Category: limnetic

Stickleback camouflage

July 8, 2011 / pcwainwr

This week, the Wainwright blog returns to a topic of perennial interest, the threespine stickleback. I will discuss a recent paper from the Schluter lab at UBC on color plasticity and background matching in stickleback.

To set the stage, it’s important to realize that from a stickleback’s perspective, “bird” is a four-letter word. Predation by diving birds like grebes and coots is commonplace in many freshwater stickleback populations. Unlike predatory dragonfly larva, which detect prey by vision and by water movement, diving birds generally detect their prey by sight alone. In other words, if you’re a freshwater stickleback, it’s very important that the top of your body blends in with your surroundings.

This stickleback didn't get the memo. (http://www.lifeontheslea.co.uk )

In this paper, Jason Clarke and Dolph Schluter tried to assay background matching capability between limnetic and benthic sticklebacks in Paxton Lake, British Columbia. First, they used a spectrometer to record the background color in the limnetic and benthic habitats. The open-water limnetic habitat was a bluish color, but the benthic habitat, which has more aquatic vegetation, tended to be more greenish. Additionally, the benthic habitat showed much more variation in color than the limnetic habitat.

After checking the background color, the authors painted two sets of cups, one designed to look like the limnetic background, and one designed to look like the benthic background. Then they put benthic and limnetic sticklebacks on each background, let them adjust their color for 15 minutes, photographed each fish, then measured how well each fish matched its background. They also did the same experiment again, but this time taking pictures every 20 seconds.

What did they find? Limnetic fish and benthic fish were equally good at matching the blue limnetic background, but limnetic fish were not as good at matching the green benthic background as benthics were. The time trial experiment helped to clear up what was going on: benthics rapidly adapted their colors to match the background, but limnetics were doing something different. Limnetic fish were cycling through different colors instead of fixing a particular color. Limnetics were more variable in color when viewed with a benthic background, but even on their “home turf” in the limnetic background, they still showed variation in color, but to a lesser degree.

The authors suggest that the patterns of color chance exhibited by benthics and limnetics are probably adaptive. Their spectrometer data indicates that the benthic habitat is more variable in color, and their background experiments show that benthics are better at rapidly changing their colors to match the background. The limnetic habitat, on the other hand, is much more uniform, so there would be little incentive for limnetics to evolve rapid color matching. However, limnetics may be adapting to their light environment in an entirely different way: the “flickering” exhibited by limnetics could be an adaptation to fluctuating light intensity in open water.

After reading this paper, I’m particularly curious what the color-matching abilities of the ancestral marine sticklebacks are like. If they resemble the limnetic, then this color matching ability will be another interesting benthic stickleback adaptation. It will be cool to see if it is possible to discern the genetic basis for this shift in plasticity.

Clark JM, Schluter D. Colour plasticity and background matching in a threespine stickleback species pair. Biological Journal of the Linnean Society. DOI: 10.1111/j.1095-8312.2011.01623.x

Stickleblog: 80s stickleback (but without the crazy hair)

June 5, 2009 / pcwainwr

Back in the early 80s, Don McPhail worked on sticklebacks in Vancouver Island, and specifically in some intriguing lakes that had not one but two different species of sticklebacks in them. Ten years later, McPhail and Schluter would build on this research and help to catapult stickleback to the forefront of evolutionary biology.

But for now, let’s go back to the 80s and look at a little paper with big implications.

The Enos Lake stickleback species pair consisted of a benthic species and a limnetic species, though they are sadly no longer with us due to an invasive species introduction. As is generally the case with these species pairs, benthics are larger, with deep bodies and small short gill rakers, whereas limnetics are smaller with slim bodies and lots of long filamentous gill rakers.

However, morphological differences do not necessarily translate into ecological differences, so the authors tested the performance of the different species on its preferred habitat. Three experiments were performed: a prey size trial, a feeding trial on benthic substrate, and a plankton feeding trial.

In the size trial, benthics ate significantly larger prey than either limnetics or hybrids between the two forms. In the feeding trial on the benthic substrate, limnetics and benthics made similar numbers of strikes, but benthics were significantly more successful at capturing prey. In the zooplankton trial, the stomachs of limnetic stickleback contained a much higher number of prey items than than the stomachs of benthic stickleback.

The performance data from these three experiments supports the hypothesis that the Enos Lake stickleback pair does have ecological as well as morphological differentiation, though there are some interesting issues with limnetic stickleback in particular. When the authors allowed female limnetics to feed on the benthic substrate, the sticklebacks did not, though male limnetics fed freely, and made just as many feeding strikes as benthics of both sexes.

A possible reason might be that male limnetic stickleback have to spend time near the benthos to construct their nests, so it would make sense to eat benthic prey items, whereas a female only has to approach the benthos to find a suitable male, and can spend the rest of her time in the water column eating zooplankton.

When Stickleblog returns, we’ll continue our look into the species pairs with a Schluter paper that examines hybrid performance relative to limnetics and benthics.

