Benthic Macroinvertebrate Sample Identification Protocol

The described protocol is only applicable for benthic macroinvertebrate samples that have been previously sorted and or processed. See the NAMC sorting protocol for more information on the sorting process NAMC uses. This protocol is also compatible with any benthic invertebrate study. However; taxonomy should only be performed by accredited taxonomists certified by the Society of Freshwater Science or a similar entity. Proof of NAMC’s taxonomists’ credentials are available upon request.

Required Equipment

  • Dissecting microscope with 10x magnification eyepieces
  • Ocular micrometer (either built into the eyepiece or a separate attached piece)
  • Light source (either built into the microscope or a “goose neck” external fiber-optic light source)
  • Forceps
  • Ethanol (75%-95%)
  • Scintillation vials or similar (glass is preferred, but plastic works, too)
  • Petri dishes of various sizes (2-inch diameter is usually sufficient)
  • Genus/species level taxonomic keys (Merritt, Cummins, and Berg [MC&B] is the most widely accepted resource for freshwater taxonomists. Other, more specialized, keys do exist). There must be a dichotomous key in the book/resource for it to be an accepted resource.
  • Click counter (multiple clickers on one apparatus is most ideal)

Starting a Sample

Begin by transferring the contents of a vial (or whatever receptacle the invertebrates are in) into an appropriately sized petri dish. Ideally, the invertebrates in the vial are all in the same taxonomic order (if following NAMC’s protocol, this would be the case). Place the dish in the focus of the microscope. There are two methods to begin identifying a sample:

  1. Physically isolate the morphologically distinct individuals from one another in the dish and identify each clump of individuals of
  2. Find the first morphologically distinct individual in the dish, identify it, and search for other individuals that match that identification.

The method will depend on each taxonomist and how they prefer to work. Regardless of the method, it is best to identify all individuals of the same genus, then move onto the next genus, to avoid confusion and data entry errors.

Identifying an Organism

Choose the organism you will be identifying. Using the appropriate dichotomous key (whether it is a chapter in MC&B or a specialized key for a certain order), start at couplet number 1. Using the characteristics of the organism, follow the corresponding couplets that describe the organism the best. Note that the first sentence in a couplet is the most important defining feature for the organism whereas all following sentences are less defining. It is imperative that the couplet is followed correctly to avoid misidentifications. If needed, view the cited figures in the couplet that describe certain characteristics. Once an identification has been made, double check the distribution of the organism using either MC&B, a specialized key, or websites like bugguide.net or encyclopedia of life (EOL). Note: websites can be constantly updated and may have incorrect or incomplete information. It is best to rely on published data, such as MC&B, and only use websites for special cases. Move the organism to a new vial filled with ethanol using the forceps. Using the counter, enumerate the total number of this unique organism in the vial. Repeat for all organisms in the dish, then enter the taxonomic data with corresponding counts into a database, spreadsheet, or another data storage interface. Clear the counter, empty the contents of the next vial into the petri dish, and repeat the identification process until the same is complete.

Closing a Sample

Once all vials in a sample have been completed, use Rite-in-the-Rain paper to make an internal label for the final sample vial. Be sure the label includes: sample name, sample ID, subsample (Split) percent, sample date, identification date, and final organism count. Place the vial in a safe place, such as an empty vial box with separators inside. It is best to keep all vials from a sample effort together to avoid conflation with other samples. To further reduce confusion, write the sample ID or another unique value on the sample vial lid.

Exceptions to Genus-Level Identifications

If an individual is damaged or immature, leave its identification at its family or even order level. There are also some taxa, mainly the Corixidae, where a specific gender is required for genus-level identification. If a female Corixid is in a sample, it cannot be identified to genus and must be left at family level. Pupae are also extremely difficult to identify to genus and should be left at family (or even order) level based on their maturity.