aI chose to experiment with the effects of salinity changes on the polychaete, Nereis succinea. Along with the other members of the group, Patty and Jeremy, I was curious to see whether the worms would engage in adaptive behavior when placed in a tank of water of foreign salinity, or whether they would simply continue changing osmotically until they reached equilibrium with the environment.
The first step in our experiment was to simply observe the worms and get a “feel” for the ways in which they act. We did this on Wednesday, May 7, 1997 from 9:30am to 10:30am. Also on this day we learned how to mix and measure salinity, practiced weighing the worms, and deciding our exact schedule as far as when we would come in and for how long, etc.
From what I observed, the polychaete is a salt-water worm that has adapted to live in estuaries. We kept the control tank at 20 parts per thousand to 24 parts per thousand, and the worms seemed very content and healthy at that level. The worms on which we experimented ranged in size from approximately four inches to approximately six inches. They weighed from 1.8 grams to 4.6 grams at the beginning of the experiment. They have a pinkish, almost salmon color to them, and on two opposite sides, they have these crimson hairs lined up in a row, stretching the entire length of their bodies (the hairs are less than an eighth of an inch long). If we were to call the two lines of hair “east and west”, then on the “north and south” sides, there were dark lines that also stretched the entire length of their bodies. These were their primary blood vessels, and though we tried to locate the pulse that is supposed to conspicuously travel up and down this vessel, we were not able to l!
ocate it, except once on one worm for less than 30 seconds. Also I often was not able to tell the difference between the head and the tail.
Their actions were very basic. They seemed to like to stay still for the most part, hiding underneath the little bit of seaweed we put in the tank. We also put a glass tube at the bottom of the tank, thinking that they might try to crawl in there for safety, but we never saw them in there. Basically, they remained very still, except for certain instances in which they seemed to start flailing uncontrollably. They would start swimming around in circles or in figure eights or in some other odd pattern. It was actually quite hilarious to watch. I was not quite sure why they did that, but I guessed that they were looking for something. I later found out that that was true, that they were looking for some sort of protection (like the seaweed).
I made another very shocking and interesting discovery the first time I took a worm out to weigh it. I took it out with a net and put it on a paper towel, and as I was walking to the scale, this “thing” jumped out at me from inside the worm (I literally almost dropped the poor guy!). The only way I can really explain it is if you take a sock and turn it inside-out. The worm basically extended its body by “unfolding” this unknown thing from inside. After the initial scare, I later come to realize that this is called the “reversible probascis” or something to that effect. I learned that the worm uses it to catch small fish when it is hiding in some seaweed. I also observed it later and found little teeth on the end of the probascis. That basically sums up the activity that I noticed.
After observing the worms, I formulated the hypothesis that, when facing a change in salinity, the worms would adapt osmotically to the environment and their volumes would change, but they would not make any efforts to re-adapt back to their original volumes. The reasons I formulated this hypothesis were quite frankly less than scientifically stable. When I looked at the worms, I saw a very basic physiology, and I suppose I figured that a basic physiology like that would be less capable of engaging in re-adaptive processes. I know that this hypothesis was based on a whim, but that is honestly how I came to it. I really do not have an excessively scientific background, so I am not overly aware of all the factors that go into a process like this. So my hypothesis was based on a general conjecture. Also I had heard that some of these worms have a tendency to lacerate under low salinity conditions, so I figured that would not support a re-adaptive hypothesis.
We began the experiment on Thursday, May 8, 1997. We came in at 7:30am to mix the salts and set everything up. The control tank was at 24 parts per thousand. We decided to put three worms (named Goliath, Louie, and Pedro) in 32 parts per thousand and three worms (named Boris, Jenny, and Dopey) in ten parts per thousand. We started weighing at 8:10am. I picked them up with my bare hands (what a stud I am!), Jeremy dried them off with a paper towel and put them in the container on the scale, and Patty recorded the time and weight. We also made sure to dry off the container after every use to make sure that the excess water did not get calculated with the worm’s weight. We weighed all the worms every half hour until approximately 10:45am, when Jeremy and I had to leave. Patty stayed and continued to weigh the worms, but only every hour rather than half hour, because the rate of their changing had begun to slow down. She stayed and weighed the last worm at 1:45pm. Then !
she returned at 4:00pm to weigh them once more. By this time, of the ten parts per thousand worms, Goliath continued growing (he was a whopping 8.2 grams), Louie had leveled off at 3.4 grams, and Pedro was dead. All the 32 parts per thousand worms had basically leveled off.
