Tier 3 Benthos

Tier 3 Benthos  – Meiofauna

1. Meiofauna abundance and biomass

Interstitial sediment fauna typically termed meiofauna includes  Copepoda, Nematoda, Foraminifera, Thecamoebans, juvenile macrofauna (all taxa), smaller adult fauna typically considered macrofauna (ie; some crustaceans, oligochaetes, polychaetes in particular) Kinorhyncha, Tardigrada, and miscellaneous non-unicellular fauna.  A typical meiofaunal sample may also include large plant-based resting spores or the tests of planktonic organisms.  These latter are not considered to be meiofauna and should not be enumerated in a Tier 3 assessment. 

The Tier 3 methods for SSAMEx recommend (in addition to macrofaunal analyses as described in Tier 1 and Tier 2) abundance and biomass estimates for size fractions of interstitial sediment organisms.  The purpose of this method in the SSAMEx program is to provide estimates of relative importance of the biomass and production contributions from the interstitial organisms to the overall benthic community function. 

For marine environments, Gray et al. (1992) noted that collection of meiofauna or foraminifera is most appropriately performed with core samplers.  However, small cores can be inserted into grab samples with at least 10 cm depth of sediment.  Screening of the sample debris in the field prior to sample preservation is not recommended.  Rather, the small volume samples should be processed in their entirety.  An ~ 150 mL  sub-sample should be extracted from the upper 2 cm of the grab or core, with volume measured carefully.  

Meiofauna require particular attention to careful sample preservation and handling, in order to ensure that biomass estimates are as accurate as possible. Some of these organisms are extremely fragile, particularly the foraminifera. The samples should be preserved directly in 10% buffered formalin.  All samples should be transferred to 70% Ethanol within 2-6 days of initial sampling.  Samples are first rinsed in water (retaining formalin for safe disposal) on the appropriate screen size. Meiofaunal samples are fractioned on stacked sieves measuring 1 mm (to extract macrofauna), 500 µm, 250 µm, and 125 µm. After the sample is thoroughly rinsed on each screen (avoiding sample loss by ensuring each fraction is washed into the sieve below), each fraction is stored separately in 70% ethanol, labelled with the sample number, and stained with Rose Bengal. Rose Bengal staining should be deep pink.

A carefully measured fraction of each sample is examined in solution (wet picked), and specimens counted. At least 300 specimens should be counted and identified from each sub-sample, so that samples with small populations require examination of the entire sample. Both foraminifera and Thecamoebians should be identified. However, Tier 3 for SSAMEx does not describe or recommend formal taxonomic identification past phylum or order for most of these organisms. More detailed taxonomy and analysis should follow Loeblich and Tappan (1988).  The unexamined remainder of each sample is dried. Each dried sample can be sieved into size fractions (125 um, 250 um, 500 um) and archived for future analysis.

Meiofauna Sorting

For the 500 µm and 250 µm fractions, the following 5 major groups are counted (although each group may not be present in all fractions), pulled out with fine forceps, and stored in different vials (in 70% ETOH).

1. Molluscs

2. Annelids

3. Harpacticoid copepods

4. Other Crustacea

5. Foraminifera 

6. Nematodes

7. Miscellaneous (all remaining)

The 500 µm and 250 µm fractions are sorted in their entirety whenever possible. If the count in the 250 µm fraction is >200, this sample may be split. For the 125 µm fractions, this sample is generally split. See ‘splitting’ below.

All samples are sorted with a regular sorting dish (500 and 250 µm) or a Bogorov tray (125 µm). For the 500 and 250 µm fractions, all sorted organisms are removed from the sample and placed in a vial. For the 125 µm fraction, it is not feasible to remove all of the organisms from the sample due to their small size, thus these are counted directly. This MUST be done on a Bogorov tray to obtain accurate counts. However, representative specimens of all taxa must be measured to obtain bio-volume estimates. The number of specimens required is project dependent.

 (c) Splitting

Because of the large number of organisms in meiofaunal samples, splitting is required. The best way to do this is to use a Folsom splitter, ensuring at each step that the samples remain in suspension and splits are as equal as possible.  You will be working with a very small amount of sediment, thus it is of upmost importance to perform the splitting carefully and consistently.  

Each fraction is handled differently:

The 500 µm fraction is processed as a WHOLE. No splitting, regardless of numbers (in general these are likely to be <300).

The 250 µm fraction is split to achieve a count of at least 200 organisms total.   Generally split to ½ to 1/4; retain additional splits so they can be processed as needed to attain a target count of 200. 

The 125 µm fraction is split to achieve a count of at least 200 organisms total (across all groups).  Generally this is also split to 1/8 to 1/16; additional splits may need to be processed to attain a target count of 200.

(d) Archiving sorted sediment

It is necessary that you retain an archived equivalent split for quality control purposes (250 and 125 µm fractions only).  For example, if you process 1/8 of the sample, a further 1/8 should be archived.  You may be asked to process this archived split in the future to ensure the splitting results in accurate estimates of the total numbers.

3. Preservation of organisms

All organisms must be stored carefully, bearing in mind the need to retain these samples for biomass measurements in the best condition possible.  It is important that these samples are stored in appropriate containers (shell vials) and treated carefully to prevent the loss of sample. All samples must be organized by TAXON and SAMPLE so these may be accessed easily for biomass estimates.

All organisms should be stored in 2 mL shell dram vials and placed in a Cornell Tray so that they will stay in an UPRIGHT POSITION to minimize the possibility of the sample drying out.  

Each vial should be no more than half full of ethanol; this makes it easier to retrieve organisms for biomass measurements.

4. Labeling

Be sure to label each vial with the Project Title, Station number, total sample processed (e.g. 1/8, 1/16) and the Major Group (see 7 groups above).

5. Biomass

The contents of each vial (all individuals of a particular group) is weighed on a balance accurate to at least 5 decimal points (hundredth of a milligram).  These must be weighed on non-glass fibre filters (e.g. paper, to prevent static).  Mass is recorded once the reading is stabilized (i.e. excess ethanol is evaporated), and thus represents a wet weight of the organisms.  

For the 125micron samples, biomass may be calculcated via biovolume estimation, by converting length and width measurements into a biovolume (depending on the shape of the organism) and using standard equations (e.g. Giere, 2009).


Gray, J.S., Rudolf Shiu-sun Wu, R., Or, Y.Y. 1992. Effects of hypoxia and organic enrichment on the coastal marine environment. Marine Ecology Progress Series 238, 249-279.

Giere O.  2009. The microscopic motile fauna of aquatic sediments. Meiobenthology. Berlin/Heidelberg, Germany: Springer.

Loeblich, A. R. J., and H. Tappan. 1988. Foraminiferal genera and their classification. Van Nostrand Reinhold, New York.