stage of G. oceanica. We found no significant ultrastructural differences between E. huxleyi and G. oceanica either in the calcifying diploid stage or the haploid phase. The similarities between these two morphospecies demonstrated a high degree of conservation of cytological features. We discuss the significance of these results in the light of the evolution of the Noelaerhabdaceae.

Feeding rates and prey selectivity of planktonic decapod larvae in the Western English Channel

Meroplankton are seasonally important contributors to the zooplankton, particularly at inshore sites, yet their feeding ecology is poorly known relative to holoplankton. While several studies have measured feeding in decapod larvae, few studies have examined the feeding rates of decapod larvae on natural prey assemblages throughout the reproductive season. We conducted 8 feeding experiments with Necora puber, Liocarcinus spp. and Upogebia spp. zoea larvae collected from the L4 monitoring site off Plymouth (50°15.00′N, 4°13.02′W) during spring–summer 2009 and 2010. This period spanned moderate-to-high food availability (0.5–1.6 µg chl-a L−1), but a great range in food composition with small cells <20 µm dominating in 2010. Daily rations averaged 17, 60 and 22 % of body C for the 3 respective decapod species. Clearance rates differed according to prey type, and all 3 decapod genera showed evidence of selection of dinoflagellates. Importantly, small cells including nano- and pico-plankton were ingested, this being demonstrated independently by flow cytometric analysis of the feeding experiments and molecular analysis. PCR-based analysis of the haptophyte portion of the diet revealed ingestion of Isochrysis galbana by decapod larvae in the bottle incubations and Isochrysis galbana and Phaeocystis globosa by decapod larvae collected directly from the field. This study has shown that pico- and nano-sized plankton form an important supplement to the diverse and variable diet of decapod larvae.

Feeding selectivity of bivalve larvae on natural plankton assemblages in the Western English Channel

Meroplankton, including bivalve larvae, are an important and yet understudied component of coastal marine food webs. Understanding the baseline of meroplankton ecology is imperative to establish and predict their sensitivity to local and global marine stressors. Over an annual cycle (October 2009–September 2010), bivalve larvae were collected from the Western Channel Observatory time series station L4 (50°15.00′N, 4°13.02′W). The morphologically similar larvae were identified by analysis of the 18S nuclear small subunit ribosomal RNA gene, and a series of incubation experiments were conducted to determine larval ingestion rates on natural plankton assemblages. Complementary gut content analysis was performed using a PCR-based method for detecting prey DNA both from field-collected larvae and those from the feeding experiments. Molecular identification of bivalve larvae showed the community composition to change over the course of the sampling period with domination by Phaxas in winter and higher diversity in autumn. The larvae selected for nanoeukaryotes (2–20 µm) including coccolithophores (<20 µm) which together comprised >75 % of the bivalve larvae diet. Additionally, a small percentage of carbon ingested originated from heterotrophic ciliates (<30 µm). The molecular analysis of bivalve larvae gut content provided increased resolution of identification of prey consumed and demonstrated that the composition of prey consumed established through bottle incubations conferred with that established from in situ larvae. Despite changes in bivalve larvae community structure, clearance rates of each prey type did not change significantly over the course of the experiment, suggesting different bivalve larvae species may consume similar prey.

The Impact of Polystyrene Microplastics on Feeding, Function and Fecundity in the Marine CopepodCalanus helgolandicus

Microscopic plastic debris, termed “microplastics”, are of increasing environmental concern. Recent studies have demonstrated that a range of zooplankton, including copepods, can ingest microplastics. Copepods are a globally abundant class of zooplankton that form a key trophic link between primary producers and higher trophic marine organisms. Here we demonstrate that ingestion of microplastics can significantly alter the feeding capacity of the pelagic copepod Calanus helgolandicus. Exposed to 20 μm polystyrene beads (75 microplastics mL–1) and cultured algae ([250 μg C L–1) for 24 h, C. helgolandicus ingested 11% fewer algal cells (P = 0.33) and 40% less carbon biomass (P < 0.01). There was a net downward shift in the mean size of algal prey consumed (P < 0.001), with a 3.6 fold increase in ingestion rate for the smallest size class of algal prey (11.6–12.6 μm), suggestive of postcapture or postingestion rejection. Prolonged exposure to polystyrene microplastics significantly decreased reproductive output, but there were no significant differences in egg production rates, respiration or survival. We constructed a conceptual energetic (carbon) budget showing that microplastic-exposed copepods suffer energetic depletion over time. We conclude that microplastics impede feeding in copepods, which over time could lead to sustained reductions in ingested carbon biomass.

Does predation control adult sex ratios and longevities in marine pelagic copepods?

We assess the causes of adult sex ratio skew in marine pelagic copepods by examining changes in these ratios between the juveniles and adults, sexual differences in juvenile stage durations, and mortality rates of adults in the field and laboratory (when free from predators). In the field, late copepodite stages (CIV and CV) commonly have sex ratios that are either not significantly different from equity (1 : 1), or slightly male biased. By contrast, in adults, these ratios are commonly significantly biased toward female dominance. Sex ratio skews are therefore primarily attributable to processes in adults. Members of the non-Diaptomoidea have especially skewed adult ratios; in the members Oithonidae and Clausocalanidae this is not generated from differences between male and female adult physiological longevity (i.e., laboratory longevity when free of predators). In the genera Acartia, Oithona, and Pseudocalanus, we estimate that predation mortality contributed ≥ 69% of the field mortality rate in adult males, whereas in Acartia, Oithona, and Calanus adult females, this is ≥ 36%.We conclude that (1) adult sex ratio skew in pelagic copepods is primarily due to differential mortality of the sexes in the adult stage and not in juveniles, (2) mortality rates of adult Acartia, Pseudocalanus, and Oithona are dominated by predation mortality rather than physiological longevity (except under extreme food limitation), and (3) in Pseudocalanus and Oithona, elevated mortality rates in adult males to females is predominantly due to higher predation on males. Our work demonstrates that we now need to develop a more comprehensive understanding of the importance of feeding preferences in predators. Continue reading full article