Hello! I'm a hobbyist-level microscopist, and I've been relying pretty heavily on Free-Living Freshwater Protozoa: A Colour Guide by D.J. Patterson for identification of the protozoa I'm finding. This guide uses a binary key to sort the protozoa it identifies, which isn't my favorite method, but the photographs it has are just great, so I've been pretty hooked on it.
I tend to get tripped up on one of the binary sorting steps, though - and it's an early one, so it affects a lot of my observations. In identifying ciliates, Step 131 differentiates between "Cells that normally crawl against the substrate" versus "Cells that do not normally crawl over the substrate, but either swim freely or are embedded in the substrate." I have a very hard time choosing which path to follow in this key. Most of the ciliates I observe seem to swim freely, with a few obvious exceptions, but that path in the key reduces the possible ciliates considerably. Meanwhile, ciliates that I frequently see swimming freely, such as Stylonychia or Climacostomum, are behind the "crawling against the substrate" branch.
Can anyone who's familiar with the Patterson key help me understand better what distinction is being made here, and how to recognize which path I should follow when looking at a given ciliate on the slide? Thank you!
Question about the D.J. Patterson key
-
Bruce Taylor
- Posts: 1002
- Joined: Fri Sep 04, 2015 11:34 am
Re: Question about the D.J. Patterson key
You've chosen a very good introductory book. David "Paddy" Patterson is a major figure in protist taxonomy (now retired, but still helping amateur microscopists on iNaturalist and the Facebook microscopy page), and his book was ahead of its time in providing good microphotographs of the organisms described. The book has limitations, of course. The taxonomy is a bit outdated, now. And, like any field guide, it can only give a brief sketch of the real diversity that is out there (at the time it was written there were already nearly 7,000 valid species names of ciliates!).
That said, his key is a bit of a mess in some places, and I wouldn't try to follow it too strictly. In ciliatology "crawling on the substrate" usually refers to a style of movement found among spirotrich ciliates (hypotrichs and euplotids) which possess thick bundled cilia called "cirri" that can be used like legs, enabling the organism to walk around like an insect. However, Patterson is using the word "crawl" more loosely, to refer to the habit of keeping close to the substrate while the organism is feeding. So, he includes creatures like Climacostomum and Loxodes, which do not walk but swim along the substrate while browsing for algae, bacteria and other prey.
There are also many long, thin ciliates that can crawl through debris by wriggling like worms. However, this adaptation has evolved independently in many groups of ciliates that are not closely related to one another, so is not too useful for narrowing down an identification.
To add to the confusion, many ciliates that "normally" walk along the substrate are also accomplished swimmers. As you've already noted, when detached from the substrate Stylonychia swims very capably, zipping around in the water with the help of the "membranelles" that surround its "mouth". Most spirotrichs can do the same...and many can also "crawl" by wriggling (worm style).
In short, crawling/swimming isn't a very helpful distinction for a key like this one.
Dichotomous keys (requiring the user to navigate branches with a series of choices) can be very frustrating, especially for beginners. I've discussed keys with Patterson a few times, and he often expresses a liking for a type called a "matrix key," where users narrow down the identification by selecting from a broad menu of characters (like online shopping sites, where you can limit your purchasing options by selecting certain colours, price ranges, manufacturers, etc.). Obviously, that would have been difficult to implement on paper, in the 90s!
That said, his key is a bit of a mess in some places, and I wouldn't try to follow it too strictly. In ciliatology "crawling on the substrate" usually refers to a style of movement found among spirotrich ciliates (hypotrichs and euplotids) which possess thick bundled cilia called "cirri" that can be used like legs, enabling the organism to walk around like an insect. However, Patterson is using the word "crawl" more loosely, to refer to the habit of keeping close to the substrate while the organism is feeding. So, he includes creatures like Climacostomum and Loxodes, which do not walk but swim along the substrate while browsing for algae, bacteria and other prey.
There are also many long, thin ciliates that can crawl through debris by wriggling like worms. However, this adaptation has evolved independently in many groups of ciliates that are not closely related to one another, so is not too useful for narrowing down an identification.
To add to the confusion, many ciliates that "normally" walk along the substrate are also accomplished swimmers. As you've already noted, when detached from the substrate Stylonychia swims very capably, zipping around in the water with the help of the "membranelles" that surround its "mouth". Most spirotrichs can do the same...and many can also "crawl" by wriggling (worm style).
In short, crawling/swimming isn't a very helpful distinction for a key like this one.
Dichotomous keys (requiring the user to navigate branches with a series of choices) can be very frustrating, especially for beginners. I've discussed keys with Patterson a few times, and he often expresses a liking for a type called a "matrix key," where users narrow down the identification by selecting from a broad menu of characters (like online shopping sites, where you can limit your purchasing options by selecting certain colours, price ranges, manufacturers, etc.). Obviously, that would have been difficult to implement on paper, in the 90s!
Re: Question about the D.J. Patterson key
Thanks for the help. Yes, I've actually had more use out of the key by following it backwards - locating the picture that looks most like what I'm looking for, then seeing what questions lead to it to determine if it's a match.
That kind of matrix key would be very useful, and definitely possible in an online format!
That kind of matrix key would be very useful, and definitely possible in an online format!