Category: armor plates

Stickleback attack (part 1)

July 29, 2011 / pcwainwr / 1 Comment

Since our last video posting, many of the videos on our lab’s Youtube channel have gone viral. As of this blog post, the video of Inermia vitatta has accrued over 120,000 hits and has been featured on TV programs and newspaper articles around the globe. Not bad for a small fish!

[youtube=http://www.youtube.com/watch?v=psdLbN7skg4]

Today’s video features the threespine stickleback, Gasterosteus aculeatus, feeding on a cladoceran (Daphnia pulex). If you have a short attention span like me, one of the first things you’ll notice from the video is how shiny the fish is. The reflective armor plates and large spines are a clue that this is a threespine stickleback from an anadromous population. Anadromous stickleback have a life history similar to a miniature salmon – they are born in freshwater, travel to the ocean, then return to freshwater to breed. Unlike salmon, anadromous stickleback do not necessarily return to their home stream to breed. Anadromous stickleback also look very similar to each other – an Alaska anadromous fish looks very similar to a California anadromous fish.

Sometimes, these anadromous stickleback will travel to a newly-formed lake or river, and instead of returning to the ocean, some fish will stay in freshwater, founding a new population of freshwater stickleback. Over time, this freshwater population will evolve to better match its new freshwater habitat.

These anadromous and freshwater populations are one of the reasons stickleback are such a good system for studying evolutionary biology. We can study the result of rapid evolution in the freshwater populations, and then turn around and study the anadromous fish that resemble the fish that founded the freshwater population. Studying ancestral and derived populations is one of the few ways – short of a time machine – that we can learn the dynamics of adaptation in natural populations.

If we study how this anadromous stickleback captures prey, and then study how freshwater stickleback catch prey, we can learn a lot about the process of adaptation. I’ve devoted much of my PhD work to studying this system, and I’ll be talking more about it in future posts.

Stickleblog: Caught in the act

May 22, 2009 / pcwainwr / 0 Comments

This week, I’m going to discuss a cool paper that came out of Dolph Schluter’s lab in 2008. The paper zooms in on a particularly interesting part of stickleback evolution, the transition between an ancestral marine form that breeds in fresh water to a population that lives in freshwater year-round.

Usually, (and this is one of the “color-coded for your convenience” things that make stickleback a fantastic model system) you can get a good idea where a stickleback is from by looking at its armor plates. Stickleback from marine habitats tend to have a full complement of plates, whereas sticklebacks from freshwater habitats will have few to no plates:

Stickleback armor plate phenotypes: fully plated (top), partially plated(middle), low plated(bottom)

The authors sorted through hundreds of marine stickleback to find fish that had intermediate numbers of plates, which signified that they were heterozygotes for the gene that governs plate number, Eda. These fish were placed in experimental ponds and allowed to breed. Because the fish were heterozygotes for Eda, they produced offspring with high, medium, and low plates, which gave the authors a chance to observe if natural selection favored the low-plated form in freshwater.

In each pond, the frequency of the low allele increased over time, and in a similar way. There was a slight dip when fish were very young, but then frequency increased until the fish reached breeding condition. Interestingly, fish carrying the low allele grew faster and reached breeding condition sooner than fish carrying the high allele, probably because building armor plates takes energy that could be spent on growing more quickly.

The story is more complicated than that, though – not only is there a period early in life where the high allele appears to be favored, but there is also a point where fish with intermediate plates have the highest fitness, which is difficult to explain. The authors raise the possibility that the Eda gene that controls plates in stickleback may affect other traits (pleiotropy). Either way, it looks like even the most well-understood stickleback phenotype has more to tell us.

Barrett, R., Rogers, S., & Schluter, D. (2008). Natural Selection on a Major Armor Gene in Threespine Stickleback Science, 322 (5899), 255-257 DOI: 10.1126/science.1159978

Stickleblog: What happens when you put a stickleback and a trout together?

April 24, 2009 / pcwainwr / 0 Comments

One of the most striking features of marine stickleback is the row of bony armor plates that run along the side of the body. These “armor plates” are actually enlarged and ossified lateral line scales, and they’re a unique feature of threespine stickleback; other sticklebacks (and tubesnouts) just have a tiny row of lateral scales at the most.

Threespine stickleback, fully armored form
(illustration from Wikimedia Commons)

Freshwater stickleback populations will often have few to no armor plates, which has prompted biologists to look into the both the genetic basis of armor loss and the effect of natural selection on plate number.

In 1992, Canadian ecologist Tom Reimchen published a paper in Evolution that shed some light on the latter question.

Tom captured wild stickleback from a freshwater lake and then put them in an enclosure with one of their chief lake predators, the cutthroat trout. Predictably, the trout would bite the stickleback and try to eat it; whenever a bitten stickleback escaped or was spit out, Tom caught it. The first 153 fish were simply preserved, and the last 143 fish were placed in aquariums and monitored for several days to see if their injuries were fatal.

Then, Tom took a look at what sort of injuries all 296 stickleback had sustained from the trout attack. In particular, were stickleback with more armor plates injured less frequently than stickleback with fewer plates? It turned out that puncture wounds from trout teeth were significantly less common in more armored stickleback.

Top graph: plate number versus puncture wounds sustained
Bottom graph: plate number versus survival

In the second group of 143 fish that had been monitored for survival, over half of the fish died, many of whom did not survive the first 24 hours (for those wondering, Tom did have a control tanks of non-injured fish in the same room – they all survived). Fish with more plates survived significantly longer than fish with fewer plates; in addition, fish with injuries exhibited significantly lower survival.

Taken together, the results suggest that having more armor plates results in fewer injuries sustained from predators, which increases the fish’s chances of survival if it escapes being eaten.

There is one interesting caveat, though: all of these fish would still qualify as “low-plated” freshwater stickleback. Most of the plate variation involved the presence of a few additional plates closer to the head – does this mean that fully-plated marine fish get the same sort of protective benefit from having armor closer to the tail?

Reimchen, T. (1992). Injuries on Stickleback from Attacks by a Toothed Predator (Oncorhynchus) and Implications for the Evolution of Lateral Plates Evolution, 46 (4) DOI: 10.2307/2409768