Selective Feeding of Bdelloid Rotifers in River Biofilms

<div><p><i>In situ</i> pigment contents of biofilm-dwelling bdelloid rotifers of the Garonne River (France) were measured by high performance liquid chromatography (HPLC) and compared with pigment composition of surrounding biofilm microphytobenthic communities. Among pigments that were detected in rotifers, the presence of carotenoids fucoxanthin and myxoxanthophyll showed that the rotifers fed on diatoms and cyanobacteria. Unexpectedly, while diatoms strongly dominated microphytobenthic communities in terms of biomass, HPLC results hinted that rotifers selectively ingested benthic filamentous cyanobacteria. In doing so, rotifers could daily remove a substantial fraction (up to 28%) of this cyanobacterial biomass. The possibility that the rotifers hosted symbiotic myxoxanthophyll-containing cyanobacteria was examined by localisation of chlorophyll fluorescence within rotifers using confocal laser scanning microscopy (CLSM). CLSM results showed an even distribution of quasi–circular fluorescent objects (FO) throughout rotifer bodies, whereas myxoxanthophyll is a biomarker pigment of filamentous cyanobacteria, so the hypothesis was rejected. Our results also suggest that rotifers converted β-carotene (provided by ingested algae) into echinenone, a photoprotective pigment. This study, which is the first one to detail <i>in situ</i> pigment contents of rotifers, clearly shows that the role of cyanobacteria as a food source for meiobenthic invertebrates has been underestimated so far, and deserves urgent consideration.</p> </div

Stable isotope values of European catfish and the putative prey.

<p>δ¹³C and δ¹⁵N values (‰) of each individual (n = 14) and the putative aquatic (fish, n = 9 and crayfish, n = 3) and terrestrial (pigeon, n = 6) prey are displayed. The large symbols for each prey represent the mean value (± SD).</p

European catfish displaying beaching behavior to capture land birds.

<p>Several individuals were observed swimming nearby the gravel beach in shallow waters where pigeons regroup for drinking and cleaning (large picture). One individual is seen approaching land birds and beaching to successfully capture one (small pictures).</p

Predicted contribution of putative prey to the diet of each European catfish.

<p>Putative prey are (a) fish (in red), (b) crayfish (in blue) and (c) pigeons (in green). Reported values are the lower and upper 50, 75 and 95% Bayesian credibility intervals predicted by the mixing models.</p

Comparison of pigment proportions between rotifer gut contents (rotifers) and their habitat (biofilm).

<p>(A) Fucoxanthin: Chl<i>a</i>-eq; (B): Myxoxanthophyll:Chl<i>a</i>-eq; Error bars are SD (n = 3). Fucoxanthin (Fuco) and myxoxanthophyll (myxo) are biomarkers for diatoms and filamentous cyanobacteria respectively.</p

Examples of HPLC absorbance chromatograms obtained at 440 nm from field samples.

<p>(A) biofilm, (B): rotifer (946 individuals). 1: chlorophylls <i>c</i>; 2: fucoxanthin-like; 3: pheophorbide <i>a</i>; 4: fucoxanthin; 5: diadinoxanthin; 6: myxoxanthophyll; 7: cis-fucoxanthin; 8: zeaxanthin-like; 9: zeaxanthin; 10: lutein; 11: chlorophyll b; 12: echinenone; 13: chlorophyll a-like; 14: chlorophyll <i>a</i>; 15: pheophytin <i>a</i>; 16: α-carotene; 17: β-carotene 18: echinenone-like1; 19: echinenone-like 2.</p

Effects of flooding and <i>Aechmea bracteata</i> on the number of spiders (modeled in a linear mixed-effects statistical framework where the individual samples were set as a random variable).

<p>Points refer to the observed numbers, vertical lines to the model prediction.</p

Effects of flooding and <i>Aechmea bracteata</i> presence (A) and of flooding and contact with <i>A. bracteata</i> foliage (B) on the number of spiders (modeled in a linear mixed-effects statistical framework where the individual samples were set as a random variable).

<p>Effects of flooding and <i>Aechmea bracteata</i> presence (A) and of flooding and contact with <i>A. bracteata</i> foliage (B) on the number of spiders (modeled in a linear mixed-effects statistical framework where the individual samples were set as a random variable).</p

Using the Self-Organizing Map algorithm (SOM) to establish congruent patterns between spiders, ants and the bromeliad, <i>Aechmea bracteata</i>.

<p>Distribution of the <i>A. bracteata</i> individuals on the SOM during the dry (Fig. 4A, May 2011) and the flooding period (Fig. 4B, January 2012) according to their spider and ant assemblages. The numbers of <i>A. bracteata</i> shoots were given a null weight during the ordination process, and therefore act as an explanatory variable. In the large map, <i>A. bracteata</i> individuals that are neighbors within hexagons (or output neurons) are expected to have similar spider-ant assemblages, while those separated by a large distance from each other have different spider-ant assemblages. Clusters A–D (May 2011) and A–E (January 2012) were delineated by applying Ward’s algorithm to the weight vectors of the spider and ant species in the various hexagons. Each small map representing the number of shoots or one taxa can be compared to (or superimposed on) the corresponding large map representing the distribution of <i>A. bracteata</i>. They thus show gradients in the number of shoots (Nshoots), the probability of occurrence of each ant species (first line in the small maps), and the abundance of each spider taxa (second and third lines in the small maps) within the SOM (in shades of grey; dark  =  high values, light  =  low values). Codes (e.g., AB9) correspond to individual plants (sampling units).</p

Effects of flooding and contact with <i>Aechmea bracteata</i> on the number of spiders (modeled in a linear mixed-effects statistical framework where the individual samples were set as a random variable).

<p>Points refer to the observed numbers, vertical lines to the model prediction.</p