I came in the next day (Friday, May 9, 1997) and started weighing them at 11:03am. Of the low salinity worms, Goliath popped and died, Pedro was still dead (obviously), and Louie decreased one-tenth of a gram. Of the high salinity worms, Jenny and Dopey remained the same as the day before and Boris decreased one-tenth of a gram.
I then came in on Monday, May 12, 1997, and weighed them at 10:35am. Over the weekend, the last remaining low salinity worm, Louie, looked as if he was dead too. He was all bloody, and the water in his bowl was murky, so I figured he was dead, but then I saw him moving. He was in bad shape but still alive. So I weighed him, and he had decreased 1.1 grams. Of the high salinity worms, Boris was dead, and Jenny and Dopey had continued to decrease in volume (Jenny: -0.3 and Dopey: -0.6). Then I put all the worms that were still alive back in the control tank. I then threw away the dead worms and rinsed out all the bowls. We were planning on repeating the experiment on Tuesday, May 13, 1997, but most of the worms were dead when we got there.
So what happened? The changes in volumes were caused by osmosis. Osmosis is the passing of water through a semi-permeable membrane in order to reach equilibrium. Equilibrium is something that is naturally strived for, and when a polychaete’s body weight remains constant, equilibrium has been reached. When a worm is in a constant salinity, say 24 parts per thousand, the level of solute in the worm’s body is the same as the level of solute of the water it occupies; there is equilibrium. When the worm is removed from that environment and placed in a different one, that equilibrium is no longer present, and by laws of nature, something must happen to re-equalize. That is where osmosis comes in. When we put the worms in the water with low salinity (ten parts per thousand), they increased in volume. This happened because there was more solute on the inside of the worm than in the water. Solute cannot escape the semi-permeable membrane, so the only option is for water to en!
ter the worm to dilute it, to make the solute concentration less dense. When the concentration of solute is the same in the worm and in the water, no more water will enter, and equilibrium will have been reached. In this case, equilibrium was never reached. The salinity was so low that water kept entering the worms, and the worms got bigger and bigger, until they popped, because their epidermis could not expand any further.
The opposite is true of the worms placed in higher salinity. The concentration of solute in their bodies was less than that of the water, so it expelled water to make its own concentration more dense. Again, this happens until equilibrium is reached, and in this experiment, it appeared for a moment as if that occurred, but the worms either died or continued decreasing in volume.
Looking at the data, Goliath met his demise in a very basic way. First of all, he was huge to begin with (4.6 grams), and he just continued increasing in volume until he exploded. Pedro continued increasing, and then right before he died, his weight decreased half a gram. I am not sure why that happened; it is possible that right before he died, he lost some fluid from a laceration. Louie really confused me. For almost four hours (and probably more) on Thursday, Louie remained constant at 3.4 grams. It looked like he had reached equilibrium, and then on the next day, he decreased one tenth of a gram, so maybe he was re-adapting. Then on Monday, he decreased 1.1 grams. So then I figured that he was definitely re-adapting. But I also realized that he was definitely lacerated and very bloody and the water was murky, and I came to the conclusion that he had lost a good amount of body fluid and blood.
As for the higher salinity worms, they basically acted how I suspected them to act. Their volumes continued decreasing. Both Boris and Jenny did have one measurement in which their weights actually increased, and I honestly do not know how to explain that. They all looked at one point as if they reached equilibrium (especially Dopey), but none of them did.
So according to these data that we collected in this experiment, it looks as if Nereis succinea, when placed in an environment with a different salinity, goes through a process of osmosis to reach equilibrium, but does not control processes to return back to its original volume.
I very much enjoyed this project, and I truly, honestly did learn a lot from it (and I’m not just saying that). If I were to do it again, I would not have made the change in salinity so great. It would have been interesting to see what would have taken place if the change in salinity were only, say, six parts per thousand higher and six parts per thousand lower. Maybe next time we’ll do that.