P. Bentzen, & J. D. McPhail (1984). Ecology and evolution of sympatric sticklebacks (Gasterosteus): specialization for alternative trophic niches in the Enos Lake species pair Can. J. Zool, 62 (11), 2280-2286 DOI: 10.1139/CJZ-62-11-2280

Stickleblog: Sticklebacks (in) rock

May 15, 2009 / pcwainwr

There are millions of sticklebacks across the globe, but you can also find sticklebacks in fossil form. The scientific name for most fossil sticklebacks is Gasterosteus doryssus, but morphologically this fossil “species” belongs within the threespine stickleback complex.

One Miocene fossil site has offered up some fascinating insights into the pace of evolution in threespine stickleback. Today I’ll be focusing on a paper that examines evolution in diet type in this unique stickleback “population”.

A few weeks ago, I mentioned “limnetic” and “benthic” stickleback – two different morphs of freshwater stickleback that live in different places within a lake and eat different things. Limnetic stickleback generally swim in the open areas of the lake and feed on zooplankton like calanoid copepods. Benthic stickleback stay close to the lakebed and feed on insect larva and small crustaceans like gammarids and ostracods.

In an earlier paper, it was shown that you can identify whether a stickleback is benthic or limnetic just from tiny scratches on the teeth. That technique was applied to fossil sticklebacks, with some striking results: at different periods in time, the population changed from limnetic to benthic and back again to limnetic.

Most stickleback in this lake were limnetic, which makes a lot of sense – in order for the stickleback to be preserved in anoxic sediment, the lake had to be fairly deep, which opens up a lot of potential habitat for limnetic stickleback. In addition, the substrate the sticklebacks are buried in is called diatomaceous earth – basically, millions and millions of dead diatoms, a type of phytoplankton. Lots of phytoplankton swimming around suggests there was zooplankton that ate them, which would provide a perfect source of food for limnetic stickleback.

Fossil sticklebacks (photo courtesy of Michael Bell)

So what about the point in time where the population changed from limnetic to benthic? The authors suggest that because of the speed of the change – and because there are few sticklebacks from these rocks that are halfway between benthic and limnetic – it might be the case that the limnetic sticklebacks went extinct and were replaced by a new population of invading benthic stickleback.

Still, even if we can’t say for sure whether the limnetics were replaced by benthics or whether they evolved into benthics, we can say that the benthic population evolved into a limnetic population over a few thousand years, because the pattern of tooth wear changes from the heavy markings typical of a benthic to the lighter markings typical of a limnetic.

It’s rare that we can use fossils to examine how a specific population changes over time, but because we can take our understanding of modern stickleback and apply it to the fossils, we can learn a lot about the dynamics of evolutionary change.

Purnell, M., Bell, M., Baines, D., Hart, P., & Travis, M. (2007). Correlated Evolution and Dietary Change in Fossil Stickleback Science, 317 (5846), 1887-1887 DOI: 10.1126/science.1147337

Stickleblog: Sticklebacks at work

May 1, 2009 / pcwainwr

Today’s Stickleblog deals with a recent paper in the journal Nature by Luke Harmon(a contributor on the blog Dechronization – check it out!), Dolph Schluter, and a number of other folks.

The paper features the threespine stickleback species pairs, which have become a famous evolutionary model system in the last several decades. In a few British Columbia lakes, you can find not one but two different kinds of stickleback – a small slim “limnetic” form that eats zooplankton in open areas of the lake, and a large deep-bodied “benthic” form that eats small invertebrates on the lake bottom.

A lot of work has already been done on the stickleback species pairs, but Harmon and the others took things in a new direction and examined whether these two specialized sticklebacks could affect the lake environment itself — in other words, are sticklebacks ecosystem engineers?

To answer the question, the researchers set up large outdoor tanks using sediment and small invertebrates from an actual stickleback lake. Then, they added fish: one set of tanks received only the limnetic, another received only the benthic, another received both limnetic and benthic, and the last set received generalist sticklebacks from a single-species lake.

The type of sticklebacks added to the tank had an effect on the invertebrate community – if the tank had limnetic sticklebacks(or limnetic+benthic), there were far fewer calanoid copepods. There were also large differences between the generalist stickleback tanks and the species-pair tanks in primary production.

The most striking finding was that sticklebacks had an effect on the clarity of water; generalist sticklebacks had significant more transparent water than any of the other treatments, and species-pair treatments had the least clear water.

A lot more work will be required to uncover exactly how the sticklebacks are producing these effects, but it seems that the difference between one generalist stickleback and an adaptive radiation of two specialist sticklebacks can have important consequences for the habitats they live in.

Harmon, L., Matthews, B., Des Roches, S., Chase, J., Shurin, J., & Schluter, D. (2009). Evolutionary diversification in stickleback affects ecosystem functioning Nature, 458 (7242), 1167-1170 DOI: 10.1038/nature07974