On page 56, Carroll discusses stickleback fish and their evolution pertaining to armor plates. The oceanic sticklebacks typically have 30 armor plates or more as a part of their body skeleton. However, in freshwater environments, "this number has been reduced to a range of from 0 to 9 plates". According to Carroll, "the selective advantage of plate reduction in lakes and streams may be due to greater body flexibility and maneuverability while swimming".
My question is why would freshwater fish need greater body flexibility and maneuverability as compared to the marine fish? Do the stickleback fish occupy a different niche in the ocean than fresh water? Also, the armor plates provide protection for the fish against predators. Why does flexibility and maneuverability outweigh the advantage of protection in the case of the freshwater sticklebacks?
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The three-spined stickleback would lose its armor plates when making the adaption to the freshwater environment in order to avoid predation, mainly. Armored plates, though they provide protection, slow down the stickleback, which allows it to be preyed upon by both birds and larger fish, which have the same type of plate reduction according to page 570 of Developmental Plasticity and Evolution.
ReplyDeleteThe plate reduction in larger fish allow them to outswim their prey, as the prey would be hindered in maneuvering streams due to the lack of a flexible stem, which their predators would have. Thus, the stickleback shed their armor plates, allowing for them to swim just as fast, if not faster than their predators, or avoid being caught by a bird, in the closed environments which freshwater fish are bound to.
The oceanic stickleback need not fear this threat- out in the open, the oceanic stickleback retains its armor plates for protection, as it doesn't need to be as flexible or maneuverable in the ocean- there would be no impediments in its path while it is being chased by a predator, and birds are less likely to pick off this stickleback, as the shot of the bird actually catching the stickleback would be remarkably low.
Since stickleback fish have no scales, they must have some other anatomical feature that protects them for predators. Bony armor plates of oceanic stickleback fish are needed for protection for many reasons. First of all, the intense parental care of three-spined stickleback fish means that they must be able to protect their young from more powerful fish. Male sticklebacks defend their territories containing the nests with their young, and by protecting their young, the parents have a greater ability to pass on their genes to their children and their children’s children. Also, their courtship behaviors are very aggressive and they need to be able to protect themselves from members of their own species. As well as having a fixed action pattern when seeing the sign stimulus of a red underbelly of other species members, three-spined sticklebacks display aggressive behavior in order to obtain and mate and have a higher chance of reproducing. For these reasons, oceanic stickleback fish need their armor plates.
ReplyDeleteAlthough, as Carroll stated, freshwater stickleback fish have few or none armor plates that their oceanic relatives have. One reason for this is that freshwater sticklebacks have longer and heavier bodies, along with smaller eyes. For this reason, these fish have lost their armor plates in order to have some anatomical feature that helps them catch prey. Heavier bodies and smaller eyes usually give fish a lesser ability to maneuver in the water and find plankton to eat; therefore, by losing their armor plates, freshwater sticklebacks are able to maneuver just as easily as their oceanic form that has shorter and slimmer bodies.
There is also variation between stickleback fish in deeper lakes and shallower lakes: those that live in deeper lakes more resemble oceanic stickleback fish, as their bodies are also smaller and lighter. Those in shallower lakes, as described earlier, have larger and heavier bodies. One reason for this might be that fish in shallow lakes usually feed on the lake bed, where there are usually more plants and fish. Because of this, shallower sticklebacks would need a way of maneuvering more easily in the water, and the loss of armor plates has allowed them to do so. Sticklebacks in deeper lakes usually feed on plankton, which are usually found in more open waters and in large groups. Therefore, oceanic sticklebacks and freshwater sticklebacks that live in deeper waters are able to retain more of their armor plates because they don’t need to be able to maneuver as easily in the water as those that live in shallower waters.
Sources:
http://en.wikipedia.org/wiki/Three-spined_stickleback
The reducing of the plate size of a fish allows for greater manueverabillity, not only in pursuing prey, but also in evading predators. For the latter reason reason, stickleback fish have rapidly evolved to shrink the size of their plates. According to Michael Bell, the reason for plate size reduction clearly is due to the absence of predators. Through experimentation, he is able to confirm these results.
ReplyDeleteThe reason for stickleback fish located in the ocean to not need a reduction in plate size is that in the ocean, there is far more room to manuever, meaning that the fish would not have to fear the possibility of its plates hindering its ability to evade potential predators. Because of this, ocean stickleback were able to retain their armor, which contributed to their defense.
http://www.jstor.org/stable/pdfplus/2410193.pdf