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Geographical variation in morphology: natural selection or random drift? |
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Starry flounder are one of about 550 species of flatfish (Order Pleuronectoformes, Family Pleuronectidae), and are found in the North Pacific from central California to Japan. All flatfish hatch as symmetrical larvae, but then go through metamorphosis during which one eye migrates across the dorsal side of the head and ends up on the other side of the body. This 'eyed side' then becomes pigmented, and the juvenile flatfish lies down on the substrate on the 'blind side', which is usually white. This is a photo of the eyed side of a starry flounder... and this is a photo of the blind side of the same fish. |
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One of the many interesting things about starry flounder is that they are one of only seven species of flatfish that are POLYMORPHIC for which side of the body the eyes end up on. Below are photos of the two forms of starry flounder, left eyed and right eyed. |
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LEFT EYED |
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RIGHT EYED |
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Interestingly, these two
forms are not distributed randomly across the range of the species, but
instead show a
I am using a variety of methods to determine what factors maintain this cline, and am largely interested in resolving how important a role natural selection plays. Comparative landmark morphometric analyses I have found that left-sided (sinistral) and right-sided (dextral) starry flounder are not mirror images of each other. Rather, they differ subtley but consistently among samples in a number of other morphological characters. One of these is the number of gill rakers, which are small cartilagenous extensions from the gill bars that filter out food particles. Across fish species, the number of gill rakers is an indication of the trophic niche the fish occupies: few, widely spaced, short rakers filter out larger food particles and usually indicate a fish that eats larger prey and is therefore higher on the trophic food web, while many, tightly packed, long rakers filter out smaller particles and indicate fish that occupy a lower trophic niche. Consistently along the coast of north American, sinistral starry flounder possess greater numbers of gill rakers than dextral starry flounders, indicating that they may be slightly lower on the trophic food web. Dextral starry flounder also tend to have deeper, rounder bodies and longer, thicker caudal peduncles. Whether these differences are genetic or plastic responses to the environment is still unknown, but they do show that sinistral and dextral starry flounder occupy different ecological landscapes, and therefore may be resonding to divergent selective forces. Rearing experiments to assess heritability and environmental effects on metamorphosis and body asymmetry When investigating the evolutionary significance of variation in phenotype, it is crucial to understand the genetic basis of that phenotype. Preliminary work by D. Policansky in the 1980's showed that heritability of eyed-side in starry flounder is low to moderate. I am replicating his findings but am using a larger number of crosses in a balanced design in order to be able to quantify heritability of this triat more accurately. In addition, I am testing the effect of temperature on the development of asymmetry in this species. During the larval phase and metamorphosis in early spring, there is a marked temperature cline across most of the range of this species. Since temperature affects development of many triats in fish and other taxa, I am raising full siblings in different temperatures in order to tease out the effects of temperature, if any, from the heritability of body asymmetry in starry flounder. Population structure and morphological variation Adaptation to local habitats by natural selection is enhanced when populations are relatively genetically isolated from each other. For this reason, I am investigating the genetic population structure of starry flounder. In collaboration with John Nelson at the University of Victoria, I am using microsatellite markers to estimate gene flow patterns within this species from samples collected from southern Washington state to Pacific Russia. Comparative behavioural experiments I am investigating the ways that sinistral and dextral starry flounder deviate from being mirror images of each other, not only morphologically, but also behaviourally. Preliminary studies show that juvenile starry flounder tend to turn toward their dorsal side when foraging on large plankton. Since the dorsal direction is actually 'left' for dextral starrys and 'right' for sinistrals, this means that these two lateral morphs turn in opposite directions when picking out small prey. Other experiments have shown that they also tend to turn dorsally when faced with a visual predator stimulus. This is interesting because is implies an asymmetry in behaviour during ecological interactions that differs in direction between sinistral and dextral starry flounders. If there are also behavioural asymmetries in either their prey or their predators, this could put sinistral or dextral flounders at a relative advantage in some ecological interactions.
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