I want to study if two intertidal red macroalgae (of genera Bostrychia and Catenella) are likely to be grazed by Hydrobia ulvae (Pennant), the most abundant gastropod in the temperate estuary where they were collected.
I’ve gathered some papers about the feeding behaviour of H. ulvae that mainly focus on microalgae and particles feeding, while other experiments that were performed in red macroalgae used other type of grazers.
As far as I’ve read, palatability can be determined by using dry powdered macroalgae. However, I’m concerned about the different thallus toughness of the two macroalgae, which would be ecologically relevant.
Would you have any suggestions about the experimental design, either in field or under laboratory controlled conditions?
You could do a lab experiment using fresh macroalgae material, with pots with or without the H. ulvae and measuring the weight of your alga before and after leaving the gastropod the opportunity to eat the macroalga. You should look at literature on choice experiment for help on experimental setup, but remember that these experiments will only give you a 'potential' feeding link as feeding choices can vary greatly depending on the total variety of food sources available to the consumers plus many other factors.
If you have money and access to the technique, you could add stable isotope measurements of all sources present (including your macroalgae) and H. ulvae to check if it is possible at all that these H. ulvae rely on these macroalgae in your estuary or not.
I fully agree with Clarie Golléty!
A traditional method would be to use, for instance, 50 mL flasks with aeration with algae, with Hydrobia and with both and control the weight variations during a certain period.
Carbon and nitrogen stable isotopes are also a good choice by themselves or complementary to the weight variation experiments can be used. Don't forget that everyone is what they eat
I think that the isotopic ratio between algae and Hydrobia is the most effective method to test the consumption.
However, I suppose that it is possible try to find some traces of red algae photosintetic pigment (phycoerythrina) inside the stomachs of Hydrobia by chromatography. Take a look at this pubblications:
1) Maccubbin, A. E., Black, P., Trzeciak, L., & Black, J. J. (1985). Evidence for polynuclear aromatic hydrocarbons in the diet of bottom-feeding fish. Bulletin of environmental contamination and toxicology, 34(1), 876-882.
2) Moriarty, D. J. W. (1973), The physiology of digestion of blue-green algae in the cichlid fish, Tilapia nilotica. Journal of Zoology, 171: 25–39.
Thank to everyone for your helping suggestions. I will think about them all.
Best regards
The suggestions to do feeding experiments with the actual algae are certainly a straight forward, low cost way to see if there is any willingness by the snails to feed on the algae. If you have access to a chemistry lab, thin layer chromatography (TLC) with algal extracts and gut content extracts would also be a low cost way to get some additional clues. A more high-tech (chemistry) approach would be to use gas chromatography/mass spectroscopy (GCMS), which would give you greater ability to compare the chemistry.
Raquel, if you perform also fatty acid analysis of both sources and consumer, with a semiquantitative approach (using FA as biomarkers) you will improve your ability to distinguish among the organic matter sources and/or bacteria for Hydrobia
I also use stable isotopes and fatty acids as trophic tracers, these papers might be relevant for you (and are available on researchgate):
- De Troch, M., Chepurnov, V.A., Vincx, M. & Ólafsson, E. (2008). The effect of Fucus vesiculosus on the grazing efficiency of harpacticoid copepods on diatom biofilms. Journal of Sea Research 60: 139-143. --> 13 C labelling of algae and their biofilm
- De Troch, M., Boeckx, P., Cnudde, C., Van Gansbeke, D., Vanreusel, A., Vincx, M. & Caramujo, M.J. (2012). Bioconversion of fatty acids at the basis of marine food webs: insights from a compound-specific stable isotope analysis. Marine Ecology Progress Series 465: 53-67.
There are several approaches to test this (other than the ones already mentioned and referenced). One includes no-choice experiments, in which the gastropods are offered only one food item (e.g., one of the two red algae mentioned, OR another, but not both simultaneously). I would suggest several sets, each set with one gastropod and one of the algae naturally available in the study site. This has the advantage that, once starving, the gastropod will be compelled to feed on those algae, if it can. Also for statistical analysis, the procedure is more simple to analyse. Consumption rates can be weight (it seems to me as not the appropriate, depending on the thallus type and size), area (if the algae are foliaceous), both, or size (for branched algae, it would be a straighforward measurement). However, this approach doesn't reflect the natural environment, where several algae are usually available simultaneously. Therefore you can also perform multiple-choice experiments, in which you offer to each gastropod the whole spectrum of algal species available in the field. This usually results in lower feeding rates per algal species (as opposed to single/no-choice experiments), but provides a more ecologically relevant portrait of the feeding preferences. On the other hand, the results obtained are dependent, i.e., the consumption rate of one alga depends on the consumption of the other, as if a gastropod feeds mainly on one alga, the consumption of the other species will necessarily be lower. Although not with algae, I strongly suggest you read Cueto et al. (2003) Seed preferences by birds: effects of the design of feeding-preference experiments. Journal of Avian Biology 32: 275-8. I hope this helps!
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