PVIK
Polynomial Visual Identification Key (PVIK)
A smart, modern tool for phytoplankton identification
Identifying plankton can be challenging: many species look almost identical.
The Polynomial Visual Identification Key (PVIK) is a modern digital tool for phytoplankton species identification.
It is based on pairwise classification and coupling – a principle developed by Jerome H. Friedman (1996) and widely applied in
advanced identification technologies, including face recognition and autonomous vehicles. This powerful methodology enables
PVIK to evaluate several morphological features simultaneously and rapidly narrow down the number of potential species.
PVIK has been developed from more than 25 years of professional experience in plankton identification, bringing that expertise into an intuitive digital environment. Although PVIK is primarily designed for professional identifiers, it can also be used by anyone with sufficient taxonomic experience to recognise the groups covered by the key. Species are organised at genus level, keeping the identification process focused and manageable.
New groups are being added continuously, with approximately 15 new groups planned each year from 2026 onwards.
PVIK provides a clear visual overview of morphological size ranges. This is a deliberate advantage: rather than working through a long sequence of text-based choices, you can immediately see how species compare in size and where your specimen fits within the group. Drawings and photos of algae will be added in future versions – don’t worry, they are on the way!
Why use PVIK?
Because it is fast. But why is it faster?
1. Start with a smaller group
PVIK starts with a much smaller number of species than most traditional identification keys. Species are divided into morphological groups that typically contain only 15–20 species.
That makes a considerable difference. For example, traditional keys for Closterium, such as those by Růžička and Coesel, may require you to work through 70 or more species.
Instead of starting with a large and sometimes intimidating list, PVIK gives you a smaller, more manageable group from the outset.
2. Use several characters at the same time
Traditional dichotomous keys are essentially linear: at each step, you are required to choose between two alternatives.
PVIK works differently. You can combine several morphological features simultaneously, making the identification process considerably faster and more flexible.
Select the appropriate group, enter the length and width of your specimen, and then refine the result using additional morphological filters. Within seconds, PVIK produces a shortlist of species that match the characteristics you have observed.
The final identification should, of course, still be verified against the relevant taxonomic literature. PVIK is not intended to replace standard identification literature. Its purpose is to save valuable time by reducing the number of plausible candidates to a manageable few – while also helping you learn and compare species more effectively.
How does PVIK work?
Start at genus level
PVIK begins at genus level. This means that the user needs some basic taxonomic experience: you should already be able to recognise, for example, a Closterium or a Phacus.
Within each genus, species are divided into smaller morphological groups based on easily recognisable characters. For example:
- Closterium species with parallel sides
- nearly round Phacus species
Each group typically contains 15-20 species – still challenging, but manageable.
Enter the size
First, select the appropriate group of algae using the menu in the top right-hand corner.
You can then enter the size of your specimen using the sliders or by clicking directly on the graph. For fine adjustment, you can also use the arrow keys.
In most groups, length is shown on the y-axis and width on the x-axis. There are exceptions where another measurement is more useful for identification.
For example, in the cyanobacterial genus Dolichospermum, the y-axis represents akinete length rather than cell length. This is because akinetes are considerably more informative for identification than the vegetative cells. This approach allows PVIK to combine one of the most useful measurements of the vegetative cell – its width – with the highly variable size of the akinetes.
See the size distribution at a glance
The graph shows the size distribution of the entire group, allowing you to see the ranges of all species simultaneously.
At a glance, you can see:
- species with exceptionally large or small dimensions
- clusters of overlapping species
- where your specimen falls within the overall size distribution
This visual overview also helps you develop an intuitive feel for the size differences between species. Over time, that familiarity makes future identifications easier.
Narrow down the candidates
As soon as you enter the measurements, PVIK generates a preliminary candidate list by highlighting the species that could have that size.
In a matter of seconds, you might go from 20 potential species to just 4 or 5.
Two shades are used to indicate the reliability of the size match:
- Darker shading indicates that the measurement falls within the normal reported range of the species.
- Lighter shading indicates an outlier – a size reported in the literature outside the normal range, usually given in brackets.
Not all species have an outlier range. Such ranges are common in genera such as Closterium and Dolichospermum, where considerable size variation can occur.
You can also choose not to use size as a filter by deactivating “Filter by measurements”. This allows you to see which morphological features are associated with which species, making the tool particularly useful for learning and training.
This option can also be useful when identifying cyanobacteria without akinetes. If no akinetes are visible in your specimen, you can simply switch off the measurement filter and continue using the other morphological characters.
Combine multiple features
The next step is where PVIK really comes into its own.
You can apply multiple morphological filters simultaneously to further narrow down the candidate list – again, within seconds.
Instead of repeatedly going back and forth through a dichotomous key, you can simply select the features you observe in your specimen and let PVIK combine them to identify the most likely candidates.
This is the core advantage of the system: several characters can be considered at the same time rather than one after another.
Learn while you identify
The same tools can also be used for training.
Explore the different morphological features and see which species possess them and which do not. Adjust the measurements, activate and deactivate filters, and explore how the candidate list changes.
In other words: play with the key and learn.
The more you use it, the more familiar you become with the morphological variation within each group – and the easier it becomes to recognise species in your routine work.
Get involved!
The next version of PVIK is now online – please take a look!
Five groups are currently available to try free of charge, and more are being added all the time. You can follow the development on the Groups page.
In autumn 2026, a larger version will become available to subscribers for €36 per year (0% VAT). If you subscribe part-way through the year, you will only be charged for the remaining months.
Is there a group you’d like to see?
Would you like to have a PVIK identification key for a particular phytoplankton group?Or perhaps there is a morphological filter you would like to see added to one of the groups already available?
Get in touch!
I can add your group or filter to the development list, or we can work together to create the next PVIK group.
PVIK – built for identifiers.
Try PVIK and discover a faster, more flexible way to identify phytoplankton.
The app is written by Daniel Blackburn, Startle Company.
