Behold the sieve.
It is a marine biologist's best friend, saving hours of sorting and enabling us to quantify ecosystems. In fact, you can get these miracle workers at McMaster-Carr, the field biologists' version of died-and-gone-to-heaven, for a mere $40-50. Take good care of these puppies and they will last through several generations of graduate student! I prefer the 500 micron mesh size myself, but usually on top of the 64, because it filters out all those damn limpets (Scheißeschnecke!!).
Limpets always foul things up, and in hydrothermal vent ecosystems, there are A LOT of limpets. In 2002, Governar and colleagues found up to nearly 100,000 of these half-centimeter bastards per square meter in tubeworm clumps at the Juan de Fuca Ridge, which is off of Washington state. Sorting tens of thousands of limpets can be quite dreary and when you find something that is not a limpet, it is often a moment of silent joy.
The sieve size I use at the bottom, though, is the most important. It is my cut off. Essentially, I am saying I'll ignore anything that can fall through this size hole. Ideally, this should be as low as possible, but I'm often limited by what sieve size my colleagues have used in past studies. This is important because our results need to be compared to each other. Any methodological misstep makes broad comparisons about pattern and process less tangible and lessens the context the study is placed within. Thus, interpretations are also limited by sieve.
Gage and colleagues published an important methodological paper in 2002 describing the influence of sieve size on characterizing a deep sea community. The graph to the left, from this paper, clearly shows that biomass, numbers of individuals and numbers of species significantly increase for 2 independent box-core samples based on sieve mesh size. About 20 more species were recovered by winnowing down from 500 micron to 250 micron mesh. Twenty species is not a laughing matter, especially when it makes up nearly 20% of your sample. That could easily make the difference in deciding if your treatments have a significant effect, or not.
In 2009 paper, Pavithran and colleagues took it a step further and asked if it mattered what type of animal was being shaken down the sieve gauntlet. Using a replicated transect of box-cores in the Indian Ocean they looked at the effect a 200 micron difference in mesh size (between 500 and 300 microns) had in characterizing 7 very different animal groups that live in marine sediments: nematodes, polychaete worms, tanaids (a type of crustacean), a family of copepods, isopods, bivalves and nemertines (another worm-like animal).
The authors found the greatest difference in biomass occurred among polychaetes, up to 90% reduction of biomass using the 500 micron mesh, followed by 78% reduction of nematode biomass. But a reduction in biomass doesn’t necessarily mean a reduction in species present between mesh sizes. After all, it could be smaller individuals of a couple predominant species that is retained on the smaller mesh. This was not the case, though, in Pavithran's study. The smaller mesh retained 66 species, while the larger mesh only 40. Additionally, there were nearly twice as many individuals on the smaller mesh sieve. Overall, they saw a loss of 43% of the species from a simple methodology choice alone.
What does this mean for interpretation? As mentioned above, one of the important things in designing a study is make sure your work will be comparable to the work of others. But if other researchers have been ignoring a certain size fraction of the animal community, should you continue to ignore it too? Meiobenthologists would respond very vocally with a NO! In fact, they would argue that the majority of benthic studies are just plain wrong, or at best misleading, since they have ignored a potentially important component of the seafloor. Some of the world’s best nutrient recyclers are in that under 200 micron size class. Potentially half of deep sea species could have been thrown overboard during the last 50 years of intensive deep-sea research! The fact remains, though, there wouldn't have been enough taxonomists to describe them all, especially the among the handful of meiobenthologists.
Gage, J., Hughes, D., Gonzalez Vecino, J. (2002). Sieve size influence in estimating biomass, abundance and diversity in samples of deep-sea macrobenthos. Marine Ecology Progress Series 225, 97-107. DOI:10.3354/meps225097
Govenar, B, Bergquist, D.C., Urcuyo,I.A., Eckner, J.T., Fisher, C.R. (2002). Three Ridgeia piscesae assemblages from a single Juan de Fuca sulphide edifice: structurally different and functionally similar. Cahiers Biologie Marine 43, 247-252
Pavithran, S., Ingole, B., Nanajkar, M., Goltekar, R. (2009). Importance of sieve size in deep-sea macrobenthic studies. Marine Biology Research 5 (4), 391-398. DOI: 10.1080/17451000802441285