Tuesday, May 5, 2015

Finally! An evasive prey for guppies!

Finding a challenging prey type for the guppies has posed quite a challenge for me!  I have spent the past few months trying to work out the kinks in my data collection, and a big one was finding a prey that guppies like, that is relatively evasive, but that I can also successfully culture in the lab.  To determine differences in feeding between populations of guppies, the prey need to be challenging to capture so that each fish has to work to catch them.  Alternatively, I also want to film them with a prey type that requires a completely different type of capture, so will also be using frozen flake paste spread onto a die that they have to scrape to remove.  But this second prey type was easier to figure out.

Early trial of a guppy capturing brine shrimp nauplii (way too easy!)

In terms of an evasive prey, I have used other, smaller fish as prey in the past, but this doesn't work with guppies since they're not piscivorous.  At SICB I was listening intently to Jeannette Yen's talk on copepod evasive behaviors, and was convinced this is what I needed.  However, at this time all the lakes in N. CO were still frozen, so it was not really possible to check out the local stocks.  So I tried my hand at both freshwater (ordered online) and saltwater (bought locally) copepods, but couldn't get those cultures to last more than a week for some reason.  I needed to try something else.

By this time the lakes had thawed, so my next tactic, because why not, was to gather my undergraduate helper Travis, some plankton sieves, and my kayaks and spend the day at Riverbend Ponds Natural Area to see what we could catch.  Fortunately, we did come back with some plankton (though the water was FREEZING!), and upon inspection under the microscope, saw that indeed it was mostly copepods!  It also had lots of rotifers and daphnia but we tried to sieve out what we could to keep the copepods.  So we put them in our culture bucket, fed them some phytoplankton paste, and let them "soak" for about a week.

Riverbend Ponds Natural Area, Long's peak in the distance

To our surprise, what was a culture of copepods a week earlier was now a culture of Daphnia!  Since it seems to be the only zooplankton I can keep alive, we decided to go with it.  The Daphnia are also larger than the copepods, so they show up on the camera more easily (a plus!).  However, I was worried that they would be too easy for the guppies to catch.  So I gave Travis the task of filming some guppies to see what would happen.

Travis doing some filming
(note the foil to keep the camera from melting!)

To our surprise, the guppies loved to eat them, and appeared calm and willing to eat for us, which was a first!  Even more of a surprise was that, when we examined the video, it looks like Daphnia can evade guppies from a good distance, and so are a relatively evasive prey.  Here's a video (Daphnia at the bottom left):

... and a close-up of the predator-prey interaction:

I'm also working on digitizing a few trials to get some preliminary data.  Here are the locomotor data for the trial in the videos above:

Prey velocity, prey acceleration, predator velocity, and predator acceleration
It's pretty clear from this figure that the Daphnia actually does 3 escape responses, and that the guppy steadily approaches its prey and rapidly decelerates during capture.  I only digitized the first capture attempt so the successful one isn't shown on the graphs above.

Long story short: Daphnia single-handedly rescue my post-doc!